Temperature dependence of surface layering in a dielectric liquid

Abstract

The temperature dependence of the density oscillations ͑layers͒ at the free surface of tetrakis͑2-ethylhexoxy͒silane, a nonmetallic molecular liquid, was investigated using x-ray reflectivity. Below ϳ215 K, the layer parameters weakly vary with temperature, if at all. Above this temperature, the layer spacings and intrinsic layer widths increase continuously, until there is no identifiable layering above 230 K. This transition occurs at T / T c Ϸ 0.23, a temperature region that is usually accessible in metallic liquids but is preempted by freezing in many dielectric liquids.

Ionic multilayers at the free surface of an ionic liquid, trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide, probed by x-ray reflectivity measurements

“…Aside from the deviation from R F , more importantly, one can see a convex feature around Q = 0.4 Å −1 . A similar convex feature was observed at the free surface of liquids that form surface multilayers, [60][61][62][63][64][65][66][67][68][69][70] indicating the existence of an oscillatory electron density multilayers at the surface of ͓TOMA + ͔͓C 4 C 4 N − ͔ with an interlayer distance of 2 / 0.4ϳ 16 Å. Another hypothetical reflectivity incorporating the surface roughness from the capillary wave theory ͑see below for the detail͒ but without any surface structures is also shown in Fig.…”

Surface order in cold liquids: X-ray reflectivity studies of dielectric liquids and comparison to liquid metals

“…Since does not appear to have a significant temperature dependence in any given material in the layered liquid phase, 12,15,[20][21][22]30 available values have been used irrespective of the temperature at which they were measured. Figure 10, top panel shows the plot of / d 0 for different dielectric and metallic liquids, showing surface layering, 12,15,21,30 with their defined or estimated 24,36,37 critical temperature ͑T c ͒. In and Sn, which also have surface layers, are not shown because their T c is not known.…”

“…The single exception prior to the present work is tetrakis͑2-ethylhexoxy͒silane ͑TEHOS͒, which remains liquid in the bulk at the threshold temperature and clearly shows surface layers developing when cooled to ϳ0.23T c . 20,21 In this paper, we use "cold liquid" to mean any liquid that is at or below ϳ0.2T c but is still a bulk fluid and not a solid or glass. We will describe in detail our studies of the surface profiles of two other cold liquids, pentaphenyl trimethyl trisiloxane ͑PPTMTS͒, 22 and pentavinyl pentamethyl cyclopentasiloxane ͑PVPMCPS͒.…”

“…The viscosity of PVCMCPS is low, and therefore uniform liquid films of thickness ϳ5000 Å were prepared simply by putting a few drops of liquid on the substrate, allowing the liquid to spread, and then draining the excess. 20,21 For one film of PPTMTS, we used the same sample preparation method as PVPMCPS, and we saw no differences in x-ray results due to sample preparation. The thickness is much larger than relevant length scales ͑surface roughness, molecular dimensions, etc.͒.…”

What x rays can tell us about the interfacial profile of water near hydrophobic surfaces

“…X-ray and neutron reflectivity have been extensively used to determine the density profiles at the free surfaces of many liquids [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] including water, 22,25 and also at liquid-solid [26][27][28] and liquidliquid [29][30] interfaces. These probes are sensitive to electron density and scattering length density respectively, and for a known material these are both measures of the local mass density.…”