Surfactants bulk solution

More rigorous thermodynamic relations valid for adsorption layers which undergo a phase transition could be derived based on the requirement that the chemical potentials in either phase should be equal to each other. The phases are represented by the surfactant bulk solution, the non-condensed (surface solution) and the condensed part of the surface layer. The dependence of p- on the composition of a surface layer is given by the Butler equation (2.7). The chemical potential of the i component in the condensed phase comprised of the given component only (f x = 1) can be derived from Eq. (2.7) as... 

Effects of Surfactants on Solutions. A surfactant changes the properties of a solvent ia which it is dissolved to a much greater extent than is expected from its concentration effects. This marked effect is the result of adsorption at the solution s iaterfaces, orientation of the adsorbed surfactant ions or molecules, micelle formation ia the bulk of the solution, and orientation of the surfactant ions or molecules ia the micelles, which are caused by the amphipathic stmcture of a surfactant molecule. The magnitude of these effects depends to a large extent on the solubiUty balance of the molecule. An efficient surfactant is usually relatively iasoluble as iadividual ions or molecules ia the bulk of a solution, eg, 10 to mol/L. 

The expected surfactant distribution is also portrayed qualitatively in Figure 2. At low Ca, recirculation eddies in the liquid phase lead to two stagnation rings around the bubble, as shown by the two pairs of heavy black dots on the interface (18>19). Near the bubble front, surfactant molecules are swept along the interface and away from the stagnation perimeter. They are not instantaneously replenished from the bulk solution. Accordingly, a surface stress, rg, develops along the interface... 

If the supply of surfactant to and from the interface is very fast compared to surface convection, then adsorption equilibrium is attained along the entire bubble. In this case the bubble achieves a constant surface tension, and the formal results of Bretherton apply, only now for a bubble with an equilibrium surface excess concentration of surfactant. The net mass-transfer rate of surfactant to the interface is controlled by the slower of the adsorption-desorption kinetics and the diffusion of surfactant from the bulk solution. The characteris-... 

In dilute solutions of surfactants adsorption processes are controlled by transport of the surfactant from the bulk solution towards the surface as a result of the concentration gradient formed in the diffusion layer the inherent rate of adsorption usually is rapid. For non-equilibrium adsorption the apparent (non-equilibrium) isotherm can be constructed for different time periods that are shifted with respect to the true adsorption isotherm in the direction of higher concentration (Cosovic, 1990) (see Fig. 4.10). 

Application of Activity at cmc. The above consideration suggested us to propose a new treatment for ionic micelle formation. According to thermodynamics, the micelle-monomer equilibrium is achieved when the chemical potential of surfactant in the micelle is equal to that in the bulk solution. The free energy of micelle formation can be represented by the use of the critical micelle activity, cma, which is the activity of surfactant at the cmc, as... 

LDAO/SDS Interaction. Mixing of cationic and anionic surfactant solutions results In the formation of a mixed species that Is more surface active than the Individual species. The enhanced synergistic effect has been explained (2,3) by showing that a close-packed adsorption of electroneutral R R takes place (R" " and R represent the long chain cation and anion respectively). In the case of Ci2 and C14-DAO, a 1 1 LDAO/SDS molar ratio produces a minimum In surface tension and Is accompanied by an Increase In pH In the bulk solution the association seems to be of the type R R", and the absence of visible precipitate may be attributed to the solubilization of the R R" complex In the solution. In the region where LDAO Is In excess, the structure Is probably [cationic (LDAOH ) anionic (SDS)] nonlonlc (LDAO), while [cationic (LDAOH anionic (SDS)] anionic (SDS) Is formed when SDS Is In excess. Equal molar concentration results In cationic (LDAOH ) anionic (SDS) complex which should favor precipitation. However, at pH >9, there Is no Indication of precipitation (even when the total solute concentration Is 0.35 M). When the pH Is below 9, then precipitation will take place. 

This transition may j-.e. reducing the specific surface energy, f. The reduction of f to sufficiently small values was accounted for by Ruckenstein (15) in terms of the so called dilution effect". Accumulation of surfactant and cosurfactant at the interface not only causes significant reduction in the interfacial tension, but also results in reduction of the chemical potential of surfactant and cosurfactant in bulk solution. The latter reduction may exceed the positive free energy caused by the total interfacial tension and hence the overall Ag of the system may become negative. Further analysis by Ruckenstein and Krishnan (16) have showed that micelle formation encountered with water soluble surfactants reduces the dilution effect as a result of the association of the the surfactants molecules. However, if a cosurfactant is added, it can reduce the interfacial tension by further adsorption and introduces a dilution effect. The treatment of Ruckenstein and Krishnan (16) also highlighted the role of interfacial tension in the formation of microemulsions. When the contribution of surfactant and cosurfactant adsorption is taken into account, the entropy of the drops becomes negligible and the interfacial tension does not need to attain ultralow values before stable microemulsions form. 

In general, the adsorption of a surfactant on particles with previously adsorbed polymer can be influenced by (i) a reduction of surface area available for adsorption as a result of the presence of adsorbed polymer, (ii) possible interactions between polymer and surfactant in the bulk solution or in the interfacial region (that is, surfactant with loops, tails or trains of adsorbed polymer molecules), (iii) the steric effect of adsorbed polymer, preventing approach of surfactant molecules for adsorption at the surface, or (iv) possible electrostatic effects if polymer and/or surfactant are charged species. 

On the basis of the above experimental results, the expected conformations of polymer-surfactant complexes at the oil-water interface are depicted in Fig. 2.19. In case I, the added polymer associates with excess surfactants present in the bulk solution, but the complexes prefer to remain in the bulk phase. Alternately, the polymer-surfactant complexes are unable to displace the adsorbed surfactant molecules from the liquid-liquid interface. Irrespective of the amount of polymer-surfactant concentration in the bulk, the experimental decay length values remain comparable to the Debye lengths, corresponding to the concentration of ion species in the bulk solution (Eq. (2.11)). This means that the force profile is... 

We commence with the adsorption of nonionic surfactants, which does not require the consideration of the effect of the electrical double layer on adsorption. The equilibrium distribution of the surfactant molecules and the solvent between the bulk solution (b) and at the surface (s) is determined by the respective chemical potentials. The chemical potential /zf of each component i in the surface layer can be expressed in terms of partial molar fraction, xf, partial molar area a>i, and surface tension y by the Butler equation as [14]... 

Fig. 8 Results of the regression analysis of Eq. 56 for surface potential of the air-water interface with the adsorption of alkali dodecyl sulfate molecules as a function of the surfactant concentration in the bulk solution...

The polarity within a surfactant assembly will be quite different from that of the bulk solution. It is useful to know the micropolarity of these assem-bhes for such applications where different substrates are compartmentalized inside these surfactants. The micropolarity of the surfactant assembhes can be determined using any fluorescence probe whose emission characteristics change with solvent polarity. The emissions of the probe are measured in solvents of known polarities and the polarity of the surfactant assembhes is determined by comparison. 

The fluidity (nanoviscosity) in an organized surfactant assembly on soUds can be substantially different from that in the bulk aqueous phase and hence, the diffusional resistance experienced by the probe in the micelle will be considerably different from that faced in the bulk solution [ 145]. Measurement of the viscosity or fluidity of the interior of a micelle is based on measurement of fluorescence properties that depend on the mobihty of the probe in the interior. A commonly used method for such studies involves the intramoleciflar... 

The change in a, is caused by the change in bulk solute concentration. This is the Gibbs surface tension equation. Basically, these equations describe the fact that increasing the chemical potential of the adsorbing species reduces the energy required to produce new surface (i.e. y). This, of course, is the principal action of surfactants, which will be discussed in more detail in a later section. 

Figure 2. The calculated surfactant composition in the micelles, x , as a function of the surfactant composition in the bulk solution, a. The curves are calculated from Equation 28 with X = 0 (ideal case, solid line) and X (dashed line). The experimental points are calculated from Equation 31.

Table II presents the experimental data, obtained from using bulk solutions of different NP-EO q/SDS ratios. Figure 6 shows the surfactant composition on the polystyrene latex surface as a function of the surfactant composition in the bulk solution at concentrations corresponding to the onset of micellization. If the surfactant composition on the surface were the same as that in the bulk solution, the experimental points would fall on the dashed line in the figure. Thus, the...

Figure 5. Dependence of the surfactant composition in the bulk solution on the total surfactant concentration, a) cmc curve and b) in the presence of latex. The crossing of the curves gives the solution composition and concentration at close packing of the surfactants on the PS surface.

In particular, we would like to point out two conclusions of practical importance. Firstly, a surface analysis of the serum (bulk solution) cannot give direct information on either the surfactant composition on the latex surface or in the total system. This is an important conclusion since such analyses are frequently carried out in industrial laboratories. Secondly, Figure 6 shows that if NP-EOiq is added to a system stabilized with SDS, the latter will desorb. In practice, this causes foaming problems. Such problems can be predicted, as is shown below. 

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