A Novel Method for Surface Free-Energy Determination of Powdered Solids

Interfacial solid/liquid interactions play a crucial role in wetting, spreading, and adhesion processes. In the case of a flat solid surface, contact angle measurements are commonly utilized for the determination of the solid surface free energy and its components. However, if such a surface cannot be obtained, then the contact angle can not be measured directly. Usually methods based on imbibition of probe liquids into a thin porous layer or column are applied. In this paper a novel method, also based on the capillary rise, is proposed for the solid surface free-energy components determination. Actually, it is a modification of the thin column wicking method; similar theoretical background can be applied together with that appropriate for the capillary rise method of liquid surface tension determination. The proposed theoretical approach and procedure are verified by using single glass capillaries, and then alumina and ground glass powders were used for the method testing. Thus obtained surface free-energy components for these solids, for both glass and alumina, agree well with the literature values.     

Acid base contributions to interfacial free energy: self-consistency considerations

The evaluation of interfacial free energies is a subject of much discussion. Of recent times, the role of acid/base interactions has been much debated and a mathematical model devised by van Oss et al. is now commonly used. In this work, we study several polymers using recognised probe liquids and, exploiting an extended matrix analysis, obtain values of solid surface characteristics in agreement with literature values. However, by ‘inverting’ the method, and using accepted solid surface characteristics to ‘rederive’ surface free energy parameters for probe liquids, inconsistencies become apparent. Despite the conceptual interest of acid/base interactions, clearly the mathematical formulation of van Oss et al. is incomplete and further development is required.     

Surface Heterogeneity of Passively Oxidized Silicon Carbide Particles: Hydrophobic-Hydrophilic Partition

The surface of silicon carbide (SiC) particles previously subjected to passive oxidation has been characterized using various techniques such as adsorption of positively and negatively charged surfactants in aqueous solution, immersion microcalorimetry in three probe liquids, and flow microcalorimetry in organic media. Adsorption data show that around one quarter of the surface appears negatively charged and thus hydrophilic, while three quarters appear uncharged and hydrophobic. This is attributed to dissociation of silanols groups. Immersion calorimetry in liquids having well-defined polar and nonpolar components of surface energy shows that the Lifshitz-van der Waals component of SiC is very high and that the acid and basic components are weak. The experimental results appear to be consistent with both computations of surface energy using Lifshitz theory and experimental data previously obtained with other minerals. The three indexes are discussed and it is argued that they represent different terms of the solid surface energy. Copyright 2000 Academic Press.     

New Prediction Model of Surface and Interfacial Energies Based on COSMO-UCE**

The nature and strength of intermolecular and surface forces are the key factors that influence the solvation, adhesion and wetting phenomena. The universal cohesive energy prediction equation based on conductor-like screening model (COSMO-UCE) was extended from like molecules (pure liquids) to unlike molecules (dissimilar liquids). A new molecular-thermodynamic model of interfacial tension (IFT) for liquid-liquid and solid-liquid systems was developed in this work, which can predict the surface free energy of solid materials and interfacial energy directly through cohesive energy calculations based on COSMO-UCE. The applications of this model in prediction of IFT for water-organic, solid (n-hexatriacontane, polytetrafluoroethylene (PTFE) and octadecyl-amine monolayer)-liquid systems have been verified extensively with successful results; which indicates that this is a straightforward and reliable model of surface and interfacial energies through predicting intermolecular interactions based on merely molecular structure (profiles of surface segment charge density), the dimensionless wetting coefficient R_A/C can characterize the wetting behavior (poor adhesive (non-wetting), wetting, spreading) of liquids on the surface of solid materials very well.     

MMA-天然橡胶接枝聚合物表面的酸碱作用

<正> 聚合物-基体界面的酸碱配位作用是界面粘合力形成的重要因素之一,Fowkes,Drago,Matsunaga等曾对参照液体-聚合物体系粘附功的酸碱作用成份的测算方法进行了研究,并测定了乙烯、丙烯酸、纤维素等聚合物表面的酸碱属性。本文应用Fowkes的基本方法,对MMA(甲基丙烯酸甲酯)-天然橡胶接枝聚合物表面的酸碱属性及其对基体的粘合力的影响进行了研究。     

Correlation between mechanical adhesion and interfacial properties of starch/biodegradable polyester blends

Biopolymers are preferred ingredients for the manufacture of materials because they are based on abundantly available and renewable raw materials that have benign environmental problems associated with their production, fabrication, use, and disposal; however, the wide use of biopolymers in engineering applications has not been achieved, mainly because of the inferior quality of many biopolymer‐based products. To overcome this limitation, studies have been initiated on blends of biopolymers and biodegradable synthetic polymers. We used the contact angle of probe liquids to measure the surface energy of polystyrene, the biodegradable polyesters polycaprolactone, poly(hydroxybutyrate‐co‐hydroxyvalerate), polylactic acid, polybutylene adipate terephthalate, and adipic poly(hydroxy ester ether), and normal starch. The surface energies were used to estimate the starch/polymer interfacial energy and work of adhesion. The calculated starch/polyester work of adhesion showed mixed correlation with published starch/polyester mechanical properties, indicating that factors other than interfacial properties might be dominant in determining the mechanical properties of some starch/polyester blends. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 920–930, 2001     

Recent progress in the determination of solid surface tensions from contact angles

Advancing contact angles of different liquids measured on the same solid surface fall very close to a smooth curve when plotted as a function of liquid surface tension, i.e., gamma(lv)costheta versus gamma(lv). Changing the solid surface, and hence gamma(sv), shifts the curve in a regular manner. These patterns suggest that gamma(lv)costheta depends only on gamma(lv) and gamma(sv). Thus, an "equation of state for the interfacial tensions" was developed to facilitate the determination of solid surface tensions from contact angles in conjunction with Young's equation. However, a close examination of the smooth curves showed that contact angles typically show a scatter of 1-3 degrees around the curves. The existence of the deviations introduces an element of uncertainty in the determination of solid surface tensions. Establishing that (i) contact angles are exclusively a material property of the coating polymer and do not depend on experimental procedures and that (ii) contact angle measurements with a sophisticated methodology, axisymmetric drop shape analysis (ADSA), are highly reproducible guarantees that the deviations are not experimental errors and must have physical causes. The contact angles of a large number of liquids on the films of four different fluoropolymers were studied to identify the causes of the deviations. Specific molecular interactions at solid-vapor and/or solid-liquid interfaces account for the minor contact angle deviations. Such interactions take place in different ways. Adsorption of vapor of the test liquid onto the solid surface is apparently the only process that influences the solid-vapor interfacial tension (gamma(sv)). The molecular interactions taking place at the solid-liquid interface are more diverse and complicated. Parallel alignment of liquid molecules at the solid surface, reorganization of liquid molecules at the solid-liquid interface, change in the configuration of polymer chains due to contact with certain probe liquids, and intermolecular interactions between solid and liquid molecules cause the solid-liquid interfacial (gamma(sl)) tension to be different from that predicted by the equation of state, i.e., gamma(sl) is not a precise function of gamma(lv) and gamma(sv). In other words, the experimental contact angles deviate from the "ideal" contact angle pattern. Specific criteria are proposed to identify probe liquids which eliminate specific molecular interactions. Octamethylcyclotetrasiloxane (OMCTS) and decamethylcyclopentasiloxane (DMCPS) are shown to meet those criteria, and therefore are the most suitable liquids to characterize surface tensions of low energy fluoropolymer films with an accuracy of +/-0.2 mJ/m2.     

Importance of surface tension for physical paint properties

Surface tension is a parameter of decisive importance for characterizing painted and unpainted surfaces related to wetting and adhesion phenomena. This article presents measurements of the surface tension of solids by means of an automatic contact angle measurement device. The evaluation of the surface tension is based on a separation into polar and disperse components. In addition, this paper briefly touches on other, more far-reaching approaches (acid/base) and discusses a method for the determination of dynamic surface tension of liquids.     

The surface energy of various biomaterials coated with adhesion molecules used in cell culture

This study calculates the surface energy of polystyrene tissue culture plastic, silicon, silicon dioxide and indium tin oxide, all of which have applications in tissue culture. The adhesion molecules: collagen, fibronectin, poly-L-ornithine and poly-D-lysine, were coated onto these various surfaces, and the surface energy of the coated substrates calculated. Coating with fibronectin was found to produce a monopolar acidic surface while poly-D-lysine, poly-L-ornithine and collagen coatings were found to produce monopolar basic surfaces. The calculated surface energy components of the coated materials were then used to give a quantitative determination of the magnitude of their hydrophobicity. It was concluded that collagen, polylysine and polyornithine could provide a hydrophobic or hydrophilic surface depending on the underlying substrates they were coated on. The measurement obtained for fibronectin, unlike the other adhesion molecules, was independent of the underlying surface and remained hydrophobic on all substrates tested. Wetting experiments were carried out on the coated substrates, using the tissue culture medium Dulbeccos modified eagles medium, both containing and not containing serum proteins, and saline solution. These liquids that are commonly used in tissue culture, were then used to provide information how these liquids behave on various substrates coated with the adhesion molecules. Results show that fibronectin coated surfaces represent the most phobic surface for all three liquids. The findings of this study can be used in cell manipulation studies and provide a valuable data set for the biomedical and research industries.     

Analysis of the Silica Surface Free Energy by the Imbibition Technique

The surface free energy of silica and its components have been evaluated from imbibition experiments performed with liquids of differing surface properties by the distance–time method. Data were analyzed by a parabolic fit to Washburn's equation, because of the uncertainty in the exact position and time at which penetration begins in these kinds of experiments. In addition to the mathematical treatment of the experimental results, the influence of the components and parameters of the surface tension of the liquids used on the values of the solid surface free-energy components has been analyzed.     

Equilibrium contact angles of liquid droplets on ideal rough solids

This work proposes a theoretical model for predicting the apparent equilibrium contact angle of a liquid on an ideal rough surface that is homogeneous and has a negligible body force, line tension, or contact angle hysteresis between solid and liquid. The model is derived from the conservation equations and the free-energy minimization theory for the changes of state of liquid droplets. The work of adhesion is expressed as the contact angles in the wetting process of the liquid droplets. Equilibrium contact angles of liquid droplets for rough surfaces are expressed as functions of the area ratios for the solid, liquid, and surrounding gas and the roughness ratio and wetting ratio of the liquid on the solid for the partially and fully wet states. It is found that the ideal critical angle for accentuating the contact angles by the surface roughness is 48°. The present model is compared with existing experimental data and the classical Wenzel and Cassie-Baxter models and agrees with most of the experimental data for various surfaces and liquids better than does the Wenzel model and accounts for trends that the Wenzel model cannot explain.     

The effect of trimethylsilyl substituents in the monomer unit on the energy characteristics of surfaces of polynorbornenes obtained via metathesis polymerization

The surface properties of polynorbornenes obtained via metathesis polymerization are investigated via the wetting method. The incorporation of trimethylsilyl groups into the monomer unit causes decreases in the specific free surface energies of the investigated polymers. It is found that the dispersion terms of the specific free surface energies correlate with the free-volume and gas-permeability values of the polymers. As shown through the methods of differential scanning calorimetry and FTIR spectroscopy, during heating in air, the considered polynorbornenes undergo oxidative crosslinking, which increases their specific free surface energies. The interphase energies of crosslinked polynorbornenes are determined at their interfaces with liquids modeling polar and nonpolar phases, and the values of the work of adhesion of the crosslinked polynorbornenes to the model liquids are calculated. It is shown that the work of adhesion of the polymer to the polar phase decreases after introduction of trimethylsilyl groups, while the combination of polar and nonpolar phases give rise to an increase in the work of adhesion.     

Determination of contact angles on microporous particles using the thin-layer wicking technique

The properties of particle-stabilized emulsions, especially with regard to phase inversion, are very dependent on the contact angle that the particles experience at the oil-water interface. For the very small particles used for such emulsions (often a few tens of nm), it is impossible to measure this contact angle directly. Its value could be calculated if it were possible to determine the components of the solid surface free energy. To establish a method suitable for such particles, we have investigated the imbibition of five probe liquids into a porous bed of silica (commercial TLC plates) using the thin-layer wicking technique. For all liquids, the difference between wicking rate for bare plates and for those pre-contacted with the vapors is large but it is not due to an advancing angle effect on bare plates. Our analysis shows that it is due to the diversion of flowing liquid into blind pores which are already filled in the pre-contacted case. Thus a new model is proposed describing wicking in a porous medium with very small blind pores by introducing a parameter into the Washburn equation that corrects for this capillary condensation effect. The parameter needed is determined independently using gravimetric adsorption measurements. When this modified Washburn equation is used, the difference between advancing and receding contact angle is actually quite small. When the averages are used as the Young's contact angles, values for the surface energy components of silica are obtained that are completely consistent between the five liquids and have magnitudes expected for this type of silica surface.     

Hydrate-phobic surfaces: fundamental studies in clathrate hydrate adhesion reduction

Clathrate hydrate formation and subsequent plugging of deep-sea oil and gas pipelines represent a significant bottleneck for deep-sea oil and gas operations. Current methods for hydrate mitigation are expensive and energy intensive, comprising chemical, thermal, or flow management techniques. In this paper, we present an alternate approach of using functionalized coatings to reduce hydrate adhesion to surfaces, ideally to a low enough level that hydrodynamic shear stresses can detach deposits and prevent plug formation. Systematic and quantitative studies of hydrate adhesion on smooth substrates with varying solid surface energies reveal a linear trend between hydrate adhesion strength and the practical work of adhesion (γ(total)[1 + cos?θ(rec)]) of a suitable probe liquid, that is, one with similar surface energy properties to those of the hydrate. A reduction in hydrate adhesion strength by more than a factor of four when compared to bare steel is achieved on surfaces characterized by low Lewis acid, Lewis base, and van der Waals contributions to surface free energy such that the practical work of adhesion is minimized. These fundamental studies provide a framework for the development of hydrate-phobic surfaces, and could lead to passive enhancement of flow assurance and prevention of blockages in deep-sea oil and gas operations.     

Wetting study of imidazolium ionic liquids

In this work, we present a systematic contact angles study of a series of 1-alkyl, 3-methyl-imidazolium ionic liquids (ILs) on well-defined polar and nonpolar monolayer surfaces supported on Si wafers. The advancing and receding contact angles of ILs were used to determine the surface energy of the monolayer surfaces using Neumann's equation-of-state and Zisman's critical surface tension approaches. In parallel, the contact angles of conventional probe fluids (molecular liquids) including water, formamide, methylene iodide, ethylene glycol, and hexadecane were determined on the same surfaces. The results obtained showed a great deal of similarity in wetting behavior of ionic vs molecular probe fluids: the contact angles of both sets of liquids followed the same patterns in accord with the surface tension of the fluid. A good agreement was found between the surface energy determined by different sets of liquids.     

Chemical Modifications on Human Hair Studied by Means of Contact Angle Determination

Wetting properties of a solid surface can change as a consequence of chemical treatment. There is a relationship between the molecular structure of a surface and the macroscopic properties of this surface such as wetting and adhesion. Information on the surface energy of a solid was obtained by calculating polar and dispersion force contributions by means of contact angle determination. The superficial modification undergone by human hair treatments with or without hydrogen peroxide at alkaline pH was studied by means of wetting force measurements. The wetting increase in treated human hair fibers was analyzed following the Hüttinger method using contact angle data, taking into account the acid-base and dispersion components of the total wetting adhesion work. The hydrogen peroxide treatment at alkaline pH leads to a partial removal of hydrocarbon chains and to the formation of ionic groups (cysteic acid residues) on the outer scale cell surface. The latter phenomenon was observed by means of an increase in the acid-base adhesion work versus water wetting liquid at alkaline pH. Copyright 2001 Academic Press.     

New Approach to the Study of Particle-Surface Adhesion Using Atomic Force Microscopy

The colloidal probe technique is commonly employed to determine the adhesion force between a particle and a solid surface. Characterization of the adhesion of a particle across a surface can be as important, if not more so, as the determination of an average value for the adhesion. Unfortunately, the measurement of the variation in adhesion can be difficult at best. A new approach for studying particle-surface adhesion based on the force-volume technique is presented. Upon combining the force-volume technique with a colloidal probe, not only is it possible to determine the average adhesion force, but an image of the spatial variation of the adhesion can also be obtained. This method is envisioned to have great potential for examining and analyzing the adhesion behavior in complex natural and technological systems. Copyright 2000 Academic Press.     

Principal component analysis and multicomponent surface free energy theories

The same underlying mathematical structure characterizes some of the most popular multicomponent models for the prediction of surface free energies and adhesion works. After a brief illustration of the general methods for the computation of liquid and solid components in typical multicomponent theories, it is shown that both model definition and component estimate may take great advantage from application of Principal Component Analysis techniques, owing to the very peculiar structure of adhesion work equations. It is also put into evidence that a problem of scale multiplicity arises as a consequence of the symmetries involved in the model equations for adhesion work and surface free energy. A special discussion is devoted to the specific cases of van Oss–Chaudhury–Good acid–base theory, Qin–Chang model and extended Drago theory, which constitute the most common multicomponent models usually applied in the analysis of adhesion phenomena.      

Validity and accuracy in evaluating surface tension of solids by additive approaches

The validity and the accuracy of both the Owens and Wendt and the Lifshitz-van der Waals/acid-base (LW/AB) methods for the determination of surface tensions of solids have been examined for a wide variety of situations. In each case, the allowed range of contact angles that result in positive values of all the square roots of the surface tension components of the solid has first been determined. Then the maximum relative errors in the surface tensions of solids that result from errors in contact angle measurements have been calculated within the allowed range. For both methods, it has been found that the maximum relative errors are minimal if one of the liquids is apolar. In the case of the LW/AB method, minimal errors are obtained if, in addition, the other two liquids are monopolar with different polarities. However, the more similar are the properties of the liquids, the narrower is the allowed range, and the larger are the maximum relative errors.     

Reconciling slip measurements in symmetric and asymmetric systems

In the past decade, the slip of simple liquids on solid surfaces has been demonstrated by many groups. However, the slip of liquids on wettable surfaces is heavily debated. Using colloid probe atomic force microscopy (AFM), we found the slip length of di-n-octylphthalate in a symmetric wettable system (silica) to be around 11 nm, which raises the question of what the measured slip length in an asymmetric hydrophilic-hydrophobic system would be. To answer this question, we investigated liquid slip in one symmetric nonwettable system (hydrophobic DCDMS or OTS) and in one asymmetric hydrophilic (silica)-hydrophobic (DCDMS) system by the same method at driving velocities of between 10 and 80 μm/s. The slip results obtained from the three systems are in agreement with each other, and this comparison provides a means to self-assess the accuracy and reproducibility of the measured force curves and the fitted slip length in our systems. Furthermore, this method provides access to reliable values of the actual slip length on any investigated flat surface in an asymmetric system, avoiding the difficulty of preparing a symmetric probe/flat surface system in a colloid probe AFM force measurement.