Adsorption and wetting : experiments, thermodynamics and molecular aspects

Abstract

Adsorption and wetting are related phenomena. In order to improve knowledge of both and their relations, experiments, thermodynamics and a theoretical interpretation have been connected, starring n-alkanes.Starting from the Gibbs adsorption equation thermodynamic relations between vapour adsorption and wetting are derived. The surface pressure of a film, formed by vapour adsorption on a solid surface, is calculated by integrating the vapour adsorption isotherm. The surface pressure at the saturated vapour pressure determines, together with the interfacial tension of the liquid, the difference between the interfacial tension of a clean solid and a solid- liquid interface. Moreover, the surface pressure is related to the spreading tension and contact angle in a solid-liquid-vapour system. The thermodynamic equations derived are generally valid and the approach covers wetting on both flat and powdered solids. From the individual surface pressure values of two immiscible liquids, wetting and displacement in a solid-liquid-liquid system can be assessed. The procedure is illustrated for a silica-water-octane system. Silica is one of the most abundant minerals on earth and the oil/water wettability of silica can be considered a model for oil displacement in reservoir rocks.By combining the Young equation with the Gibbs adsorption equation, the contact angle and the work of adhesion of an aqueous electrolyte solution on a charged solid is investigated as a function of the solid surface charge density or the electrolyte concentration. In the case of partial wetting, the solid-solution-vapour contact angle is a maximum at the point of zero charge of the solid. The contact angle decreases and the work of adhesion increases with increasing absolute value of the surface charge. The derived equations are used to study the wettability of silica under changing electrolyte conditions. The surface charge density of silica Aerosil OX-50 at a number of indifferent KCl concentrations, ranging from 0.01 M to 1M, is determined as a function of pH by potentiometric titrations. The silica surface charge increases with increasing ionic strength and increasing pH. At its point of zero charge ( p H 0 ≈ 3) silica is already completely wetted by water. Charging of the surface results in an even better water wettability although this can not be observed experimentally. At pH = 9 and 1 M KCl the silica surface charge equals -0.25 C/m 2. Compared to the uncharged silica, this surface charge decreases the silica-water interfacial tension by 22 mJ/m 2. Under usual conditions the electrolyte adsorption at the solid-vapour interface will be less than at the solid-water interface. With respect to an uncharged silica ( pH =3), the silica surface charge of -0.25 C/m 2decreases the silica-(water) vapour interfacial tension by maximally 22 mJ/m 2whereas it increases the work of adhesion by maximally 22 mJ/m 2. By combining the present approach with theoretical equations describing the adsorption of charge determining ions on solids with different kinds and amounts of chargable groups the wettabililty of such solids as a function of their charging behaviour can be described theoretically. This remains a task for the future.The vapour adsorption of different n-alkanes, cyclohexane, toluene and water on bare and methylated pyrogenic silica (Aerosil OX-50) has been studied gravimetrically. Linear adsorption isotherms of the n-alkanes and of cyclohexane on both substrates are found until high relative vapour pressures. The same holds for toluene on methylated silica. The linearity of the isotherms indicates relatively weak lateral interactions between adsorbed molecules. On bare silica, the adsorption of the n-alkanes studied (C7-C9) is, expressed in moles/m 2, independent of the chain length. The adsorption strongly increases after the coverage corresponding to a monolayer of alkanes, oriented perpendicular to the surface, has been reached (at p/p 0 ≈0.8). Methylation of the silica decreases the adsorption of all adsorptives studied. Until just before saturation the octane adsorption on methylated silica is below that of a monolayer parallel to the surface. The shape of these adsorption isotherms indicates that on bare silica n-alkanes predominantly adsorb end-on, perpendicular to the surface, whereas on methylated silica, the adsorption is rather parallel. From the adsorption data surface pressure isotherms are constructed and the work of adhesion is obtained. The work of adhesion reveals that the Lifshits-van der Waals part of the silica surface tension is reduced from 44 mJ/m 2for bare (pyrogenic) silica to 30 mJ/m 2for methylated silica. The adsorption data are also converted to disjoining pressure isotherms. At low film thicknesses, these can be described by an exponential short-range interaction. The classical macroscopic models are not very suited for the description of such thin films for which the molecular organization and the discrete character of the adsorbed layer are extremely important. However, thin adsorbed layers can be described on the basis of microscopic models for adsorption. Also surface pressures of simple systems can be obtained from classical adsorption equations (e.g., Langmuir, Volmer, BET, Polyani). However, for chain molecules like n-alkanes these models are inadequate as they are unable to describe the structure of the molecules and their adsorbed layers. This problem can be overcome by using, for instance, a more recent self-consistent-field (SCF) theory, orginially developed by Scheutjens and Fleer and extended by Leermakers; and others.The SCF theory is applied for the description of chain molecular fluids and their interfaces. Hereto a fluid is considered as a mixture of chains and monomeric vacancies. The latter account for the free volume in the system. Intermolecular interactions are described in terms of Flory- Huggins (FH) parameters. For the homologous series of linear alkanes, these parameters are generalized and assessed from a fit to vapour pressure data. In the SCF lattice-fluid theory, each alkane is described as a chain of segments with a volume of 0.027 nm 3each. The segment- segment interactions (for which the FH parameter is zero by definition) are reflected in a non zero FH interaction parameter for a chain segment-vacancy contact χAO .Under the conditions mentioned χ AO equals 580/ T ( T in K) for all n-alkanes. With these parameters n-alkane bulk properties such as the vapour pressure, density, critical point and heat of vaporization can be obtained together with structural and thermodynamic properties of the liquidvapour ( LV ) interface. The calculations reveal that chain ends are the major constituents on the vapour side of the (alkane) LV interface. For longer chains and lower temperatures the (relative) preference of the chain ends to protrude into the vapour phase is more pronounced. The calculated variation of the n-alkane LV interfacial tension (γ) with temperature and chain length agrees quantitatively with experimental data. If the theory is applied for temperatures below the (experimental) n-alkane freezing points, positive dγ/d T values occur and a maximum in the LV interfacial tension is found at T/T C ≈0.12, irrespective of the chain length of the molecule. In experimental studies close to the n-alkane freezing points similar observations have been made. However, a comparion of these experimental observations with our theoretical predictions should be performed with some reservation as the theory describes a frozen phase as an isotropic supercooled fluid.The lattice fluid theory description of the n-alkane interfacial properties may be improved by considering chain-flexibility constraints, such as trans-gauche conformations and/or by distinghuishing (the parameters of the) CH 3 and CH 2 segments. It is rather straightforward to extend the present theory to more complex systems such as fluid mixtures, or fluids (vapour and liquid) at solid surfaces. The interfacial tensions of such interfaces can be inferred from the theory so that the work of adhesion and contact angles on these interfaces can be investigated as a function of temperature and chain length of the (wetting) liquid. Some of these aspects are elaborated in the last Chapter.The adsorption, structure and thermodynamics of (aliphatic) chain molecular fluids at rigid surfaces and at solids with thermally grafted (aliphatic) chains is also investigated. Vapour adsorption isotherms, inclusive the meta- and unstable regions, of an octameric fluid on various substrates are calculated. The octameric molecules are modelled as B-A 6 -B chains where A represents a CH 2 segment and B a CH 3 segment. On a bare solid, the influence of adsorption energy differences between the A and B segments of the chain molecule is investigated together with the influence of the chain flexibility. For semi-flexible chains with high chain end-adsorption energies the shape of the calculated isotherm qualitatively agrees with the linear vapour adsorption isotherms measured for n-alkanes on bare silica. With the theory adsorption isotherms resembling the ones measured for n-alkanes on methylated silica can be obtained. This requires semi-flexible chains with interaction parameters that favour a rather parallel adsorption of the chain-molecules with respect to the surface. A reduction of the chain flexibility, for instance by applying the RIS scheme, increases the tendency of the adsorbed molecules to line up. In general, this increases the adsorbed amounts when the interaction parameters favour end-on adsorption whereas this reduces the adsorbed amounts when the interaction parameters are in favour of parallel adsorption. On a poorly wetted rigid solid, a decreasing contact angle was calculated for increasing chain length of the (aliphatic) wetting liquid An . The contact angles and the (Zisman) critical surface tension for wetting decrease with increasing temperature. When the temperature approaches the critical temperature of the wetting fluid, complete wetting occurs. Furthermore, it is established that the temperature dependence of the contact angle mainly results from the influence of the temperature on the liquid-vapour and solid-liquid interfacial tensions.On solids with grafted chains, octamer ( A8 ) adsorption isotherms and contact angles are calculated for different grafting densities and grafted chain lengths. Grafting aliphatic chains on a very poorly wetted (bare) solid decreases the contact angle of the octameric liquid. On such a solid, the contact angle as a function of the grafting density passes through a minimum. The rise beyond the minimum has an entropic origin. Grafting of chains on a completely wetting (bare) solid eventually results in a finite contact angle; higher grafting densities give rise to higher contact angles. When longer chains are grafted, lower contact angles result for both substrates. The calculations provide insight into the wettability of a substrate by chain-molecular fluids on a molecular level. Partial wetting of chain molecules can be explained from an autophobic effect: due to the ordering (anisotropy) of the molecules present in the thinnest adsorbed layer that can form at saturation, molecules of the isotropic liquid are repelled. The liquid does not spread on the thin film and droplet formation results. The calculations reveal that the partial wetting of chain molecular liquids on grafted solids is largely due to the enrichment of the grafted layer by middle segments of the liquid molecules.In this thesis the thermodynamics and molecular aspects of adsorption and wetting have been investigated and coupled by means of vapour adsorption isotherms and a lattice fluid theory. At present a reasonable (semi) quantitative agreement between theory and experiments has been achieved. For the (near) future, some other investigations based on the present theory are challenging. Firstly, a better quantitative agreement with experimental data is feasible by optimizing the description of the aliphatic molecules, for instance by incorporating differences between end and middle segments and their mutual interactions, or, in our case, their interactions with a vacancy. Secondly, the theory is able to describe random, heterogeneous surfaces and rough substrates so that contact angles of chain molecular liquids on such substrates can be inferred and compared to theories such as developed by Cassie and Wenzel. Moreover, the theory can be extended to a twodimensional SCF approximation instead of the one dimension we used in this work. This renders calculation of the (density) contour plot of a droplet feasible. By comparing the contact angle of this contour plot with the equilibrium contact angle, calculated in onedimensional SCF, the effect of the drop size, its curvature and line tension on the contact angle can be studied. Finally, a "dynamic" version of the SCF theory is currently being developed in the department of Physical and Colloid Chemistry. In the future, this theory will be suited to investigate the dynamics of evaporation of chain molecular liquids as well as the dynamics of their adsorption, spreading and contact angles on solid substrates