Surfactants in the micellar phase

Various approaches have been employed to tackle the problem of micelle formation. The most simple approach treats micelles as a single phase, and this is referred to as the phase-separation model. In this model, micelle formation is considered as a phase-separation phenomenon, and the cmc is then taken as the saturation concentration of the amphiphile in the monomeric state, whereas the micelles constitute the separated pseudophase. Above the cmc, a phase equilibrium exists with a constant activity of the surfactant in the micellar phase. The Krafft point is viewed as the temperature at which a solid-hydrated surfactant, the micelles, and a solution saturated with undissociated surfactant molecules are in equiUbrium at a given pressure. 

For binary surfactant mixtures, one might expect that micelles formed at the CMC of the mixture would be enriched in the less hydrophilic surfactant (i.e., the one with the lower CMC). Analysis based on the phase separation model confirms this expectation. The simplest approach is to assume an ideal mixture in the micellar phase (i.e., activity of each species equal to its mole fraction). With this assumption one obtains the following expression for the ratio x Jx2 of the two surfactants in the micellar phase at the CMC (see Problem 4.3) ... 

If and //m are the chemical potentials per mole of the unassociated surfactant in the aqueous phase and associated surfactant in the micellar phase, respectively, then since these two phases are in equilibrium... 

KP and v can, in contrast to kp, not be determined via the concentration gradient for binary and ternary mixed micelles, because for the calculation of the Nemstian distribution a constant CMC and an almost constant partial molar volume must be assumed. The calculation of aggregation constants of simple bile salt systems based on Eq. (4) yields similar results (Fig. 8b). Assuming the formation of several concurrent complexes, a brutto stability constant can be calculated. For each application of any tenside, suitable markers have to be found. The completeness of dissolution in the micellar phase is, among other parameters, dependent on the pH value and the ionic strength of the counterions. Therefore, the displacement method should be used, which is not dependent on the chemical solubilization properties of markers. For electrophoretic MACE studies, it is advantageous for the micellar constitution (structure of micelle, type of phase micellar or lamellar) to be known for the relevant range of concentrations (surfactant, lipids). 

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 basis for the phase-separation model (Mukerjee, 1967) assumes that the occurrence of thf phase change happens at the CMC. This model uses the chemical potentials of the free surfactant ii the aqueous phaseassociated surfactant in the micellar pha 0,Gu,... 

Surfactants are frequently used in detergents and food products to alter the properties of solution interfaces, mediating between immiscible phases because of their hydrophobic and hydrophilic moieties. The addition of surfactants increases the concentration of hydrophobic compounds in the water phase by solubilization or emulsification above a specific threshold, the critical micellar concentration (CMC), where surfactant molecules aggregate to micelles [130]. Two widely utilized nonionic surfactants, Tween 80 and Triton X-100, were evaluated in terms of enzyme interaction, by calculating the inactivation coefficient (kA) under static conditions. Concentrations lower than CMC were studied in order to avoid diffusional limitations in the interaction of the enzyme and the PAH in the micellar phase. The concentration 0.25 CMC was considered the most favorable for the enzyme, with Triton X-100 being the surfactant that led to the lowest inactivation coefficients for all the concentrations tested and was 2.5 times lower than kd in control experiment. 

C is the concentration of surfactant in the micellar form (i,e, the total surfactant concentration minus the CMC), and are the dimensionless solute partition coefficients between micelles and the bulk water and between the stationary phase and the bulk water, respectively. 

When both the micellar phase and the aqueous phase were completely saturated, the observed rate constant was found to increase up to a surfactant concentration of 0-04 m, however at higher concentrations it was found to be independent of the benzaldehyde and the surfactant concentration and also of the chain length of the non-ionic amphiphile. Under these conditions the concentration of the aldehyde in the micellar phase, is given by... 

Head Space Analysis. A plot of peak area as a function of concentration for aqueous solutions of MMA was found to be a straight line as shown in Figure 4. This plot provides a Henry s Law relationship between the concentration of MMA in solution and its corresponding vapor pressure. In addition to these standard systems, the vapor pressure over samples from the Lj and microemulsion phases was also determined for the 14.7 wt% SLS aqueous solution. At low concentrations, the peak area is again linearly related to MMA concentration. As the saturation point is approached, however, the peak area increases more slowly. For any of the surfactant systems, the concentration of MMA in the continuous aqueous phase can be determined by constructing a horizontal line from the surfactant curve to the standard curve and then dropping a vertical line down to the concentration axis. The intercept is the concentration in the continuous phase and the amount of MMA in the micellar phase then follows from mass balance. Figure 4 shows that the concentration of MMA in the aqueous phase at the L j phase boundary and in the microemulsions is approximately 0.15 M. This is also the solubility limit of MMA in water. 

For low concentrations of MMA, where the surfactant system areas can be fitted to a straight line, the mole fraction of MMA in the aqueous phase, X, and in the micellar phase, X, can be calculated from a m... 

The micelle is simply a charged aggregate of surfactant ions plus an equivalent number of counterions in the surrounding atmosphere, and is treated as a separate phase. At any given temperature the chemical potential of the surfactant in the micellar state is assumed to be constant, and this may be adopted as the standard chemical potential, by analogy to a pure liquid or a pure solid. Considering the equilibrium between micelles and monomer, then... 

By treating the Krafft point as the melting point of the hydrated solid surfactant, the partition coefficient of the solute can be calculated from its effect on the Krafft point. Simple thermodynamic considerations lead to the following relationship at low mole fractions of solute in the micellar phase ... 

The experimental techniques vary in the sense that some will determine the partition coefficients directly and some will determine the fraction of solute solubilized in the micellar phase. The data are easily converted if the surfactant and solute concentrations are given. 

Initially, the vapor pressure measurements appear to be the most direct, but even here some assumptions are needed. The amount of alcohol in the micellar phase needs to be determined. To do this the difference in vapor pressure between a pure aqueous and a micellar solution is measured. If the ions of the surfactant salts out alcohol, the vapor pressure of pure water is not the correct comparison, and this could lead to lower partition coefficients. Thermodynamic data are well suited for model calculations, and both the models of DeLisi et al. ° ° and Hetu et al. fit the data well. Although in reasonable internal agreement, the partition coefficients calculated from partial molar volumes differ from those calculated from enthalpies the first is 927 or 944, the latter... 

The mechanism by means of which hydrotropes operate in surfactant solutions was elucidated by Friberg and coworkers (1970, 1971 Cox, 1981), who showed that it is due to the inhibition of the formation of surfactant liquid-crystalline phases by the hydrotrope. Since they have structures similar to those of surfactants, hydrotropes can form mixed micellar structures with surfactants. However, since their hydrophilic heads are large and their hydrophobic groups are small (their V)/ Icdo ratio [Chapter 3, Section II] is liquid-crystalline structures and thus inhibit the formation of the latter. This destruction or inhibition of the liquid-crystalline phase increases the solubility of the surfactant in the aqueous phase and the capacity of its micellar solution to solubilize material. Hydrotropic action occurs at concentrations at which the hydrotrope self-associates to form these mixed structures with the surfactant (Gonzalez, 2000). 

Congo Red and Curcumin containing MCM-41 materials have been synthesized and characterized by means of X-ray dif action, UV-Visible and FTIR spectroscopy. Dyes are incorporated in the micellar phase of silica-surfactant mesophase and their spectroscopic properties confirm that they are in a solvated state, where both surfactant and silica wall may act as a solvent. Dyes maintain their pH indicator properties and are accessible to ionic species, such as H3O and Cu. ... 

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