MEASUREMENT OF FORCES IN LAMELLAR AND HEXAGONAL PHASES OF ALKYL ESTERS OF ACYLCARNITINE BY OSMOTIC-STRESS TECHNIQUE

It has been recently reported that alkyl esters of acyl-L-carnitine sh ow a lyotropic liquid crystalline polymorphism. Depending on the relat ive length of the hydrocarbon chains, two different phase behaviours w ere detected: in addition to the lamellar L alpha phase, compounds bea ring a relatively short alkyl chain exhibit hexagonal H and cubic Ia3d type I phases. In this paper, X-ray diffraction and osmotic stress ex periments have been performed in order to directly measure as a functi on of distance the forces between structure elements, i.e., lamellae i n the L alpha phase and rods in the H phase. In both cases, exponentia lly decaying repulsive forces dominate the interactions at close separ ation. The magnitude and decay length of these forces are very similar . The exponential nature of the repulsion and the value of the decay l ength led us to conclude that the dominant force experienced by the st ructure element surfaces approaching contact is hydration repulsion. T his force represents the work of removing water from the vicinity of t he approaching surfaces.