On the determination of the surface energetics of porous polymer materials

The solid surface tension γsv of hydrophobic polymer powders has been determined using the capillary penetration technique. By plotting Kγlv cos ζ, where K is a geometric factor, versus the liquid surface tension γlv, the following values of γsv were directly derived from the curves: poly(tetrafluoroethylene) γsv = 20.4 mJ/m², polypropylene γsv = 30.2 mJ/m², polyethylene γsv = 34.4 mJ/m², and polystyrene γsv = 27.5 mJ/m². These values are in good agreement with the γsv values obtained from contact angle measurements on flat and smooth solid surfaces of the same materials. If the contact angles were first calculated from the capillary penetration experiments, which is the usual procedure applied in the literature, distinctly higher contact angles were obtained. Obviously these angles are affected by the powder morphology and are therefore meaningless contact angles in terms of a surface energetic interpretation.     

Low-rate Dynamic Contact Angles on Poly(methyl methacrylate/n-butyl methacrylate) and the Determination of Solid Surface Tensions

Low-rate dynamic contact angles of 12 liquids on a poly(methyl methacrylate/n-butyl methacrylate) P(MMA/nBMA) copolymer are measured by an automated axisymmetric drop shape analysis-profile (ADSA-P). It is found that 6 liquids yield non-constant contact angles, and/or dissolve the polymer on contact. From the experimental contact angles of the remaining 6 liquids, it is found that the liquid- vapour surface tension times the cosine of the contact angle changes smoothly with the liquid-vapour surface tension, i.e., γ_iv cos θ depends only on γ_iv for a given solid surface (or solid surface tension). This contact angle pattern is in harmony with those from other inert and noninert (polar and non-polar) surfaces [34-42, 51 -53]. The solid-vapour surface tension calculated from the equation-of-state approach for solid -liquid interfacial tensions [14] is found to be 34.4 mJ/m², with a 95% confidence limit of \pm 0.8mJ/m², from the experimental contact angles of the 6 liquids.     

Low-rate dynamic contact angles on polystyrene and the determination of solid surface tensions

Low-rate dynamic contact angles of 13 liquids on a polystyrene polymer are measured by an automated axisymmetric drop shape analysis – profile (ADSA-P). It is found that 7 liquids yielded non-constant contact angles, and/or dissolved the polymer on contact. From the experimental contact angles of the other 6 liquids, it is found that the liquid-vapor surface tension times cosine of the contact angle changes smoothly with the liquid-vapor surface tension, i.e. γ_lvcosθ depends only on γ_lv for a given solid surface (or solid surface tension). This contact angle pattern is in harmony with those from other inert and non-inert (polar and non-polar) surfaces (7–13, 24–26). The solid-vapor surface tension calculated from the equation-of-state approach for solid-liquid interfacial tensions (33) is found to be 29.8 mJ/m², with a 95% confidence limit of ±0.5 mJ/m² from the experimental contact angles of 6 liquids.     

Analysis and surface energy estimation of various model polymeric surfaces using contact angle hysteresis

Wetting of hydrophobic polymer surfaces commonly employed in electronic coatings and their interaction with surfactant-laden liquids and aqueous polymer solutions are analyzed using a contact angle hysteresis (CAH) approach developed by Chibowski and co-workers. In addition, a number of low surface tension acrylic monomer liquids, as well as common probe liquids are used to estimate solid surface energy of the coatings in order to facilitate a thorough analysis of surfactant effects in adhesion. Extensive literature data on contact angle hysteresis of surfactant-laden liquids on polymeric surfaces are available and are used here to estimate solid surface energy for further understanding and comparisons with the present experimental data. In certain cases, adhesion tension plots are utilized to interpret wetting of surfaces by surfactant and polymer solutions. Wetting of an ultra-hydrophobic surface with surfactant-laden liquids is also analyzed using the contact angle hysteresis method. Finally, a detailed analysis of the effect of probe liquid molecular structure on contact angle hysteresis is given using the detailed experiments of Timmons and Zisman on a hydrophobic self-assembled monolayer (SAM) surface. Hydrophobic surfaces used in the present experiments include an acetal resin [poly(oxymethylene), POM] surface, and silane, siloxane and fluoro-acrylic coatings. Model surfaces relevant to the literature data include paraffin wax, poly(methyl methacrylate) and a nano-textured surface. Based on the results, it is suggested that for practical coating applications in which surfactant-laden and acrylic formulations are considered, a preliminary evaluation and analysis of solid surface energy can be made using surfactant-laden probe liquids to tailor and ascertain the quality of the final coating.     

On the interpretation of contact angle hysteresis

The determination of solid surface free energy is still an open problem. The method proposed by van Oss and coworkers gives scattered values for apolar Lifshitz-van der Waals and polar (Lewis acid-base) electron-donor and electron-acceptor components for the investigated solid. The values of the components depend on the kind of three probe liquids used for their determination. In this paper a new alternative approach employing contact angle hysteresis is offered. It is based on three measurable parameters: advancing and receding contact angles (hysteresis of the contact angle) and the liquid surface tension. The equation obtained allows calculation of total surface free energy for the investigated solid. The equation is tested using some literature values, as well as advancing and receding contact angles measured for six probe liquids on microscope glass slides and poly(methyl methacrylate) PMMA, plates. It was found that for the tested solids thus calculated total surface free energy depended, to some extent, on the liquid used. Also, the surface free energy components of these solids determined by van Oss and coworkers' method and then the total surface free energy calculated from them varied depending on for which liquid-set the advancing contact angles were used for the calculations. However, the average values of the surface free energy, both for glass and PMMA, determined from these two approaches were in an excellent agreement. Therefore, it was concluded that using other condensed phase (liquid), thus determined value of solid surface free energy is an apparent one, because it seemingly depends not only on the kind but also on the strength of interactions operating across the solid/liquid interface, which are different for different liquids.     

Hydrophilic/electron-acceptor surface properties of metallic biomaterials and their effect on osteoblast cell activity

Mice osteoblast cells were cultured on samples of 316L stainless steel and titanium alloy (Ti-6Al-4V) with different surface treatments. The resulting cell differentiation was correlated with the solid surface tension and electron-acceptor surface tension parameter of the biomaterial samples. Both of these characteristics were determined through contact angle measurements, using the Lifshitz–van der Waals/Lewis acid–base interaction model. Before calculating the surface tension of the biomaterials, the experimental contact angles were corrected for the effect of roughness using the Wenzel equation. For this calculation, the roughness characteristics of the solid surfaces were determined using atomic force microscopy and interferometric profilometry at the nano-scale and micro-scale, respectively. It was found that osteoblast cell differentiation directly related to the implant surface tension and electron-acceptor properties. The alkaline phosphatase activity increased both with increasing surface tension and increasing electron-acceptor surface tension parameter of the implant materials. These results suggest that the formation of surface hydroxyl groups with acidic character gives rise to enhanced attachment of osteoblast cells.     

Determination of dynamic contact angles in solid–liquid–liquid systems using the Wilhelmy plate apparatus

The purpose of this work was to carry out a systematic study of the effects of brine composition and rock mineralogy on rock-oil-brine interactions taking place in petroleum reservoirs. These terms are generally lumped into a single term called wettability in petroleum engineering. The extent of wetting of the rock surface by water or oil depends on the dynamic contact angles measured in such a mode as to enable movements of the three-phase contact line. The Wilhelmy plate technique has been used in this study to measure adhesion tension (which is the product of interfacial tension and cosine of the contact angle) at the solid-liquid interface. The water-advancing and water-receding contact angles have been calculated from the adhesion tensions by making independent measurements of the liquid-liquid interfacial tensions using a du Noüy ring tensiometer. The water-advancing and receding angles have been measured in this study for pure hydrocarbons against synthetic brines of different concentrations. Polished surfaces of glass slides and dolomite have been used to simulate the reservoir rock surfaces. A nonionic surfactant (ethoxy alcohol), which is being used in Yates reservoir in West Texas for enhancing oil recovery, was used to quantify its wettability effects. The results of the systematic experimental investigation of the effects of practical variables on wettability are presented. It is found that interactions between surface-active agents at the interface of two liquids have an effect on wettability alteration. The composition and concentrations of different organic and inorganic chemical species have a major effect in making a reservoir oil-wet or water-wet.     

Contact Angle Hysteresis on Polymer Surfaces: An Experimental Study

In order to characterize a solid surface, the commonly used approach is to measure the advancing and receding contact angles, i.e., the contact angle hysteresis. However, often an estimate of the average wettability of the solid–liquid system is required, which involves both the dry and wetted states of the surface. In this work, we measured advancing and receding contact angles on six polymer surfaces (polystyrene, poly(ethylene terephthalate), poly(methyl methacrylate), polycarbonate, unplasticized poly(vinyl chloride), and poly(tetrafluoroethylene)) with water, ethylene glycol and formamide using the sessile drop and captive bubble methods. We observed a general disagreement between these two methods in the advancing and receding contact angles values and the average contact angle determined separately by each method, although the contact angle hysteresis range mostly agreed. Surface mobility, swelling or liquid penetration might explain this behaviour. However, we found that the 'cross' averages of the advancing and receding angles coincided. This finding suggests that the cross-averaged angle might be a meaningful contact angle for polymer–liquid systems. Hence, we recommend using both the sessile drop and captive bubble methods.     

Spreading of liquid drops over solid substrates: 'like wets like'

The classic hydrodynamic wetting theory leads to a linear relationship between spreading speed and the capillary force, being determined only by the surface tension of the liquid and its viscosity. Both equilibrium and dynamic processes of wetting are important in adhesion phenomena. The theory appears to be in good agreement with the results generated from experiments conducted on the spreading of poly(dimethylsiloxane) (PDMS) on soda-lime glass substrate and fails to account for the behavior of other liquids. In this study, the spreading kinetics of four different liquids (hexadecane, undecane, glycerol and water) was determined on three different solids, namely, soda-lime glass, poly(methyl methacrylate) (PMMA) and polystyrene (PS). Droplets from the same liquid allowed to spread under identical conditions on three different substrates produce distinctly different behaviors. The results show that the equilibrium contact angles are qualitatively ranked in accordance with the critical surface tension of wetting (γ _c) of the respective solid, i.e., high-γ _c solids caused the low surface tension liquids to assume the least equilibrium spreading (largest contact angle). On the other end, low-γ _c solids with the lowest surface tension liquid produce the most wetting (smallest contact angle). The results suggest that equilibrium spreading could be explained on the basis of the axiom 'like wets like'; in other words, polar surfaces tend to be wetted by polar liquids and vice versa.     

Wetting Behavior of Aqueous Solutions of Binary Surfactant Mixtures to Poly(tetrafluoroethylene)

Measurements of the surface tension (γ _LV) and advancing contact angle () on poly(tetrafluoroethylene) (PTFE) were carried out for aqueous solutions of cetyltrimethylammonium bromide (CTAB), cetylpyridynium bromide (CPyB), sodium decylsulfate (SDS), sodium dodecylsulfate (SDDS), p-(1,1,3,3-tetramethylbutyl) phenoxypoly(ethylene glycol)s, Triton X-100 (TX100) and Triton X-165 (TX165) and their mixtures. The results obtained indicate that the values of the surface tension and wettability of PTFE depend on the concentration and composition of the surfactants mixture. In contrast to Zisman finding, there was no linear dependence between cos and the surface tension of aqueous solutions of surfactants and their mixtures for all studied systems, but a linear dependence existed between the adhesional tension and solution surface tension for PTFE in the whole concentration range, the slope of which was –1, indicating that the surface excess concentration of surfactant at the PTFE–solution interface was the same as that at the solution–air interface for a given bulk concentration. It was also found that the work of adhesion of aqueous solutions of surfactants and their mixtures to PTFE surface did not depend on the type of surfactant, its concentration and composition of the mixture. This means that for the studied systems the interaction across PTFE–solution interface was constant, and it was largely of Lifshitz–van der Waals type. On the basis of the surface tension of PTFE and the Young equation and thermodynamic analysis of the work of adhesion of aqueous solutions of surfactants to the polymer surface it was found that in the case of PTFE the changes of the contact angle as a function of the total mixture concentration in the bulk phase resulted only from changes of the polar component of the solution surface tension.     

A new model to determine contact angles on swelling polymer particles by the column wicking method

The contact angle determination on swelling polymer particles by the Washburn equation using column wicking measurements may be problematic because swelling occurs during the wicking process. The objective of this research was to develop a new model to more accurately determine contact angles for polymer particles that undergo solvent swelling during the column wicking process. Two phenomena were observed related to the swelling effect during the wicking process: (1) a temperature rise was detected during the wicking process when the swelling polymer particles interacted with polar liquids, and (2) a smaller average capillary radius (r) was obtained when using methanol (polar liquid) compared to using hexane (non-polar liquid). The particle swelling will induce both particle geometry changes and energy loss which will influence the capillary rise rate. The model developed in this study considered the average pore radius change and the energy loss due to the polymer swelling effect. Contact angle comparisons were conducted on wood with formamide, ethylene glycol, and water as test liquids, determined by both the new model and the Washburn equation. It was shown that the contact angles determined by the new model were about 4-37° lower than those determined by the Washburn equation for water, formamide, and ethylene glycol. Todetermine whether the polymer particles are swelling, two low surface tension liquids, one polar (methanol) and the other non-polar (hexane), can be used to determine the average pore radius (r values) using the Washburn equation. If the same r values are obtained for the two liquids, no swelling occurs, and the Washburn equation can be used for the contact angle calculation. Otherwise, the model established in this study should be used for contact angle determination.     

Use of the combined Lifshitz–van der Waals and Lewis acid–base approaches in determining the apolar and polar contributions to surface and interfacial tensions and free energies

Recently, a number of authors have been rearranging the various combinations and permutations of the different apolar and polar liquids with which contact angles can be measured on polar surfaces and in so doing have arrived at bizarre results. The rational order and procedures to be followed in the determination of the apolar and polar surface tension properties of polar materials, according to the van Oss-Chaudhury-Good components and parameters of the approach, are reiterated.     

Wettability of Polymeric Solids by Aqueous Solutions of Anionic and Nonionic Surfactant Mixtures

Measurements of the surface tension (γ _LV) and advancing contact angle () on poly(tetrafluoroethylene) (PTFE) and poly(methyl methacrylate) (PMMA) were carried out for aqueous solutions of sodium decyl sulfate (SDS) and p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycol) (TX100) and their mixtures. The results obtained indicate that the values of the surface tension and contact angles of solutions of surfactants on PTFE and PMMA surfaces depend on the concentration and composition of the surfactant mixtures. Calculations based on the Lucassen-Reynders equation indicate that for single surfactants and their mixtures at a given concentration in the bulk phase the values of surface excess concentration of surfactants at water–air and PTFE–water interfaces are nearly the same, so the adsorption of the surfactants at water–air and PTFE–water interfaces should also be the same. However, the adsorption of TX100 and its mixtures with SDS at water–air interface is higher than that at PMMA–water interface, which is confirmed by the ratio of absolute values of molecular interaction parameters at these interfaces calculated on the basis of Rosen approach. If we take into account the hydration of the poly(ethylene oxide) chains of TX100 and acid and base parameters of the surface tension of water it appears that the PMMA surface is covered by the 'pure' water molecules from the solution or molecules connected with the chain of nonionic surfactant. On the other hand, the lack of SDS molecules at the PMMA–water interface may result from the formations of its micelles which are connected with the TX100 chain.     

The multiple roles of interfacial tension in fluid phase equilibria and fluid–solid interactions

The tension at the interfaces separating the three phases of matter is a unique property in that it can reveal a great deal of information about the phases in contact, including the direction and extent of mass transfer of components, their proximity to equilibrium, the nature of fluids distribution relative to one another, the contact angle, and the spreading and adhesion behavior of liquids on solid surfaces. In this paper we examine, with supporting experimental data, the multitude of roles played by interfacial tension in establishing (1) the phase behavior characteristics of solubility, miscibility, and the associated mass transfer mechanisms in multicomponent fluid systems, (2) the nature of fluids distribution in gas–oil–water systems in porous solid substrates and (3) the spreading and adhesion characteristics in solid–liquid–liquid systems through dynamic contact angles.     

Influence of effective porosity in the determination of contact angles in porous solids by imbibition techniques

Distance-time and weight-time imbibition measurements with liquids having different densities, surface tensions, and viscosities have been carried out on silica gel plates in order to determine contact angles, by employing the parabolic form of the Washburn equation and the physical model that considers the difference in pressure at the liquid-vapor interface in terms of the Laplace equation. A comparison of the contact angles obtained by these two experimental techniques showed that the results are the same if the effective porosity of the porous solid determined independently by each capillary penetration experiment is utilized to analyze the weight-time experimental data.     

Recent theoretical and experimental advancements in the application of the van Oss–Chaudhury–Good acid–base theory to the analysis of polymer surfaces II. Some peculiar cases

—According to the general guidelines presented in the accompanying paper, some relevant examples of common polymer surfaces are analysed and discussed; a number of polymers commercially available or laboratory synthesized have been analysed. In particular, the case of poly(vinyl chloride) (PVC), often considered as peculiar in the literature, is fully analysed on the basis of a new set of well-prepared samples, whose compositions were checked by high-vacuum spectroscopies. 'Equilibrium' contact angles, obtained by a new experimental technique, are presented. The results are, however, preliminary, because the final set of liquids used is not so 'well equilibrated' as possible, from the point of view of acid–base properties. The contact angle data obtained are analysed in a non-linear way to calculate the acid–base components of all the liquids and solids. The results are discussed and compared with those obtained from liquid–liquid interfaces presented in the accompanying paper. The physico-chemical features of these samples have also been compared with the adhesion properties of some bacterial cells, commonly found as infective agents on biomaterials surfaces of medical devices, in order to rationalize these results within the theoretical framework of acid–base theory.     

Acid–base surface free energies of solids and the definition of scales in the Good–van Oss–Chaudhury theory

The overwhelming basicity of all analysed surfaces strongly dependent on the choice of liquid triplet used for contact angle measurements and the negative values sometimes obtained for the square roots of the acid-base parameters can be summarized as the main problems arising from the application of the Good-van Oss-Chaudhury (GvOC) theory to the calculation of Lewis acid-base properties of polymer surfaces from contact angle data. This paper tries to account for these problems, namely: (1) the Lewis base, or electron donor component, is much greater than the Lewis acid or electron-acceptor component because of the reference values for water chosen in the original GvOC theory. A direct comparison of the acidic component with the basic one of the same materials has no meaning. A new reference scale for water which is able to overcome this problem is suggested. For the calculation of acid-base components, a best-fit approach is proposed which does not require any starting information about the liquids or polymers and can yield estimates of the acid-base parameters for both the liquids and the polymers involved; (2) the strong dependence of the value of the acid-base components on the three liquids employed is due to ill-conditioning of the related set of equations, an intrinsic and purely mathematical feature which cannot be completely cured by any realistic improvement in experimental accuracy. To reduce or eliminate the effect, one only needs a proper set of liquids, representative of all kinds of different solvents; (3) the negative coefficients appear as a simple consequence of measurement uncertainty, combined with the possible ill-conditioning of the equation set. We cannot exclude, however, that in some cases they could have a different origin.     

Effects in Surface Free Energy of Sputter-Deposited VNx Films

Vanadium nitride (VN _x ) thin films have attracted much attention for semiconductor integrated circuit (IC) packaging molding dies, and forming tools due to their excellent hardness and, thermal stability. VN _x thin films with VN_0.45, VN_0.83, VN_1.22, VN_1.73, VN_2.06 were prepared using a radio frequency (RF) sputter technique. The experimental results showed that the contact angle at 20°C increases with increasing nitrogen content of the VN _x films, to 101.4° corresponding to VN_1.73 and then decreased. In addition, the contact angles decreased with increasing surface temperature, because an increase of the surface temperature disrupts the hydrogen bonds between water and the films and the water gradually vaporizes. The total surface fee energy (SFE) at 20°C decreased with nitrogen content of the VN _x films to 29.8 mN/m (VN_1.73) and then increased. This is because a larger contact angle means weaker hydrogen bonding which results in a lower SFE. The polar SFE component had the same trend as the total SFE, but the dispersive SFE component had the opposite trend. The polar SFE component is also lower than the dispersive SFE component. This is because hydrogen bonds are polar. The total SFE, dispersive SFE and polar SFE of the VN _x films all decrease with increasing surface temperature. This is because with increasing temperature, water evaporates from the surface, disrupting hydrogen bonds and hence increasing surface entropy. The film roughness has an obvious effect on the SFE and there is tendency for the SFE to increase with increasing film surface roughness. As a result the SFE and surface roughness can be expressed in terms of a simple ratio function.     

The effect of polymer surface on the wetting and adhesion of liquid systems

Young's equation describes the wetting phenomenon in terms of the contact angle between a liquid and a solid surface. However, the contact angle is not the only parameter that defines liquid–solid interactions, an additional parameter related to the adhesion between the liquid drop and the solid surface is also of importance in cases where liquid sliding is involved. It is postulated that wetting which is related to the contact angle, and interfacial adhesion, which is related to the sliding angle, are interdependent phenomena and have to be considered simultaneously. A variety of models that relate the sliding angle to the forces developed along the contact periphery between a liquid drop and a solid surface have been proposed in the literature. Here, a modified model is proposed that quantifies the drop-sliding phenomenon, based also on the interfacial adhesion that develops across the contact area of the liquid/solid interface. Consequently, an interfacial adhesion strength parameter can be defined depending on the mass of the drop, the contact angle and the sliding angle. To verify the proposed approach the adhesion strength parameter has been calculated, based on experimental results, for a number of polymer surfaces and has been correlated with their composition and structure. The interaction strength parameter can be calculated for any smooth surface from measurements of the contact and the sliding angles.