Surface Freezing Transition Research Articles

Effect of Surface Freezing of Sodium Hexadecyl Sulfate - Hexadecanol Mixed Adsorbed film on OW Emulsion Stability.

The adsorbed film of Sodium Hexadecyl Sulfate (SHS) at the dodecane - water interface showed a first-order phase transition to a surface frozen monolayer upon cooling by the lateral van der Waals attraction between their hydrophobic tails and those of hexadecanol (C16OH) incorporated from the dodecane phase. The surface freezing transition of the SHS - C16OH monolayer was then utilized to stabilize an oil-in-water (OW) emulsion. The obtained results were compared to those examined previously for the cetyltrimethylammonium chloride (CTAC) - C16OH surface frozen monolayer. The main conclusion of this study was that the interfacial density of SHS significantly increased at the surface freezing by the cooperative adsorption with C16OH which gave rise to a higher surface freezing temperature (35°C) compared to CTAC (25°C). The formation of the surface freezing monolayer in the ambient temperature range could have a significant importance when it is applied to practical applications.

Demulsification of Silica Stabilized Pickering Emulsions Using Surface Freezing Transition of CTAC Adsorbed Films at the Tetradecane-Water Interface.

The adsorbed film of cetyltrimethylammonium chloride (CTAC) at the tetradecane (C14) - water interface undergoes a first-order surface transition from two-dimensional liquid to solid states upon cooling. In this paper, we utilized this surface freezing transition to realize a spontaneous demulsification of Pickering emulsions stabilized by silica particles. In the temperature range above the surface freezing transition, the interfacial tension of silica laden oil-water interface was lower than CTAC adsorbed film, hence, stable Pickering emulsion was obtained by vortex mixing. However, the interfacial tension of CTAC adsorbed film decreased rapidly below the surface freezing temperature and became lower than the silica laden interface. The reversal of the interfacial tensions between silica laden and CTAC adsorbed films gave rise to Pickering emulsion demulsification by the desorption of silica particles from the oil-water interface. The exchange of silica particles and CTAC at the surface of emulsion droplets was also confirmed experimentally by using phase modulation ellipsometry at the oil-water interface.

Surface Freezing of Cetyltrimethylammonium Chloride-Hexadecanol Mixed Adsorbed Film at Dodecane-Water Interface.

The surface freezing transition of a mixed adsorbed film containing cetyltrimethylammonium chloride (CTAC) and n-hexadecanol (C16OH) was utilized at the dodecane-water interface to control the stability of oil-in-water (O/W) emulsions. The corresponding surface frozen and surface liquid mixed adsorbed films were characterized using interfacial tensiometry and X-ray reflectometry. The emulsion samples prepared in the temperature range of the surface frozen and surface liquid phases showed a clear difference in their stability: the emulsion volume decreased continuously right after the emulsification in the surface liquid region, while it remained constant or decreased at a much slower rate in the surface frozen region. Compared to the previously examined CTAC-tetradecane mixed adsorbed film, the surface freezing temperature increased from 9.5 to 25.0 °C due to the better chain matching between CTAC and C16OH and higher surface activity of C16OH. This then renders such systems much more attractive for practical applications.

Effect of Surface Freezing on Stability of Oil-in-Water Emulsions.

Penetration of alkane molecules into the adsorbed film of a cationic surfactant gives rise to a surface freezing transition at the alkane-water interface upon cooling. In this paper, we show that surface freezing of hexadecyltrimethylammonium chloride (CTAC) at the tetradecane-water interface stabilizes oil-in-water (OW) emulsions. For concentrations of CTAC near the critical micelle concentration, an OW emulsion coalesced readily above the surface freezing transition whereas the OW emulsion was stable in the surface frozen state. There was a discontinuous change in the stability of the OW emulsion at a temperature very close to the surface phase transition temperature as determined by interfacial tensiometry and ellipsometry on a planar oil-water interface. The mechanical elasticity of the surface frozen layer opposes film drainage and density fluctuations that could lead to rupture and is the most likely cause of the enhanced emulsion stability.

Crystallization of soft matter under confinement at interfaces and in wedges

The surface freezing and surface melting transitions that are exhibited by a model two-dimensional soft matter system are studied. The behaviour when confined within a wedge is also considered. The system consists of particles interacting via a soft purely repulsive pair potential. Density functional theory (DFT) is used to calculate density profiles and thermodynamic quantities. The external potential due to the confining walls is modelled via a hard wall with an additional repulsive Yukawa potential. The surface phase behaviour depends on the range and strength of this repulsion: when the repulsion is weak, the wall promotes freezing at the surface of the wall. The thickness of this frozen layer grows logarithmically as the bulk liquid–solid phase coexistence is approached. Our mean-field DFT predicts that this crystalline layer at the wall must be nucleated (i.e. there is a free energy barrier) and its formation is necessarily a first-order transition, referred to as ‘prefreezing’, by analogy with the prewetting transition. However, in contrast to the latter, prefreezing cannot terminate in a critical point, since the phase transition involves a change in symmetry. If the wall–fluid interaction is sufficiently long ranged and the repulsion is strong enough, surface melting can occur instead. Then the interface between the wall and the bulk crystalline solid is wetted by the liquid phase as the chemical potential is decreased towards the value at liquid–solid coexistence. It is observed that the finite thickness fluid film at the wall has a broken translational symmetry due to its proximity to the bulk crystal, and so the nucleation of the wetting film can be either first order or continuous. Our mean-field theory predicts that for certain wall potentials there is a premelting critical point analogous to the surface critical point for the prewetting transition. When the fluid is confined within a linear wedge, this can strongly promote freezing when the opening angle of the wedge is commensurate with the crystal lattice.

Effect of alkane chain length and counterion on the freezing transition of cationic surfactant adsorbed film at alkane mixture - water interfaces.

Penetration of alkane molecules into the adsorbed film gives rise to a surface freezing transition of cationic surfactant at the alkane-water interface. To examine the effect of the alkane chain length and counterion on the surface freezing, we employed interfacial tensiometry and ellipsometry to study the interface of cetyltrimethylammonium bromide and cetyltrimethylammonium chloride aqueous solutions against dodecane, tetradecane, hexadecane, and their mixtures. Applying theoretical equations to the experimental results obtained, we found that the alkane molecules that have the same chain length as the surfactant adsorb preferentially into the surface freezing film. Furthermore, we demonstrated that the freezing transition temperature of cationic surfactant adsorbed film was independent of the kind of counterion.

Surface Freezing and Molecular Miscibility of Binary Alkane–Alkane and Fluoroalkane–Alkane Liquid Mixtures

The surface freezing (SF) of liquid n-heptadecane (C17)-n-octadecane (C18) and 1-perfluorooctyl decane (F8H10)-C18 mixtures were studied by surface tension and external reflection absorption FTIR (ERA-FTIR) measurements. The surface tension versus temperature curves of all pure liquids show a sharp break point at Ts corresponding to a surface liquid (SL)-SF transition. The entropy of surface formation is very negative, indicating a well-ordered structure of the SF layer. The ERA-FTIR spectra in the SF state suggested that the C18 molecules are densely packed in the solid state, while the packing of the hydrocarbon (HC) part of F8H10 is a little looser than the fluorocarbon (FC) part because of the difference in the cross-sectional area. In the C17-C18 mixture, the SL-SF transition was found at all bulk compositions. The estimation of the surface composition suggested that two components are miscible both in SL and SF states. The excess entropy of the surface is almost zero in both states, and thus, it was concluded that the two components are mixed almost ideally at the surface. In the case of the F8H10-C18 system, on the other hand, the SL layer is enriched in F8H10 with lower surface tension than C18 compared to bulk liquid. The surface composition in the SF state is almost zero or unity, indicating that F8H10 and C18 molecules are practically immiscible mainly due to the weak interaction between different components. Furthermore, the negative excess entropy in the SL layer suggests domain formation of F8H10 molecules at the surface.

Thermal stability of surface freezing films in Ga-based alloys: An x-ray photoelectron spectroscopy and scanning tunneling microscopy study

We have investigated the thickness and surface structure of surface freezing films in Ga-Bi and Ga-Pb alloys over a wide temperature range between room temperature and the respective surface freezing transitions by x-ray photoelectron spectroscopy (XPS) and scanning tunnelling microscopy (STM). For the example of a Ga-Bi alloy dilute in Bi, XPS measurements show that the surface freezing film has a nearly constant value of approximately 25 A between the surface freezing temperature of 130 degrees C and room temperature if the sample is cooled slowly (5 Kh). On heating to 130 degrees C the film thickness exhibits a clear hysteresis on melting. On quenching the alloy sample (>100 Kh) the film thickness increases by almost a factor of 10. These observations indicate that the surface freezing films are metastable. The surface structure of the surface freezing films of various Ga-rich Ga-Bi and Ga-Pb alloys has been probed for the first time by STM at different temperatures below and above the bulk eutectic point. Atomically resolved STM images show the surface structures of pure Bi (0001) and Pb (111), respectively, at room temperature. On heating above the eutectic temperature the surface structure of the films does not change significantly as judged from the size and thickness of Pb or Bi terraces. These observations together with the film thickness variation with temperature indicate that the surface freezing films behave like a metastable independent surface phase. These results together with the wetting characteristics of these alloys suggest that surface freezing in these systems is a first order surface phase transition between wetting and metastable surface freezing films. The energy barrier for nucleation is strongly reduced due to a lowering of the interfacial energy if the nucleus is completely immersed in the respective wetting layer.

Ellipsometric characterization of surface freezing in Ga-based alloys

We present results on surface freezing of Ga-based alloys, GaBi, GaPb and GaTl, above theliquidus line between the Ga-rich eutectic and the monotectic point. Spectroscopic ellipsometry(0.8 eV ≤hν≤4.2 eV) and kinetic single wavelength ellipsometry (2.75 eV) have been employed toprobe the changes of the interfacial electronic structures on surface freezing.To minimize thermal gradients across the sample a heatable cap that coversthe sample and crucible was developed. The surface freezing temperature,TSF, for the spontaneous formation of a solid-like film on top of the Ga-rich liquidon cooling the sample from the homogeneous phase region was found to beindependent of the temperature difference between the upper and lower furnace(ΔT:+10 to−10 K) and only weakly dependent on the cooling rate (: 2.5–20 K h−1). In the case of GaPb the solid film consists of solid Pb with a thicknessh≥400 Å. Comparing with GaBi we draw analogous conclusions for GaPb and GaTl and suggestthat the surface freezing transition precedes the bulk phase transition along the liquidusline as the alloy is cooled.

Electron spectroscopy and scanning tunneling microscopy study of quasi-two-dimensional freezing at the liquid/vapor interface of Ga-Bi alloys

We have studied the interfacial characteristics of Ga-rich liquid Ga-Bi alloys at various compositions and temperatures up to 300 degrees C by different electron spectroscopies under ultrahigh vacuum conditions. In particular, the thickness and thickness profiles of Bi-rich wetting and surface freezing films have been measured. It is found that at the surface freezing transition the Bi film thickness jumps from approximately 5 A to macroscopic values of over 100 A. This distinct behavior is discussed in comparison with the results in Ga-Pb. In addition, we report first scanning tunneling microscopy observations of the surface structure of a solidified Ga(0.989)Bi(0.011) alloy after passing a surface freezing transition. The topology at room temperature is characterized by extended, atomically flat Bi terraces covered by nanosized triangular two-dimensional Bi islands.

Surface freezing and a two-step pathway of the isotropic-smectic phase transition in colloidal rods.

We study the kinetics of the isotropic-smectic phase transition in a colloidal rod/polymer mixture by visualizing individual smectic layers. First, we show that the bulk isotropic-smectic phase transition is preceded by a surface freezing transition in which a quasi-two-dimensional smectic phase wets the isotropic-nematic interface. Next, we identify a two-step kinetic pathway for the formation of a bulk smectic phase. In the first step a metastable isotropic-nematic interface is formed. This interface is wetted by the surface-induced smectic phase. In the subsequent step, smectic layers nucleate at this surface phase and grow into the isotropic bulk phase.

Wetting, prewetting and surface freezing transitions in fluid Ga-based alloys: asurface light scattering study

The surface energy and entropy of liquid Ga–Bi and Ga–Pb alloys have beenstudied by means of surface light scattering measurements at various compositionsand temperatures between the respective eutectic and monotectic points. Analysisof these results using the Gibbs adsorption equation gives evidence for wettingand prewetting transitions in these alloys completely consistent with a tetra-pointwetting scenario (Dietrich S and Schick M 1997 Surf. Sci. 382 178). Surfacefreezing transitions are observed for conditions near the liquidus curves. In viewof their viscoelastic properties and their relation with the wetting filmcharacteristics, we suggest a simple explanation for the observed surface freezingphenomena in terms of nucleation of strongly undercooled wetting films.

Surface freezing transitions in liquid binary alloys

In this paper recent results on surface phase transitions at the interface of Ga–Bi alloys are reviewed. The wetting transition in the liquid–liquid demixing region is briefly summarized. Then, the surface freezing transition, which occurs in the concentration range between the eutectic ( x Bi=0.0022) and the monotectic ( x Bi=0.085) compositions, is described. This was discovered recently using second harmonic generation and plasma generation methods. The surface freezing line was found approximately 20 K above the eutectic point; it approaches the liquidus line with increasing Bi mole fraction and presumably merges at the monotectic point. Arguments for the thermodynamic stability of the surface freezing Bi-rich films and a possible correlation between the wetting transition and the surface freezing transition are discussed. Finally, we present preliminary experimental results on a study of surface freezing in Ga–Pb.

Light scattering study of surface freezing and surface viscoelasticity in a eutectic liquid Ga–Bi alloy

Surface freezing in a metallic liquid, the eutectic Ga–Bi alloy, has been studied for the first time by capillary wave spectroscopy at various wave numbers and temperatures. The surface freezing transition is marked by a sharp drop of the surface tension σ by 25 mJ m −2 and a hysteresis behaviour indicating a first-order interfacial phase transition. Analysis of the capillary wave spectra yields information on the surface excess viscoelastic modulus governing shear normal to the surface plane which is found to be consistent with a metallic solid-like surface freezing film on top of the eutectic Ga–Bi alloy.

Electron-Diffraction Study on Surface Hexatic Behavior in Free-Standing 4O.8 Liquid-Crystal Films

An unusual sequence of three-stage surface freezing transitions has been observed in free-standing films of N-(4-n-butoxybenzylidene)-4-n-octylaniline (4O.8). The possible existence of an intermediate highly correlated liquid (smectic-A′) phase may alter our understanding of two-dimensional melting. The surface smectic-A-smectic-A′ transition is found to be first order. In addition, the adjacent crystal-B and hexatic-Blayers which appear at lower temperatures show evidence of orientational epitaxy.

Surface-Freezing Transitions and Novel Tilted Hexatic Phases in Smectic Liquid-Crystal Thin Films

We report the first unambiguous identification in a thermotropic liquid crystal of a hexatic smectic-L, a phase with molecular tilt direction intermediate between those of the smectic-I and smectic-F, using electron diffraction in free-standing films of 5-(4″-hexyl, 3′-fluoro-p-terphenyl-4-oxy)-pentanoic acid ethyl ester. Our structural data directly support the proposed explanation of the unusual stripe phase observed optically. The films show a rich phase diagram as a function of thickness and temperature. A novel phase transition within the surface smectic-L phase is also observed.

Layer-Dependent Surface-Induced Tilt, Hexatic, and Positional Orders in Free-Standing 7O.7 Films

Electron diffraction in free-standing films of 4-n-heptyloxybenzylidene-4-n-heptylaniline of up to 25 molecular layers has revealed the coexistence, as a function of distance from the surface, of three distinct phases: an outermost smectic-I layer, several layers of a middle phase, and a smectic-A interior. The middle phase is an unusual highly-correlated tilted liquid, with hexaticlike positional correlations but no long-range bond-orientational order, which transforms to the smectic-I at a lower temperature. The discovery of this unusual middle phase also represents the possibility of a novel layer-dependent two-stage tilt-induced surface-freezing transition.

Surface-Freezing Transitions and Novel Tilted Hexatic Phases in Smectic Liquid-Crystal Films

We report the first unambiguous identification in a thermotropic liquid crystal of a hexatic smectic- $L$, a phase with molecular tilt direction intermediate between those of the smectic- $I$ and smectic- $F$, using electron diffraction in free-standing films of 5-(4\ensuremath{'}\ensuremath{'}-hexyl,3\ensuremath{'}-fluoro-p-terphenyl-4-oxy)-pentanoic acid ethyl ester. Our structural data directly support the proposed explanation of the unusual stripe phase observed optically. We also describe multiple surface-freezing transitions which occur at lower temperatures and involve novel tilted crystal phases.