Electrolytes micellization

Harima, Y. and K. Yamashita (1989). Electrochemical characterization of phthalocyanine thin films prepared by the electrolytic micelle disruption method. J. Phys. Chem. 93, 4184-4188. 

Surface active electrolytes produce charged micelles whose effective charge can be measured by electrophoretic mobility [117,156]. The net charge is lower than the degree of aggregation, however, since some of the counterions remain associated with the micelle, presumably as part of a Stem layer (see Section V-3) [157]. Combination of self-diffusion with electrophoretic mobility measurements indicates that a typical micelle of a univalent surfactant contains about 1(X) monomer units and carries a net charge of 50-70. Additional colloidal characterization techniques are applicable to micelles such as ultrafiltration [158]. 

The ernes of ionic surfactants are usually depressed by tire addition of inert salts. Electrostatic repulsion between headgroups is screened by tire added electrolyte. This screening effectively makes tire surfactants more hydrophobic and tliis increased hydrophobicity induces micellization at lower concentrations. A linear free energy relationship expressing such a salt effect is given by ... 

Micellar properties are affected by changes in the environment, eg, temperature, solvents, electrolytes, and solubilized components. These changes include compHcated phase changes, viscosity effects, gel formation, and Hquefication of Hquid crystals. Of the simpler changes, high concentrations of water-soluble alcohols in aqueous solution often dissolve micelles and in nonaqueous solvents addition of water frequendy causes a sharp increase in micellar size. 

An interesting change of the UV-absorbances with electrolyte concentration was observed for A18 and T18, as shown in Fig. 5. The molar extinction coefficient of A18 decreased by about 7% at 0.09 mM, and that of T18 about 10% at 0.16 mM. These concentrations may correspond to the critical micelle concentration, since the cmc observed from the surface tension measurements were about 0.1 mM for both A18 and T18. 

FIG. 1 Critical micelle concentration as a function of the number of carbon atoms in the hydrophobic rest of sodium a-sulfo fatty acid methyl esters. Methods O, surface tension +, conductivity A, solubilization of a dye x, solubility (all without electrolyte) , surface tension with a constant electrolyte concentration of 5 x 10"2 mol/L. (From Ref. 57.)... 

The same measurements also provide values of the second virial coefficient, which corresponds to the repulsive energy between micelles. The coefficient of the Na methyl a-sulfomyristate decreases from 7.30 x 10 3 to 3.05 x 10"4 ml/ g with increasing concentration of the electrolyte. The second virial coefficient of the calcium salt is small and changes to a negative value in 0.01 N Ca(N03)2. 

The secondary solubilization of electrolytes, namely, their solubilization into water solubilized in the micelle, has been widely investigated [125-127]. 

Electrolytes are obviously solubilized only in the aqueous micellar core. Adding electrolytes in water-containing AOT-reversed micelles has an effect that is opposite to that observed for direct micelles, i.e., a decrease in the micellar radius and in the intermicellar attractive interactions is observed. This has been attributed to the stabilization of AOT ions at the water/surfactant interface [128]. 

Effects on the micellar shape are also induced by electrolyte addition. It has been observed that, in decane, the water-containing AOT-reversed micelles become more spherical upon addition of salt (NaCl, CaCli) [6]. 

The addition of salts modifies the composition of the layer of charges at the micellar interface of ionic surfactants, reducing the static dielectric constant of the system [129,130]. Moreover, addition of an electrolyte (NaCl or CaCli) to water-containing AOT-reversed micelles leads to a marked decrease in the maximal solubihty of water, in the viscosity, and in the electrical birefringence relaxation time [131],... 

Stigter, D, Kinetic Charge of Colloidal Electrolytes from Conductance and Electrophoresis. Detergent Micelles, Poly(methacrylates), and DNA in Univalent Salt Solutions, Journal of Physical Chemistry 83, 1670, 1979. 

It was mentioned previously that the narrow range of concentrations in which sudden changes are produced in the physicochemical properties in solutions of surfactants is known as critical micelle concentration. To determine the value of this parameter the change in one of these properties can be used so normally electrical conductivity, surface tension, or refraction index can be measured. Numerous cmc values have been published, most of them for surfactants that contain hydrocarbon chains of between 10 and 16 carbon atoms [1, 3, 7], The value of the cmc depends on several factors such as the length of the surfactant chain, the presence of electrolytes, temperature, and pressure [7, 14], Some of these values of cmc are shown in Table 2. 

Like other emulsifiers, an EUP forms micelles at a critical micelle concentration (CMC). For comonomer-free EUP-emulsions of the (MA+HD)- type the CMC is about 5 X 10"4 g/ml [115,118]. The CMC depends on the composition and chain length of the polyester, the presence of an electrolyte [118] and the temperature. 

The incorporation of comonomers increases the mean hydrodynamic diameter of EUP-micelles, dz (Fig. 16). Contrary to CMC, the dz of micelles resp. microemulsion droplets increases with the concentration of an external electrolyte. 

Because the presence of an electrolyte increases the dimensions of micelles and microemulsion droplets [115], it may be expected that in presence of ions the size of microgels is also increased. This expectation could be confirmed external electrolyte increases Mw (Fig. 21) as well as dz and [r ] (Fig. 22) up to the limit of the emulsion stability. Therefore, the addition of an external electrolyte to the reaction mixture for the ECP of EUP and comonomers is a means to vary the molar mass, the diameter and the intrinsic viscosity of microgels from EUP and comonomers deliberately. 

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