Contact angles of various liquids and surfactant solutions on polytef and paraffin were measured. Critical surface tension values were obtained by extrapolation of plots of cosine of the contact angles versus corresponding surface tension values. Contact angles measured using polyoxyethylene octylphenols produced linear Zisman plots and yielded critical surface tensions that agreed with accepted values. This liquid series provides a reasonable approach to the measurement of critical surface tension for solid drugs that are soluble in organic liquids but relatively insoluble in water.

An absolute method for the determinalion of surface tension of liquids using the pendent drop profiles at conical tips, which has several distinct advantages, has been proposed. For systems with zero contact angle, the dimensionless governing equations for drop profiles at different conical tips have been computer-solved. and the theoretical plots of X_T and Z_T vs. their ratio, where X_T and Z_T are the dimensionless x and z coordinates of the drop profile at a plane at the conical tip perpendicular to the axis of symmetry, are statistically anaJyied to generate suitable tables for using the proposed method.

Experiments were carried out on a variety of surfactant-coated mica surfaces using the surface forces apparatus technique and contact angle measurements. The experiments were designed to clarify the molecular mechanisms underlying adhesion hysteresis (during loading-unloading cycles) and contact angle hysteresis (of advancing/receding liquids), and to explore any possible relationship between these two energy-dissipating phenomena. We found that hysteresis effects are not simply due to surface imperfections, such as roughness or chemical heterogeneity. Even surfaces that are initially smooth and chemically homogeneous can exhibit large adhesion and contact angle hysteresis effects. Our results indicate that, for such surfaces, hysteresis arises because of molecular rearrangements occurring at solid-solid or solid-liquid interfaces after they have come into contact. This results in a lower surface free energy during the approach of two surfaces (or during spreading) than during separation (or retraction). We have studied a number of factors that enhance hysteresis: (i) increasing the freedom of the surface molecules to reorder, (ii) increasing the load and time surfaces are allowed to remain in contact, and (iii) increasing the rate of separation (or retraction). These findings highlight the inherent nonequilibrium nature of most loading-unloading and wetting-dewetting cycles and suggest ways for reducing the energy-dissipating hysteresis associated with such processes. Our results further indicate that the adhesion or pull-off force F between two curved surfaces of radius R is related to the surface energy-gamma by the Johnson-Kendall-Roberts theory, for example, F = 3-pi-R-gamma for a sphere on a flat surface, but only when the separation occurs under equilibrium conditions. Preliminary results also indicate a correlation between adhesion hysteresis and friction/stiction.

Sir: Padday and Uffindell produced a well organized and readable article, but unfortunately their mathematics is incorrect (by nearly one order of magnitude) because intermolecular potentials were integrated over molecular distances, breaking a fundamental principle of integral calculus.

There is, consequently, a great need for methods for the estimation of surface and interfacial energies — even if only approximate ones. It was as an approach to these problems that the following treatment was developed. Theory of Interfacial Energies — The Berthelot relation for the attractive constants between like molecules...

A theory is proposed by which the surface free energy of certain solids can be estimated from the contact angles ofliquids on them. The method is verified using contact angle and surface tension data from the literature, for benzene and a-methylnaphthalene on liquid and solid fractions of a fluorinated lubricating oil. The method is then applied to data on the contact angles of various liquids on Teflon and on an octadecylamine monolayer. The surface tensions of these solids are estimated to be, respectively, about 28 and 30 ergs/cm…

Surface tensions of the n-alkanes and interfacial tensions between the n-alkanes and water have been calculated. The ca1culations use a modified form of the Moelwyn-Hughes' equation for the dispersion interaction between two particles, the integtation method of Hamaker to derive the total interaction across a plane surface, the geometric mean relationship of Good and Girifalco for the interaction of two dissimilar phases, and an assumption that the entropy of surface formation equals the difference between the interaction energy so calculated and the total intern&l energy of surface formation. The calculated surface tensions of the n-alkanes are compared with and agree well with experimentally determined values; also, some of their calculated interfacial-tension, contact-angle, and spreading-coefficient measurements with water all agree with the corresponding experimental values. For other systems, calculations are limited lo the contribution of the dispersion forces to the total interaction of the system.

Surface tensions of the n-alkanes and interfacial tensions between the n-alkanes and water have been calculated. The ca1culations use a modified form of the Moelwyn-Hughes' equation for the dispersion interaction between two particles, the integtation method of Hamaker to derive the total interaction across a plane surface, the geometric mean relationship of Good and Girifalco for the interaction of two dissimilar phases, and an assumption that the entropy of surface formation equals the difference between the interaction energy so calculated and the total intern&l energy of surface formation. The calculated surface tensions of the n-alkanes are compared with and agree well with experimentally determined values; also, some of their calculated interfacial-tension, contact-angle, and spreading-coefficient measurements with water all agree with the corresponding experimental values. For other systems, calculations are limited lo the contribution of the dispersion forces to the total interaction of the system.

Earlier systematic studies of the angle of contact (9) exhibited by drops of liquid on plane, solid surfaces of low surface energy have revealed a regular linear variation in cos 9 with the surface tension (ylv) of a large variety of liquids; this led to the concept of the critical surface tension of spreading (yc) and its use in characterizing the wettability of organic solids and of high energy surfaces coated with adsorbed organic films. Effects of the nature and packing of the atoms or organic radicals in the organic surface in determining the wetting of the solid are summarized. Simple and useful correlations have been found between „and the constitution of low energy solid surfaces. It is concluded that usually atoms more than a few atom diameters below the surface have no influence on wetting. The “retraction method” of preparing monomolecular films from solutions on solids is shown to be a direct consequence of the above constitutive law of wetting. The same analysis can be applied to a pure liquid also, and it results in the explanation of the behavior of the autophobic liquids at room temperature and of the process of depositing a monolayer on a solid by retraction from the melt over a range of temperatures.

Dependence of the wetting characteristics of a solid on the roughness of its surface is inherent in the fundamental theory of wetting action (R. N. Wenzel: Ind. Eng. Chem. 28, 988 (1936)). This is immediately apparent when analyses of wetting phenomena take into account the actual areas of the several inter-faces involved as well as their respective specific energy values. The same method of analysis has led to quantitative evaluationof the effects of surface heterogeneity and surface porosity (A. B. D. Cassie and S. Baxter: Trans. Faraday Soc. 40, 546 (1944)).

Zisman's critical surface tension has been widely used as an appoximate value to the surface free energy of solids. Attempts to correlate the critical surface tension with the bulk properties have been reported previously. Howeer, none of these is quantitiatively reliable. In this work, we developed a modified Hildebrand-Scott equation that permits prediction of the critical surface tension for polymers from molecular constitution with good accuracy.

The present methods of measuring contact angles all require that the solid material be obtainable in some special shape, such as a flat plate or capillary tube. Many surfaces, for example, those of plant materials, occur in irregular forms and must be dealt with in situ, because of the inhomogeneity of the body. The chief value of the method herein described is that its applicability is largely independent of the form of the solid surface. Some of the earliest determinations of contact angles were made from measurements of the dimensions of bubbles and drops. The work has been confined to large drops, but the use of very small drops may be shown to possess several advantages . . .

A sphere in continuum model, with an internal surface, is used to relate surface tension and internal pressure. The results support the previous use of this model for polar interactions. The agreement of theory and experiment is close to that obtained with a recent lattice model.

We wish to propose that the surface entropy of a liquid may be taken as a criterion of surface orientation. Orientation in the surface will lead to a lower entropy than that in the condition where the surface molecules are disordered. The question is, first, how much lower is the surface entropy of polar substances than that of non-polar substances? And second, can we set up a simple model which will account for the lower entropy of polar liquids, as resulting from surface orientation?

Ramsay and Shields4 reached the conclusion empirically that there was a “normal” value (2.1) for the Eötvös constant, which is directly related to the molar surface entropy.^6-7 From various theo-retical studies,^6-9 particularly that of Born and Courant, it might be expected that there should be a “normal” value for nearly spherical non-polar molecules. (The extension of this concept to non-spherical molecules cannot be made very simply, because the number of molecules “in the surface” per unit area depends on the degree of orientation as well as the ratio of length to thickness.⁷) The hypothesis of Ramsay and Shields, that the degree of association could be calculated from the ratio of the observed Eötvös constant to the “normal” value, 2.1, has of course long sincebeen discredited;⁷ but it persists in textbooks and the literature, probably because of the lack of a plausible alternative. We will show that surface orientation furnishes a much more reasonable explanation.

In previous investigations2 the heats of fusion and heat capacities of BiCl3, BiBrs, HgCl2, CdCl2, CdBr2 and Cdl2 near the meltingpoint were measured. However, the heat contents relative to 298.15 K. were not published for these halides at the melting point. Since it recently was pointed out3 that such data would be of value, the Hm. p.—ff29S. iK increments as well as the heat capacities (assuming they are constant) of the solid salts for the same temperature range are listed in Table I.

Although the agreement between the heat content increment of this study and that of an earlier work4 for CdCl2 is poor, the present value as well as those of all the other salts except the bismuth halides are in excellent agreement with literature estimates6 (Table I). Included in Table I with the heat capacities for the temperature range 298 K. to the melting point are those values found near the melting points of the salts.

The wettability by organic liquids containing Cl, Br or I is less affected by the amide or ester groups as might be expected from the inability of halogenated liquids to form hydrogen bonds. Reasons are given for believing that hydrogen-bonding takes placein the wetting of nylon by water, glycerol, formamide and thiodiglycol and does not take place in the wetting of polyethylene terephthalate by these liquids. The postulated mechanism of wetting led to an experiment which showed that perfluorolauric acid could be adsorbed on nylon from n-decane solution rendering thenylon surface oleophobic.

Wettability of solid surfaces containing covalent chlorine increases greatly with the chlorine content. There is no indication of hydrogen-bonding at the solid/liquid interface for surfaces containing carbon, hydrogen and chlorine. A close packed monolayer of perchloro-2, 4-pentadienoic acid adsorbed on a polished metal is shown to behave with respect to wetting like an organic surface comprising Í00 atom per cent, of chlorine substitution. Increased wettability of fluorine-containing surfaces by hydrogen-bonding liquids is reported for a number of new, partially fluorinated plastic surfaces. The wettability of fluorinated surfaces varies with the type of spreading liquid. For non-polar liquids the wettability decreases with increasing fluorine substitution. For hydrogen-bonding liquids, the wettability increases in the order: polytetrafluoroethylene, polytrifluoroethylene, polyethylene, polvvinylidene fluoride and polyvinyl fluoride. The corresponding order for the haligenated liquids is polytetrufluoroet. hylene, polytrifluoroethylene, polyvinylidene fluoride, polyethylene and polyvinyl fluoride. Explanations are offered for the relation between wettability and the atom per cent, fluorine substitution in the surface based on the electronegativity of the fluorine atoms in the surface and the molecular structure of the spreading liquid.

The interfacial tension along the boundary formed between two immiscible polymer liquids has been measured by the pendant drop method. The polymers employed for the study are polyethylene, polydimethylsiloxane, poly(ethylene oxide), polytetrahydrofuran, poly(vinyl acetate) and an ethylene-vinyl acetate copolymer. Surface tensions of these polymers (against air) were also determined by the same technique. The values of interfacial tension between polyethylene and each of the five polar polymers, together with the surface tension data, were utilized to calculate the separate contributions to the surface tension by dispersion and dipole interaction forces, in accordance with the procedure proposed by Fowkes. The interfacial tension between two polar polymers was then analyzed in terms of these separate components of forces. An empirical relation has been shown to correlate the dipole interaction term in interfacial tension with the individual dipole force components of the two polar polymers involved.

Data are given for the free surface energy of a number of fluorocarbons. These data show that fluorocarbons as a class have lower surfaceenergies than all other compounds. Curves for the lowering of surface tension as a function of concentration are given for a series ofcompletely fluorinated acids. These surface active compounds cause a greater lowering of the surface tension of water than has ever been found before. Fluorocarbon surface active compounds are shown to lower the surface tension of organic substances very materially. Some measurements of interfacial tensions between fluorocarbons and water and between fluorocarbons and organic solvents are presented. An attempt is made to show some correlation between the above effects in the formation of emulsions and in the wetting of fluorocarbon surfaces.

(1) A study hasbeen made of the wettability of adsorbed monolayers of monohydroperfluoroundecanoic acid. The results obtained on this surface (comprising CF₂H groups) are compared with previous results on adsorbed monolayers of perfluorodecanoic acid (comprising CF₃ groups) and solid polytetrafluoroethylene (comprising CF₂ groups).(2) It is shown that for “normal” liquids [ie, those for which only van der Waals forces of adhesion are operative) the contact angles on CF₂H surfaces are larger than on CF₂ surfaces and nearly as large as on CF₃ surfaces.(3) Alcohols, acids and amines are found to give abnormally low contact angles on CF₂H and CF₃ surfaces, but only amines give low angles on polytetrafluoroethylene. This can be accounted for by the ability of these polar liquids to form hydrogen bonds with the fluorinecontaining surfaces. Esters, lacking a suitable hydrogen atom, are shown to bond only to the CF₂H surface (which can supply the necessary hydrogen atom); alcohols and acids bond to CF₂H and CF₃ surfaces; primary amines bond to all three fluorinated surfaces because of their ability to form “double” hydrogen bonds. These phenomena are believed to be new examples of the “unsymmetrical hydrogen bond” described by Pauling.

It has been shown that the determining factors in the wetting of low-energy solid surfaces are the nature of the atoms in the surface and their physical packing. Of all solid organic polymers polytetrafluoroethylene is known to have the lowest surface energy and thus the lowest critical surface tension of wetting (y_c). In this study the wettability of a series of copolymers of tetrafluoroethylene with hexafluoropropylene by a variety oforganic and inorganic liquids was examined; the results show that these solid plastics have critical surface tensions which are even lower than that of solid polytetrafluoroethylene. As anticipated, the progressive increase in the proportion of perfluoromethyl side chains in the polymer introduces a higher concentration of exposed-CFs groups in the surface which in turn progressively reduces y_c. By extrapolation y_c of a polyhexafluoropropylene has been calculated to give the value of 15 dynes/cm.

The effect of surface heterogeneity on the wettability of an idealized heterogeneous surface has been studied with the aid of a digital computer. Surface heterogeneity permits the existence of a large number of metastable configurations. Contact angle hysteresis is discussed in terms of a balance between the vibrational energy of a drop and the heights of the energy barriers between allowed metastable states.

Wettability studies of aqueous solutionsof several series of pure, highly fluorinated, aliphatic acids and salts were carried out on two low-energy organic solids, polyethylene and Teflon. As anticipated, the fluorinated compounds were able to depress the surface tension of water below the critical surface tension (γ_c) of Teflon, and were therefore capable of completely wetting it. In contrast, conventional hydrocarbon wetting agents do not depress the surface tension of water to this extent. Whenever possible, wettability curves were obtained for each...

It has been suggested in a previous communication (3) that widely variant surface energies may exist at closely adjoining points on a surface. Well-substantiated theory as to the surface structure of solid catalytic materials is in accord with thisview (7). The “active patches” on the catalytic surfaces are an extreme example of irregularity in the surface energy, but it seems reasonable to suppose that such irregularities may exist to a lesser degree in nearly all ordinary surfaces. Photographic evidence in support of this proposition appears in the work of Wark and Cox (9), who found that the same air bubble under a mineral surface wet with water might have an angle of contact on the right side different from that on the left.

Instead of measuring the contact angle directly, it may be calculated from the dimensions of the drop. The angle so obtained may be regarded as the integral of the sum of all the various contact angles existing along the circumference of the drop. Thus each determination yields an average result not unduly influenced by irregularities at a given point on the surface. For precise determinations the method should have an especial advantage over the usual procedure of direct measurement, because the error in personal judgment involved in drawing the tangent to the curved drop surface at the point of contact is eliminated. This error becomes increasingly important as the contact angle approaches 180, while the dimensions of the drop may be measured with the same degree of accuracy as before.

Low contact angle hysteresis is an important characteristic of superhydrophobic surfaces for nonstick applications involving the exposure of these surfaces to water or dust particles. In this article, a relationship is derived between the surface work of adhesion and the dynamic contact angle hysteresis, and the resulting predictions are compared with experimental data. Superhydrophobic surfaces with different contact angles and contact angle hysteresis were prepared by generating silicon pillars with varying pillar size and pitch. Surfaces were subsequently treated with fluoroalkyl silanes to modify further the hydrophobicity. The three-phase contact line established for such systems was related to the Laplace pressure needed to maintain a stable superhydrophobic state.

The surface chemistry and nanotopography of low-density polyethylene (LDPE) were modified by downstream, inductively coupled, radio frequency (rf) Ar plasma without inducing surface damage. The extent of surface modification was controlled by the applied ion energy fluence, determined from the plasma ion density measured with a Langmuir probe. The treated LDPE surfaces were characterized by atomic force microscope (AFM) imaging, contact angle measurements, and X-ray photoelectron spectroscopy (XPS). Analysis of AFM surface images confirmed that topography changes occurred at the nanoscale and that surface damage was insignificant. Contact angle measurements demonstrated an enhancement of the surface hydrophilicity with the increase of the plasma power. XPS results showed surface chemistry changes involving the development of different carbon-oxygen functionalities that increased the surface hydrophilicity. Physical and chemical surface modification was achieved under conditions conducive to high-density inductively coupled rf plasma.

The chemical modification of polystyrene surfaces by low-energy (10−100 eV) SF₅⁺, C₃F₅⁺, and SO₃⁺ ions was studied by X-ray photoelectron spectroscopy and two-laser ion trap mass spectrometry. The mechanism of fluorination was found to be dissimilar for SF₅⁺ and C₃F₅⁺ ions in this energy range at fluences of 10¹⁴−10¹⁶ ions/cm². SF₅⁺ was found to induce fluorination of the polymer surface by grafting reactive F atoms upon dissociation at impact. SF_n fragments were not found to be grafted or implanted into the polymer. Sulfur was detected on the polymer surface only at incident energies above 50 eV and was found to be sulfidic in nature. In contrast, C₃F₅⁺ ions induced grafting of both reactive F atoms and molecular C_mF_n fragments from the dissociation of the incident projectile. Larger proportions of highly fluorinated sites and thicker fluorocarbon layers were found for C₃F₅⁺ at all energies and fluences. A variety of aliphatic and aromatic fluorine bonding environments were detected on both SF₅⁺ and C₃F₅⁺ modified polystyrene surfaces.