Aqueous phase critical micelle concentrations

Table 1. Aqueous Phase Critical Micelle Concentrations (erne s), Limiting Surface Tensions yeme s and Microemulsion Stability Pressures for Fluorinated Surfactants.

During Stage I the number of polymer particles range from 10 to 10 per mL. As the particles grow they adsorb more emulsifier and eventually reduce the soap concentration below its critical micelle concentration (CMC). Once below the CMC, the micelles disappear and emulsifier is distributed between the growing polymer particles, monomer droplets, and aqueous phase. 

Emulsification is the process by which a hydrophobic monomer, such as styrene, is dispersed into micelles and monomer droplets. A measure of a surfactant s abiUty to solubilize a monomer is its critical micelle concentration (CMC). Below the CMC the surfactant is dissolved ia the aqueous phase and does not serve to solubilize monomer. At and above the CMC the surfactant forms spherical micelles, usually 50 to 200 soap molecules per micelle. Many... 

The molecular collective behavior of surfactant molecules has been analyzed using the time courses of capillary wave frequency after injection of surfactant aqueous solution onto the liquid-liquid interface [5,8]. Typical power spectra for capillary waves excited at the water-nitrobenzene interface are shown in Fig. 3 (a) without CTAB (cetyltrimethy-lammonium bromide) molecules, and (b) 10 s after the injection of CTAB solution to the water phase [5]. The peak appearing around 10-13 kHz represents the beat frequency, i.e., the capillary wave frequency. The peak of the capillary wave frequency shifts from 12.5 to 10.0kHz on the injection of CTAB solution. This is due to the decrease in interfacial tension caused by the increased number density of surfactant molecules at the interface. Time courses of capillary wave frequency after the injection of different CTAB concentrations into the aqueous phase are reproduced in Fig. 4. An anomalous temporary decrease in capillary wave frequency is observed when the CTAB solution beyond the CMC (critical micelle concentration) was injected. The capillary wave frequency decreases rapidly on injection, and after attaining its minimum value, it increases... 

Oheme and co-workers investigated335 in an aqueous micellar system the asymmetric hydrogenation of a-amino acid precursors using optically active rhodium-phosphine complexes. Surfactants of different types significantly enhance both activity and enantioselectivity provided that the concentration of the surfactants is above the critical micelle concentration. The application of amphiphilized polymers and polymerized micelles as surfactants facilitates the phase separation after the reaction. Table 2 shows selected hydrogenation results with and without amphiphiles and with amphiphilized polymers for the reaction in Scheme 61.335... 

What characterizes surfactants is their ability to adsorb onto surfaces and to modify the surface properties. At the gas/liquid interface this leads to a reduction in surface tension. Fig. 4.1 shows the dependence of surface tension on the concentration for different surfactant types [39]. It is obvious from this figure that the nonionic surfactants have a lower surface tension for the same alkyl chain length and concentration than the ionic surfactants. The second effect which can be seen from Fig. 4.1 is the discontinuity of the surface tension-concentration curves with a constant value for the surface tension above this point. The breakpoint of the curves can be correlated to the critical micelle concentration (cmc) above which the formation of micellar aggregates can be observed in the bulk phase. These micelles are characteristic for the ability of surfactants to solubilize hydrophobic substances in aqueous solution. So the concentration of surfactant in the washing liquor has at least to be right above the cmc. 

In recent studies, Friberg and co-workers (J, 2) showed that the 21 carbon dicarboxylic acid 5(6)-carboxyl-4-hexyl-2-cyclohexene-1-yl octanoic acid (C21-DA, see Figure 1) exhibited hydrotropic or solubilizing properties in the multicomponent system(s) sodium octanoate (decanoate)/n-octanol/C2i-DA aqueous disodium salt solutions. Hydrotropic action was observed in dilute solutions even at concentrations below the critical micelle concentration (CMC) of the alkanoate. Such action was also observed in concentrates containing pure nonionic and anionic surfactants and C21-DA salt. The function of the hydrotrope was to retard formation of a more ordered structure or mesophase (liquid crystalline phase). 

In this system, in the aqueous phase, the micellar system, NaDDS, on addition of butanol would change in free energy due to mixed micelle formation (i. je. NaDDS+n-Butanol), as we showed in an earlier study (23). The cahnge in free energy is also observed from the fact that both the critical micelle concentration (c.m.c.) and the Krafft point of NaDDS solution change on addition of n-Butanol (23,... 

Partition coefficients of surfactants have been reported to remain constant below the critical micelle concentration (CMC), and to increase with concentration above the CMC (2,9,10). The effect of surfactant concentration in the aqueous phase (C ) on K was investigated with Makon 14 (14 mol% ethylene oxide, NPEj ), the results are given in Fig. 2. These data indicate a CMC of about O.lg/1, or 12 piM, in close agreement with the value obtained by surface tension measurements (our data and ref. 22). In subsequent determinations of Kp, C was just below the CMC to minimize the effects of micellization (15,23). 

In MEKC, the supporting electrolyte medium contains a surfactant at a concentration above its critical micelle concentration (CMC). The surfactant self-aggregates in the aqueous medium and forms micelles whose hydrophilic head groups and hydrophobic tail groups form a nonpolar core into which the solutes can partition. The micelles are anionic on their surface, and they migrate in the opposite direction to the electroosmotic flow under the applied current. The differential partitioning of neutral molecules between the buffered aqueous mobile phase and the micellar pseudostationary phase is the sole basis for separation as the buffer and micelles form a two-phase system, and the analyte partitions between them (Smyth and McClean 1998). 

A micelle is an aggregate of surfactants. In water, the hydrophobic tails form clusters that are, in effect, little oil drops insulated from the aqueous phase by the ionic head groups. At low concentration, surfactant molecules do not form micelles. When their concentration exceeds the critical micelle concentration, spontaneous aggregation into micelles occurs.8 Isolated surfactant molecules exist in equilibrium with micelles. Nonpolar organic solutes... 

In MLC, the mobile phase consists of surfactants at concentrations above their critical micelle concentration (CMC) in an aqueous solvent with an alkyl-bonded phase (52). Retention behavior in MLC is controlled by solute partitioning from the bulk solvent into micelles and into stationary phase as well as on direct transfer from the micelles in the mobile phase into the stationary phase. Eluent strength in MLC is inversely related to micelle concentration. A linear relationship exists between the inverse of retention factor and micelle concentration. Similar to what is observed in RPLC, a linear relationship exists between retention in MLC and , the volume fraction of the organic modifier. Modeling retention in MLC is much more complicated than in RPLC. The number of parameters is important. Micelles are obviously a new domain in both liquid chromatography and electrophoresis. Readers interested in the topic will appreciate Ref. 53, a special volume on it. 

---