A thin–thick transition in the surface-frozen layer of a binary alcohol mixture

Salt-induced stability and modified interfacial energetics in self-faceting emulsion droplets

HYPOTHESIS

The counterintuitive temperature-controlled self-faceting of water-suspended, surfactant-stabilized, liquid oil droplets provides new opportunities in engineering of smart liquids, the properties of which are controllable by external stimuli. However, many emulsions exhibiting self-faceting phenomena have limited stability due to surfactant precipitation. The emulsions' stability may be enhanced, and their inter-droplet electrostatic repulsion tuned, through controlled charge screening driven by varying-concentration added salts. Moreover, in many technologically-relevant situations, salts may already exist in the emulsion's aqueous phase. Yet, salts' impact on self-faceting effects has never been explored. We hypothesize that the self-faceting transitions' temperatures, and stability against surfactant precipitation, of ionic-surfactants-stabilized emulsions are significantly modified by salt introduction.

EXPERIMENTS

We explore the temperature-dependent impact of NaCl and CsCl salt concentration on the emulsions' phase diagrams, employing optical microscopy of emulsion droplet shapes and interfacial tension measurements, both sensitive to interfacial phase transitions.

FINDINGS

A salt concentration dependent increase in the self-faceting transition temperatures is found, and its mechanism elucidated. Our findings allow for a significant enhancement of the emulsions' stability, and provide the physical understanding necessary for future progress in research and applications of self-faceting phenomena in salt-containing emulsions.

Surface nanocrystallization of an ionic liquid

Surface crystallization at the vapor-liquid interface of the ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate) is observed in synchrotron x-ray diffraction studies. Sharp Bragg reflections emerge in grazing-angle x-ray diffraction patterns 37 °C above the bulk melting temperature, indicating the presence of a long-range ordered phase at the surface in coexistence with the bulk parent liquid. The unique structure of the vapor-liquid interface where butyl chains attached to the cations are expelled to the vapor side facilitates interionic electrostatic interactions that lead to the crystallization. Our results demonstrate the complexity of ionic-liquid structure with their tendency to spontaneously phase separate into nanodomains with finite correlation lengths in coexistence with the liquid phase. By virtue of interfacial boundary conditions, these nanodomains grow laterally to form quasi-two-dimensional crystals.

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 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.