Soaps solutions, critical concentration

This equation indicates that the log CMC falls linearly with increasing chain length and electrolyte concentration. Thus addition of electrolyte lowers the CMC but lowers the concentration of soap anion even more greatly. The addition of electrolyte thus tends to salt out the soap solution rather than cause micelle formation. Micelle formation can be induced by raising the experimental temperature. Addition of electrolytes lowers the CMC but raises the critical micellar temperature, and the latter effect is greater (41). 

Above a critical concentration, surfactant molecules form aggregates called micelles. One example of this phenomenon is a solution of the potassium or sodium salts of fatty or rosin acids. These are usually known as soaps. The addition of monomers which are insoluble in water, such as styrene or butadiene, to fte stirred soap solution results in droplets of monomer stabilised by soap molecules being formed. These droplets are approximately 1000 nm in diameter. 

Study of anomalies in a large number of physicochemical properties of solutions of soaps and detergents, e.g., equivalent conductivity, osmotic coefficient, viscosity, and transference numbers, has revealed that the detergents exist both as simple ions and as colloidal aggregates known as micelles. The transition from simple strong electrolytes to micelles begins rather abruptly in dilute solution at a critical concentration char-... 

We call the centre of the concentration range the critical micelle concentration (CMC). As an over-simplification, we say the solution has no colloidal micelles below the CMC, but effectively all the monomer exists as micelles above the CMC. As no micelles exist below the CMC, a solution of monomer is clear - like the solution of dilute soap in the bath. But above the CMC, micelles form in solution and impart a turbid aspect owing to Tyndall light scattering. This latter situation corresponds to washing the face in a sink. 

Long-chain fatty acids are insoluble in water, and their titration curves are concentration-dependent because of the formation of organized aggregates (acid soaps, soap micelles, fatty acid precipitates) which concentrate protons at the surface. At concentrations above the critical micellar concentration, solutions of long-chain fatty acid soaps manifest a diprotic curve when they are titrated from pH 10 to 4 (23). The first... 

Surface tension measurement. Adsorption titration, also called soap titration, (2.3) was carried out by the drop volume method at different polymer concentrations. The equivalent concentration of salt was held constant. The amount of emulsifier necessary to reach the critical micelle concentration (CMC) in the latex was determined by each titration. The total weight of emulsifier present in the latex is the weight of emulsifier in the water plus the weight of emulsifier adsorbed. The linear plot of emulsifier concentration (total amount of emulsifier corresponding to the end-point of each titration) versus polymer concentration gives the CMC as the intercept and the slope determines the amount of emulsifier adsorbed on the polymer surface in equilibrium with emulsifier in solution at the CMC (E ). 

It is obvious that in the theoretical treatment sketched above, S, the amount of soap, refers to the amount of micellar soap. If in a polymerization mixture a soap is used with a relatively high critical micelle concentration (C.M.C.), it is necessary to correct for the amount of soap which is molecularly dissolved and does not contribute to particle formation, at least in the mechanism considered by Smith and Ewart. Consequently, the number of particles and hence the rate of polymerization decrease sharply with increasing C.M.C., as was observed by Staudinger (59), who reported initial polymerization rates of 0.041, 0.12, and 0.225% per minute for reactions in 3% solutions of potassium caprate, laurate, and stearate, where the C.M.C.s are about 2.1, 0.60, and 0.17%, respectively. 

Fig. 1. The fatty acid soap-water phase diagram of McBain (58) modified (1) to show the molecular arrangement in relation to aqueous concentration (abscissa) and temperature (ordinate). Ideal solution, i.e., true molecular solution, is to the left of the vertical dashed line, indicating the critical micellar concentration (CMC), which varies little with temperature. At concentrations above the CMC, provided that the temperature is above the critical micellar temperature (CMT), a micellar phase is present. At high concentrations, the soap exists in a liquid crystalline arrangement, provided that the solution is above the transition temperature of the system, i.e., the temperature at which a crystalline phase becomes liquid crystalline. The Krafft point is best defined (D. M. Small, personal communication) as the triple point, i.e., the concentration and temperature at which the three phases (true solution, micelles, and solid crystals) coexist, but in the past the Krafft point has been equated with the CMT. The diagram emphasizes the requirement for micelle formation (a) a concentration above the CMC, (b) temperature above the CMT, and (c) a concentration below that at which the transition from micelles to liquid crystals occurs. Modified from Hofmann and Small (1).

Fig. 10. Phase equilibria of the bile acid (as sodium salt)-fatty acid soap-water phase diagram at constant water concentration in relation to temperature. Mixtures with varying molar ratios of bile acid/sodium soap (total concentration 40 mM) were incubated, and the temperature at which the system became clear was plotted solutions were buffered top i 12. The curves indicate the critical micellar temperature of the system and have also been termed mixed Krafft points (46). The CMT of the bile acids is extremely low.

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