A molecular dynamics study of freezing in a confined geometry

Ma, Wen-Jong; Banavar, Jayanth R.; Koplik, Joel

doi:10.1063/1.463594

Cited by 62 publications

(20 citation statements)

References 41 publications

(4 reference statements)

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“…This structure was first considered [20] in conjunction with the structure of Cobalt and observed in systems of hard colloid spheres [21]. Molecular dynamics simulations of Lennard-Jones liquids solidifying in narrow channels bounded by molecular walls observed a solid with a hexagonal lattice within each layer [22]. However, no periodicity was observed in the stacking of the layers, reminiscent of the dhcp structure that we have assigned to the confined rare gas solids.…”

mentioning

confidence: 87%

New Disorder Induced Phase Transitions of Classical Rare Gases in Porous Vycor Glass

1998

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We report the results of x-ray diffraction studies of Ar and Kr confined in Vycor glass. The freezing and melting temperatures of both Ar and Kr were suppressed well below their bulk freezing points. On solidification, both samples crystallized in a disordered hexagonal close packed structure similar to that observed in molecular dynamics simulations of confined solids. A new solid-solid phase transition is observed at a reduced temperature of T ͞T f ഠ 0.5 when confined to Vycor. Below this temperature the disordered hexagonal closed packed structure coexists with the fcc structure.[S0031-9007 (98)06675-7] PACS numbers: 61.43.Gt, 61.10.NzThe effects of finite size and confinement on gases, liquids, and solids adsorbed in mesoporous materials, such as porous glasses and zeolites, is a topic of current interest for classical and quantum systems. For example, in classical systems the liquid-gas transition is enhanced with respect to the bulk [1,2] while the liquid-solid transition is suppressed [2,3]. Hysteresis, which appears to be a stable thermodynamic behavior, is also introduced into these transitions. Quantum transitions show equally dramatic behavior. For example, the superfluid transition of 4 He in Vycor is substantially suppressed while the critical exponents remain unchanged. However, confinement in aerogel results in only a small suppression but substantially different critical exponents [4,5]. 3 He, on the other hand, has superfluid phases in high porosity aerogels [6,7], but not in lower porosity confining geometries.Microscopic structural studies of materials confined to mesoporous media have only recently been undertaken, and a variety of different behaviors have been reported. In some cases, such as Hg in Vycor [8], the structure of the confined solid is identical to that of the bulk while in other cases, such as H 2 O and D 2 in Vycor [9,10], confinement stabilizes new structures not present in the bulk. Confinement can also suppress crystalline order, such as O 2 in xerogel [11], resulting in a glassy solid phase. Confinement has not, however, been observed to introduce new phase transitions in any systems that have been studied to date.In this Letter, we report x-ray diffraction studies of Ar and Kr adsorbed in Vycor. As in many other systems, a suppression of the liquid-solid transition hysteresis between freezing and melting is observed. On freezing, both Ar and Kr are observed to form a disordered hexagonal close packed (dhcp) structure, characterized by random plane stacking, in contrast to the bulk fcc structure. A new phase transition, which is absent from the bulk phase diagram, occurs at roughly half of the freezing temperature below which both Kr and Ar exhibit coexistence of the dhcp and fcc phases. This solid-solid structural phase transition is repeatable on cycling and no appreciable hysteresis is observed. Thus, the coexistence phase appears to be a true disorder-induced thermodynamic phase, rather than a metastable phase.The confining media used in these studies was Vycor glass. Vy...

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confidence: 87%

New Disorder Induced Phase Transitions of Classical Rare Gases in Porous Vycor Glass

1998

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“…A complete study of the influence of the Ar-wall interaction parameters and of the nature of the pore itself (structureless, crystalline or amorphous) is certainly necessary for a complete ''microscopic understanding'' of the behaviour of the geometry of (mesoscopic) porous systems on adsorption, capillary condensation and freezing transitions can be found in the literature (see for instance [11][12][13][14]). In this work, the pore in which the cluster is confined is structureless.…”

Section: Molecular Dynamics Simulationmentioning

confidence: 99%

Melting of Lennard-Jones clusters in confined geometries

Celestini

Pellenq

Bordarier

et al. 1996

Z Phys D - Atoms, Molecules and Clusters

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The melting of Lennard-Jones (argon) clusters of various size (N"500 to 10000 atoms), confined in a rigid matrix, is studied by molecular dynamics simulations. For spherical clusters we show the existence of a cluster size below which the dependence of the melting temperature cannot be described by the classical GibbsThompson equation. We also provide evidence of the formation of a quasi-liquid layer at the surface of mesoscopic clusters. A good agreement is found between the theoretical model due to Celestini et al. and the simulation results obtained in this work. The melting of an ellipsoidal cluster is also investigated. We observe, in agreement with recent experimental and theoretical work, that the thickness of the molten layer is larger in the region of higher local curvature.

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“…Therefore, it is suggested that the solid invasion will be stopped at the throat' 2 ( the smallest pore size of the media). Based on the neutron scattering investigation of liquid 02 and D 2 in porous vycol glass by Sokol et al 4 and the computer simulation by Ma et al 6 , on the other hand, the results suggest that the fluids inside the pores freeze layer by layer initiating from the wall. The results for the TNT confined 20 nm pores with bulk on the outer surface strongly suggests that the freezing of the bulk TNT indeed can trigger the freezing transition of the confined TNT inside the pores because at the bulk freezing temperature Tbf , all the confined TNT molecules are in the supercooled state, i.e., Tbf < T,.…”

Section: Tnt Physically Confined In 10 and 20 Nm Pores With Bulk On Omentioning

confidence: 99%

“…In the case of freezing and melting transitions, three assumptions must be made with this model: 1) The substrate wall is more favorable a liquid-wall interface than a solid-wall interface, 2) nucleation occurs at the pore center, and 3) nucleation process is restricted in the reduced dimension. A computer simulation study 6 of molecular freezing dynamics of a Lennard-Jones liquid in a confined geometry has predicted the time development of ordering and a novel freezing mechanism. Upon cooling, the confined liquid forms layers near the pore wall and a subsequent in-plane ordering within a layer is accomplished by a sharpening of the layering in the transverse direction, which is also qualitatively supported by neutron scattering experiments of Sokol et al 4 On the other hand, the simulation can not provide information on supercooling effects for confined fluids.…”

Section: Introductionmentioning

confidence: 99%

An Undercooling Effect in Porous Glass: From Bulk to the Confined

Xue

Henderson

et al. 1994

MRS Proc.

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The Thermodynamic properties of TNT physically confined in 5, 10, and 20 rnm pores were investigated by Differential Scanning Calorimetry (DSC). Depending upon the pore size and the sample condition, different aspects of the thermodynamic properties of the confined TNT were illustrated. The freezing transition of the confined TNT in 10, 20 nm pores can be triggered by excess bulk TNT on the outer surface. The sharp freezing transition of the TNT in 20 nm pores without the presence of the bulk suggests that the confined TNT maintains its interconnectivity during the transition. The confined TNT in 10 nm pores without bulk on surface failed to freeze during the cooling run up to 100 K with cooling rates ranging from 10 K/min. -I K/min. However, a sharp crystallization peak was observed upon heating. When the TNT is confined in 5 nm pores, the confined TNT is incapable of freezing.

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A molecular dynamics study of freezing in a confined geometry

Cited by 62 publications

References 41 publications

New Disorder Induced Phase Transitions of Classical Rare Gases in Porous Vycor Glass

New Disorder Induced Phase Transitions of Classical Rare Gases in Porous Vycor Glass

Melting of Lennard-Jones clusters in confined geometries

An Undercooling Effect in Porous Glass: From Bulk to the Confined

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