Monomeric surfactant solutions

Spectra of Monomeric Surfactant Solutions. The next step was to study alkyldimethylamine oxides having methylene chains of sufficient length such that the molecule is surface active. The FT-IR spectra of monomer solutions of CgAO at various degrees of protonation (Z) are shown in Figure 3. These spectra are much more complex than those of C AO due to the introduction of deformation modes of the CH2 and the terminal CH3 groups. 

The term A/y i,] accounts for the dilution enthalpy observed upon titration of lipid vesicles into the surfactant solution and can be detennined in a separate experiment by titrating lipid vesicles into water. When the reversed experiment is performed, i.e. the titration of a monomeric surfactant solution to a vesicles suspension in the cell, then equation (16) has to be differentiated with respect to D, and one obtains ... 

Most studies of micellar systems have been carried out on synthetic surfactants where the polar or ionic head group may be cationic, e.g. an ammonium or pyridinium ion, anionic, e.g. a carboxylate, sulfate or sulfonate ion, non-ionic, e.g. hydroxy-compound, or zwitterionic, e.g. an amine oxide or a carboxylate or sulfonate betaine. Surfactants are often given trivial or trade names, and abbreviations based on either trivial or systematic names are freely used (Fendler and Fendler, 1975). Many commercial surfactants are mixtures so that purity can be a major problem. In addition, some surfactants, e.g. monoalkyl sulfates, decompose slowly in aqueous solution. Some examples of surfactants are given in Table 1, together with values of the critical micelle concentration, cmc. This is the surfactant concentration at the onset of micellization (Mukerjee and Mysels, 1970) and can therefore be taken to be the maximum concentration of monomeric surfactant in a solution (Menger and Portnoy, 1967). Its value is related to the change of free energy on micellization (Fendler and Fendler, 1975 Lindman and Wennerstrom, 1980). 

Monomeric surfactant and added solutes rapidly diffuse into the micelles, so that transfer of material between water and micelle is much faster than most activated thermal chemical reactions. This generalization cannot be applied to photochemical reactions where some steps of the reaction may be very rapid and therefore faster than solute transfer (Fendler, 1982 Thomas, 1984). 

Key questions in these treatments are the constancy of a (or P) and the nature of the reaction site at the micellar surface. Other questions are less troubling for example the equations include a term for the concentration of monomeric surfactant which is assumed to be given by the cmc, but cmc values depend on added solutes and so will be affected by the reactants. In addition submicellar aggregates may form at surfactant concentrations near the cmc and may affect the reaction rate. But these uncertainties become less important when [surfactant] > cmc and kinetic analyses can be made under these conditions. In addition, perturbation of the micelle by substrate can be reduced by keeping surfactant in large excess over substrate. 

Except for some anionic/cationic surfactant mixtures which form ion pairs, in a typical surfactant solution, the concentration of the surfactant components as monomeric species is so dilute that no significant interactions between surfactant monomers occur. Therefore, the monomer—mi celle equilibria is dictated by the tendency of the surfactant components to form micelles and the interaction between surfactants in the micelle. Prediction of monomer—micelle equilibria reduces to modeling of the thermodynamics of mixed micelle formation. 

With these considerations in mind, the chemical potential of the ith free monomeric surfactant component in solution is given by... 

The Pseudo-Phase Model Consider a process in which surfactant is added to water that is acting as a solvent. Initially the surfactant dissolves as monomer species, either as molecules for a non-ionic surfactant or as monomeric ions for an ionic surfactant. When the concentration of surfactant reaches the CMC, a micelle separates from solution. In the pseudo-phase model,20 the assumption is made that this micelle is a separate pure phase that is in equilibrium with the dissolved monomeric surfactant. To maintain equilibrium, continued addition of surfactant causes the micellar phase to grow, with the concentration of the monomer staying constant at the CMC value. This relationship is shown in Figure 18.14 in which we plot m, the stoichiometric molality,y against mj, the molality of the monomer in the solution. Below the CMC, m = m2, while above the CMC, m2 = CMC and the fraction a of the surfactant present as monomer... 

The Mass Action Model The mass action model represents a very different approach to the interpretation of the thermodynamic properties of a surfactant solution than does the pseudo-phase model presented in the previous section. A chemical equilibrium is assumed to exist between the monomer and the micelle. For this reaction an equilibrium constant can be written to relate the activity (concentrations) of monomer and micelle present. The most comprehensive treatment of this process is due to Burchfield and Woolley.22 We will now describe the procedure followed, although we will not attempt to fill in all the steps of the derivation. The aggregation of an anionic surfactant MA is approximated by a simple equilibrium in which the monomeric anion and cation combine to form one aggregate species (micelle) having an aggregation number n, with a fraction of bound counterions, f3. The reaction isdd... 

The primary mechanism for energy conservation is adsorption of surfactant molecules at various available interfaces. However, when, for instance, the water-air interface is saturated conservator may continue through other means (Figure 12.3). One such example is the crystallization or precipitation of the surfactant from solution, in other words, bulk phase separation. Another example is the formation of molecular aggregates or micelles that remain in solution as thermodynamically stable, dispersed species with properties distinct from those of an isotropic solution containing monomeric surfactant molecules (Myers, 1992). 

Micellar Solutions. At concentrations above the cmc, where the amine oxide molecules are distributed between the micellar and monomeric forms, the situation is more complex since the degree of protonation of the micellar surfactant can be quite different than that of the monomeric surfactant. The mass balance for the surfactant is... 

Solutions below the cmc contain only monomeric surfactant molecules. To a first approximation these dissolved molecules do not influence the viscosity which is the same as for pure water (1 mPa s at room temperature) [37]. 

Surfactant-Mediated Chromatographic Separations. The selective interaction of surfactants with a variety of solutes (as ion pairs with monomeric surfactant molecules or as bound ("associated") species... 

The experiments prove this assumption in practice. In Figure 7a, the solubilization of Eosin in the monomeric aqueous surfactant solution indicates the CMC at 1.55.10 mol/1 while the corresponding polysiloxane (r=95) (Figure 7b) solubilizes Eosin without an indication of a CMC (24). For this polymer r... 

An important aspect to be considered concerns the chemical constitution of the polysurfactants. While monomeric surfactants associate to form micelles in aqueous solutions regardless of whether the hydrophilic group is an anionic, cationic, zwitterionic or non-ionic group, it is more complex for the ionic compared to the non-ionic polysurfactants. This is clearly reflected in the rheological properties of ionic polysurfactants(26). 

In sufficiently dilute aqueous solutions surfactants are present as monomeric particles or ions. Above critical micellization concentration CMC, monomers are in equilibrium with micelles. In this chapter the term micelle is used to denote spherical aggregates, each containing a few dozens of monomeric units, whose structure is illustrated in Fig. 4.64. The CMC of common surfactants are on the order of 10 " -10 mol dm . The CMC is not sharply defined and different methods (e.g. breakpoints in the curves expressing the conductivity, surface tension, viscosity and turbidity of surfactant solutions as the function of concentration) lead to somewhat different values. Moreover, CMC depends on the experimental conditions (temperature, presence of other solutes), thus the CMC relevant for the expierimental system of interest is not necessarily readily available from the literature. For example, the CMC is depressed in the presence of inert electrolytes and in the presence of apolar solutes, and it increases when the temperature increases. These shifts in the CMC reflect the effect of cosolutes on the activity of monomer species in surfactant solution, and consequently the factors affecting the CMC (e.g. salinity) affect also the surfactant adsorption. 

The totality of micelles represents a colloidal phase, into which a substance is dissolved in the aqueous phase partitions. The capacity of the micellar phase to solubilize a solute can therefore be expressed as a partition coefficient A n,. Hence, a linear relationship can be expected between the concentration of substance solubilized by micelles and the concentration of the surfactant Cs in the system. Because only micelles contribute to the solubilizing effect but not the monomeric surfactant molecules, the critical micelle concentration Ccmc must be subtracted from the total of the surfactant concentration. The resulting total concentration of solute in the micellar solution is then ... 

Therefore, micelle-forming surfactant molecules (e.g., SDS) will be present in three different forms, namely, on the lipid surface, as micelles, and as monomeric surfactant molecules in solution. Lecithin will form liposomes, which have also been detected in nanoemulsions for parenteral nutrition [77], Mixed micelles have to be considered in glycocholate/lecithin-stabilized and -related systems. Micelles, mixed micelles, and liposomes are known to solubilize drugs, and are therefore attractive alternative drug-incorporation sites (especially with respect to the low incorporation capacity of lipid crystals). 

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