Water-surfactant interactions

Phase diagrams of water, hydrocarbon, and nonionic surfactants (polyoxyethylene alkyl ethers) are presented, and their general features are related to the PIT value or HLB temperature. The pronounced solubilization changes in the isotropic liquid phases which have been observed in the HLB temperature range were limited to the association of the surfactant into micelles. The solubility of water in a liquid surfactant and the regions of liquid crystals obtained from water-surfactant interaction varied only slightly in the HLB temperature range. 

In the phenomenological model of Kahlweit et al. [46], the behavior of a ternary oil-water-surfactant system can be described in terms of the miscibility gaps of the oil-surfactant and water-surfactant binary subsystems. Their locations are indicated by the upper critical solution temperature (UCST), of the oil-surfactant binary systems and the critical solution temperature of the water-surfactant binary systems. Nonionic surfactants in water normally have a lower critical solution temperature (LCST), Tp, for the temperature ranges encountered in surfactant phase studies. Ionic surfactants, on the other hand, have a UCST, T. Kahlweit and coworkers have shown that techniques for altering surfactant phase behavior can be described in terms of their ability to change the miscibility gaps. One may note an analogy between this analysis and the Winsor analysis in that both involve a comparison of oil - surfactant and water-surfactant interactions. 

Smiechowski M, Lvovich V (2002) Electrochemical monitoring of water-surfactant interactions in industrial lubricants. J Electroanal Chem 534 171-180. doi 10.1016/S0022-0728(02)01106-3... 

The size and shape evolution of reversed micelles as a function of the water and surfactant concentrations are system-specific. The micellar size is mainly controlled by the strong tendency of the surfactant to be located at the interface between water and apolar solvent, which involves an enormous value of the interfacial surface and micelles of nanometric size. Spherical micelles result fl om a minimization of the micellar surface-to-volume ratio, i.e., a minimization of water-surfactant interactions less favorable than water-water and/or surfactant-surfactant interactions, while rodlike micelles, characterized by a greater surface-to-volume ratio, result from water-surfactant interactions more favorable than... 

As an alternative (albeit similar) interpretation we would suggest that a plateau in a 71J vs. C plot means that the strength of the water-surfactant interaction is approximately the same within the respective concentration subinterval. The difference between the plateaus just reflects the difference between various grades of such interactions, which may not be directly related to definite microstructures. 

It was demonstrated for system A that although pentanol enhances water solubilization and is present at the interface, its interaction with water or surfactant is not revealed by SZT-DSC. This point has been investigated in the same way as the water-surfactant interaction [10]. We determined the concentrations of pentanol from the measured enthalpy change associated with the pentanol peak and the enthalpy change associated with pure pentanol. The derived concentrations were compared with the actual concentrations determined gravimetrically (see Table 2) [45]. These results suggest that no evidence for interaction of pentanol with water or surfactant can be found in SZT-DSC measurements [10]. 

We have discussed this problem in previous publications [8,9,11] and more systematically in our review [2] in an attempt to show that reliable information on water-surfactant interactions can be obtained from SZT-DSC experimental results. The gist of our argument is that any (measurable) amount of free water formed in a microemulsion sample will necessarily be detected as a melting peak at about 0°C in the endothermic mode of SZT-DSC. Thus, free water cannot be formed in the system and concurrently not revealed by this peak The same conclusion should, of course, also be valid if boimd (or interphasal) water formed at ambient temperature was detached... 

It is of particular interest to be able to correlate solubility and partitioning with the molecular stmcture of the surfactant and solute. Likes dissolve like is a well-wom plirase that appears applicable, as we see in microemulsion fonnation where reverse micelles solubilize water and nonnal micelles solubilize hydrocarbons. Surfactant interactions, geometrical factors and solute loading produce limitations, however. There appear to be no universal models for solubilization that are readily available and that rest on molecular stmcture. Correlations of homologous solutes in various micellar solutions have been reviewed by Nagarajan [52]. Some examples of solubilization, such as for polycyclic aromatics in dodecyl sulphonate micelles, are driven by hydrophobic... 

Electrolytes are obviously solubilized only in the aqueous micellar core. Adding electrolytes in water-containing AOT-reversed micelles has an effect that is opposite to that observed for direct micelles, i.e., a decrease in the micellar radius and in the intermicellar attractive interactions is observed. This has been attributed to the stabilization of AOT ions at the water/surfactant interface [128]. 

AOT is an anionic surfactant complexed to the counterion, usually sodium. The water molecules in the intramicellar water pool are either free or bound to the interface. The bound water can interact with various parts of the surfactant. These interactions include hydrogen-bonding interactions with oxygen molecules on the sulfonate and succinate groups, ion-dipole interactions with the anionic surfactant headgroup and counterion, dipole-dipole interactions with the succinate group, and dispersive forces with the hydrocarbon tails. 

This effect can be of great importance, because it is susceptible to considerable alteration of the surfactant interaction between oil and water, and the solubilization in microemulsion (as in the so-called lipophilic and hydrophihc linker mechanisms). The role of the linker molecules is to extend the reach of the surfactant in the bulk phase and in practice to somehow modify the oil and water phases close to the interface, so that their characteristic parameters are altered [66-69]. 

This represents the difference in the second adsorption free energy term in Equation 21, i.e. the two terms on the right hand side each represent the change in free energy when a water-surface molecular contact is replaced with a surfactant-surface molecular contact. It is very reasonable to assume that, at close packing, both surfactants adsorb with only their hydrocarbon moieties (or part of these moieties) in direct contact with the surface. Hence, the two surfactants interact with the latex surface with the same strength and the last term in Equation 17 is equal to zero. 

Where this factor plays a role, the hydrophobic interaction between the hydrocarbon chains of the surfactant and the non-polar parts of protein functional groups are predominant. An example of this effect is the marked endothermic character of the interactions between the anionic CITREM and sodium caseinate at pH = 7.2 (Semenova et al., 2006), and also between sodium dodecyl sulfate (SDS) and soy protein at pH values of 7.0 and 8.2 (Nakai et al., 1980). It is important here to note that, when the character of the protein-surfactant interactions is endothermic (/.< ., involving a positive contribution from the enthalpy to the change in the overall free energy of the system), the main thermodynamic driving force is considered to be an increase in the entropy of the system due to release into bulk solution of a great number of water molecules. This entropy... 

Figure 6.8 Sketch of proposed molecular mechanism of protein-surfactant interaction for CITREM + sodium caseinate (0.5 % w/v in aqueous medium (pH = 7.2, ionic strength = 0.05 M) at 293 K. Picture (I) shows the water molecules bound with polar groups of the protein and surfactant, as w ell as w ater molecules structured as a result of hydrophobic hydration around the hydrocarbon chain of the surfactant. (For clarity, the free w ater molecules are not shown.) Picture (H) demonstrates the release of bound and structured water molecules resulting Rom the predominantly hydrophobic interactions between protein and surfactant. Reproduced Rom Semenova et al. (2006) with permission.

When conventional surfactants are used in emulsion polymerization, difficulties are encountered which are inherent in their use. Conventional surfactants are held on the particle surface by physical forces thus adsorption/des-orption equilibria always exist, which may not be desirable. They can interfere with adhesion to a substrate and may be leached out upon contact with water. Surfactant migration affects film formation and their lateral motion during particle-particle interactions can cause destabilization of the colloidal dispersion. 

Low internal phase emulsions typically result when high shear conditions are used for emulsification, while low shear mixing can lead to high internal phase, or concentrated, emulsions [435]. There are several conditions needed to form a concentrated emulsion. Low shear mixing is required while the internal phase is slowly added to the continuous phase, and the surfactants used to create the emulsion need to be able to form elastic films [435—438]. The formation of concentrated emulsions has also been linked to surfactant-oil phase interactions [436] and therefore the oil-water interfacial tension and the potential for surfactant-surfactant interactions [439]. 

For small amounts of solubilized water, as a polar additive, the stability of the micelle is markedly increased, as shown by a decrease in the CMC. On the other hand, large amounts of water as a polar additive decrease the stability of the micelle. It is known that a solution of AOT in iso-octane solubilized up to 50 moles of water per mole of surfactant. As the concentration of water increases, the isotropic reverse micellar solution changes to a water-in-oil microemulsion. A clear understanding of the complex analyte-micelle-water pool interactions, especially analyte concentration and pH at the head group interfacial region, is under intensive study (Cline Love and al., 1984 Little and Singleterry, 1964 Luisi and Straub, 1984 Mclntire, 1990). 

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