A SURFACE TENSION THEORY OF MICELLES Part I. Critical micelles: dimensions and surface tension
Titel:
A SURFACE TENSION THEORY OF MICELLES Part I. Critical micelles: dimensions and surface tension
Auteur:
Sørensen, Torben Smith
Verschenen in:
Chemical engineering communications
Paginering:
Jaargang 20 (1983) nr. 1-2 pagina's 93-125
Jaar:
1983-02-01
Inhoud:
A completely consistent theory of the cmc (critical micelle concentration) for nonionic and ionic micelles is described. Micellization is treated as a critical phenomenon using classical, thermodynamic nucleation theory modified to take into account a variable surface tension. The micelles are described as small oil droplets which have to surpass a critical size (the critical micelle) in order to have micellization. The driving force is the “supersaturation” of hydrocarbon chains in aqueous solution, i.e. hydrophobic forces. Real phase separation is prevented, however, because of the accumulation of head groups on the micellar surface. This accumulation leads eventually to zero or negative interfacial tension and dissolution of the micelles by Brownian motions. Indeed, the interfacial tension of critical nonionic and ionic micelles is found to be lower than the interfacial tension of a plane and pure oil-water interface. The lowering of the interfacial tension is well described as a product of a surface pressure effect and a Tolman correction for curvature in the case of nonionic micelles. For ionic micelles we have in addition the surface tension lowering of the electric double layer. The real surface potential as calculated from special plots of experimental cmc data seems to be higher than the surface potential calculated from the nonlinear, spherical Poisson-Boltzmann equation. This is in accordance with Sanfeld's local thermodynamic theory of charged and polarized layers. The higher surface potentials are explained by the decrease in dielectric constant due to the presence of counterions and by an increase in the space charge near the micelles due to volume exclusion of counterions. Large, hydrated cationic counterions are more efficient in both respects than small, unhydraied anions. Therefore, the deviations are greater for anionic micelles at high concentrations of added electrolyte. The dimension of the critical micelle varies slightly with the length of the hydrocarbon chains (wc). For nonionic micelles, the radius decreases from 13.8 Å at nc = 4 to 12.5 Å at nc = 16. The cationic micelles studied have critical radii around 11.6 Å and the anionic around 14.2 Å. The radii of equilibrium micelles are greater than the radii of the critical micelles
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