Observation of a prewetting transition during surface melting of caprolactam

Abstract

The surface-induced melting of the close-packed (100) face of the anisotropic molecular crystal caprolactam has been studied using x-ray reflectivity. A thin-to-thick film prewetting transition is observed at about 13 K below the bulk melting point. Only above this transition does the thickness of the quasiliquid layer increase continuously with temperature. We speculate that initially the surface melting proceeds via layering transitions. PACS numbers: 64.70.Dv, 68.35.Rh, 68.45.GdThe experimental observation of a quasiliquid film wetting the solid-vapor interface below the melting temperature TM [1-5] has confirmed the old idea that the surface initiates the melting of a solid. Theoretically surface melting can be regarded as the wetting of the solid by the melt [5,6] and can result in either the quasiliquid film thickness remaining finite (incomplete wetting) or diverging (complete wetting) as the temperature approaches TM-In the latter case the divergence is governed by the range of the dominant interactions in the system. While long-range van der Waals interactions result in the thickness increasing as {TM ~ T)

Critical examination of prewetting and surface melting on caprolactam

“…3 The second is an x-ray study of the ͑001͒ surface of caprolactam ͑C 6 H 11 ON͒, which exhibited behavior similar to surface melting. 4 In the present investigation, we have repeated the original experiment under more controlled conditions, and shown that the observed phenomenon is inconsistent with surface melting. Further, we propose a surface hydration mechanism that can account for all the empirical observations.…”

“…In order to reconcile these findings with those of Ref. 4, we must consider other explanations for the observed behavior of the wetting layer, in addition to surface melting. Effects related to sample impurity, for example, may be a possible alternative interpretation of the data.…”

“…4, typically have 0.1-0.2 % of H 2 O contamination in the form of filled crystallographic inclusions, which are ϳ10 m in diameter. 7 The mean distance between such inclusions is ϳ100 m, and can be even smaller near the surface compared to the bulk, so that the effective concentration of such inclusions near the surfaces of as-grown samples can be larger than that for cleaved samples.…”

Molecular Dynamics Simulation of Prewetting

“…The density-functional models have been refined and extended [3][4][5][6][7] and supported by computer simulations [8][9][10][11][12][13][14][15] as well as experiments. [16][17][18] For a review of wetting and prewetting, see ref 19.…”

Surface melting of methane and methane film on magnesium oxide

“…X-ray reflectivity measurements were used for high-temperature studies of the (100) face of caprolactam (C 6 H 11 ON) [5,6]. Initially a series of layering transition followed by prewetting was suggested, but later a surface hydration mechanism was proposed for temperature disordering of this surface.…”