Critical micelle concentration hydrophilic ions

Figure 20 shows the plot of the surface tension vs. the logarithm of the concentration (or-lg c-isotherms) of sodium alkanesulfonates C,0-C15 at 45°C. In accordance with the general behavior of surfactants, the interfacial activity increases with growing chain length. The critical micelle concentration (cM) is shifted to lower concentration values. The typical surface tension at cM is between 38 and 33 mN/m. The ammonium alkanesulfonates show similar behavior, though their solubility is much better. The impact of the counterions is twofold First, a more polarizable counterion lowers the cM value (Fig. 21), while the aggregation number of the micelles rises. Second, polarizable and hydrophobic counterions, such as n-propyl- or isopropylammonium and n-butylammonium ions, enhance the interfacial activity as well (Fig. 22). Hydrophilic counterions such as 2-hydroxyethylammonium have the opposite effect. Table 14 summarizes some data for the dodecane 1-sulfonates.

With alkali halide-TBA-W or alkali halide-PD-W systems, the parameters Bne are negative for volumes and heat capacities (see Figures 1-5 and 10). This sign seems to be the one usually observed for the interaction of a hydrophobic with a hydrophilic solute (6). At intermediate cosolvent concentration, AYe°(W — W + TBA) and AYe°(W — W + PD) deviate in the direction we would expect for hydrophobic association the volume increases sharply, and the heat capacity decreases further. Inorganic electrolytes lower the critical micelle concentration of surfactants by salting out the monomers, thus favoring micellization (25) in a similar way, in the co-sphere of a hydrophilic ion, the hydrophobic bonding between the cosolvent molecules may be enhanced. 

McBain pointed out that this seemingly anomalous behaviour could be explained in terms of organised aggregates, or micelles, of the surfactant ions in which the lipophilic hydrocarbon chains are orientated towards the interior of the micelle, leaving the hydrophilic groups in contact with the aqueous medium. The concentration above which micelle formation becomes appreciable is termed the critical micelle concentration (c.m.c.). 

When sodium stearate is placed in water, the hydrophilic carboxylate group encourages the formation of a solution the lipophilic alkyl chain discourages it. The compromise achieved is to form a colloidal dispersion of aggregates called micelles (Figure 18.5). Micelles form spontaneously when the carboxylate concentration exceeds a certain minimum value called the critical micelle concentration. Each micelle is composed of 50-100 individual molecules, with the polar carboxylate groups directed toward its outside where they experience attractive forces with water and sodium ions. The nonpolar hydrocarbon chains are directed toward the interior of the micelle, where individually... 

Micellar liquid chromatography is the use of a surfactant such as sodium dodecyl sulfate (SDS) in the mobile phase at a concentration above the critical micelle concentration (CMC) of about 10 M. At the CMC, aggregation of60-100 surfactant monomers occurs with the hydrophobic part of the molecule oriented toward the center of the assembly and the hydrophilic tail exposed to the solution. Other surfactants used have been cationic or nonionic in nature, such as cetyltrimethylammonium ion and Brij-35, respectively. For reversed-phase HPLC, the surfactant can... 

The intermicellar BS concentration (IMC) is a key parameter in such systems since it is decreased in proportion to swelling amphiphile to BS ratios [99,100], is critically influenced by micelle size [102], and determines the swelling amphiphile-BS phase limit since it controls the lipid ratio in the micellar particles [102]. As a corollary, it is greatly influenced by BS species, being much higher with the more hydrophilic C and UDC than the less hydrophilic DC and CDC [99,100,254]. The actual values have been measured directly by surfactant ion electrodes [16,144], QLS-monitored dilution effects on preserving [ 02], Sephadex G-10 exclusion... 

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