Adsorption and Micellar Behavior of Aqueous Ionic Surfactant Systems
1.1 BEHAVIOR OF SURFACTANTS IN AQUEOUS SOULTION
Surfactants are known to play a vital role in many processes of both fundamental and applied aspects1. They have a characteristic molecular structure consisting of a polar or changed head group that possesses strong affinity for water and a hydrophobic alkyl chain that does not. This unique duality towards an aqueous environment leads them to a wide variety of complex self-assembly in the bulk of aqueous solution. 1-9.On the other hand surfactant molecules dissolved in the bulk of the aqueous solution can form monolayers upon spontaneous adsorption at the air-water interface due to their preferential surface active nature. The spontaneous adsorption of surfactant molecules results in an increase the two-dimensional surface pressure with a consequent increase in surface density of the adsorbed molecules. If the delicate balance in the interactions between the hydrocarbon chains and polar head-groups permits, then at a definite temperature the two-dimensional adsorbed monolayer can undergo a pressure-induced phase transition showing a variety of patterns at the air-water interface4. However, high-density condensed-phase formation in adsorbed monolayers sometimes becomes difficult due to electrostatic repulsion, bulkiness as well as strong hydration of the polar head group. In such a case, hydrophobic interactions among the alkyl chains make it more favorable to remain in the bulk of the aqueous solution by forming micellar aggregates when the surfactant concentration attains a minimum value known as the critical micelle concentration (CMC). The CMC is a narrow concentration range over which surfactants show an abrupt change in a number of physical properties1. The occurrence of the CMC results from a delicate balance of thermodynamic forces between the favorable interaction between the hydrophobic alkyl chains and the opposing repulsive interaction between the head groups which depend on various factors such as temperature, dielectric constant of the medium, length of the alkyl chain, presence of additives and relative size and charge of the headgroup.7,8 The formation of micelles and its dependence on different factors such as temperature, additives, dielectric constant of the medium, the extent of counter-ion binding (for ionic surfactants), solubilization etc. are important physicochemical aspects that need detailed and intensive attention for both fundamental understanding and industrial applications. The dominance of the favorable interaction between alkyl chains of the surfactant favors micellization and lead CMC to lower values by stabilizing micelles while the opposing repulsive interaction between the polar/charged head groups disfavor micellization and leads CMC to higher values7.Micelles are known to have an antisotropic water distribution within structure. In other words, the water concentration decreases from the bulk towards the interior of the micelle, with a completely hydrophobic-like interior. Thus, micellar solution consist of special medium in which hydrophobic organic compounds can be solubilized in aqueous surfactant solution, which are otherwise insoluble in water 10-17. However, below the CMC surfactant molecule exist as monomers and have only little or no influence on the solubility of water-insoluble compounds. In other words, micellar solubilization occurs when the concentration is equal to or above the CMC value. Micelle-enhanced solubilization of nonpolar organic compounds is one of the most significant applications of surfactant. It provides the basis for detergency, micellar catalysis and extraction, and formation of microemulsion15. The extent of solubilization depends on many factors such as the structure of the surfactant, aggregation number, micellar geometry, and temperature, ionic strength of the medium and the nature of the solubilizate. The locus of solubilization of poorly water-soluble compounds in micellar systems depends on the polarity of solubilizate. Non-polar molecules are solubilized in the micelle core and substances with intermediate polarity are distributed along surfactant molecules in certain intermediate position15. An increase in surfactant concentration in solution thus increases the extent of solubilization of hydrophobic solutes because of an increase in the number of micelles in the bulk. Studies of the solubilization of poorly water-soluble compounds in non-aqueous and aqueous system have revealed a lot of application in the practical fields such as drug carrier10,13, drug solubilization16, seperation16, toxic waste removal14,17 etc. The solubilizing capacity of a surfactant is usually expressed quantitatively by molar solubilization ratio (MSR). The MSR can be expressed as the number of moles of the substance solubilized per mole of the surfactant in solution17. The potential value of surfactant led to research on their use in drug delivery as drug carrier. Besides, surfactant micelles have been used as model systems for bio-membranes to study the interaction of different compounds including drug molecules with bio-membrane. A better understanding of interactions between surfactant allow for the more rational design and use of surfactants for biomedical application as well as understanding the biological system. Proper micelles, as defined above, do not occur in living systems to a great extent. The hydrophobic interaction, which is the main driving force for the formation of micelles from monomeric amphiphiles, is of fundamental importance for the spatial organization of chemical process in living systems. The basic building blocks of biological membranes are phospholipids. Due to the hydrophobic interaction, these amphiphiles spontaneously form lamellar structures when dispersed in water. Moreover, these extended lamellar structures can rather easily be disrupted so that a globular closed aggregate is formed, a so called vesicle, where only a single phospholipid bilayer can constitute a diaphragm between the outside and the inside water solutions. This might be the most spectacular example of the behavior of biological amphiphilic substances but the hydrophobic effect is of great importance in a number of other cases as for instance in determining protein conformation. A careful study of micellar solutions is one way of attracting the general problem of the hydrophobic interaction. This for example, the point of view adopted by Tanford in his book “The hydrophobic effect”18, where one can find a lot of additional cases where the hydrophobic interaction plays an important role in biological processes. In this short survey we will only point out some of the specific biological implications of the different aspects of micelle formation that will be presented in the upcoming sections. The general thermodynamic principles guiding the formation of micelles are equally valid for membrane formation or protein folding. There are good reasons to believe that the model expressions for the chemical potential developed by Tanford19. This model is valid also for phospholipid system. An interesting possibility would be to try describing phospholipid mixtures where phenomena like a lateral phase separation might occur. Much effort has been devoted to determining the physical state of the alkyl chains in the membrane bilayer. As for micelles, one normally finds a typical liquid like interior, which makes rapid molecular processes possible within the bilayer structure. However, with some phospholipids one can possibly have more solid-like structure under certain physiological conditions. It seems also settled that the interior of large globular proteins has some liquid-like properties and it is not as rigid as one might infer from the x-ray diffraction structure determinations. The process of solubilization is of tremendous importance for a number of physiological and pharmaceutical phenomena. It is well established that many types of membranes has a high content of solubilized cholesterol. The role of the cholesterol in the membranes is not clear. One relevant aspect of a nonpolar pharmaceutical substance is its non-specific ability to be solubilized in a membrane which is a complication that has to be considered when discussing physiological effects on the basis of studies on model systems. As for micelles, the hydrations of pure phospholipid bilayers do not seem to extend beyond the polar head-groups. Consequently, such a bilayer constitutes an effective barrier for transport of polar substances in general and ions in particular. A transport through the bilayer can be made possibly by the use of a carrier or by the formation of a hydrophilic channel.It seems that ionic interactions are of considerable importance in controlling the functioning of biological membranes. For example, divalent cations as Mg2+ and Ca2+ have several important regulatory effects. Transport protein can bind electrostatically to the membrane surface which always contains some charged groups. It is clear that a study of ion binding properties of amphiphiles have important implications for these phenomena. The survey of biological implications of micelle formation is only to give some ideas to interpret studies regarding the present findings in wider range of area.
1.2 TYPES OF SURFACTANTS
Surfactants are amphiphilic compounds with well-segregated polar and a polar domains that have measurable aqueous solubility as both aggregates and as monomers. Surfactants belong to a class of compounds that reduce interfacial surface tension (in oil, water or both) by adsorbing to interfaces. The ability of a surfactant to participate in a specific biological/biochemical function is related to its structure; the polar hydrophilic portion of the surfactant molecule is referred to as the “hydrophilic head group” while the nonpolar hydrophobic, portion is referred to as the “tail” (Figure 2).The chemical com-position of surfactants can vary greatly as alterations can be made to either the hydrophobic “tail” or hydrophilic “head” depending on the desired application. Surfactants
are generally classified by the nature of their head group and the main classes include anionic, cationic, zwitterionic (amphoteric), non-ionic, and combinations of the above. A summary of the main classes, some examples, and their uses can be seen in Figure 1.4.
Figure1.1: Surfactants in the aqueous medium
1.3 MICELLE
The solubility pattern with respect to solvent properties of a non-polar compound like alkane is in sharp contrast to that of a charged or otherwise strongly polar chemical species. If these two features occur simultaneously in the same chemical entity, interesting comprises are observed. For aqueous solutions, one well known situation is that the polar group is located in the solution while the nonpolar part seeks to avoid the aqueous environment by stretching into the gas phase or into an adjacent non-polar liquid phase. Except for this adsorption at gas –liquid, liquid-liquid or liquid-solid interfaces there is an alternative possibility to avoid the unfavorable contact between non-polar groups and water and between polar groups and non-polar solvent, i.e. by self-association into various type of aggregates. The term micelle is introduced by the pioneer in the field J.W McBain in 1913 to describe the formation of colloidal properties by detergents and soaps. The word “micelle” has also been used in biology and in colloid chemistry for other phenomena. Important features of the micelle are the high aggregation number and effective separation of hydrophilic and hydrophobic part. It was established at an early stage that micelle formation displays peculiar concentration dependence. Thus at low concentration an aqueous ionic surfactant solution behaves essentially as a strong electrolyte. On the other hand, an increased amphiphile concentration leads to a corresponding increase in the amount of micelles while the monomer concentration stays roughly independent of the total amphiphile concentration. The quite pronounced change in the concentration dependence of a large number of properties in the region where micelle formation starts and it is called critical micelle concentration (CMC). There are two common approaches to the theoretical treatment of amphiphile aggregation. In one, the so-called phase separation model, micelle formation is considered as analogous to a phase separation. The CMC is then the saturation concentration of the amphiphile in the monomeric state and micelles constitute the separated pseudo-phase. According to other approach, the equilibrium model, micelle formation is treated analogous to a chemical equilibrium. There is now general agreement that the equilibrium model provides a correct description of micelle formation. Analysis of the equilibria shows that for the cooperative formation of large aggregates, the onset of micelle formation effectively takes place in quite narrow concentration range. This observation makes the term CMC from a practical point of view since it gives an approximate figure well characterizing the self-association pattern of ascertain amphiphile. Although due caution must be exercised in making use of CMC data, their variation with various factor such as alkyl chain group, polar head-group, counter-ion, added electrolyte etc. needs considerable attention and has been most illuminating for acquiring of an understanding for several aspects of micelle formation.
| Sodium dodecylsulfate (SDS)
(Anionic) |
| C16TABr – cetyltrimethylammonium bromide C16H33N(CH3)3Br
(cationic) |
| An <href=”#13″>amine oxide
(Zwitterionic) |
| Polyoxyethylene (4) lauryl ether (Brij 30)
(Nonionic) |
Figure 1.2: Various types of surfactant and corresponding head-group (red color) and carbon tail(blue color)
1.4 THERMODYNAMICS OF MICELLE FORMATION
The occurrence of the CMC results from a delicate balance of intermolecular forces. The interplay among these forces is also responsible for the structural organization in living systems as for example, in bio-membranes. A thermodynamical description of micellar solutions has thus much wider implications than an understanding of the micellar system itself. As micelles are formed by readily available, easily purified and usually well-defined chemicals they are suitable model systems in experimental investigations of hydrophobic and hydrophilic effects. Different theoretical treatments can then be tested against experimental quantities like free energies of formation or CMC values, heat of formation, micellar size and shape and their variation with temperature. The effect of additives such as electrolyte or non-polar substances accounted for a theoretical description. The first attempt of a quantitative treatment of micellization was made by Debye20,21. Although quantitatively incorrect his work becomes a starting point but for refinement, four different approaches have been used in the thermodynamic description of surfactant aggregation. These are (pseudo) phase separation model, the mass action model, the small system model and the multiple equilibrium models. The two former approximations as such while the two latter can be used in a rigorous description.
1.4.1 Thermodynamic Models
1.4.1 (a) The phase separation model
A number of properties such as osmotic pressure, surface tension, equivalent conductivity show a change in concentration dependence around a particular concentration called the critical micelle concentration. This behavior resembles very much what one finds for a transition into a two-phase region. This suggests that one might treat the micellar solution as a two phase system where the CMC is the concentration where the system enters the two- phase region. This is the so-called phase separation assumption. Although micellar systems are one-phase systems, the micellar association is cooperative to the extent that the phase separation model can be very useful. Through this approximation one renounces the possibility of describing properties of the micellar aggregates such as size and shape but the model is often important for the conceptual understanding of micellar systems. It is also very convenient in a quantitative analysis of the variation of molecular properties with concentration. When the micelles are regarded as a separate phase the chemical potential of the surfactant in the aqueous phase is
………………………………………………………………..(1.1)
Where expresses standard chemical potential, denotes the activity coefficient and the monomer mole fraction. At a certain critical concentration the chemical potential in the aqueous phase equals that of micellar (pseudo) phase
…………………………………………………………………(1.2)
and a phase separation occurs. The critical concentration then identified as the critical micelle concentration, Xcric=CMC. Below the CMC only monomers and possibly non-micellar aggregates exist, while above the CMC the concentration of non micellar molecules is constant. This result has important consequences. The mean value of any molecular property should vary linearly with concentration above the CMC and this should also be the case for the solubility of additive. These predictions are experimentally found to be only approximately correct. The discrepancies occur mainly for two reasons. At the CMC there is no true phase change and molecular properties change more gradually without a discontinuity in the rate of change. Furthermore, the monomer activity does not stay quite constant above the CMC and this leads in some cases to important changes in micelle size and shape with concomitant change in molecular properties.
1.4.1 (b) The mass action model
In the case of ionic surfactants the equilibrium model is preferable because it is possible to take in consideration, in an explicit way, the effect of the counter-ion dissociation. The equilibrium model considers that the micellization process can be described by equilibrium between monomers, counter-ions, and mono-disperse micelles. In the case of a cationic surfactant this equilibrium can be represented by
……………………………………………………………(1.3)
where S+ represents the surfactant cations, C?the corresponding counter-ions, and Mp+ the
micelle formed by n monomers with an effective charge of p. The standard free energy of micellization per mole of surfactant, , is given by
………………………………..(1.4)
Where a is the activity of the respective species. For large n values the first term of the parenthesis is negligible and both and can be replaced by the activity at the CMC. Moreover, since the micellar formation occurs in dilute solutions, the activity can be replaced by the surfactant concentration (expressed in mole fraction) at the CMC. Considering these approximations, Eq. [1.4] can be expressed as
………………………………………………………….(1.5)
Where ( = p/n) is the degree of counter-ion dissociation. If the change in with temperature is small, as occurs in our case over the temperature range studied, enthalpy yields
……………………………………………………(1.6)
In this way, the enthalpy of micellization can be evaluated from the slope of a tangent to a plot of ln(XCMC) versus T at a particular temperature. In all the cases the best fit can be found to be a second-order polynomial. In addition, once and have been obtained, the entropy of micellization can be estimated from
………………..……………………………………………………..(1.7)
In the mass action model one has a description of the system through only one parameter and yet a smooth transition in the CMC region.
1.4.1(c) The multiple equilibrium models
The obvious extension of the mass action model is to introduce aggregates of different sizes which are in equilibrium with each other. These multiple equilibria can be formally written in two equivalent ways. Either one has a stepwise growth of the micelle according to the scheme
M1+Mn-1 Mn n=2, 3……..……………..……………………………………(1.8)
Or one can regard each aggregate to be formed directly from the monomers
nM1 Mn, n=2,3…………………………………………………………(1.9)
For the first equation the equilibrium constant is
Kn= ………………………………………….(1.10)
And the aggregation process is determined through the values of the constants Kn. An alternative formulation is obtained by writing the chemical potential of the aggregate Mn as
+kT ………………………………………………………………..(1.11)
Where is the standard chemical potential per monomer in the micelle. The chemical potentials of the monomer in the micelle and in the aqueous solution are equal at equilibrium and from eqn(1.1) and (1.11) for all n
……………………………………………..(1.12)
The mole fraction of aggregation n is
n/ ……………………………………………….(1.13)
Together with the expression for the total concentration S of surfactant molecules
……………………………………………………………………………….(1.14)
Equation (1.13) determines the size distribution in the micellar solution. The eqns. (1.10) and (1.12) are related through
……………………………………………………….(1.15)
Depending on the actual application either eq. (1.10) or (1.12) is the most convenient one to use in a description of surfactant aggregation to micelles. No distinction between ionic and non-ionic amphiphiles has been made by this model. For the ionic amphiphiles it is possible to include the counter-ions explicitly in the chemical equilibria. The number of chemical species is then greatly enhanced, which makes the approach less useful. Instead the counter-ions are usually regarded as members of the ion cloud surrounding the charged micelle. Changes in ion binding have then to be accounted for through changes in the activity coefficients or in the standard chemical potentials.
1.5 MICELLE AS A MICRO-SYSTEM
When one wants to account for the concentration dependence of micellar properties the phase separation or mass action model are usually sufficiently accurate, but as soon as changes in micelle size and shape have to be accounted for one must introduce refinements in the models. The phase separation model can be extended into a formally rigorous framework. The method was developed by Hill22and called thermodynamics of small systems. In this approach, the micelle is regarded as surrounded by a bath which defines the so called environmental variables. Hill showed that the maximum number of independent intensive variables is more for the small system than for a corresponding macroscopic system. Micelles of different size are in dynamic equilibrium with each other and for such a case the relevant intensive environmental variables are the temperature, T, the pressure, P and the chemical potential of the monomers in the aqueous solution . These intensive variables determine properties like micelle size distribution. However, the work by Hall has not been followed up by other workers in the field and it seems that the small system thermodynamics involves an unnecessarily complicated formalism for most applications.23
1.6 LITERATURE SURVEY (FACTORS RESPONSIBLE FOR MICELLE SELF-AGGREGATION
1.6.1 Hydrophobic Interaction
One of the important features that make water unique as a solvent is its response to a-polar solutes. These have low solubility in water. These have low solubility is caused mainly by entropy effects24. This suggests that the cause of interaction between a-polar molecules and solvent water is to be found in peculiarities in the structure of liquid water. The tendency for a-polar molecules or molecular fragments to avoid contact with water is said to be due to the hydrophobic interaction, which thus gives rise to a thermodynamic force rather than a mechanical force. The hydrophobic interaction has been extensively studied and for recent survey of the subject one can consult a review by Franks24.The mechanism of surfactant self-assembly has been studied extensively. However, it is still unclear. From a thermodynamic point of view, surfactant self-assembly is entropy driven process. When temperature is increased, entropy of water is increased due to the destruction of structured water around the hydrophobic tail and entropy of surfactant is decreased a little compared to the water. Even though it is an endothermic process, the free energy of the whole process is negative which suggests micelle formation is a spontaneous process. Generally, the water molecules are arranged in an ordered way around the monomeric units of micelles, which can be defined as ‘iceberg’. During micellization, due to the destruction of the iceberg a positive entropy change occurs. Despite this micellization-favoring phenomenon, a negative change can occur if the ordering of the randomly oriented amphiphile molecules from the solvated form into a micelle structure is more pronounced than disordering effect due to the destruction of icebergs around the alkyl chains. At the same time, the motion of the water molecules bound to the hydrophilic heads become more restricted, contributing to the decrease in entropy.
Figure 1.3: Micelle formation from monomeric surfactants
1.6.2 Hydration
Water has several peculiar properties like a density increase on melting and a high boiling point and these are now rather well understood on a molecular basis. In liquid water, the fraction of broken hydrogen-bond is rather small and the water-water distances are only slightly longer than that in ice. The coordination number is slightly larger than four and the coordination is approximately tetrahedral. The structure of liquid is a very open one and according to the X-ray scattering studies25 it is closely similar to that of ice but with some non-hydrogen bonded water molecules in interstitial positions. Due to its highly structured nature, water as a solvent displays a very complex behavior. Thus in addition to direct ion-molecule interactions also the effect of a solute on the hydrogen bonded network is of paramount importance. In the present context it is most significant to note that non-polar solutes have particularly great influences on water structure. Thus alkyl groups markedly reduce both the rotational and the translational mobility of the water molecules26 and marked “Structure-stabilizing” effect is evident also from a large decrease in partial molal entropy and an increase in partial molal heat capacity on introduction of alkyl containing solutes24. The entropically unfavorable solution of nonpolar molecules or group in water has been termed “hydrophobic hydration” to distinguish it from enthalpy-driven process due to ion-dipole interactions and hydrogen-bonding. (Hydrophobic interaction leading to a gain in entropy is the partial reversal of the hydrophobic hydration) The exact structural nature of the hydrophobic hydration is known but a location of the solute in “holes” in the open solvent structure in an often discussed idea which is supported by negative excess partial molar volumes(-15 cm3 per mole of CH2 groups) 24. Crystalline hydrates27 of many non-polar compounds show a striking stability even for high hydration numbers, X-ray diffraction studies have established their structure to be of the clathrate type, with the solute surrounded by a layer of hydrogen-bonded water molecules forming, for example, pentagonal dodecahedra. Thus even if the detailed structure is not presently established, it can be assumed that alkyl chain of an amphipile monomer in water is surrounded by a hydrogen-bonded organized water layer. Except for thermodynamic observation24, this is supported by observations of large effects of surfactant molecules on the molecular motion of water (rotational and translational) below the CMC28,29. The polar heads of the monomer interact with water in away similar to simple polar solutes and electrolytes through hydrogen-bond, dipole-dipole and ion-dipole interactions. It is evident that these hydration features are affected when the amphiphile enters a micelle but the question of the extent and nature of the changes has been given rise to considerable controversy in recent year. It is clear that an understanding of micellization must involve a detailed geometrical description of the hydration of the different parts of the micelle, but it will be seen that merely a global hydration number can be quite informative in eliminating certain of the hypotheses advanced. The concept of a single hydration number to describe the solute-water interaction is of course a simplification and it is well known that the definition of the hydration number is dependent on the experimental approach30. A suitable definition for the present purpose is to take the micelle hydration water number as the number of water molecules moving with the micelle as a kinetic entity in solution. Then the hydration number can be determined from the transport properties, e.g. viscosity data is given by Mukherjee31. The procedure involves determination of the intrinsic viscosity and comparing it with the partial specific volume of the amphiphile. The micelle hydration numbers per amphiphile obtained by Mukharjee were 9 for SDS and hydration number 5 for CTAC and TTAC. A similar procedure was used by Ekwall and Holmberg32 to obtain a hydration number of 8.5-8.9 for sodium octanoate micelles and by Courchene33 to obtain hydration number 10 for dimethyldodecylamineoxide micelles. Tokiwa and Ohki34 used viscosity data as well as combined sedimentation and diffusion data to obtain micellar hydration numbers. For SDS, hydration number 8 was combined while for a series of sodium dodecylpolyoxyethylene sulfates, the hydration number increases strongly with the number of oxyethylene units. The study of the water self-diffusion coefficient as a function of micellar concentration is another efficient way of obtaining hydration number and in this way the hydration number was determined to be 8.7 for sodium octanoate micelles29. The hydration numbers given are somewhat approximate and are affected by sources of error in the evaluation but it seems that any correction should lower these numbers which can therefore serve as rather reliable upper limits. As noted by Mukherjee31 these values are smaller than those estimated for a uniform monolayer of water at the micellar surface. Mukherjee also observed that the hydration numbers can be approximately understood in terms of hydration of the bound counter-ions and the polar heads alone. Water contact of the alkyl chain is not suggested by these data. On the other hand, partial molar volume data were taken to imply that also the ?-CH2 group adjacent to the polar head is in contact with water39 but this interpretation has been criticized36. Hydration of non-ionic surfactants has been studied to a small extent; one reason is that difficulty in studies hydrodynamic parameter for systems where micellar size and shape are so sensitive to temperature and concentration effects. Important contributions are due to Elworthyet al37-39 who performed viscosity and water vapor pressure studies of solutions of the compound. Hydration was found, firstly according to expectation to increase with the number of oxyethylene groups and secondly, in contrast to what is often stated in the literature, with the temperature. However, in regard to the latter point information is rather sparse. For lamellar liquid crystals of water and CH3(CH2)8C6H4(OCH2CH2)xOH (x=6 or 10) the observation of deuteron quadruple splitting which change only little with temperature40 suggests an increased hydration with increasing temperature since the order parameter is expected to decrease markedly. For lecithin mesophase a marked increase in hydration with increasing temperature has been observed41.There are also various spectroscopic methods for the study of amphiphile hydration. Deuteron quadruple splitting studies may provide information on the number of water molecules influenced in their orientation by the amphiphile aggregates in liquid crystals. For the lamellar phase of the systems alkali octanoate-decanol-water, for example, at most about 5 water molecules per octanate are appreciably oriented42. The order parameter of hydration water indicates a high mobility; correspondingly the water molecules at the surface of a micelle are certainly quite mobile. Proton NMR chemical shifts and relocation rates of water in sodium alkyl sulfate solutions change at a lower rate after the CMC than before demonstrating the decreased hydration43,28; however, conclusion are difficult to make. Various spectroscopic method, and in particular magnetic resonance techniques, should in principle be helpful in this respect but hitherto few adequate studies have been presented. Proton NMR of the alkyl chain suffers very much from the low resolution and the small chemical shift range but a study at 220 MHz of proton chemical shifts and relaxation times by Podo44 on some non-ionic polyoxyethylene containing compounds was informative. Thus the alkyl groups were found not to be in study by Clemett45 of n-decylpentaoxyethyleneglycol mono-ether led to the same conclusion while proton chemical shifts were interpreted differently46. Concluding our discussion on the hydration of micellized amphipile and the degree of water penetration we may state that the polar head-groups certainly are hydrated although to a varying extent and there seems to be no evidence for any water inside the micelle core formed by the alkyl chains. It is safe to assume that there is only a small water penetration beyond the ?-CH2 group and there is furthermore, no good experimental demonstration of a marked contact between these groups and there is furthermore, no good experimental demonstration of a marked contact between this group and water. Even if the ?-CH2 water contact thus seems to be small some must probably occurs solely for geometrical reasons. Turning away from the micellar field it is easy to find strong support for the adopted view of a negligible alkyl chain-water contact besides the low solubility of water in hydrocarbon. Thus very low water content in the non-polar region is demonstrated by the slow passage of water through lipid bilayers47 or between reversed micelles48.
1.6.3 Counter-ion Binding
Although the formation of micelles from ionic and nonionic surfactants is qualitatively similar, there are important quantities differences. For nonionic amphiphiles the micelles form at much lower concentrations and hence a larger tendency to aggregate compared to ionic ones. The size of ionic amphiphile micelles increases on addition of electrolyte and if affected, decreases on increasing temperature. For non-ionics, on the other hand, the micelle size relatively unaffected by added electrolytes. These differences are basically due to the importance of electrostatic interactions, for which a quite detailed picture of counter-ion binding is required. We can estimate the gross number of counter-ion binding to the micelle, so we also need information on geometric features of the counter-ion binding, on modes of interaction, and ion specificity effects, on counter-ion hydration etc. In the case of counter-ion binding to micelles, there is unambiguous distinction between bound and free counter-ions. Instead, the counter-ion concentration as a function of the distance from the micelles show a gradual decrease in going outwards with no well-defined transition point. There are a large number of experimental methods which are useful for studying counter-ion binding to micelles such as freezing point depression, vapor pressure lowering, and change of CMC with salt addition, electrical conductance, ion activity measurements, light scattering and self-diffusion49,50 . Different methods may make quite different distinctions between free and bound counter-ions and it is of no surprise, therefore, that data on counter-ion binding from different types of studies may be very different. This is especially well exemplified in the study of Mukharjee51. On the other hand, trends in counter-ion binding with ionic radius are faithfully reproduced by most experimental approaches. It appears that methods where one monitors directly the counter-ion itself should be advantageous and one such method which has been found to work well is based on the translational self-diffusion coefficient of the counter-ion29. Comparing this with the micellar self-diffusion co-efficient gives the degree of counter-ion association, provided the self-diffusion coefficient of the ions not bound is known. The value so obtained corresponds to the number of counter-ions moving with the micelle as a kinetic entity. Such a definition has been found to be most useful in the theoretical treatment of counter-ion binding to micelles. Among the results obtained, it can be mentioned that ? close to 0.6 for sodium ion binding to a number of anionic surfactant micelles. For CTAB, ? is 0.7 and both in this case and in the case of sodium octanate, a slight increase in ? with increase concentration was noted. Except for this, the invariance of ? is notable and the similarity of ? for different cases is also evident from other types of studies.Mukharjee51 has inferred slightly larger degree of dissociation of Li+ than of Na+ from dodecylsulfate micelles but otherwise there is little information available on ion specific effects on the micellar charge. It has been reported51that the CMC of alkali dodecylsulfates increases with decreasing atomic number showing that the counter-ion interaction follows the effective radius of the hydrated ion. For tetraalkylammonium counter-ions, CMC decreases with increasing ion size and this was attributed to a balancing of hydrophobic and electrostatic interactions. For the carboxylate end group there is no systematic study available on the variation of CMC with counter-ion. In general, counter-ion specificity is more pronounced in the case of cationic surfactants than in the case of anionic ones, and this can certainly to a great extent to be explained by a weaker hydration of typical counter anions. CMC of n-dodecyltrimethylammonium salts follows the sequence NO3?<Br?<Cl??and this sequence persists in the presence of added sodium salt of the anion52. Likewise the CMC of CTAB is considerably lower than of the corresponding chloride (CTAC).By using ion-specific electrodes, Larson and Magid53showed NO3? to displace Br? from the micelles in CTAB solution. A peculiar difference between micelles of CTAB and CTAC is that at room temperature a pronounced transition to very long micelles takes place in the former case while the micelles remain approximately spherical up to the highest concentration in the latter case54,55. Similarly, a marked viscoelastic behavior can occur for the same amphiphile with certain anions already at low concentration but has been observed over a wide concentration range for others56. In solution containing both CTAB and CTAC, where spherical and long rod-like micelles seem to co-exist, Br? ions show preferential binding to rod-like micelles and Cl? to spherical ones57. For cationic amphiphiles, counter-ion specificity is also indicated in phase diagram58 but systematic studies of the counter-ion dependence have not yet been reported. Because of the possibility of charge transfer, interactions between polar head and halide ions, ion specific interactions can be expected. In addition, CMC varies appreciably with counter-ion by the following order I?<Br?<Cl?59 and the same sequence applies to counter-ion dissociation60. Charge transfer complexes of dodecylpyridium iodide micelles were examined spectroscopically by Mukharjee and Ray60-62 who also discussed in detail the specificity of the counter-ion adsorption. The size of hexadecylpyridium micelles is very sensitive to the anion of added salt, aggregation being promoted according to the sequence F?<Cl?<Br?<NO3?<I?63. Decreasing CMC with increasing counter-ion size for dodecylsulfate51 and tetradecylpyridium salts as well as an increased surfactant ion residence time in the micelle64 are two types of the observation pointing to a micelle stabilizing effect due to surfactant counter-ion and hydrophobic interactions. Fundamental to our picture of counter-ion binding to micelles is the knowledge of whether counter-ions retain their hydration sheaths or not on binding. Mukharjee65 concluded from partial molar volume data that it is the interaction of the hydrated alkali ion with the micelle retains low water contents in surfactant system. An early 23Na NMR relaxation study66 indicated that the Na+ ion retains its inner hydration layer down to quite low water contents in surfactant systems. Also the marked counter-ion specificity observed in certain cases is difficult to understand if the counter-ion retained completely their hydration layer. It is thus possible that at least Br– and I– ions become partially dehydrated when bound to micelles but there exists certainly no conclusive evidence. Stigter67and Mukharjee68considered that Cl? anion retains its hydration water on binding to micelles. To find out a theoretical description of ionic interaction in micellar solution, the contributions of Stigter67, 69, 70 are most important. A natural starting point is to make use of electrical double layer theory approximating the micelle as a uniformly charged sphere with the counter-ion forming a surrounding Gouy-Chapman diffuse double layer. The distribution of counter-ions and the values of thermodynamic quantities can then be deduced by using the Poisson-Boltzmann equation. To eliminate the Gouy-Chapman approach, Stigter67 introduced a more detailed model involving a stern layer inside the shear surface in addition to a diffuse Gouy-Chapman approach; Stigter67introduced a more detailed model involving a stern layer inside the shear surface in addition to a diffuse Gouy-Chapman layer outside the shear surface. For the stern layer, the discrete nature and size of the counter-ion and of the ionic head-groups are taken into account, and furthermore, the possibility of specific counter-ion adsorption is introduced. In treating the specific adsorption energy, account was taken for “image forces” resulting when the counter-ion approaches the micellar core with its low dielectric constant67. Image forces can also well rationalize the variation of CMC with charge distribution of the polar head in decyloyridium bromides71. Stigter69,70 has developed a more detailed model of the stern layer and discussed the distribution therein of head-groups and counter-ions using lattice and cell theories of two component system. Although the theoretical work has added much to our knowledge of electrostatic effect of micellar systems, the limitations stand out clearly; deficiencies concern, for example, the discontinuity in the ion distribution between the stern and Gouy-Chapman layers and the use of macroscopic dielectric constants. As a general conclusion it may perhaps be said that the gross features of the counter-ion distribution between the kinetic micelle and the bulk solution can be understood in simplified electrostatic models, while the variation of it with the system characteristics, the exact location of counter-ions and head-groups in the Stern layer etc. require sophisticated treatments. A striking observation is that the counter-ion association is roughly constant when such features as surfactant concentration, electrolyte addition, solubilization, phase structure and head group structure are varied. Similar observations apply for polyelectrolyte systems, where the phenomena is termed counter-ion condensation, implying that the net charge density of poly-electrolytes is neutralized by counter-ion binding to essentially the same constant value irrespective of other system parameters72. The counter-ion condensation for linear poly-ions has got a theoretical basis in solutions of the Poisson-Boltzmann equation for point charges in the presence of a continuous line charge of high charge density making two-phase approximation72. It seems most probable that a consideration of the counter-ion condensation model should be profitable not only for rod-like micelle but also for spherical ones as well as for lamellar meso-phase.
1.6.4 Krafft Point for Ionic Surfactants
The solubility of a surfactant is not linearly related to solvent temperature, but rather a temperature exists at which there is a sharp increase in the solubility of a surfactant. At this temperature, the concentration of the surfactant becomes equal to the CMC and is defined as the Krafft temperature or point (Tk), which varies for each surfactant. Most surfactants are used above this temperature to ensure the maximum surface tension reduction by overcoming the CMC. So, increase of the krafft point in presence of electrolyte solution of cationic surfactant (For example, CTAB) which bears same anioic part (For example, NaBr) or electrolyte (For example, NaCl) with Hexapyridinium Chloride (CPC)73,74was reported by some investigators. The increase of the Krafft point is not suitable for industrial use. Since in application, surfactant generally use above the krafft point, the decrease of krafft point in the study of surfactant bears great importance but the related work is very few in number.
1.7 APPLICATION OF SURFACTANTS
Surfactant and its application: Surfactants are used in numerous applications including:
1.7.1 Detergents and Cleaners
The primary traditional application for surfactants is their use as soaps and detergents for a wide variety of cleaning processes. Soap has been used in personal hygiene for well over 2000 years with little change in the basic chemistry of their production and use. New products with pleasant colors, odors, and deodorant and antiperspirant activity have creptin to the market since the early twentieth century. On the other hand, the synthetic detergents used in cleaning our clothes, dishes, houses, and so on are relative newcomers. ‘‘Whiter than white’’ and ‘‘squeaky clean’’ commercials notwithstanding, the purpose of detergents is to remove unwanted dirt, oils, and other pollutants, while not doing irreparable damage to the substrate. In the past, due primarily to the shortcomings of available surfactants, such cleaning usually involved energy-intensive treatments very hot water and significant mechanical agitation. Modern surfactant and detergent formulations have made it possible for us to attain the same or better results with much lower wash temperatures and less mechanical energy consumption. Improved surfactants and detergent formulations have also resulted in less water use and more
Table 1.1 Summary of main classes of surfactants.
| Class | Head Group | Applications |
| Anionic | -CO2– Na+
-SO3–Na+ -O-SO3– Na+ -O-PO3–Na+ -(OCH2CH2)n-O-SO3– Na+ |
Soaps
Synthetic detergents Detergents, personal care products Corrosion inhibitors, emulsifiers Liquid detergents, toiletries, emulsifiers |
| Cationic | -N(CH3)3 + Br –
>N(CH3)2+ Br – |
Bitumen emulsions
Bactericides, antistatic agents Fabric and hair conditioners |
| Zwitterionic | -N+(CH3)2-CH2-CO2–
-N+(CH3)-CH2-SO3– |
Shampoos, cosmetics |
| Non-ionic | -(OCH2CH2)nOH | Detergents, emulsifiers |
efficient biological degradation processes that help protect our environment. Even with lower wash temperatures and lower energy consumption, extensive studies have shown that equivalent or improved hygiene is maintained. It is only in instances where particularly dangerous pathogenic agents are present, as in hospital laundries, for example, that additional germicidal additives become necessary to obtain efficient cleaning results.
1.7.2. Cosmetics and Personal Care Products
Cosmetics and personal care products make up a vast multi-billion-dollar market worldwide, a market that continues to grow as a result of improved overall living standards in areas such as Asia and Latin America and continuing cultural driving forces in the already developed economies. Traditionally, such products have been made primarily from fats and oils, which often are perceived to have the advantage of occurring naturally in the human body and therefore present fewer problems in terms of toxicity, allergenicity, and so on. The perception is, of course, totally false, as shown by the large number of quite nasty allergens and toxins that come from the most ‘‘natural’’ of sources. Nonetheless, natural surfactants and other amphiphilic materials have been used in cosmetics since their ‘‘invention’’ in ancient Egypt (or before). It is probably safe to say that few, if any, cosmetic products known to women (or men, for that matter) are formulated without at least a small amount of a surfactant or surface-active component. That includes not only the more or less obvious creams and emulsions but also such decorative products as lipstick; rouge; mascara; and hair dyes, tints, and rinses. An important aspect of such products is, of course, the interaction of the components of the cosmetic formulation with the human skin, membranes, and other tissues or organs with which it will come into contact during use. As mentioned above, merely because a product is ‘‘natural’’ or is derived from a natural source does not guarantee that it will not produce an adverse reaction in some, if not all, users. The possible adverse effects of surfactants in cosmetics and personal care products must, of course, be studied in depth for obvious safety reasons as well as for questions of corporate liability and image. Unfortunately, our understanding of the chemical reactions or interactions among surfactants, biological membranes, and other components and structures is not sufficiently advanced to allow the formulator to say with sufficient certainty what reaction an individual will have when in contact with a surfactant.
1.7.3. Textiles and Fibers
Surfactants have historically played an important role in the textile and fibers industry. The dyeing of textiles is an obvious application of surfactants. The added surfactants serve to aid in the uniform dispersion of the dyes in the dying solution, the penetration of the dying solution into the fiber matrix, the proper deposition of the dyes on the fiber surface, and the proper ‘‘fixing’’ of the dye to that surface. For natural fibers, the role of surfactants begins at the beginning with the washing and preparation of the crude fiber in preparation for spinning. Once the crude material is ready for spinning, the use of surfactants as internal lubricants and static discharge agents allows the industry to produce yarns in extremely long and fine filaments that would be impossible to handle otherwise. Extremely fast modern spinning and weaving equipment requires that the fibers pass through the process without breaking or jamming, events that would produce very expensive production line stoppages. Sewing equipment that may work at more than 6000stitches per minute requires that the fibers and needles pass in the night with a minimum of friction that could produce a significant amount of frictional heat and even burn the fibers. That interaction is controlled by the use of the proper surfactant. Synthetic fibers also require surfactants at various steps in their evolution from monomeric organic chemicals to finished cloth. Depending on the type of polymer involved, the process may require surfactants beginning with the polymer synthesis, but certainly once the first extrusion and spinning processes begin. Even after the textile is ‘‘finished’’ it is common to apply a final treatment with a surface-active material to define the final characteristics of the product. In woven polyester rugs, for example, a final finish with an antistatic surfactant reduces or eliminates problems with static discharge (those shocking doorknobs in winter) and retards the adhesion of dirt to the fibers. The applications of fluorinated materials produces the stain repelling ‘‘Scotch Guard’’ effect by coating the fibers with a Teflon like armor.
1.7.4. Leather and Furs
Surfactants are an important part of the manufacture of leather and furs, starting with the original untreated skin or hide and ending with the finished product. In leather tanning, for example, it is normal to treat the leather with a surfactant to produce a protective coating on the skin and hide fibers. This helps prevent the fibers from sticking together and keeps the fiber network flexible or supple while increasing the tensile strength of the finished leather product. Surfactants may also help the penetration of dyes and other components into the fiber network thereby improving the efficiency of various stages of the tanning process, saving time, energy, and materials while helping to guarantee a higher-quality, more uniform finished product. The final surface finish of leather goods is now commonly applied in the form of lacquer like polymer coatings that can be applied as emulsions and suspensions, using suitable surfactants, of course. Similar applications are found in the fur industry.
1.7.5 Paints, Lacquers, and Other Coating Products
It is probably not surprising to find that surfactants are required in many capacities
in the production of paints and lacquers, and in related coating systems. In all paints that carry pigment loads, it is necessary to prepare a uniform dispersion that has reasonable stability to flocculation and coalescence. In addition, the preparation of mineral pigments involves the process of grinding the solid material down to the desired particle size, which is an energy-intensive process. In general, it is found that a smaller, more uniform particle size results in a higher covering power for the same weight load of pigment, that is, a more efficient use of material and consequently a reduction in cost always a nice effect in commerce. The grinding process is helped by reducing the surface energy of the solid pigment, an effect achieved by the addition of surfactants. Since pigment solids are far from smooth surfaces at the molecular level, the raw material will have small cracks and holes that serve as initiation points for the rupture of the structure. In the presence of the proper surfactant, the molecules penetrate into the cracks and crevices, adsorb onto the solid surface, and significantly reduce the surface energy of newly exposed solid, facilitating the continued breaking of the large particles into smaller units. The adsorbed surfactant molecules also create a barrier like coating that helps prevent the small particles from adhering or agglomerating. It is estimated that the use of surfactants in the grinding process can save up to 75% of the energy needed to achieve the same result without added surfactant. Once the pigment is properly ground, it must be mixed into the basic liquid carrier and maintained stable or easily dispersible for an extended period of time, much against the natural driving force of thermodynamics. For the dispersion of the pigment in the final coating formulation, it may be necessary to add additional surfactant of the same or another class. In organic coating systems, the surfactant may in fact be a polymeric system that doubles as the final dried binder for the pigment. On the other hand, there is available low-molecular-weight surfactants specifically designed to act in organic solvents. In aqueous or latex paints, the surfactant is important not only in the pigment grinding process but also in the preparation of the latex polymer itself. The chemistry of emulsion polymerization (i.e., latex formation) is a complex and interesting phenomenon and cannot be treated here. Very few emulsion polymers are produced without the addition of surfactants, and most of those so prepared are interesting laboratory novelties that never see the light of commercial exposure. In addition to surfactants for pigment grinding and dispersion and latex preparation, they are also important in the control of the wetting and leveling characteristics of the applied paint. In painting applications that use lacquers such as the automobile industry, application and drying times are important. In such situations wetting and leveling are also important. In powdered lacquers, the presence of the proper surfactants produces a net electrical charge on the surface of the particles, which allows them to be applied quickly and evenly by electrophoretic processes. A potential drawback to rapid paint or lacquer application is that such speed can facilitate the introduction of air into the material resulting in foam formation at the time and point of application. If foam is produced, the drying bubbles on the painted surface will produce indentations and perhaps even bare spots that will significantly degrade the aesthetic and protective properties of the coating. To help prevent such foaming it is sometimes useful to add surfactants that also serve as antifoaming agents. Although it is common to relate surfactants with increased foam as in beer, shaving cream, whipped toppings, and firefighting foams.
1.7.6 Paper and Cellulose Products
Surfactants play several important roles in the papermaking industry. Several components of paper such as pigments for producing white or colored paper and sizing agents, often emulsion polymers that bind the cellulose fibers in the finished product and incorporate strength and dimensional stability, require surfactants in their preparation. In addition, the water-absorbing capacity of paper is often controlled by the addition of the proper surfactants. Surfactants are also important in the process of recycling paper. A major step in the process is the removal of the ink and pigments present (deinking). That process is what is termed a flotation process in which a surfactant is added to aqueous slurry of old paper. The surfactant is chosen so that it will adsorb on the surfaces of pigment particle and ink droplets, causing them to become very hydrophobic. Air is then bubbled through the slurry. As the bubbles rise through the system, they become preferentially attached to the hydrophobic pigment and ink particles, acting like lifejackets and causing the particles to rise to the surface. At the surface they are skimmed off and separated from the cellulose slurry.