MELTING OF SHORT 1-ALCOHOL MONOLAYERS ON WATER - THERMODYNAMICS AND X-RAY-SCATTERING STUDIES

From surface tension measurements we extract the melting entropy Delta S-2D of fatty-alcohol monolayers on water. Delta S-2D is found to be 4(kB)/mol lower than in the bulk. Because of the role of the conformat ional entropy, the melting transition is discontinuous for long chains , but tends to be continuous for molecules shorter than 1-nonanol. For 1-decanol Delta S-2D is sufficiently small to allow observation of cr itical fluctuations; the diffraction peak, measured with a high resolu tion synchrotron experiment, is described by a power-law singularity w hich broadens as the temperature approaches the melting point.