PHASE-TRANSITIONS IN GIBBS FILMS - STAR TEXTURAL DEFECTS IN TILTED MESOPHASES

The formation of adsorbed films at the free surface of aqueous solutio ns of sodium hexadecanoate was observed by microscopy at the Brewster angle. A phase transition between a gas and a tilted phase occurs duri ng these slow formations. The tilt of the molecules creates an observa ble optical anisotropy. We observed circular domains separated into re gions of uniform tilt direction by straight defect lines, i.e., ''star '' textural defects very similar to those observed in freely suspended liquid crystal thin films and in Langmuir films of esters. The coales cence of domains leads to large regions of uniform tilt direction sepa rated by straight or kink lines. A second transition towards a nontilt ed phase is observed. Quantitative information about the coefficients of the continuum elastic theory can be derived from our experiments.