MICELLAR PROPERTIES OF ZWITTERIONIC PHOSPHOBETAINE AMPHIPHILES IN AQUEOUS-SOLUTION - INFLUENCE OF THE INTERCHARGE DISTANCE

Zwitterionic amphiphiles of the general formula H(CH//2)//y ** plus N(CH//3)//2(CH//2)//n PO//2C//6H//5, where the number of intercharge methylenes n is varied, were studied in dilute aqueous solution. Their critical micellar concentrations show a peculiar variation with n, first increasing as n varies from 1 to 4 and then slowly decreasing as methylenes are added up to 10. This behavior is interpreted as being the consequence of two opposite contributions. The first is the classical CMC lowering due to the increase of hydrophobic character with the total number of methylene groups in the surfactant molecule. The second contribution is the increase in the dipole moment of the zwitterionic headgroup with n, leading to stronger dipole-dipole repulsions between headgroups at the micellar surface. Experimental results suggest that the dipole moment does not increase linearly with n because of the polymethylene chain flexibility. This is supported by **1**3CNMR relaxation experiments.