Surface freezing of chain molecules at the liquid-liquid and liquid-air interfaces

Surface freezing (SF) is the formation of a crystalline monolayer at the free surface of a melt at a temperature T_s, a few degrees above the bulk freezing temperature, T_b. This effect, i.e. T_s > T_b, common to many chain molecules, is in a marked contrast with the surface melting effect, i.e. T_s ≤ T_b, 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, |Δn|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 T_i > T_b. The measurements indicate that a single frozen monolayer is formed, with a temperature-existence range of up to 10°C, much larger than the 1.2°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, δT = T_mix - T_b ≤ T_i - T_b by cooling the system below T_i. 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 T_i is also discussed and a tentative theoretical explanation is suggested.