Some micellar properties of long-chain acylcarnitines

The acid dissociation constants of long-chain esters of carnitine (β-hydroxy-γ-trimethylammonium-butyrate) above the critical micelle concentration were determined potentiometrically at several concentrations of added KCl. As the degree of protonation β increases the apparent pK values decrease owing to the increased positive charge on the micelle. The difference in pK between the neutral (zwitterionic) micelle and the value at any given β was used to determine the surface potential of the micelle Ψ at that degree of protonation. At each degree of protonation the measured surface potential was related to the surface charge density σ with the aid of the calculations of Loeb, Wiersema, and Overbeek for a spherical impenetrable particle. The surface potentials and surface charge densities of lauryl-, myristyl-, and palmitylcarnitine are nearly identical at a given degree of protonation and ionic strength, and, as expected, increasing the ionic strength produces a decrease in the surface potential. From the partial molal volume of each surfactant in the micelle and the calculated surface charge density it was possible to calculate the aggregation number n of the micelle. Good agreement was found between the calculated values of n and values obtained from light-scattering experiments at several ionic strengths and degrees of protonation.