Mixed surfactant solutions

Small micelles in dilute solution close to the CMC are generally beheved to be spherical. Under other conditions, micellar materials can assume stmctures such as oblate and prolate spheroids, vesicles (double layers), rods, and lamellae (36,37). AH of these stmctures have been demonstrated under certain conditions, and a single surfactant can assume a number of stmctures, depending on surfactant, salt concentration, and temperature. In mixed surfactant solutions, micelles of each species may coexist, but usually mixed micelles are formed. Anionic-nonionic mixtures are of technical importance and their properties have been studied (38,39). 

Mixed surfactant systems are of importance from a fundamental and practical point of view. Therefore, many recent papers have reported on the micellar properties of mixed surfactant solutions. For example, Tokiwa et al. have measured the NMF spectra W Ingram has measured surface tension ( 5). Previously, we have reported the solution properties of anionic-nonlonlc surfactant mixed systems from the point of view of electrical (., 7) and surface tension measurements (8-10), and investigated the mixed micelle formation. 

Adsorption of Two Surfactants. We now denote a quantity valid at the onset of micellization in the equilibrium mixed surfactant solution by the superscript c. Thus, the chemical potential of surfactant i in the mixed solution or in the mixed surface phase at the onset of micellization is given by... 

Critical Micelle Concentration. In order to demonstrate the analogy between our treatment of mixed adsorption and earlier treatments of mixed micellization, we will briefly review the thermodynamics of mixed micelles. The thermodynamics of formation of ideal mixed micelles by two surfactants has been treated by Lange and Beck (9 ) and Cling (10). Rubingh ( ) extended the treatment to account for interactions between the surfactants, essentially by writing the cmc in the mixed surfactant solution as. 

For flow through porous media studies, the sandpacks used as porous media were flushed vertically with carbon dioxide for an hour to replace interstitial air. Distilled water was pumped and the pore volume (PV) of the porous medium was determined. By this procedure, the trapped gas bubbles in the porous media can be easily eliminated because carbon dioxide is soluble in water. For determining the absolute permeability of the porous medium, the water was pumped at various flow rates and the pressure drop across the sandpack as a function of flow rate was recorded. After the porous medium was characterized, the mixed surfactant solutions of known surface properties were injected. This was followed by air injection to determine the effect of chain length compatibility on fluid displacement efficiency, breakthrough time and air mobility in porous media. 

Table I. Effect of Properties Chain Length Compatibility on of Mixed Surfactant Solutions Surface ...

Wang, W. and Kwak, J.C.T., Adsorption at the alumina-water interface from mixed surfactant solutions. Colloids Surf. A, 156, 95, 1999. 

More or less systematic studies have been carried out on nonionic surfactants below the CMC but there is lack of systematic studies on micellar and mixed surfactant solutions. Moreover, there is an almost complete lack of studies on ionic surfactants, as discussed in Chapter 7. It seems that comprehensive experiments on adsorption dynamics can be performed on the basis of the recent theories and considerably improved experimental technique in order to understand the formation and action of dynamic adsorption layers better. This of course applies unrestrictedly to proteins, and mixed surfactant/protein systems where the level of imderstanding is even lower than for surfactants solutions (de Feijter Benjamins 1987, Serrienetal. 1992). 

To model the exchange of matter at the interface of a mixed surfactant solution the same principle can be used as for the system containing only one surfactant. However, one term for each of the r surface active compounds in the system is needed. Garrett Joos (1976) derived the complex elasticity modulus which is given by... 

Equilibrium Adsorption Properties of Single and Mixed Surfactant Solutions... 

This method has been used to prepare core-shell particles by use of mixed surfactant solutions. The core-shell structure was observed for certain pairs of surfactants which differed greatly in their surface activity in the initial ethanolic solution so that phase separation occurs during the... 

Worm-Like Micelles in Diluted Mixed Surfactant Solutions Formation and Rheological Behavior... 

There are different models developed in the past to describe the adsorption from mixed surfactant solutions, for example recently by Siddiqui and Franses (1997), Ariel et al. (1999), Mulqueen and Blankschtein (1999, 2000), Penfold et al. (2003). The simplest model is obviously a generalised Langmuir isotherm (for ideal behaviour in the bulk and at the interface) for mixtures of two surfactants 1 and 2 with similar partial molar surface area (O can be presented in the form (Fainerman et al. 2001)... 

When surfactant mixtures of practical interest containing multiple species were used (e.g., commercial nonionic surfactants or mixtures of anionic and nonionic surfactants), a maximum in hydrocarbon removal from polyester/cotton fabric similar to that in Figure 4.32 was again seen. For situations where the surfactant oil ratio in the system is large, the typical case for household washing, the maximum occurred at the PIT of a system for which surfactant composition in the films separating oil and water domains of the microemulsion phase was the same as the initial surfactant composition (Raney and Miller, 1987). This result is reasonable since the small amount of hydrocarbon present can dissolve only a small portion of the total surfactant, leaving the remainder, which has neariy the initial composition, to make up the films. It should be noted that here too the PIT is well above the cloud point temperature of the mixed surfactant solution. 

Figure 3. Comparison of wetting behaviour of ionic surfactant solution and ionic-non-ionic mixed surfactants solution. Different symbols refer to the different surface coatings superhydrophobic (filled), bare polymer (open). The first group on the left refers to pure ionic SDS (circles) and CTAB (triangles) solutions (at supramicellar concentration -h 20 mM NaCl). Left to right mixtures of SDS and CTAB with different non-ionic surfactants at c = 2 x 10 M.

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