Phase transition of<i>n</i>-alkane layers adsorbed on mica

Maeda, Nobuo; Kohonen, Mika M.; Christenson, Hugo K.

doi:10.1103/physreve.61.7239

Cited by 26 publications

(30 citation statements)

References 20 publications

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“…This feature did not allow accurate measurements of their refractive index profiles, and, therefore, we concentrated here instead on the mean refractive index of the n-hexadecane bridges. This effect may be related to the large difference in the rates of evaporation of the two liquids (12)(13)(14)(15). Second, the much slower rates of evaporation and condensation of n-hexadecane allowed for more accurate measurements to be made, especially during the final stages of evaporation, than was previously possible with cyclohexane.…”

Section: Resultsmentioning

confidence: 99%

Evaporation and instabilities of microscopic capillary bridges

Maeda

Israelachvili

Kohonen

2003

Proc. Natl. Acad. Sci. U.S.A.

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129

113

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The formation and disappearance of liquid bridges between two surfaces can occur either through equilibrium or nonequilibrium processes. In the first instance, the bridge molecules are in thermodynamic equilibrium with the surrounding vapor medium. In the second, chemical potential gradients result in material transfer; mechanical instabilities, because of van der Waals force jumps on approach or a Rayleigh instability on rapid separation, may trigger irreversible film coalescence or bridge snapping. We have studied the growth and disappearance mechanisms of laterally microscopic liquid bridges of three hydrocarbon liquids in slit-like pores. At rapid slit-opening rates, the bridges rupture by means of a mechanical instability described by the Young-Laplace equation. Noncontinuum but apparently reversible behavior is observed when a bridge is held at nanoscopic surface separations H close to the thermodynamic equilibrium Kelvin length, 2r Kcos , where rK is the Kelvin radius and is the contact angle. During the course of slow evaporation (at H > 2r Kcos ) and subsequent regrowth by capillary condensation (at H < 2r Kcos ), the refractive index of the bridge may vary continuously and reversibly between that of the bulk liquid and vapor. The evaporation process becomes irreversible only at the very final stage of evaporation, when the refractive index of the fluid attains virtually that of the vapor. Measured refractive index profiles and the time-dependence of evaporating neck diameters also seem to differ from predictions based on a continuum picture of bridge evaporation far from the critical point. We discuss these findings in terms of the probable density profiles in evolving liquid bridges.T he equilibrium and dynamic aspects of phase changes are often influenced by the presence of surfaces and interfaces. For example, in the case of homogeneous nucleation of a liquid phase, a significant supersaturation must be reached before vapor starts to condense to form droplets because of the activation barrier in the free energy that hinders the nucleation of clusters smaller than a certain size. Interfaces can lower this activation barrier and speed up the nucleation process. From a practical point of view, the nucleation, condensation, and evaporation of fluids in confined geometries play a major role in adhesion, the behavior of moist granular materials, and the transport of fluids through porous media.Much less attention has been paid to the process of evaporation or disappearance of capillary-held liquids than to their formation (condensation or nucleation). The surface force apparatus (SFA; refs. 1-3) provides a powerful means with which to study the behavior of highly confined fluids and has been used to study various aspects of capillary condensed liquids. Recently, we reported some preliminary results on the evaporation of microscopic cyclohexane bridges (4). The major result from this study was the observation of lateral refractive index gradients across the evaporating bridges once they shrink below a cert...

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Section: Resultsmentioning

confidence: 99%

Evaporation and instabilities of microscopic capillary bridges

Maeda

Israelachvili

Kohonen

2003

Proc. Natl. Acad. Sci. U.S.A.

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129

113

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“…1,5,6 Although rotator phases have been studied in bulk crystals of alkanes, there are few reports of these phases in molecularly thin films. [7][8][9][10][11]16 Solid C32 films grown on SiO 2 surfaces have been found to follow the Stranski-Krastanov growth mode. At the substrate-alkane interface, there is a complete bilayer of molecules with their long axis parallel to the surface.…”

Section: Introductionmentioning

confidence: 99%

Crystalline-to-plastic phase transitions in molecularly thin n-dotriacontane films adsorbed on solid surfaces

Cisternas

Corrales

Campo

et al. 2009

The Journal of Chemical Physics

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Crystalline-to-rotator phase transitions have been widely studied in bulk hydrocarbons, in particular in normal alkanes. But few studies of these transitions deal with molecularly thin films of pure n-alkanes on solid substrates. In this work, we were able to grow dotriacontane ͑n-C 32 H 66 ͒ films without coexisting bulk particles, which allows us to isolate the contribution to the ellipsometric signal from a monolayer of molecules oriented with their long axis perpendicular to the SiO 2 surface. For these submonolayer films, we found a step in the ellipsometer signal at ϳ331 K, which we identify with a solid-solid phase transition. At higher coverages, we observed additional steps in the ellipsometric signal that we identify with a solid-solid phase transition in multilayer islands ͑ϳ333 K͒ and with the transition to the rotator phase in bulk crystallites ͑ϳ337 K͒, respectively. After considering three alternative explanations, we propose that the step upward in the ellipsometric signal observed at ϳ331 K on heating the submonolayer film is the signature of a transition from a perpendicular monolayer phase to a denser phase in which the alkane chains contain on average one to two gauche defects per molecule.

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“…Between T m and T s the surface of liquid n-octadecane is more ordered than the bulk (the surface entropy is negative) and consists of a monomolecular layer of parallel molecules essentially perpendicular to the interface. A similar surface phase transition also occurs with n-octadecane (and other long-chain n-alkanes adsorbed from vapor on mica), [16,17] and this changes the wetting properties of the surface. Above T s the adsorbed layer is disordered, and the surface is a mixture of methyl and methylene groups.…”

Section: Freezing and Melting Of Capillary Condensates Of N-octadecanementioning

confidence: 82%

“…Studies of the initial growth rate of capillary condensates of n-octadecane as well as measurements the adsorbed layer thickness have shown that the adsorbed film undergoes a structural transition about 28C above the melting point. [16,17] As this temperature T s is approached from above, the layer thickens and becomes more ordered. This transition is akin to the surface freezing transition found at the liquid-vapor interface of n-octadecane (and other long-chain n-alkanes [18] ).…”

Section: Freezing and Melting Of Capillary Condensates Of N-octadecanementioning

confidence: 98%

“…[16,17] The properties of this adsorbed layer have a decisive influence on the behavior of the condensates after snapping. Studies of the initial growth rate of capillary condensates of n-octadecane as well as measurements the adsorbed layer thickness have shown that the adsorbed film undergoes a structural transition about 28C above the melting point.…”

Section: Freezing and Melting Of Capillary Condensates Of N-octadecanementioning

confidence: 99%

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Phase Behavior in Confinement Studied with a Surface Force Apparatus

Christenson

2006

Journal of Dispersion Science and Technology

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The free energy of a surface has a major influence on the phase behavior of systems where the area-to-volume ratio is large. These include finely dispersed solids and liquids as well as substances confined to meso-and microporous solids. In a surface force apparatus (SFA) mica surfaces at or close to contact form a single model pore in which surface energy effects on phase behavior may be conveniently studied. Capillary condensation of liquid from undersaturated vapor is the most familiar example of a surface-induced shift of bulk phase behavior, and this has been extensively studied with the SFA. I will here describe three additional examples of phase behavior in confinement studied with the SFA. The first is the observation of a surface-induced phase separation in a binary liquid mixture. The second example concerns the freezing and melting behavior of a long-chain n-alkane in confinement. The last example is a quantitative study of capillary condensation below the bulk melting point, i.e., from vapor in equilibrium with solid. These examples illustrate how the SFA may be used to study different aspects of surface effects on phase behavior and how the results complement experiments on phase behavior in porous solids.

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Phase transition ofn-alkane layers adsorbed on mica

Cited by 26 publications

References 20 publications

Evaporation and instabilities of microscopic capillary bridges

Evaporation and instabilities of microscopic capillary bridges

Crystalline-to-plastic phase transitions in molecularly thin n-dotriacontane films adsorbed on solid surfaces

Phase Behavior in Confinement Studied with a Surface Force Apparatus

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