Surface Freezing Research Articles

Surface supercooling and stability of n-alkane films

The surface tension of n-octadecane was studied in the vicinity of the bulk melting point using both the maximum bubble pressure and Wilhelmy plate methods. The bubble surfaces were found to be supercooled below the surface freezing point. The onset of surface freezing is indicated by a sharp drop in surface tension at a constant temperature. This transition is accompanied by an increased film stability resulting in longer bubble lifetimes at the liquid surface. Variations in bubble lifetime reflect changes in the interfacial mechanical properties of the film from liquidlike to solidlike.

Effect of long-range forces on surface freezing

We examine the effect of long-range van der Waals interactions on surface freezing (SF) in linear hydrocarbon chain molecules, and the wetting criteria of the bulk liquid by the crystalline surface phase. We find that although the effect of van der Waals interactions is small for SF of normal alkanes, it is important for SF of dry and hydrated alcohols. We also find that the long-range interactions should not be ignored in the interpretation of wetting phenomena in recent experimental results. The results are in good agreement with recent experiments.

Surface structure tuning in alcohol melts by bulk additives

Surface freezing, i.e. the formation of a solid bilayer at the surface of a pure alcohol melt above the freezing temperature, was recently discovered. We demonstrate here that the structure of the bilayer can be influenced by specific bulk additives. Two different additives, able to form hydrogen bonds (water) and both hydrogen bonds and van der Waals interactions (α,ω-diols), are used. The surface layer’s structure is studied by surface-specific X-ray methods. We find that water swells the bilayer by intercalation into its center at a molecular water:alcohol ratio of ∼1:2. For the diolated alcohols a reversible monolayer–bilayer surface phase transition is observed, the first of its kind for surface-frozen layers. A continuous decrease of the molecular tilt with increasing diol concentration is also found for the tilted bilayer phases.

Immersion chilling of trays of cooked products

Cooking systems producing complete, or ingredients for, chilled meals are designed to achieve temperatures of 70°C for 2 min in the centre of the food, which should cause a 10 6-fold destruction of Listeria monocytogenes. However, spore-forming micro-organisms will survive this process and multiply if the food is not rapidly reduced to a temperature below 10°C. Experimental investigations have been carried out to study the cooling of different thicknesses of cooked solid–liquid mixtures under a range of immersion chilling conditions. An iterative computer model of the process has been developed and used to extend these data to cover alternative processing conditions and a range of cooked meals. If surface freezing was to be avoided then it was found that only the shallowest (10 mm) tray could be chilled within the limits of the most severe European Guidelines. However, the model also predicted that the 40 mm trays of food would cool within the limits of the Guidelines if oriented differently.

Replicas obtained after rapid freezing and deepetching of inner surfaces of adipocyte plasma membranes

Replicas obtained after rapid freezing and deepetching of inner surfaces of adipocyte plasma membranes

Field Data from a Capillary Barrier and Model Predictions with UNSAT-H

Water balance data are presented from a capillary barrier test section located on the final cover of a municipal solid waste landfill in a semiarid region (E. Wenatchee, Washington, U.S.). Water balance and meteorological data were collected from November 1992 to August 1995. Estimates of the water balance were made using the program UNSAT-H, with input consisting of meteorological data, soil properties, and vegetative information. Estimates of evapotranspiration and soil-water storage by UNSAT-H agree reasonably well with the field data. Peak soil-water storage was underestimated during the winter and evapotranspiration was overestimated in late winter. Water contents were estimated reasonably, although the changes in water content of the sand obtained from UNSAT-H were not as large as, and occurred less quickly than, that in the field. Percolation was generally overestimated, with the greatest overestimation occurring during Winter 1993, which had substantial snowfall. Surface runoff was underestimated; no runoff was obtained from UNSAT-H, whereas 7.4 cm of runoff was measured in the field. The overestimates in percolation appear to be closely related to underestimates in runoff and extra storage in the sand layer caused by the geocomposite drain used in the test section. Snowmelt, freezing of the soil surface, and hysteresis in soil hydraulic properties also appear to have had an effect on the differences between estimated and measured water balances.

Differential thermal analysis and ultraviolet photoemission study on the surface freezing effect of n-alkane

Abstract We have performed differential thermal analysis (DTA) and surface-specific ultraviolet (UV) photoemission measurements to investigate the surface freezing effect of n -alkanes. The samples used in the experiments were pentacontane ( n -C 50 H 102 : 650 A) and tetratetracontane ( n -C 44 H 90 : 600A) films evaporated on a copper substrate. We have observed a monolayer phase transition by DTA and proved that the transition is of first order. Since the temperature dependence of the surface-specific UV photoemission measurement showed a stepwise increase and decrease of the photoemission current at the transitions observed in DTA, we could conclude that the monolayer phase transition peak originates from the surface freezing effect.

A differential thermal analysis and ultraviolet photoemission study on surface freezing of n-alkanes

The surface-freezing effect of pentacontane ( n-C 50H 102) and tetratetracontane ( n-C 44H 90) films evaporated on a copper substrate has been investigated by differential thermal analysis (DTA) simultaneously with measurements of surface-specific ultraviolet (UV) photoemission. Two anomalies in the DTA curve were observed near the bulk melting temperature, one of which has been attributed to bulk melting. Since the temperature dependence of the surface-specific UV photoemission measurement showed a corresponding stepwise increase and decrease in the photoemission current at the two anomalies observed in the DTA, we have concluded that the other phase transition peak originates from surface freezing.

Surface freezing in chain molecules

“2D or not 2D, that is the question” —An Anonymous Surface Scientist

Evolution of two-layer thermohaline systems under surface cooling

This paper presents the results of a series of laboratory experiments aimed at understanding the processes associated with surface freezing of a two-layer fluid. The flow configuration consists of a layer of cold, salty water overlying a relatively deep bottom layer of warm, saltier water. This situation is common in high-latitude oceans during periods of rapid ice formation. The experiments were conducted in a tank with well-insulated side and bottom walls, placed in a walk-in freezer with air temperatures from −12 to −20°C. A system of thermocouples was used to measure the temperatures at fixed levels in water, ice and air. Microscale conductivity and temperature probes were used to obtain vertical profiles of temperature and salinity in the water. In general, when external uxes of heat and salt are absent, such a system enhances static stability, in the sense that the net density difference between the layers increases with time. When external uxes of heat (because of surface cooling) and salt (rejected during ice formation) are applied, however, this fluid system may become unstable and overturning of fluid layers is possible. In addition, heat transport from the warmer bottom layer to the colder upper layer may be important, possibly leading to a reduction in the rate of ice formation compared to that of a homogeneous fluid with temperature and salinity identical to the upper layer. Descriptions of such physical processes are given using laboratory experiments, and quantitative measurements of salient parameters are compared with the predictions of a theoretical model developed to explicate the flow evolution.

The surface-ordered phase of liquid heptadecane: a simulation study

The unusual surface-ordered state of liquid n-alkanes was investigated by simulations of a slab of liquid heptadecane with two free surfaces. The results confirm that the surface-ordered state consists of a monolayer of molecules aligned perpendicular to the surface which are mostly in an all trans conformation. The local arrangement of molecules within the monolayer is hexagonal and the density is higher than in the bulk phase. Good agreement was found between the simulation results and X-ray diffraction and reflection data. The simulations showed that there is lateral diffusion within the surface ordered layer, albeit reduced compared to the bulk diffusion, and that there is considerable freedom of motion in the direction perpendicular to the surface. The surface-ordered layer is more like a monolayer of a smectic liquid crystal than a monolayer of solid. A disordered liquid surface at a higher temperature shows no preferred molecular orientation near the surface. The transition between the disordered and ordered phases differs from classical surface freezing in that it is apparently first-order with hysteresis and the thickness does not diverge as the bulk freezing temperature is approached. The simulation results are related to various theories of this transition.

The surface-ordered phase of liquid heptadecane: a simulation study

The unusual surface-ordered state of liquid n-alkanes was investigated by simulations of a slab of liquid heptadecane with two free surfaces. The results confirm that the surface-ordered state consists of a monolayer of molecules aligned perpendicular to the surface which are mostly in an all trans conformation. The local arrangement of molecules within the monolayer is hexagonal and the density is higher than in the bulk phase. Good agreement was found between the simulation results and X-ray diffraction and reflection data. The simulations showed that there is lateral diffusion within the surface ordered layer, albeit reduced compared to the bulk diffusion, and that there is considerable freedom of motion in the direction perpendicular to the surface. The surface-ordered layer is more like a monolayer of a smectic liquid crystal than a monolayer of solid. A disordered liquid surface at a higher temperature shows no preferred molecular orientation near the surface. The transition between the disordered and ordered phases differs from classical surface freezing in that it is apparently first-order with hysteresis and the thickness does not diverge as the bulk freezing temperature is approached. The simulation results are related to various theories of this transition.

Surface freezing in chain molecules. II. Neat and hydrated alcohols

Surface freezing is studied in dry and hydrated normal-alcohol melts by x-ray scattering and surface tensiometry. A single crystalline bilayer forms at the surface, for even carbon numbers only, at temperatures up to 1{degree}C (dry) or 2{degree}C (wet) above the bulk freezing, and persists without change down to bulk freezing. The packing is hexagonal, with untilted molecules for short chains and tilted molecules for long chains. The lattices of the upper and lower monolayers are shifted along the next-nearest-neighbor direction. Hydration is found to swell the bilayer by {approximately}2.5 {Angstrom} due to water intercalation into the bilayer, at a molecular water:alcohol ratio of {approximately}1:2. It also increases the transition temperatures, and the temperature and chain-length ranges for which surface crystallization is observed. These effects are accounted for quantitatively by considering the surprising {ital increase} in hydration upon freezing, and taking into account the Gibbs-rule-predicted water depletion at the surface in the liquid surface phase. {copyright} {ital 1998} {ital The American Physical Society}

Surface Freezing in Hydrated Alcohol Melts

Surface freezing in hydrated alcohol melts was studied by surface tension and x-ray methods. An $\ensuremath{\sim}2.5\AA{}$ swelling of the surface-frozen bilayer is observed at saturated humidity, due to water intercalation into the bilayer. The concomitant upshifts of 2.5 and 3--6 \ifmmode^\circ\else\textdegree\fi{}C, observed in the bulk and surface freezing temperatures, respectively, are traced to the surprising increase in hydration upon freezing. For the liquid surface phase, the Gibbs-rule-predicted water depletion at the surface, relative to the bulk, accounts well for the unequal upshifts. It also explains the observed increases in the chain length and temperature ranges over which surface freezing exists.

Influence of the Molecular Ordering on the Wetting ofSiO2/Air Interfaces by Alkanes

Alkanes from 16 to 50 C atoms show three wetting topologies at ${\mathrm{SiO}}_{2}/\mathrm{air}$ interfaces. Below the bulk freezing temperature ${T}_{\mathrm{b}}$, small crystallites (frozen droplets which can be annealed into mesas of uniform height) are observed. A monolayer of homeotropically oriented molecules covers the air/substrate interface in between. At ${T}_{\mathrm{b}}$, the crystallites melt into droplets. The ordered monolayer remains up to ${T}_{\mathrm{S}}^{\mathrm{SV}}\ensuremath{\approx}{T}_{\mathrm{b}}+3\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, probably due to fluctuation-stabilized ``surface freezing.'' Above ${T}_{\mathrm{S}}^{\mathrm{SV}}$, a liquid alkane film wets the surface. The decoupling of ${T}_{\mathrm{b}}$ and ${T}_{\mathrm{S}}^{\mathrm{SV}}$, and the difference between the ordered and liquid surface energies, causes the droplet-to-film transition.

Surface Crystallization and Phase Transitions of the Adsorbed Film of F(CF2)12(CH2)16H at the Surface of Liquid Hexadecane

The interplay of the surface freezing phenomenon of hexadecane (abbreviated as H 16 ) with the monolayer phase behavior of a semifluorinated alkane, 1-(perfluorododecyl)hexadecane F(CF 2 ) 12 (CH 2 ) 16 H (abbreviated as F 12 H 16 ), at the air/hexadecane solution interface was studied by measuring the surface tension of hexadecane solutions of F 12 H 16 as a function of temperature and bulk concentration under atmospheric pressure. Above the surface freezing temperature of H 16 , our system F 12 H 16 + H 16 exhibits gaseous, expanded, and condensed monolayer phases of F 12 H 16 . In the domain of the gaseous film the entropy of surface formation per unit area, Δs, is almost independent of temperature and bulk concentration, and its magnitude is similar to that of pure H 16 . In contrast, in the domain of the expanded film Δs decreases sharply with increasing bulk concentration. Thus, the expanded film of F 12 H 16 in the present system manifests a unique behavior, indicating the formation of an ordered structure comparable to that of a condensed film. At lower temperature, this evolution of surface phases of the amphiphile (F 12 H 16 ) is cut off by the surface freezing of H i6 . It appears that F 12 H 16 is effectively insoluble in the solid monolayer of H 16 and that the F 12 chains of the F 12 H 16 molecules form a close-packed film in the condensed state. The point at which the gaseous film coexists with the condensed phase of F 12 H 16 and the solid monolayer phase of H 16 represents a surface heteroazeotrope (surface eutectic) and is located at about 17.4 °C, i.e., ca. 0.2 K below the surface freezing of pure H 16 (17.64 °C).

Surface freezing in mixtures of molten alkanes and alcohols

Surface freezing is studied in molten binary mixtures of alkanes and alcohols of different lengths using X-ray surface scattering and surface tension measurements. A crystalline monolayer (for alkanes) or bilayer (for alcohols) is formed at the surface a few degrees above the bulk freezing temperatures. The behaviour is found to be dominated by the length difference of the two components, Δn. For small Δn the surface properties and structure vary continuously with concentration between those of the pure components. For large Δn, however, the variation is discontinuous, exhibiting surface segregation. Several new phenomena, not observed in the pure components, are also found: a new surface crystalline structure in alkanes, a suppression of surface freezing for some compositions in both materials, and the inducement of surface freezing in alcohols which do not show the effect when pure. A Flory-Huggins theory based on competition between entropic mixing and a repulsive interaction due to chain length mismatch accounts well for the observed phenomena in alkanes, but requires modifications when applied to alcohols, probably due to the more complex additional headgroup interactions.

What Drives the Surface Freezing in Alkanes?

A Comment on the Letter by Alexei V. Tkachenko and Yitzhak Rabin, Phys. Rev. Lett. 76, 2527 (1996). The authors of the Letter offer a Reply.

NEXAFS Studies on the Surface Structure of Liquid n-C50H102(Pentacontane)

The surface freezing effect of n-alkane has been studied by Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. Samples used in the experiment were n-C 50 H 102 (pentacontane) 2500A thick films evaporated on Cu substrate. An ordered structure has been observed at a 1.3°C higher temperature than the bulk melting point. Angle resolved measurement has revealed that the molecular long axis of pentacontane in the surface freezing layer is almost perpendicular to the surface.

Surface freezing in chain molecules: Normal alkanes

A rare surface freezing phenomenon is observed in molten normal alkanes, using x-ray and surface tension measurements. An ordered monolayer forms on the surface of the liquid alkane at temperatures up to 3 \ifmmode^\circ\else\textdegree\fi{}C above the bulk freezing temperature ${\mathrm{T}}_{\mathrm{f}}$. The structure of the monolayer was studied in detail for a wide range of molecular lengths and temperatures. The single layer formed persists down to ${\mathrm{T}}_{\mathrm{f}}$. The rare surface phase exists only for carbon numbers of 16\ensuremath{\leqslant}n\ensuremath{\leqslant}50. The molecules in the layer are hexagonally packed and show three distinct ordered phases: two rotator phases, with molecules oriented vertically (16\ensuremath{\leqslant}n\ensuremath{\leqslant}30) and tilted towards nearest neighbors (3044) and one crystalline phase with molecules tilted towards next-nearest neighbors (n\ensuremath{\geqslant}44). The temperature dependence of the surface tension and the range of existence vs carbon number are satisfactorily accounted for within a simple theory based on surface energy considerations.