DETERMINATION OF SOLID SURFACE TENSION FROM CONTACT ANGLES: THE ROLE OF SHAPE AND SIZE OF LIQUID MOLECULES

Accurate surface tension of Teflon® AF 1600 was determined using contact angles of liquids with bulky molecules. For one group of liquids, the contact angle data fall quite perfectly on a smooth curve corresponding to γ_sv = 13.61 mJ/m², with a mean deviation of only ±0.24 degrees from this curve. Results suggest that these liquids do not interact with the solid in a specific fashion. However, contact angles of a second group of liquids with fairly bulky molecules containing oxygen atoms, nitrogen atoms, or both deviate somewhat from this curve, up to approximately 3 degrees. Specific interactions between solid and liquid molecules and reorientation of liquid molecules in the close vicinity of the solid surface are the most likely causes of the deviations. It is speculated that such processes induce a change in the solid-liquid interfacial tension, causing the contact angle deviations mentioned above. Criteria are established for determination of accurate solid surface tensions.     

Hydrogen Bonding and the Interfacial Component of Adhesion: Acid/Base Interactions of Corona-Treated Polypropylene

The effect of activation of the surface of polypropylene sheet, by a corona discharge, upon the contact angles of liquids and on the surface free energy parameters γ^LW, γ^⊕ and γ^⊖, was determined. Both advancing and retreating contact angles were measured. The “acid/base” theory of the components of surface free energy was employed.

The contact angles of water and glycerol were initially lower by as much as 30°, after treatment, and that of diiodomethane was lower by about 5°. With time, the advancing angles rose, and the γ^⊕ and γ^⊖ parameters fell, towards the values on the untreated solids, and attained more or less steady values after 5 to 10 days. The basic component, γ^⊖, was the most strongly affected by the corona treatment; it rose, typically, from 2.2 to as high as 25 mJ/m². The acidic component, γ^⊕, rose from zero to as high as 1.9 mJ/m². Its decay with time was only qualitatively the same as that of γ^⊖. The retreating angles, and the corresponding energy components, were changed in the same direction, and somewhat more strongly, than were the “advancing” data.

The well-known improvement in the property of forming strong joints or adherent coatings, after corona treatment, is no doubt due to the formation of sites or areas on the polymers where hydrogen bonds can be formed. The decay of the strength of adhesion with time is, no doubt, due to the decay of these sites or areas.     

Contact angle measurements and interpretation: wetting behavior and solid surface tensions for poly(alkyl methacrylate) polymers

Low-rate dynamic contact angles of a large number of liquids were measured on a poly(ethyl methacrylate) (PEMA) polymer using an automated axisymmetric drop shape analysis profile (ADSA-P). The results suggested that not all experimental contact angles can be used for the interpretation in terms of solid surface tensions: eight liquids yielded non-constant contact angles and/or dissolved the polymer on contact. From the experimental contact angles of the remaining four liquids, we found that the liquid-vapor surface tension times the cosine of the contact angle changes smoothly with the liquid-vapor surface tension, i.e. γlv cos ζ depends only on γlv for a given solid surface (or solid surface tension). This contact angle pattern is again in harmony with those from other methacrylate polymer surfaces of different compositions and side-chains. The solid-vapor surface tension of PEMA calculated from the equation-of-state approach for solid-liquid interfacial tensions was found to be 33.6 ± 0.5 mJ/m2 from the experimental contact angles of the four liquids. The experimental results also suggested that surface tension component approaches do not reflect physical reality. In particular, experimental contact angles of polar and nonpolar liquids on polar methacrylate polymers were employed to determine solid surface tension and solid surface tension components. Contrary to the results obtained from the equation-of-state approach, we obtained inconsistent values from the Lifshitz-van der Waals/acid-base (van Oss and Good) approach using the same sets of experimental contact angles.     

Influence of different alkyl side chains on solid surface tension of polymethacrylates

Low‐rate dynamic contact angles on poly(t‐butyl methacrylate) (PtBMA) were measured by an automated axisymmetric drop shape analysis profile (ADSA‐P). The solid surface tension of PtBMA is calculated to be 18.1 mJ/m², with a 95% confidence limit of ±0.6 mJ/m². This value was compared to previous results with different homopolymeric polymethacrylates [poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), and poly(n‐butyl methacrylate) (PnBMA)] and with copolymeric polymethacrylates {poly(methyl methacrylate/ethyl methacrylate, 30/70) [P(MMA/EMA, 30/70)] and poly(methyl methacrylate/n‐butyl methacrylate) [P(MMA/nBMA)]}. It was found that increasing length and size of the alkyl side chain decrease the solid surface tension, as expected. Comparison with pure alkyl surfaces suggests that the surface tension of PtBMA is dominated by the very hydrophobic t‐butyl group. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2493–2504, 2000     

Influence of the pH of Water on its Electron-Accepticity and Donicity

By means of contact angle measurements on dry layers of electrostatically neutral dextran with pure water (pH 6.1), water acidified with HCl (to pH 1.94) and water made alkaline with NaOH (to pH 12.8), it could be shown that there was essentially no change as a function of pH in the ratio of γ⁺/γ^- of water as compared with the aqueous acid and alkaline solutions. (Here γ⁺ is the Lewis acid parameter of the polar surface tension component of water and γ^- is its Lewis base parameter). In contrast, with contact angles measured with the same liquids on negatively charged clean glass, a significant decrease in contact angle was observed with water at pH 12.8, which was caused by the fact that at this alkaline pH an increase in surface hydrophilicity took place. This is because surfaces that have a given surface electrical potential at neutral pH generally acquire an even higher surface potential under more alkaline conditions which, concomitantly, also gives rise to an increase in surface hydrophilicity, and thus to lower contact angles with water. Finally, contact angles with acid water, pure water, and alkaline water, deposited on hydrophobic Parafilm surfaces, were exactly the same.     

Measurement of the Surface Free Energy of Amorphous Cellulose by Alkane Adsorption: A Critical Evaluation of Inverse Gas Chromatography (IGC)

Surface energies of amorphous cellulose “beads” were measured by IGC at different temperatures (50 to 100°C) using n-alkane probes (pentane to undecane). The equation of Schultz and Lavielle was applied which relates the specific retention volume of the gas probe to the dispersive component of the surface energy of the solid and liquid, γ^d_s and γ^d_l, respectively, and a parameter (“a”) which represents the surface area of the gas probe in contact with the solids. At 50°C, γ^d_s was determined to be 71.5 mJ/m², and its temperature dependence was 0.36 mJ m^-2 K^-1. Compared with measurements obtained by contact angle, IGC results were found to yield higher values, and especially a higher temperature dependence, d(γ^d_s)/dT. Various potential explanations for these elevated values were examined. The surface energy, as determined by the Schultz and Lavielle equation, was found to depend mostly on the parameter “a”. Two experimental conditions are known to affect the values of “a”: the solid surface and the temperature. While the surface effect of the parameter “a” was ignored in this study, the dependence of the surface energy upon temperature and probe phase was demonstrated to be significant. Several optional treatments of the parameter “a” were modeled. It was observed that both experimental imprecision, but mostly the fundamental difference between the liquid-solid vs the gas-solid system (and the associated theoretical weakness of the model used), could explain the differences between γ^d_s and d(γ^d_s)/dT measured by contact angle and IGC. It was concluded that the exaggerated temperature dependence of the IGC results is a consequence of limitations inherent in the definition of parameter “a”.     

On the Predominant Electron-Donicity of Polar Solid Surfaces

The reasons for the predominant electron-donicity of almost all solid polar surfaces and its implications are discussed in this paper. By contact angle or interfacial tension measurements, the electron-accepting as well as the electron-donating properties of polar liquids can be ascertained, through the interplay between their energies of adhesion and cohesion. For the solid-liquid interface, direct interfacial tension measurements are not possible, but indirectly, solid/liquid interfacial tensions of polar systems can be obtained by contact angle measurement. However, as the energy of cohesion of a solid does not influence the contact angle formed by a liquid drop placed upon its surface, one can only measure the solid surface'ks residual polar property, manifested by the energy of adhesion between solid and liquid. This residual polar property is of necessity the dominant component; in most cases this turns out to be its electron donicity. When, by means of contact angle measurements with polar liquids, both electron-accepting and electron-donating potentials are found on a polar solid, it is most likely still partly covered with a polar liquid: usually water. The amount of residual water of hydration of a polar solid follows from its polar (Lewis acid-base) surface tension component (γ^AB). The degree of orientation of the residual water of hydration on a polar solid can be expressed by the ratio of the electron-donating to electron-accepting potentials (γ^⊖/γ^⊕), measured on the hydrated surface.     

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.     

Interfacial Interactions in Polymers: The Dependence of the Measured Surface Tension of Solid Polymer on the Surface Tension of Wetting Liquid

Careful measurements of the surface tension of solid polymers, y_s, based on the data on contact angles for wetting liquids with various surface tension, y_L, allows one to establish the functional dependence of y_s = f(y_L). This dependence is divided into three zones: one zone, where there is no dependence of y_s on y_L and two zones where y_s changes linearly with y_L.     

DETERMINATION OF SOLID SURFACE TENSION FROM CONTACT ANGLES: THE ROLE OF SHAPE AND SIZE OF LIQUID MOLECULES

Accurate surface tension of Teflon® AF 1600 was determined using contact angles of liquids with bulky molecules. For one group of liquids, the contact angle data fall quite perfectly on a smooth curve corresponding to γ_sv = 13.61 mJ/m², with a mean deviation of only ±0.24 degrees from this curve. Results suggest that these liquids do not interact with the solid in a specific fashion. However, contact angles of a second group of liquids with fairly bulky molecules containing oxygen atoms, nitrogen atoms, or both deviate somewhat from this curve, up to approximately 3 degrees. Specific interactions between solid and liquid molecules and reorientation of liquid molecules in the close vicinity of the solid surface are the most likely causes of the deviations. It is speculated that such processes induce a change in the solid–liquid interfacial tension, causing the contact angle deviations mentioned above. Criteria are established for determination of accurate solid surface tensions.     

WETTABILITY AND SURFACE ENERGETICS OF ROUGH FLUOROPOLYMER SURFACES

Hydrophobic solid surfaces with controlled roughness were prepared by coating glass slides with an amorphous fluoropolymer (Teflon® AF1600, DuPont) containing varying amounts of silica spheres (diameter 48 μm). Quasi-static advancing, θ_A, and receding, θ_R, contact angles were measured with the Wilhelmy technique. The contact angle hysteresis was significant but could be eliminated by subjecting the system to acoustic vibrations. Surface roughness affects all contact angles, but only the vibrated ones, θ_V, agree with the Wenzel equation. The contact angle obtained by averaging the cosines of θ_A and θ_R is a good approximation for θ_V, provided that roughness is not too large or the angles too small. Zisman's approach was employed to obtain the critical surface tension of wetting (CST) of the solid surfaces. The CST increases with roughness in accordance with Wenzel equation. Advancing, receding, and vibrated angles yield different results. The θ_A is known to be characteristic of the main hydrophobic component (the fluoropolymer). The θ_V is a better representation of the average wettability of the surface (including the presence of defects).     

两种碳纤维上浆剂的比较

测试并比较了某国产上浆剂和进口上浆剂的黏度、表面张力、粒径等性能,研究了两种碳纤维上浆剂对聚丙烯腈基碳纤维表面形貌、耐磨性、水接触角、表面能、拉伸强度等性能的影响。结果表明,国产上浆剂固含量高、黏度高、粒径小,进口上浆剂黏度低、表面张力小、粒径分布窄。采用国产上浆剂上浆后的碳纤维,接触角为56.701°,毛丝量为0.15 mg/m,表面能为41.32 mJ/m²,耐磨次数为447次,拉伸强度为2.86 GPa;采用进口上浆剂上浆后的碳纤维表面光滑,接触角为51.063°,毛丝量为0.08 mg/m,表面能为47.26 mJ/m²,耐磨次数为461次,拉伸强度为2.95 GPa。     

Wettability and Adhesion Characteristics of Plasma-Treated Carbon Fibers

The surface free energy (γ_s) of modified carbon fibers was determined by tensiometry and effects of CF₄-O₂ plasma treatment were evaluated. The treatment with the gas mixture in which oxygen was above 40% accelerated preferentially the oxidation of fiber surfaces and the nondispersive component of the surface free energy, γ^P_S, increased to about three times that of the untreated fiber. On the other hand, the treatment with the gas containing CF₄ above 80% induced fluorination and surface species such as - CF, - CF₂, or - CF₃ were formed. The γ^P_S values decreased to almost zero and the dispersive component became about 18 mJ/m². The calculated work of adhesion between various fibers and the epoxy resin was well correlated with the interfacial shear strength of the composites formed with these materials.     

The static contact angle hysteresis obtained by different experiments for the system PTFE/water

Different experimental methods have been used to determine the static contact angle hysteresis of the system polytetrafluoroethylene/water and the results compared. While the Wilhelmy plate method is not influenced by methodical variations, contact angles determined by the sessile drop and the pendant bubble methods vary with the drop or bubble diameter up to a minimal diameter d_K of the contact area with the solid. This condition seems to be a universal one and should always be checked to ensure that the measured values are comparable. Contact angles calculated from the geometrical parameters of a drop or bubble should be used with care. The surface energetic characters for the PTFE/water are δθ = 19.5°, θ_{a, e} = 108.5° and θ_{r, e} = 89°.     

On the importance of some surface and interface characteristics in the formation of the properties of adhesive joints

The importance of some relative surface characteristics which determines the strength of adhesive joints: specific surface of substrate , relative contact area β and specific contact area β in the adhesion interaction process were emphasised. Existing and potential methods of experimental evaluation of these characteristics were shortly analysed. The durability of the adhesive joints in water media significantly increases with growth of specific surface ^* of chemically treated substrate evaluated from the SEM micrographs. Specific surface calculated from the experimental data of hexane adsorption measurements for iron particles (particulate model of steel substrate) is more then ten times greater than respective ^* values. The relative contact area β of the Al₂O₃ particles (in wide range of ) with PE melt was in a roundabout way evaluated by experimental determination of the affect of on kinetic of peel strength formation of adhesive joints: Al₂O₃ filled PE-steel. The speculation was based on the ability of Al₂O₃ to adsorb low-molecular products of contact oxidation of PE which takes place in the process of formation of adhesive joints and determines their strength. The ability of sorption in its turn is proportional to efficient value of β. The availability of the Al₂O₃ surface was evaluated.     

Wilhelmy Technique and Solidification Front Technique to Study the Wettability of Fibres

The strength of fibre-reinforced materials depends heavily on the adhesion between the fibre and the resin. To predict the bond strength of the adhesion, it is desirable for the surface tension of the fibre to be known. Two independent methods, the Wilhelmy balance method and the solidification front method, were investigated. The fibres used for this investigation included a carbon fibre, Thornel 300®, and an aromatic poiyamide fibre, Kevlar.

In the Wilhelmy experiments three liquids, ethylene glycol, glycerol and distilled water were employed to measure the surface tensions of the test fibres. They were found to be 42.4 mJ/m² and 43.7 mJ/m² for the carbon fibre and Kevlar, respectively. These values agreed very well with the results obtained from the solidification front method, from which the carbon fibre was found to have a surface tension value of 41.8 mJ/m² while that for Kevlar was 46.4 mJ/m². Furthermore, error analysis has shown that the error limits of the experiments are within 5% of the resulting values. The reproducibility and accuracy of these two techniques indicate that they are viable for determining the surface tension of small diameter fibres.     

SURFACE TENSION EFFECTS IN THE SEDIMENTATION OF COAL PARTICLES IN VARIOUS LIQUID MIXTURES

The sedimentation volume, V_sed, of coal particles in mixtures of pairs of non-polar and more polar organic liquid combinations as well as in aqueous solutions was determined at 20°C.

The liquid combinations were chosen such that the surface tension, γ_SV, of the coal particles fell between the surface tensions, γ_LV, of the two liquids. A constant mass of a given coal sample was suspended in constant volumes of liquid mixtures of different concentrations.

It was found that the sedimentation volume changed with varying composition of the liquid mixtures, as did the surface tension. A maximum or a minimum occurred in V_sed when the surface tension of the suspending liquid was equal to that of the coal particles, i.e. when γ_LV = γ_SV. Maxima occurred in more polar and minima in the non-polar or less polar liquid mixtures. The position of the extrema, and hence the surface tension, γ_sv, of the particles, was found to change with particle size, in agreement with findings from other independent techniques.

It was found that the surface tension of coal, γ_sv, depends on the surrounding liquid, i.e. coal is hydrophobic in organic and hydrophilic in aqueous media, again in agreement with the results from other techniques.     

The Rule of Interfacial Equilibrium

The detailed analysis of the well-known relation γ₁₂ = γ₁ -γ₂ has been done (γ₁₂, γ₁ and γ₂ are interfacial and surface tensions of phases 1 and 2). The analysis of equilibrium of a liquid drop at the interface with another liquid allowed us to prove that this relation should be modified by including in it the value γ₂* which is considered as the result of all interactions at the interface and is accepted depending on γ₁. The value γ₂* represents the surface tension of a phase in the ternary point solid-liquid-gas, i.e. in the zone of interfacial nonuniformity. The modified form of the relation, called the rule of interfacial equilibrium, allows us to show that thermodynamic work of adhesion is equal to the cohesion energy of the interphase formed by phase 2.     

Tribological properties of perfluoralkylethyl methacrylate-polymethyl methacrylate copolymer thin films

Perfluoroalkylethyl methacrylate and polymethyl methacrylate copolymers (FMA/MMA) were synthesized by the atom transfer radical polymerization at various FMA/MMA mole ratios. 5 wt% copolymer solutions were spun cast onto PMMA sheet substrates. The effects of monomer ratio on tribological properties were investigated in terms of the surface energy calculated from contact angles, the dynamic friction, and the wear obtained from multiple scratchings. There is a range of optimum FMA/MMA ratios between (1–5)×10⁻³ in which we attain a minimum dynamic friction. The results obtained are discussed in terms of connections between friction, surface tension, and wear mechanisms.     

Interfacial properties between ionic-liquid-based electrolytes and lithium-ion-battery separator

For lithium salts, ionic liquids (ILs) are promising alternatives to conventional solvents in lithium-ion batteries (LIBs) due to a more favorable high-voltage operating window, and due to improved safety through reduction of flammability. Toward better understanding of wetting properties of IL-based electrolytes on a LIB separator, wetting properties affect electrochemical performance, experimental studies were made to determine the influence of solvent, lithium-salt type and salt concentration. Surface tensions and advancing contact angles were measured for two pure ILs ([C₄C₁im][BF₄] and [C₄C₁im][OTf]) and for four IL/alkylcarbonate solvent blends (1:1 mass ratio, [C₄C₁im][BF₄]/PC, [C₄C₁im][BF₄]/DMC, [C₄C₁im][OTf]/PC, and [C₄C₁im][OTf]/ DMC) with several concentrations of a lithium salt (LiClO₄, LiPF₆, and LiTFSI). A significant improvement of wettability of pure ILs was observed by adding DMC, while adding PC with surface tension higher than that of pure ILs is detrimental to wetting behavior. Contact angles decrease by adding LiTFSI but show almost no change upon addition of LiPF₆ or LiClO₄. Surface tensions follow the same trend as that for contact angles. Incorporation of TFSI⁻ anion gives favorable separator wettability. Estimates were made for interfacial properties of the separator (dispersive and polar components of the surface free energy for solid-vapor, for liquid–vapor, and for solid–liquid interfacial free energy).