Ofer E, Sloutskin E, Tamam L, Ocko B M, Deutsch M
Department of Physics, Bar-Ilan University, Ramat-Gan 52900, Israel.
Phys Rev E Stat Nonlin Soft Matter Phys. 2006 Aug;74(2 Pt 1):021602. doi: 10.1103/PhysRevE.74.021602. Epub 2006 Aug 7.
Surface freezing was detected and studied in mixtures of alcohol and alkane molecules, using surface tensiometry and surface-specific x-ray scattering methods. Considering that surface freezing in pure alkanes forms an ordered monolayer and in alcohols it forms an ordered bilayer, the length mismatch repulsion was minimized by varying the carbon number of the alkane component around 2n, where n is the carbon number of the alcohol molecule. A solutionlike behavior was found for all mixtures, where the ideal liquid mixture phase-separates upon freezing both in the bulk and the surface. The solid exhibits a herringbone crystalline phase below an alkane mole fraction phi(t) approximately 0.8 and a rotator phase above it. The surface frozen film below phi(t) is an alkane monolayer exhibiting a next-nearest neighbor molecular tilt of a composition-dependent magnitude. Above phi(t), no diffraction peaks were observed. This could be explained by the intrinsically shorter-range order of the rotator phase and a possible proliferation of defects.
利用表面张力测定法和表面特异性X射线散射方法，对酒精和烷烃分子混合物中的表面冻结现象进行了检测和研究。考虑到纯烷烃中的表面冻结形成有序单层，而在酒精中形成有序双层，通过改变烷烃组分的碳原子数使其接近2n（其中n为酒精分子的碳原子数），从而使长度失配排斥作用最小化。发现所有混合物都呈现出类似溶液的行为，即理想液体混合物在本体和表面冻结时都会发生相分离。在烷烃摩尔分数φ(t)约为0.8以下时，固体呈现出人字形晶相，在其之上则呈现出旋转相。在φ(t)以下的表面冻结膜是一个烷烃单层，表现出取决于组成的大小的次近邻分子倾斜。在φ(t)以上，未观察到衍射峰。这可以通过旋转相固有的较短程有序性以及可能的缺陷增殖来解释。
