Some micellar properties of long-chain acylcarnitines.

The acid dissociation constants of long-chain esters of carnitine (5-hydroxy-~,trimethylammonium-butyrate) above the critical micelle concentration were determined potentiometrieally at several concentrations of added KC1. As the degree of protonation 5 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 ~ was used to determine the surface potential of the mieelle ,I* at that degree of protonation. At each degree of protonation the measured surface potential was related to the surface charge density ~ 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 pMmitylcarnitine 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 mieelle 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.