The surface structure and thermodynamics of two ionic liquids, based on the 1-alkyl-3-methylimidazolium cations, were studied by X-ray reflectivity and surface tensiometry. A molecular layer of a density approximately 18% higher than that of the bulk is found to form at the free surface of these liquids. In common with surface layering in liquid metals and surface freezing in melts of organic chain molecules, this effect is induced by the lower dimensionality of the surface. The concentrations of the oppositely charged ions within the surface layer are determined by chemical substitution of the anion. The temperature-dependent surface tension measurements reveal a normal, negative-slope temperature dependence. The different possible molecular arrangements within the enhanced-density surface layer are discussed.
Liquid droplets, widely encountered in everyday life, have no flat facets. Here we show that water-dispersed oil droplets can be reversibly temperature-tuned to icosahedral and other faceted shapes, hitherto unreported for liquid droplets. These shape changes are shown to originate in the interplay between interfacial tension and the elasticity of the droplet's 2-nm-thick interfacial monolayer, which crystallizes at some T = T s above the oil's melting point, with the droplet's bulk remaining liquid. Strikingly, at still-lower temperatures, this interfacial freezing (IF) effect also causes droplets to deform, split, and grow tails. Our findings provide deep insights into molecular-scale elasticity and allow formation of emulsions of tunable stability for directed self-assembly of complex-shaped particles and other future technologies.emulsions | membranes' buckling | topological defects | two-dimensional crystals | spontaneous emulsification O f all same-volume shapes, a sphere has the smallest surface area A. Microscopic liquid droplets are, therefore, spherical, because this shape minimizes their interfacial energy γA for a surface tension γ > 0. Spontaneous transitions to a flat-faceted shape, which increases the surface area, have never been reported for droplets of simple liquids. Here we demonstrate that surfactantstabilized droplets of oil in water, of sizes ranging from 1 to 100 μm, known as "emulsions" or "macroemulsions" (1), can be tuned to sharp-edged, faceted, polyhedral shapes, dictated by the molecular-level topology of the closed surface. Furthermore, the physical mechanism which drives the faceting transition allows the sign of γ to be switched in a controllable manner, leading to a spontaneous increase in surface area of the droplets, akin to the spontaneous emulsification (SE) (1, 2), yet driven by a completely different, and reversible, process.At room temperature, the spherical shape of our emulsions' surfactant-stabilized oil droplets indicates shape domination by γ > 0 (oil: 16-carbon alkane, C 16 ; surfactant: trimethyloctadecylammonium bromide, C 18 TAB, see SI Appendix, Fig. S1). However, the observed shape change to an icosahedron at some T = T d , below the interfacial freezing temperature T s (Fig. 1A), demonstrates that γ has become anomalously low and no longer dominates the shape. This γ-decrease upon cooling starkly contrasts with the behavior of most other liquids, where γ increases upon cooling (1). Direct in situ γ-measurements in our emulsions (SI Appendix), as well as pendant drop tensiometry of millimetersized droplets, confirm the positive dγðTÞ=dT here (Fig. 2). Wilhelmy plate method γðTÞ measurements (3, 4) on planar interfaces between bulk alkanes and aqueous C 18 TAB solutions (blue circles in Fig. 2A) also demonstrate the same dγðTÞ=dT > 0 at T < T s . Thus, the anomalous positive dγ=dT below T s is confirmed for the C 16 /C 18 TAB system by three independent methodologies.To elucidate the implications of the positive dγ=dT, we note that thermodynamics equates an inte...
The structure of organic monolayers on liquid surfaces depends sensitively on the details of the molecular interactions. The structure of a stearic acid film on a mercury surface was measured as a function of coverage with angstrom resolution. Unlike monolayers on water, the molecules were found here to undergo a transition from surface-parallel to surface-normal orientation with increasing coverage. At high coverage, two condensed hexatic phases of standing-up molecules were found. At low coverage, a two-dimensional (2D) gas phase and condensed single- and double-layered phases of flat-lying molecular dimers were revealed, exhibiting a 1D longitudinal positional order. This system should provide a broader tunability range for nanostructure construction than solid-supported self-assembled monolayers.
The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Kraack, H., B. M. Ocko, Peter S. Pershan, E. Sloutskin, and M. Deutsch. 2003. Langmuir films of normal-alkanes on the surface of liquid mercury.The coverage dependent phase behavior of molecular films of n-alkanes (CH 3 CH nϪ2 CH 3 , denote Cn) on mercury was studied for lengths 10рnр50, using surface tensiometry and surface x-ray diffraction methods. In contrast with Langmuir films on water, where roughly surface-normal molecular orientation is invariably found, alkanes on mercury are always oriented surface-parallel, and show no long-range in-plane order at any surface pressure. At a low coverage a two-dimensional gas phase is found, followed, upon increasing the coverage, by a single condensed layer (n р18), a sequence of single and double layers (19рnр20; nу26), or a sequence of single, double, and triple layers (22рnр24). The thermodynamical and structural properties of these layers, as determined from the measurements, are discussed.
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