Layering transitions and solvation forces in an asymmetrically confined fluid

M. C. Stewart*, Robert Evans

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

11 Citations (Scopus)

Abstract

We consider a simple fluid confined between two parallel walls (substrates), separated by a distance L. The walls exert competing surface fields so that one wall is attractive and may be completely wet by liquid (it is solvophilic) while the other is solvophobic. Such asymmetric confinement is sometimes termed a "Janus Interface." The second wall is: (i) purely repulsive and therefore completely dry (contact angle theta = 180 degrees) or (ii) weakly attractive and partially dry (theta is typically in the range 160-170 degrees). At low temperatures, but above the bulk triple point, we find using classical density functional theory (DFT) that the fluid is highly structured in the liquid part of the density profile. In case (i), a sequence of layering transitions occurs: as L is increased at fixed chemical potential mu close to bulk gas-liquid coexistence mu(co), new layers of liquid-like density develop discontinuously. In contrast to confinement between identical walls, the solvation force is repulsive for all wall separations and jumps discontinuously at each layering transition and the excess grand potential exhibits many metastable minima as a function of the adsorption. For a fixed temperature T = 0.56TC, where T-C is the bulk critical temperature, we determine the transition lines in the L, mu plane. In case (ii), we do not find layering transitions and the solvation force oscillates about zero. We discuss how our mean-field DFT results might be altered by including effects of fluctuations and comment on how the phenomenology we have revealed might be relevant for experimental and simulation studies of water confined between hydrophilic and hydrophobic substrates, emphasizing it is important to distinguish between cases (i) and (ii). (C) 2014 AIP Publishing LLC.

Original languageEnglish
Article number134704
Number of pages14
JournalJournal of Chemical Physics
Volume140
Issue number13
DOIs
Publication statusPublished - 7 Apr 2014

Keywords

  • LATTICE-GAS MODEL
  • DENSITY-FUNCTIONAL THEORY
  • MONTE-CARLO SIMULATIONS
  • MULTILAYER ADSORPTION
  • PHASE-BEHAVIOR
  • ARGON FILMS
  • CAPILLARY CONDENSATION
  • SOLVENT STRUCTURE
  • WETTING PHENOMENA
  • ASYMPTOTIC DECAY

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