Critical micelle concentration added electrolyte

The concentration at which micelle formation becomes significant is called the critical micelle concentration (cmc). The one is a property of the surfactant and several other factors, including the temperature, pressure, and the presence and nature of additives, since micellization is opposed by thermal and electrostatic forces. A low cmc is produced by increasing the molecular mass of the lipophilic part of the molecule, lowering the temperature (usually), and adding electrolyte (usually). For exam-... 

The concentration of AOT in all experiments was 2.5 x 10-8 M, which roughly corresponds to its critical micelle concentration (cmc) in the absence of an electrolyte. When an electrolyte is added, the cmc concentration is lowered. For the ionic strengths employed in the experiments of Sentenac and Bennatar,19 the surface tension reached saturation well before 2.5 x 10-3 M.19 It will be therefore assumed that in all those experiments, the amount of AOT adsorbed did not change and that each surfactant molecule occupies in average an area of 60 A2, which is in agreement with the values (40—70 A2) reported in the literature.22... 

Here, d is the micelle hydrodynamic diameter (usually measured by dynamic light scattering) as before, CMC stands for the critical micellization concentration, C, is the total concentration of ionic surfactant 4 is the ionic strength due to added inorganic electrolyte (if any), and is the degree of ionization of the micelle surface ionizable groups (non-neutralized by bound counterions). 

Measuring the surface tension in order to determine the critical micelle concentration (cmc) is also straightforward. Figure 4 shows plots of the surface tension versus surfactant concentration for CTAB and sodium do-decyl sulfate (SDS) solutions. The cmc can be determined as the intersection of the straight lines through the two linear portions of the semilogarithmic plot the values determined are close to literature values. The familiar dip in surface tension at the cmc for SDS due to the presence of impurities is also clearly apparent. Although the data are not shown, this method can also be used effectively to measure the cmc in the presence of added electrolyte [13]. 

In order to separate neutral compounds, Terabe et al. [13] added surfactants to the buffer electrolyte. Above their critical micellar concentration (cmc), these surfactants form micelles in the aqueous solution of the buffer electrolyte. The technique is then called Micellar electrokinetic capillary chromatography, abbreviated as MECC or MEKC. Micelles are dynamic structures consisting of aggregates of surfactant molecules. They are highly hydrophobic in their inner structure and hydrophilic at the outer part. The micelles are usually... 

SDS/NaCI Mixtures. The effect of temperature on the micelles formed in 70 mM SDS + NaCl solutions is presented below. Mazer et al. (14) have found that the aggregation number, N, is at a maximum for supercooled solutions below the critical micellization temperature (cmt), and decreases towards the value expected for a spherical micelle as the temperature is increased. The variations in N with temperature are dependent on the concentration of added electrolyte, with the rodlike micelles formed in high salt (0.6 M) showing large variations, and the spherical micelles formed in little (0.3 M) or no salt showing only small variations. 

In our model study reported in this contribution, we have chosen two double-chained C-13 alkylbenzenesulphonate surfactants (SLABS) of closely-related structure, which form micelles in aqueous solution in the absence of salt. However, when small amounts of electrolyte are added (e.g., —20mM NaCl), vesicles are spontaneously formed over a time period of seconds/minutes. These vesicle structures are then reasonably stable over a period of hours/days. The onset of vesicle formation can be readily characterised by the determination of the critical salt concentration (esc), needed to induce the formation of vesicles, from smaller aggregates or monomers. This parameter is easily determined experimentally from the increase in light scattering associated with self-assembly. It has now been determined for a number of electrolyte systems. 

Successful attempts have been made to modify/minimize preeipitation in polyelectrolyte/oppositely charged surfactant systems. Laurent and Scott (65) reported such an effect with the addition of simple salts and defined a critical electrolyte concentration (c.e.c.) at which precipitation is totally inhibited. (See Chapter 5 and also Section III.E below.) Likewise, Dubin et al. (66,67) have found inhibitory effects on adding nonionic surfactants to these mixed polymer/surfactant systems, presumably a result of mixed micelle formation. 

---