Critical micelle concentration counterion

Fig. 3. Schematic diagram of anionic surfactant solution at equiUbrium above its critical micelle concentration, where M = micelle and 0 are counterions ...

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.

For a surface active betaine ester the rate of alkaline hydrolysis shows significant concentration dependence. Due to a locally elevated concentration of hydroxyl ions at the cationic micellar surface, i.e., a locally increased pH in the micellar pseudophase, the reaction rate can be substantially higher when the substance is present at a concentration above the critical micelle concentration compared to the rate observed for a unimeric surfactant or a non-surface active betaine ester under the same conditions. This behavior, which is illustrated in Fig. 10, is an example of micellar catalysis. The decrease in reaction rate observed at higher concentrations for the C12-C18 1 compounds is a consequence of competition between the reactive hydroxyl ions and the inert surfactant counterions at the micellar surface. This effect is in line with the essential features of the pseudophase ion-exchange model of micellar catalysis [29,31]. 

The specific case to be considered is that of the Cl" counterion surface in contact with a dilute salt solution—e.g., at 10 2N NaCl where 0 = 28° (Table II). Data of Stigter (33) show that the fraction of Cl" counterions outside the shear surface of dodecyl ammonium chloride micelles is about 0.4 in the range 0.02-0.05M NaCl plus surfactant at the critical micelle concentration (CMC). This value appears to rise toward 0.5-0.6 as NaCl concentration is lowered. Conductance data of Robins and Thomas (30) indicate that 73% of Cl" counterions are bound to 2-dodecyl aminoethanol hydrochloride micelles at the CMC in the absence of added salt. 

We have examined the stmcture of both ionic and nonionic micelles and some of the factors that affect their size and critical micelle concentration. An increase in hydrophobic chain length causes a decrease in the cmc and increase of size of ionic and nonionic micelles an increase of polyoxyethylene chain length has the opposite effect on these properties in nonionic micelles. About 70-80% of the counterions of an ionic surfactant are bound to the micelle and the nature of the counterion can influence the properties of these micelles. Electrolyte addition to micellar solutions of ionic surfactants reduces the cmc and increases the micellar size, sometimes causing a change of shape from spherical to ellipsoidal. Solutions of some nonionic surfactants become cloudy on heating and separate reversibly into two phases at the cloud point. 

Fig. 9 is a schematic phase diagram of a dilute aqueous cationic surfactant solution showing temperature and concentration effects on its microstructures. When the temperature is lower than the Krafft point [the temperature at which the solubility equals the critical micelle concentration (CMC)], the surfactant is partially in crystal or in gel form in the solution. At temperatures above the Krafft point and concentrations higher than the CMC, spherical micelles form in the surfactant solution. With further increase in concentration and/or on addition of counterions, the micelles form cylindrical rods or threads or worms with entangled thread-like and sometimes branched threadlike structures. 

Fraction of counterions bound to micelle, P = 1 — a Critical micelle concentration... 

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). 

Micelles are the simplest organised form of the self-assembly produced by amphiphilic molecules due to the so-called hydrophobic effect , firstly recognized by Tanford.NMR parameters experience dramatic effects as a result of the strong intermolecular interactions among the amphiphiles. In the case of isotropic liquid systems, NMR experiments can be easily performed and modelled, since many advances have been produced in the last two decades.Hence, information on critical micelle concentration (c.m.c.), molecular conformations and interactions, counterion binding, hydration can be obtained from chemical shifts, relaxation, and self-diffusion NMR measurements, also in mixed systems. 

Surfactant molecules are made up of two moieties that have antagonistic properties, a polar or electrically charged hydrophilic moiety and a hydrophobic moiety, most often an alkyl chain. In aqueous solution, most surfactants self-assemble and form micelles when their concentration becomes larger than the so-called critical micellization concentration (CMC). In micelles (fromthe Creek mica, which means "grain "), the alkyl chains are in contact and form an oily core that is coated by the polar head groups. The outer layer that contains head groups, counterions (in the case of ionic surfactants), water and the first methylene group of the alkyl chain is called the palisade layer. The formation o/micelles is a cooperative process that is spontaneous and reversible. Micelles are thermodynamically stable species that are in chemical equilibrium with free surfactants. 

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