Prewetting Lines Research Articles

Adsorption of Ne on a planar solid Mg surface revisited

In this study, we reexamined the wetting behavior of Ne adsorbed on a solid Mg surface. In a recent study, Zhou and Zhang (J. Phys. Chem. Solids, 2017, 103, 123) reported an investigation of this system. They proposed an adsorption potential, ϕext(z), and studied this system based on augmented density functional theory (DFT) to determine the wetting temperature Tw and critical prewetting temperature Tcpw. The value obtained for Tcpw differed greatly from that produced by Grand Canonical Monte Carlo simulations (Phys. Rev. E, 1999, 59, 864) and previous DFT calculations (Phys. Rev. E, 2009, 79, 011603). In the present study, we calculated the adsorption isotherms with the Kierlik–Rosinberg DFT for two different external potentials, i.e., the aforementioned ϕext(z) and the proposed ab initio Chizmeshya–Cole–Zaremba (CCZ) potential. This study had two main aims. First, we aimed to identify the main sources of the discrepancies between the results obtained by Zhou and Zhang and those produced in other studies. Second, we analyzed the new data computed with the CCZ potential by applying the so-called arc-length continuation method. The new results supported the old ones, yielding more accurate values for both characteristic temperatures and indicating interesting new features of the system. We developed a novel alternative procedure for determining an appropriate value of Tcpw for moderately strong adsorbers. Assuming that Ne is liquid in the range of Tw≤T<Tt, then the DFT calculations were extended into this region. We showed that close to Tt and after the prewetting jump, the growth of the films exhibited a sequence of layering transitions, which originated coalescent prewetting lines.

Wetting of a plane with a narrow solvophobic stripe

ABSTRACTWe present a numerical study of a simple density functional theory model of fluid adsorption occurring on a planar wall decorated with a narrow deep stripe of a weaker adsorbing (relatively solvophobic) material, where wall-fluid and fluid-fluid intermolecular forces are considered to be dispersive. Both the stripe and outer substrate exhibit first-order wetting transitions with the wetting temperature of the stripe lying above that of the outer material. This geometry leads to a rich phase diagram due to the interplay between the pre-wetting transition of the outer substrate and an unbending transition corresponding to the local evaporation of liquid near the stripe. Depending on the width of the stripe, the line of unbending transitions merges with the pre-wetting line inducing a two-dimensional wetting transition occurring across the substrate. In turn, this leads to the continuous pre-drying of the thick pre-wetting film as the pre-wetting line is approached from above. Interestingly we find that the merging of the unbending and pre-wetting lines occurs even for the widest stripes considered. This contrasts markedly with the scenario where the outer material has the higher wetting temperature, for which the merging of the unbending and pre-wetting lines only occurs for very narrow stripes.

First-order wedge wetting revisited.

We consider a fluid adsorbed in a wedge made from walls that exhibit a first-order wetting transition and revisit the argument as to why and how the pre-filling and pre-wetting coexistence lines merge when the opening angle is increased approaching the planar geometry. We clarify the nature of the possible surface phase diagrams, pointing out the connection with complete pre-wetting, and show that the merging of the coexistence lines lead to new interfacial transitions. These occur along the side walls and are associated with the unbinding of the thin-thick interface, rather than the liquid-gas interface (meniscus), from the wedge apex. When fluctuation effects, together with the influence of dispersion forces are included, these transitions display strong non-universal critical singularities that depend on the opening angle itself. Similar phenomena are also shown to occur for adsorption near an apex tip.

Mean-field phenomenology of wetting in nanogrooves

ABSTRACTIn this special issue article, we bring together our recent research on wetting in confinement, in particular planar walls, wedges, capillary grooves and slit pores, with emphasis on phase transitions and competition between wetting, filling and condensation, and highlight their similarities and disparities. The results presented are obtained with the classical density functional theory (DFT) for fluids, which is a mean-field statistical mechanical framework for including the spatial variations of the fluid density into the thermodynamic equation of state. For wetting in sculpted substrates, we solve numerically the DFT equations to obtain the fluid density profiles, wetting isotherms and phase diagrams. This allows us to contrast the wetting phenomenology of grooves, planar walls, slit and wedge-shaped pores. Of particular interest are the transitions associated with capillary condensation, planar pre-wetting and mean-field wedge pre-filling lines.

Asymmetric Profiles and Prewetting Lines in the Filling of Planar Slits with Ne

The filling of slits with identical planar walls is investigated in the frame of the density functional theory. For this kind of slit, the confining potential is symmetric with respect to its central plane. Closed and open systems are studied by applying, respectively, the canonical and grand canonical ensembles (CE and GCE). Results obtained for the confinement of fluid Ne by alkaline surfaces are reported. The behavior of these systems is analyzed by varying the strength of the Ne-substrate attraction, the temperature T, and the coverage Γ(l). It is assumed that the one-body density of the fluid, ρ(r), is uniform along the (x, y) planes parallel to the walls, becoming a function of the coordinate z perpendicular to those planes. Two sorts of solutions are found for the density profile: (i) symmetric ones that follow the left-right symmetry of the potential exerted by the walls and (ii) asymmetric ones that break the symmetry of the slit. The pores are wide enough for determining prewetting (PW) lines and wetting and critical PW temperatures, i.e., T(w) and T(cpw), from the analysis of symmetric solutions provided by both the CE and GCE schemes. Asymmetric species are examined in detail for T > T(w). It is shown that for a given Ne-substrate pair at a fixed T both the CE and GCE frames yield only one asymmetric 2-fold degenerate stable profile (formed by a "thin" wetting film at one wall and a "thick" wetting film at the other) coexisting with two symmetric profiles (formed by "thin" or "thick" wetting films at the two walls), while the remaining asymmetric states are at best metastable. This feature occurs along PW lines and disappears at T(cpw).

Full correspondence between asymmetric filling of slits and first-order phase transition lines

Adsorption on single planar walls and filling of slits with identical planar walls are investigated in the frame of the density functional theory. In this sort of slits the external potential is symmetric with respect to its central plane. Calculations were carried out by applying both the canonical and grand canonical ensembles (CE and GCE, respectively). The behavior is analyzed by varying the strength of the adsorbate-substrate attraction, the temperature T, and the coverage Γℓ. Results obtained for physisorption of Xe on alkaline surfaces are reported in the present work. Prewetting (PW) lines and wetting temperatures, Tw, are determined from the analysis of adsorption on single walls. The filling of slits is analyzed for temperatures T &amp;gt; Tw. It is found that whenever for a given Xe-substrate combination the adsorption on a single wall exhibits a first-order wetting transition then asymmetric profiles that break the left-right symmetry of the external potential appear in the filling of an equivalent slit. These spontaneously symmetry breaking (SSB) solutions occur in a restricted range of Γℓ with a T-dependent width. In the case of closed slits analyzed in the CE scheme, the obtained asymmetric profiles exhibit lower Helmholtz free energies than the symmetric species and, therefore, could be stabilized in this geometry. For open slits, the GCE scheme yields all the symmetric and SSB states in the corresponding convex regimes of the free energy. It is shown that both the CE and the GCE frames yield three coexistent states, two symmetric and one asymmetric twofold degenerate. Both a PW line and the related SSB effect terminate at the same temperature. For rather strongly attractive surfaces reentrant SSB states are found at a fixed value of T.

Correlation between asymmetric profiles in slits and standard prewetting lines

The adsorption of Ar on substrates of Li is investigated within the framework of a density functional theory which includes an effective pair potential recently proposed. This approach yields good results for the surface tension of the liquid-vapor interface. The behavior of the adsorbate in the cases of a single planar wall and a slit geometry is analyzed as a function of temperature. Asymmetric density profiles are found for fluid confined in a slit built up of two identical planar walls leading to the spontaneous symmetry breaking (SSB) effect. We found that the asymmetric solutions occur even above the wetting temperature $T_w$ in a range of average densities $\rho^*_{ssb1} \le \rho^*_{av} \le \rho^*_{ssb2}$, which diminishes with increasing temperatures until its disappearance at the critical prewetting point $T_{\rm cpw}$. In this way a correlation between the disappearance of the SSB effect and the end of prewetting lines observed in the adsorption on a one-wall planar substrate is established. In addition, it is shown that a value for $T_{\rm cpw}$ can be precisely determined by analyzing the asymmetry coefficients.Received: 17 June 2009, Accepted: 28 August 2009; Edited by: S. A. Cannas; DOI: 10.4279/PIP.010001

Adsorption of Ne on alkali surfaces studied with a density functional theory

A density functional formalism is applied to investigate the wetting behavior of Ne adsorbed on planar substrates. The study is performed over the complete range of temperatures spanned from the triple point T_{t} up to the critical one T_{c} . For this purpose, an effective attractive pair potential was built on the basis of a separation procedure. This approach yields a good description of properties of the liquid-vapor interface at coexistence in the whole range of temperatures T_{t} < or = T < or = T_{c} . The adsorption of Ne on alkali metals and the alkaline-earth metal Mg is analyzed. This sequence of substrates exhibit increasing attractive strength leading to a variety of wetting situations throughout the interval T_{t}<-->T_{c} . A comparison with experimental data and other microscopic calculations is done. The predictions of a simple model are discussed. For NeRb we were able to resolve prewetting lines. Results obtained from a density functional are reported for NeK and NeMg . In the case of the latter system the interesting behavior occurs close to T_{t} . According to our results, Ne wets surfaces of Na and Li, and this statement is in agreement with the whole picture of the analyzed substrates.

Wetting phase diagrams of a polyacid brush with a triple point

The (pre)wetting behavior of an annealed polyelectrolyte (PE) brush by an electrolyte solution that is strongly segregated from an apolar phase is analyzed. In this complex interface, there are interactions on various length scales. There are short-range interactions with the (uncharged) surface, and there are interactions on the length scale of the brush height. Using either the ionic strength or the water-surface interaction strength as the control parameters, it is possible to approach and induce a wetting transition in this system. The first-order wetting transition, promoted by favorable short-range substrate interactions with the surface, is in competition with the wetting transition controlled by the detachment of the fluid interface from the periphery of the PE brush. The electric double layer on top of the PE brush contributes with a repulsive forces to the disjoining pressure that tends to thicken the wetting film, and therefore, the transition in all cases is first order. Various phase portraits of the wetting phase diagram are envisioned. One of these features the crossing of two prewetting lines. At the crossing point three surface states coexist. This triple point is analyzed in some detail with the help of a molecular-level self-consistent field model.

Phase transitions of adsorbed fluids computed from multiple-histogram reweighting

This paper demonstrates the effectiveness of using multiple-histogram reweighting (MHR) to study phase transitions in confined fluids by examining capillary condensation, prewetting, and layering transitions for different systems. A comparison is made with previously published simulations, where available, to establish the accuracy of MHR as applied to inhomogeneous systems. Overlap between adjacent state points is assessed through single-histogram reweighting. Capillary condensation for methane adsorption in slit-like graphite pores exhibits 2D behaviour. Crossover of the effective exponent for the width of the coexistence curve from 2D Ising-like (1/8) further away from the critical point to mean-field (1/2) near the critical point is observed. The reduced critical temperature, the density and the effective value of the exponent for the model system are 0.77, 0.482, and 0.119, respectively, based on a fit to the simulation data. Prewetting transitions are observed for adsorption of Ar on solid CO2 using model potentials. The wetting temperature is estimated based on the intersection of the pre-wetting and bulk vapour-liquid lines, and also by extrapolation to zero of the difference between the saturation and prewetting chemical potentials. The reduced wetting temperature is estimated to be around 0.69. The reduced prewetting critical temperature, calculated from the disappearance of the two peaks in the density probability distribution, is estimated to be 0.92. The monolayer to bilayer (1-2) transition for propane on graphite is computed over a range of temperatures. Results for the 1–2 layering transition computed from MHR from a small system are in good agreement with grand canonical Monte Carlo simulations for a much larger system.

Symmetric binary polymer blends confined in thin films between competing walls: Interplay between finite size and wetting behavior

The phase behavior and structure of a symmetric binary polymer blend confined between two hard impenetrable walls is studied, assuming also short range forces between the monomers and the walls. Phenomenological considerations are elaborated with the self-consistent field theory of Gaussian chains, and also some Monte Carlo simulations of the bond-fluctuation model. In the case of “antisymmetric” walls (right wall attracts component A with the same strength as does left wall component B) the phase diagram is symmetric around volume fraction ϕ = 1/2. For very thin films (or very weak surface fields) one finds a single critical point at ϕc = 1/2, as in the bulk. For thicker films, or stronger surface fields, the phase diagram exhibits two critical points and two concomitant coexistence regions, down to a triple point Tt, while below the triple point there is a single coexistence region. When the film thickness D → ∞, the two coexistence regions for T>Tt shrink into the prewetting lines, while Tt approaches the wetting transition temperature. Asymmetric surface forces are also considered, studying the smooth crossover from “ antisymmetric walls” to “ symmetric walls” (that both attract the A component with the same strength). The resulting crossover between capillary-type behavior (i.e., a single critical point of the thin film) and the above behavior with two critical points is analyzed. Particular attention is paid to the behavior of interfaces between coexisting phases.

Symmetric polymer blend confined into a film with antisymmetric surfaces: interplay between wetting behavior and the phase diagram

We study the phase behavior of a symmetric binary polymer blend that is confined in a thin film. The film surfaces interact with the monomers via short-range potentials. We calculate the phase behavior within the self-consistent field theory of Gaussian chains. Over a wide range of parameters we find strong first-order wetting transitions for the semi-infinite system, and the interplay between the wetting/prewetting behavior and the phase diagram in confined geometry is investigated. Antisymmetric boundaries, where one surface attracts the A component with the same strength as the opposite surface attracts the B component, are applied. The phase transition does not occur close to the bulk critical temperature but in the vicinity of the wetting transition. For very thin films or weak surface fields one finds a single critical point at straight phi(c)=1 / 2. For thicker films or stronger surface fields the phase diagram exhibits two critical points and two concomitant coexistence regions. Only below a triple point there is a single two-phase coexistence region. When we increase the film thickness the two coexistence regions become the prewetting lines of the semi-infinite system, while the triple temperature converges toward the wetting transition temperature from above. The behavior close to the tricritical point, which separates phase diagrams with one and two critical points, is studied in the framework of a Ginzburg-Landau ansatz. Two-dimensional profiles of the interface between the laterally coexisting phases are calculated, and the interfacial and line tensions analyzed. The effect of fluctuations and corrections to the self-consistent field theory are discussed.

Finite size effects on the phase diagram of a binary mixture confined between competing walls

A symmetrical binary mixture AB that exhibits a critical temperature T cb of phase separation into an A- and a B-rich phase in the bulk is considered in a geometry confined between two parallel plates a distance D apart. It is assumed that one wall preferentially attracts A while the other wall preferentially attracts B with the same strength (“competing walls”). In the limit D→∞, one then may have a wetting transition of first-order at a temperature T w , from which prewetting lines extend into the one phase region both of the A- and the B-rich phase. It is discussed how this phase diagram gets distorted due to the finiteness of D: the phase transition at T cb immediately disappears for D<∞ due to finite size rounding, and the phase diagram instead exhibit two two-phase coexistence regions in a temperature range T trip< T< T c1 = T c2 . In the limit D→∞ T c1,T c2 become the prewetting critical points and T trip→ T w . For small enough D it may occur that at a tricritical value D t the temperatures T c1 = T c2 and T trip merge, and then for D< D t there is a single unmixing critical point as in the bulk but with T c ( D) near T w . As an example, for the experimentally relevant case of a polymer mixture a phase diagram with two unmixing critical points is calculated explicitly from self-consistent field methods.

Wetting of a fluid interface by a homopolymer: A system with a rich prewetting behavior

A lattice-based self-consistent-field theory is used to reveal the wetting characteristics of a polymeric phase at an interface between two solvent phases. Both solvents are monomeric and the polymer chains are modeled as freely jointed chains of N=100 similar segments. From the polymer point of view we limit our investigations to the symmetric case; both solvents are equally poor for the polymer. We show that the polymer wets the interface both in the limit of strongly and weakly segregated liquids, but that at intermediate values of the L/L demixing the polymer phase can be in a lens configuration (partial wetting). The wetting transitions at strong segregation of the two solvents is always first order, the ones at weak segregation can be either first or second order, depending on the polymer/solvent interactions. For a particular value of the polymer–solvent interaction parameter the two wetting transitions as generated by changing the L/L demixing (then both of the first-order type) merge. At this point the two prewetting lines still exist and obviously merge as well. It is further possible to find detached “prewetting” lines in the regime where the polymer wets the interface at all relevant (i.e., consistent with the presence of three phase coexistence) values of the L/L demixing. Physically this means that there is a range of interfacial widths for which the formation of a new (polymeric) phase is hindered by a free energy barrier at a certain film thickness, but that there is no wetting transition associated with this barrier.

On the Thermodynamics of Surface Transitions Close to the Bulk Critical Point

AbstractIn the Ginzburg‐Landau functional approximation, thermodynamics of phase transitions occuring at the surface of binary alloys, ferromagnets, and similar systems are analysed in detail, including finite bulk and surface thermodynamic fields. Equations describing the lines of the Nakanishi and Fisher surface transition diagram are derived. For all the lines and (multi‐)critical points of the diagram, complete sets of thermodynamic critical exponents are obtained and scaling properties of the free energy surface excess, δfs, are examined in detail. The classical scaling of the bulk only holds at the critical prewetting lines. At the rest of the transitions, δfs is either a sum of two scaling functions with different exponents or includes a logarithmic singularity in addition to the leading one. This kind of scaling apparently represents some general properties of systems with surface‐broken symmetry or in contact with a short‐range substrate.

Systematics of wetting at the vapor—liquid interface

We have measured transition temperatures, T w, from incomplete wetting to complete wetting at the air—liquid interface for five binary mixtures of normal chain alcohols with the fluorocarbon perfluoromethylcyclohexane (C 7F 14). Consolute temperatures, T c, contact angles, and surface tensions were measured for each mixture. T w decreases monotonically as the alcohol chain length increases from 1 (methanol) to 6 ( n-hexanol). The mixture water (chain length = 0) + C 7F 14 is anomalous with the lowest T w (below the freezing point of the aqueous phase). From these measurements and a regular solution free energy, we calculated the lengths of the prewetting lines and the discontinuity in the derivative of the vapor—liquid surface tension dσ/ dT at T = T w. The prewetting line is a line of low to high adsorption transitions extending into the one liquid phase region and has been predicted to occur in mixtures in which there is both a miscibility gap and a wetting transition. The calculated lengths ranged from 38 K (for ethanol + C 7F 14) to 99 K (for n-hexanol + C 7F 14). The calculated discontinuities in dσ/ dT ranged from 0.008 dyn/cm deg K (for ethanol + C 7F 14) to 0.039 dyn/cm deg K (for n-hexanol + C 7F 14) and are roughly correlated with the lengths of the prewetting lines. Among the mixtures studied, the most likely candidate for directly detecting the prewetting transition is n-hexane + C 7F 14 based on a comparison to a regular solution model.

The interfacial properties of a model of a nematic liquid crystal

We have used the theory developed in the previous paper to study the induced orientational order at simple models of a solid-liquid crystal (LC) interface. The LC is modelled by a Maier-Saupe type of anisotropic pairwise potential while the solid is modelled by an impenetrable wall which exerts an anisotropic external potential on the fluid molecules. Our theory treats both the solid-fluid and fluid-fluid potentials on a microscopic level which allows us to study the role played by these two types of interactions in the alignment of the fluid molecules at the solid surface. In particular, we study in detail the nature of the wetting of the solid-isotropic liquid interface by the nematic phase. We illustrate this by presenting the results of numerical calculations for the density and orientational order parameter profiles at the wetting transition for two different models of the solid substrate. We find that due to the weakness of the first order N/I bulk transition, the surface phase transition for z -3 substrate potentials is also weakly first order, in contrast with the results for isotropic systems. For exponential solid substrates the transition is found to depend on the range of the solid-fluid interaction; for short-ranged potentials the transition is continuous whereas for longer-ranged potentials the transition becomes more strongly first-order as the range of the interaction increases. This type of behaviour is in agreement with the results for isotropic solid-fluid interfaces. We calculate the pre-wetting lines for the two solid-fluid models. We find that the structure of the co-existing films as well as the surface critical point depend rather strongly on the model for the solid-fluid interaction. Finally, we compare the results of our theory with the results obtained from a phenomenological one-order parameter Landau-de Gennes theory and with some recent experimental results. We assess their validity and possible range of application.