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Phase Transitions of Long-Chain N-Alkanes at Interfaces

Maeda, Nobuo

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An experimental study of phase transitions of long-chain n-alkanes induced by the effect of interfaces is described. ¶ The phase behaviour of long-chain n-alkanes (carbon number 14, 16, 17, 18) adsorbed at isolated mica surfaces and confined between two mica surfaces has been studied in the vicinity of and down to several degrees below the bulk melting points, Tm. Using the Surface Force Apparatus we have measured the thickness of alkane films adsorbed from vapour (0.97 [equal to or...[Show more]

dc.contributor.authorMaeda, Nobuo
dc.date.accessioned2008-12-08T01:21:44Z
dc.date.accessioned2011-01-04T02:39:33Z
dc.date.available2008-12-08T01:21:44Z
dc.date.available2011-01-04T02:39:33Z
dc.identifier.otherb20820094
dc.identifier.urihttp://hdl.handle.net/1885/47795
dc.description.abstractAn experimental study of phase transitions of long-chain n-alkanes induced by the effect of interfaces is described. ¶ The phase behaviour of long-chain n-alkanes (carbon number 14, 16, 17, 18) adsorbed at isolated mica surfaces and confined between two mica surfaces has been studied in the vicinity of and down to several degrees below the bulk melting points, Tm. Using the Surface Force Apparatus we have measured the thickness of alkane films adsorbed from vapour (0.97 [equal to or greater-than] p/p[subscript o] [equal to or greater-than] 0.997), studied capillary condensation transition, subsequent growth of capillary condensates between two surfaces, and phase transitions in both the adsorbed films and the condensates. By measuring the growth rate of the capillary condensates we have identified a transition in the lateral mobility of molecules in the adsorbed films on isolated mica surfaces. This transition to greater mobility occurs slightly above Tm for n-hexadecane, n-heptadecane and n-octadecane but several degrees below Tm for n-tetradecane, and is accompanied by a change in wetting behaviour and a measurable decrease in adsorbed film thickness for n-heptadecane and n-octadecane. Capillary condensates that form below Tm remain liquid, but may freeze if the degree of confinement is reduced by separation of the mica surfaces. An increase in the area of the liquid-vapour interface relative to that of the liquid-mica interface facilitates freezing in the case of the long-chain alkanes, which show surface freezing at the liquid-vapour interface. ¶ Although thermodynamic properties of the surface freezing transition have been rather well documented, the kinetics involved in formation of such ordered monolayers has so far received very little attention. We studied the surface tension of n-octadecane as a function of temperature in the vicinity of Tm, using the static Wilhelmy plate and the dynamic maximum bubble pressure methods. The two methods give different results on cooling paths, where nucleation of the surface ordered phase is involved, but agree on heating paths, where both methods measure properties of the equilibrium surface phase. On cooling paths, the surface of bubbles may supercool below the equilibrium surface freezing temperature. The onset of surface freezing is marked by a sharp drop in the surface tension. The transition is accompanied by an increased stability of the films resulting in longer bubble lifetimes at the liquid surface, which suggests that the mechanical properties of the surfaces change from liquid-like to solid-like. Our results suggest occurrence of supercooling of the monolayer itself.
dc.language.isoen
dc.rights.uriThe Australian National University
dc.subjectphase transition
dc.subjectlong-chain n-alkanes
dc.subjectmica surfaces
dc.subjectcapillary condensates
dc.subjectmobility of molecules
dc.subjectliquid-vapour interface
dc.subjectcapillary freezing
dc.subjectcapillary condensation
dc.subjectsurface freezing
dc.subjectmonolayers
dc.subjectsupercooling
dc.titlePhase Transitions of Long-Chain N-Alkanes at Interfaces
dc.typeThesis (PhD)
dcterms.valid2001
local.description.refereedyes
local.type.degreeDoctor of Philosophy (PhD)
dc.date.issued2001
local.contributor.affiliationDepartment of Applied Mathematics, Research School of Physical Sciences and Engineering
local.contributor.affiliationThe Australian National University
local.identifier.doi10.25911/5d7a295d866c3
local.mintdoimint
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