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${\mathit{k}}_{\mathrm{B}}$/mol lower than in the bulk. Because of the role of the conformational 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 critical fluctuations; the diffraction peak, measured with a high resolution synchrotron experiment, is described by a power-law singularity which broadens as the temperature approaches the melting point.