Sloutskin Eli, Bain Colin D, Ocko Benjamin M, Deutsch Moshe
Physics Department, Bar-Ilan University, Ramat-Gan 52900, Israel.
Faraday Discuss. 2005;129:339-52; discussion 353-66. doi: 10.1039/b405969g.
Surface freezing (SF) is the formation of a crystalline monolayer at the free surface of a melt at a temperature Ts, a few degrees above the bulk freezing temperature, Tb. This effect, i.e. Ts > Tb, common to many chain molecules, is in a marked contrast with the surface melting effect, i.e. Ts < or = Tb, shown by almost all other materials. Depending on chain length, n, the SF layer shows a variety of phases, in some cases tuneable by bulk additives. The SF behaviour of binary mixtures of different-length alkanes and alcohols is governed by the relative chain length mismatch, /delta n/n/2, yielding a quasi-"universal" behaviour for the freezing of both bulk and surface. While SF at the liquid air interface was studied rather extensively, Lei and Bain (Phys. Rev. Lett., 2004, 94, 176103) have shown only very recently that interfacial freezing (IF) can be induced also at the water: tetradecane interface by adding the ionic surfactant CTAB to the water phase. We present measurements of the interfacial tension of the water: hexadecane interface, as a function of temperature and the ionic surfactant STAB, revealing IF at a STAB-concentration-dependent temperature Ti > Tb. The measurements indicate that a single frozen monolayer is formed, with a temperature-existence range of up to 10 degrees C, much larger than the 1.2 degrees C found for SF at the free surface of the melt. We also find a new effect, where the IF allows tuning of the interfacial tension between the two bulk phases to zero for a range of temperatures, deltaT = Tmix - Tb < or = Ti - Tb by cooling the system below Ti. We discuss qualitatively the factors stabilizing the frozen layer and their variation from the liquid-air to the liquid-liquid interfaces. The surfactant concentration dependence of Ti is also discussed and a tentative theoretical explanation is suggested.
表面冻结（SF）是指在温度Ts（比体相冻结温度Tb高几度）下，熔体的自由表面形成一层晶体单分子层。这种Ts > Tb的效应在许多链状分子中很常见，这与几乎所有其他材料所表现出的表面熔化效应（即Ts ≤ Tb）形成了鲜明对比。根据链长n的不同，SF层呈现出多种相态，在某些情况下可通过体相添加剂进行调节。不同长度的烷烃和醇的二元混合物的SF行为受相对链长不匹配量/δn/n/2的控制，这使得体相和表面的冻结呈现出准“通用”行为。虽然对液体空气界面的SF进行了相当广泛的研究，但Lei和Bain（《物理评论快报》，2004年，94卷，176103页）直到最近才表明，通过向水相中添加离子表面活性剂CTAB，在水：十四烷界面也能诱导界面冻结（IF）。我们给出了水：十六烷界面的界面张力随温度和离子表面活性剂STAB变化的测量结果，揭示了在高于Tb的、与STAB浓度相关的温度Ti下出现的IF。测量结果表明形成了单个冻结单分子层，其温度存在范围高达10℃，远大于在熔体自由表面发现的SF的1.2℃。我们还发现了一种新效应，即通过将系统冷却至低于Ti，IF可使两个体相之间的界面张力在一系列温度范围内（δT = Tmix - Tb ≤ Ti - Tb）调至零。我们定性地讨论了稳定冻结层的因素及其从液 - 气界面到液 - 液界面的变化。还讨论了Ti对表面活性剂浓度的依赖性，并给出了一个初步的理论解释。
