Surfactant molecular interactions

In tliis paper, I should like to cover the basic concepts involved in investigating synergy in binary mixtures of surfactants and to discuss some of the surfactant-surfactant molecular interactions that result in synergy. Some of the findings obtained from the study of surfactant-surfactant interactions can be used to suggest surfactant-pollutant interactions that can result in the enhanced removal of these contaminants from industrial wastes. 

There is no molecular interaction between nonionic surfactants with an ethylene-oxide chain, i.e. Genapol and ethoxylated nonylphenols. Indeed, research by Nishikido (6) on polyoxyethylene laurylethers (5 < E.O. number < 49) has shown the ideal behavior (p12 = 0) of their mixtures. Likewise, Xia (7) has found very low p12 values for mixtures of ethoxylated fatty alcohols. 

These are present in an immiscible two-phase system (0 and W denoting oil and water, respectively) containing a third-surfactant component with partial solubility in both bulk phases. Each surfactant molecule has a hydrophilic (denoted by H) and a lipophilic (denoted by L) section. Conceptually then Winsor views all the possible molecular interactions in such a system in terms of their cohesive energy (denoted by C). For such a system, there are then 10 possible cohesive molecular interactions (i.e., 10 unique combinations of the letters 0, W, H, and L). In the ideal case, the lipophile-oil and the hydrophile-water interaction will be the predominant interactions. The relative magnitude (R) of these two interactions... 

The deviations from the Szyszkowski-Langmuir adsorption theory have led to the proposal of a munber of models for the equihbrium adsorption of surfactants at the gas-Uquid interface. The aim of this paper is to critically analyze the theories and assess their applicabihty to the adsorption of both ionic and nonionic surfactants at the gas-hquid interface. The thermodynamic approach of Butler [14] and the Lucassen-Reynders dividing surface [15] will be used to describe the adsorption layer state and adsorption isotherm as a function of partial molecular area for adsorbed nonionic surfactants. The traditional approach with the Gibbs dividing surface and Gibbs adsorption isotherm, and the Gouy-Chapman electrical double layer electrostatics will be used to describe the adsorption of ionic surfactants and ionic-nonionic surfactant mixtures. The fimdamental modeling of the adsorption processes and the molecular interactions in the adsorption layers will be developed to predict the parameters of the proposed models and improve the adsorption models for ionic surfactants. Finally, experimental data for surface tension will be used to validate the proposed adsorption models. 

The standard deviation has been determined as ct = j where v is the number of degrees of freedom in the fit. The parameters for the molecular interaction /3, the maximum adsorption Too, the equilibrium constant for adsorption of surfactant ions Ki, and the equilibrium constant for adsorption of counterions K2, are thus obtained. The non-linear equations for the Frumkin adsorption isotherm have been numerically solved by the bisection method. 

It is noted that the molecular interaction parameter described by Eq. 52 of the improved model is a function of the surfactant concentration. Surprisingly, the dependence is rather significant (Eig. 9) and has been neglected in the conventional theories that use as a fitting parameter independent of the surfactant concentration. Obviously, the resultant force acting in the inner Helmholtz plane of the double layer is attractive and strongly influences the adsorption of the surfactants and binding of the counterions. Note that surface potential f s is the contribution due to the adsorption only, while the experimentally measured surface potential also includes the surface potential of the solvent (water). The effect of the electrical potential of the solvent on adsorption is included in the adsorption constants Ki and K2. 

Fig. 9 Dependence of the molecular interaction between adsorbed dodecyl sulfate ions on the concentration of alkali dodecyl sulfate surfactants...

The second factor, namely the head group interaction, can also influence the surface properties of mixed surfactant markedly. In particular, anionic/catlonic surfactant mixtures exhibit the largest effect (17,18). In nonionic/anionic surfactant mixtures, synergistic effects can still take place to a significant extent, as revealed in Figure 3 (pH 10.9, nonionic amine oxide with anionic long chain sulfate), since insertion of nonionic surfactant molecules into an ionic surfactant molecular assembly minimises electrostatic repulsion (19). 

Molecular Interaction and Synergism in Binary Mixtures of Surfactants... 

Our data, to date, show that molecular interaction between two surfactants, both in mixed monolayers at the aqueous solution/air interface and in mixed micelles in aqueous solution, increases in the order POE nonionic-POE-nonionic < POE nonionic-betaine < betaine-cationic < POE nonionic-ionic (cationic, anionic) betaine-anionic cationic-anionic. The greatest probability of synergism exists, therefore, in cationic-anionic mixtures, followed by betaine-anionic mixtures. Synergism can exist in POE nonionic-ionic mixtures only if the surfactants involved have the proper structures. 

The effect of alkyl alcohol on the surface adsorption and micellization of FC surfactant is noticeably different from HC surfactant. The molecular interactions between ROH and C7pNa in the surface layer are shown to be weaker (Smaler l jl-value) as compared with ROH-C, SNa system. 

C FNa-CioSNa and C-7FNa-Cla5Na systems. These are the mixed anionic-anionic surfactants systems. The surface tension (interfacial tension) - concentration relationships are shomn in Fig, 1 to 3. There are surfactant compositions at which uniform or homogeneous mixed micelle do not exist in these two systems due to the "mutual pho-bicity" between FC- and HC-chains of the surfactants (4,7) Therefore the molecular interaction parameterof the two surfactants in micelles can not be calculated from the Surface tension curves because this cmc has no longer the physical meaning of mixture cmc. However, we can obtain the /3(t values from the surface tension curves by means of equation 13, Table 1 and 2 show the results. 

CnSOC-CyFNa (nonionic-anionic) system. In order to avoid the complex structure and function of polyoxyethylene group.in a common nonionic surfactant (e.g. TX100), we use octylmethyl sulfoxide as a partner in the pair system to study the molecular interactions. The surface tension of the surfactants solutions (with and without adding salt) are shown in fig.6 and 7. The surface properties of 1 1 CgSOC-CyFNa system with adding salt (from Fig.6) are shown in Table 7. 

In the systems with considerable molecular interactions between the two surfactant components, such as CgNBr-CyFNa (cationic-anionic) and CsSOC-CrFNa (nonionic-anionic) systems, the "mutual phobic interaction" can be concealed entirely and there are large negative jSg. and /3m values for these systems. 

It is evident that the non-ideal solution theory of surface adsorption and micellization is a convenient and useful tool for obtaining the surface and the micelle compositions and for studing the molecular interaction in the binary surfactant system. 

By extending regular solution theory for binary mixtures of AEg in aqueous solution to the adsorption of mixture components on the surface (3,4), it is possible to calculate the mole fraction of AEg, Xg, on the mixed surface layer at tt=20, the molecular interaction parameter, 6, the activity coefficients of AEg on the mixed surface layer, fqg and f2s and mole concentration of surfactant solution, CTf=20 3t surface pressure tt=20 mn-m l (254p.l°C). The results from the following equations are shown in Table I and Table II. 

Generally, there are two approaches to the investigation of mixed adsorbed films at an oil/water interface. One way is to study mixed adsorption of surfactants from the Scime bulk phase and the other is to study adsorption from both of the bulk phases. The former has been done by many workers from the physicochemical viewpoint to clarify the difference in molecular interaction between the adsorbed state and the bulk state. The latter has been made mostly from the practical point of view, e.g., solvent extraction and complex-forming reactions that take place at the interface, though little is known concerning the thermodynamic viewpoint D). The thermodynamic study is actually useful to elucidate the behavior of film molecules in the adsorbed state. 

A situation that commonly occurs with food foams and emulsions is that there is a mixture of protein and low-molecular-weight surfactant available for adsorption at the interface. The composition and structure of the developing adsorbed layer are therefore strongly influenced by dynamic aspects of the competitive adsorption between protein and surfactant. This competitive adsorption in turn is influenced by the nature of the interfacial protein-protein and protein-surfactant interactions. At the most basic level, what drives this competition is that the surfactant-surface interaction is stronger than the interaction of the surface with the protein (or protein-surfactant complex) (Dickinson, 1998 Goff, 1997 Rodriguez Patino et al., 2007 Miller et al., 2008 Kotsmar et al., 2009). 

With monomeric molecules, the aggregation number of micelles is determined by equilibrium thermodynamics. In polymeric molecules, however, topological constraints are imposed on the system. If the degree of polymerization exceeds the aggregation number of the monomeric micelle, unsaturated sites of the polymeric molecules become available (directed to the aqueous phase) and inter-molecular interactions (agglomeration) occur. In the case of polymer with Mw= 6.23x105, typical surfactant behavior was found. 

Preservatives such as sodium benzoate, sorbic acid, and methyl and propyl parabens have been used in liquid and semisolid dosage forms. There have been reports that the parabens have been inactivated when used in the presence of various surfactants. This loss of activity was thought to be due to the formation of complexes between the preservative and the surfactant. The interaction between polysorbate (Tween) 80 and the parabens has been demonstrated by a dialysis technique (Ravin and Radebaugh, 1990). It has also been shown that molecular complexes form when the parabens are mixed with polyethylene glycol (PEG) and methylcellulose. The degree of binding was less than that observed with Tween 80. Sorbic acid also interacts with Tweens but does not interact with PEGs. The quaternary ammonium compounds are also bound by Tween 80, which reduces their preservative activity. 

The thermodynamic models discussed in the proceeding paragraph provide no insight into the underlying mechanism and molecular interactions leading to aggregational phenomena. The particular value, however, of such models is emphasized by the fact that they apply equally well to both, aqueous and nonpolar surfactant systems. 

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