W. A. Zisman

The spreading of liquids on low energy surfaces. I. polytetrafluoroethylene

1. 1. Methods of preparing surfaces of polytetrafluoroethylene (TFE) are described which permit obtaining reproducible and reliable measurements of contact angles with liquids. Polytetrafluoroethylene is found to be an ideal low energy surface for the study of the wetting relations of a solid with a wide variety of organic and inorganic liquids. 2. 2. It is shown that all but the lowest-boiling liquids studied do not adsorb on TFE sufficiently from the vapor to produce a significant spreading pressure. Hence the πE term in the expression for the work of adhesion (WA) is negligible and WA = γLV(1 + cos θ). The calculated values of WA are found to be consistent and reasonable. 3. 3. It is found that for each homologous series of liquids cos θ is a linear function of γLV. The constants of the resulting relations are given. From this WA − Wc can be expressed as a function of γLV alone. 4. 4. The observations show that there is a critical surface tension (17.5–20.5 dynes/cm.) below which liquids wet TFE. 5. 5. It is concluded that the corrected Young-Dupre relation coupled with the spreading coefficient permits a rational description of the results. The Doss and Rao relation for the surface condensation leads to such high values for high-boiling liquids as to make it unlikely that the theory is correct.

Oleophobic monolayers: I. Films adsorbed from solution in non-polar liquids☆

It was found that certain types of polar organic molecules are adsorbed from solutions in non-polar solvents to form well-oriented monolayers on polished solid surfaces. Such monolayers imparted both hydrophobic and oleophobic properties to the polished surfaces of a variety of metallic and non-metallic solids and could be formed from a large variety of solvents, those used ranging from hydrocarbons like hexadecane, mineral oils, benzene, methylnaphthalene and dicyclohexyl to other solvents such as carbon disulfide, carbon tetrachloride, bromobenzene and diphenyloxide. It is shown that the mechanism of the formation of these films on platinum was reversible adsorption from solution and that it definitely was not the accumulation of insoluble films floating at the liquid-air interface as in the Langmuir-Blodgett method. It is concluded that the adsorbed films were made up of almost vertically oriented molecules which were nearly close-packed and attached to the surface through a surface active or polar group. Attention is called to the possibility that such films do not conform to the shape of the solid surface but might bridge over those surface depressions whose areas are not too great compared to the cross-sectional area of the molecules. In order that a compound be able to adsorb as an oleophobic monolayer it appears necessary that its molecular structure be in keeping with the following requirements. First, the molecules of the compound must be capable of approximating close-packed orientation in a monolayer. Second, the surface active or polar group must be located at one extremity of the molecule and one or more methyl groups must be located at the opposite extremity. Finally, the molecules must adsorb to a fiat solid surface with sufficient close-packing so that the outermost portion of the film is essentially a plane surface, densely populated with methyl groups. It was found that compounds whose molecular configurations resemble a long rod or a flat plate, with a polar group attached to one end of the rod or rim of the plate and one or more methyl groups at the opposite end of the rod or rim of the plate, readily satisfy these requirements. Exceptions were found in the case of aliphatic, unbranched polar molecules containing one or more unsaturated bonds, for these did not form oleophobic films. It is suggested that such molecules adsorb on solids at both the polar end group and the unsaturated bonds so that, instead of orienting with their axes vertical to the surface, they are arranged more nearly horizontally. Observations were made to determine the smallest concentrations of each of the various types of oleophobic compounds which would permit the formation of oleophobic monolayers on platinum and pyrex. While weight concentrations of only 10−7 were required for primary aliphatic amines and monocarboxylic acids, roughly 1000 times more was needed for the aliphatic alcohols, esters and ketones and for cholesterol. Some of the relations of these findings to past theoretical and applied research are discussed.

The spreading of liquids on low-energy surfaces. III. Hydrocarbon surfaces

Abstract 1. 1. Contact angle measurements have been made of a wide variety of liquids on clean, smooth surfaces of polyethylene, paraffin, and surfaces of single crystals of n-hexatriacontane (C36). The calculated value of the final spreading coefficient (SLV∘/SV∘) is given and from the data there can be calculated the values of the free energy of immersion (fSL), and the work of adhesion (WA). The free energy of immersion of the solid in the liquid vapor can be neglected in these calculations since it is believed to be quite small for surfaces of low adsorptivity and low free surface energy. It is shown that the methyl-rich surfaces of C36 and paraffin are not wetted by a wide variety of organic liquids, including the n-alkanes, so that the rule that nonpolar solids are wetted by nonpolar liquids is again found to be erroneous. 2. 2. Wettability is found to decrease in the order polyethylene, paraffin, C36. This is attributed to the increase in the proportion of methyl to methylene groups in the surface. The C36 surface, like all higher n-alkane crystals, is shown to be the least wettable of all hydrocarbon surfaces since its surface comprises only methyl groups arranged in the closest possible packing. It is shown that it should be possible to estimate the packing of adsorbed monolayers of straight-chain hydrocarbon derivatives by comparing the hydrophobic contact angle to the angle on C36. Many of the variations of the hydrophobic contact angle on paraffin found in the literature are shown to be attributable to variations in the methyl methylene ratio in the surface. 3. 3. In contrast to the fluorinated polymers, paraffin and C36 are shown to have multiple curves of cos θ vs. γ. The multiplicity of these curves is attributed to differing dependencies of γSL on γLV∘ due to variation in the constitution of the liquid. Increase in adhesion of hydrocarbon surfaces to liquids is found to be in the order: liquids containing oxygen or fluorine; aliphatic hydrocarbons, aromatic hydrocarbons. 4. 4. It is shown that variation of the free energy of immersion of the n-alkane series of liquids on a given hydrocarbon surface is the resultant of two competing tendencies: i.e., increased adhesion due to increase in methylene to methyl ratio in the liquid, and decrease in adhesion due to increase in surface tension of the liquid. For the C36 surface, the latter tendency predominates; for the paraffin surface, the former tendency predominates, fSL is shown to be the upper bound of the solid surface tension for systems where γSL ⩾ 0. 5. 5. It is shown that in general there is more than one value of the critical surface tension (below which liquids spread on a given surface) for hydrocarbon surfaces, depending on the value of γSL given by a liquid of given surface tension. The fluorinated polymers are shown to be a special case where the surfaces are independent of the nature of the liquid and therefore have essentially a single value of γC.

The spreading of liquids on low-energy surfaces. II. Modified tetrafluoroethylene polymers

Abstract 1. 1. Data are presented on the equilibrium contact angles of a wide variety of liquids on specially prepared surfaces of halogenated derivatives of polyethylene. The free energy decrease of immersion (fSL), the work of adhesion (WA), and the spreading coefficients have been calculated for the liquids which do not spread. The free energy decrease on immersion of these solids in the saturated vapor of most of the liquids of this study is shown to be negligible. 2. 2. For each solid surface the contact angle and spreading coefficient follow the relation to liquid surface tension reported previously for polytetrafluoroethylene, i.e., as the liquid surface tension increases, the contact angle increases and the spreading coefficient decreases. 3. 3. Substitution of chlorine or hydrogen for fluorine in these polymers increases WA, fSL, and the spreading coefficient with respect to a given liquid, the increase depending on the proportion of nonfluorine substituent. Substitution of hydrogen for fluorine has a smaller effect in this respect than substitution of chlorine for fluorine. 4. 4. It is shown that when fSL for a given solid in a series of liquids is constant, the plots of WA υs. surface tension and spreading coefficient υs. surface tension should be straight lines with positive and negative 45° slopes, respectively. The data presented for the fluorinated polymers and the liquid n-alkanes and di-n-alkyl ethers fall on such lines. The values of fSL for the fluorinated polymers plot as a straight line against mole per cent fluorine substitution. 5. 5. There appears to be a maximum value of WA characteristic of the polymer, which is identical with the work of adhesion given by the liquid having a contact angle of 90° on the given solid. 6. 6. It is shown experimentally that there are many exceptions to the “rule” that nonpolar liquids wet nonpolar solids. 7. 7. From the data it is shown that recent attempts to measure the surface tension of solids by contact angle measurements alone involve unjustifiable assumptions.

Effect of adsorbed water on wetting properties of borosilicate glass, quartz, and sapphire

Abstract Contact angles of a variety of hydrophobic liquids were measured on clean surfaces of borosilicate glass, quartz, and sapphire at 20°C with the RH at 0.6% and 95%. The adsorption of water molecules at 0.6% RH resulted in the formation of only a fraction of a monolayer; yet it converted each high-energy solid surface to one having a low critical surface tension of wetting, γ c . The increased amount of water adsorbed at 95% RH to form a film comprising little more than a condensed monolayer further lowered γ c to a value somewhat above that of bulk water. At each relative humidity investigated the value of γ c was the same for each of these three solids; hence γ c was dependent upon the surface concentration of adsorbed water but was independent of the chemical nature of the solid substrate. The somewhat different values of γ c obtained on using several different homologous series of hydrophobic liquids are explained in terms of differences in the contribution from nondispersion interfacial forces acting between the water film and the contacting organic liquid molecules. These results reveal clearly the important influence of only a monolayer of adsorbed water upon the spreading, adhesion, and contact angles of organic liquids resting upon high-energy, hydrophilic, solid surfaces.

The spreading of liquids on low-energy surfaces. V. Perfluorodecanoic acid monolayers☆☆☆

The wettability of more than 90 pure liquids on oriented close-packed monolayers of perfluorodecanoic acid adsorbed on platinum, copper, brass, and glass has been studied. Comparison with results reported earlier for solid polyethylene, oriented monolayers of octadecylamine, and solid polytetrafluoroethylene shows that oriented, close-packed monolayers of perfluorodecanoic acid are the most nonwettable surfaces ever found and that for most liquids in contact with these surfaces the contact angle (θ) decreases in the order θCF3 > θCF2 > θCH3 > θCH2. Values of the work of adhesion, the initial and final spreading coefficients, and the free energy decrease on immersion, fSL, were computed from contact angle measurements and surface tensions using the assumption justified by experiment that fSV∘ = 0. From the values of fSL, it has been possible to estimate the free surface energy of the surfaces prepared from films of perfluorodecanoic acid. Structural differences determining the wetting properties of liquids are presented, and the effect of increase in surface tension of higher members in a homologous series is discussed. Observed modifications of the wetting properties of high-energy solids by adsorption of a single monolayer, demonstrate beyond doubt the short-range nature of the forces involved in wetting. In view of the remarkable resistance to wetting shown by surfaces of close-packed perfluoromethyl groups, surface-active compounds with —CF3 groups and hydrophilic groups at opposite ends of the molecule should have unusual and valuable properties, and they deserve to be treated as a special class of surface-active compounds.

The spreading of liquids on low-energy surfaces. IV. Monolayer coatings on platinum

The equilibrium contact angle (θE) has been measured for some sixty diverse liquids with respect to a smooth platinum surface coated with an adsorbed oriented monolayer of n-octadecylamine. Linear relations were found between cosine θE and the liquid surface tension (ggLV∘) for every homologous series. When homology was disregarded, the cos θE-υs.-γLV∘ data for all the liquids collected on three straight lines, two of which were approximately parallel. Simple curvilinear relations obtained between the work of adhesion (WA) and γLV∘ and between the final spreading coefficient (SLV∘/SV∘) and γLV∘, the constituents of each set of three curves being the same as before. The grouping onto multiple lines corresponds to differences in the solid/liquid interfacial tension, γSL, and to the relative solvent power of the liquids for the adsorbed octadecylamine. The same correlation obtained for the critical surface tension (γC), which was shown to be specific both to the homologous series and to the solid surface. Constant values of the free energy decrease on immersion (fSL) were observed for the homologous series of n-alkanes and n-alkyl ethers, while the alkylbenzene series showed a linear decrease with increasing γLV∘. From the small range and low experimental values of fSL observed for many unrelated liquids, it is concluded that the free surface energy of the monolayer-coated solid is probably not much more than 28.5 erg/cm.2. The striking similarity observed for the wetting properties of the monolayer-coated surfaces compared with those reported previously for surfaces of single crystals of n-hexatriacontane and bulk paraffin (5) demonstrates that the wetting behavior of a surface is essentially controlled by the nature and packing of the outermost group of atoms in the molecules. Intercomparison of wetting data for the monolayer with reference data obtained for a surface of methyl groups in closest packing (i.e., n-hexatriacontane single crystals) is proposed as an approach for determining, from contact angle measurements, the packing of adsorbed films at the solid/air interface.

Effect of adsorbed water on the critical surface tension of wetting on metal surfaces

Abstract Contract angles of a variety of pure nonhydrophilic liquid polar and nonpolar compounds, covering a wide range of surface tensions, were measured on clean smooth surfaces of fourteen metals and one metal oxide. At 0.6% relative humidity (RH) and 95% RH, each of these high-energy, solid surfaces was converted at 20°C to one of much lower critical surface tension of wetting (γc) by the physical adsorption of a thin film of water. The formation of only a fraction of a monolayer at 0.6% RH decreased γc to about 45 dynes/cm for each metal surface; additional water adsorption at 95% RH to form a more condensed film further lower γc to a common value of 38 dynes/cm. Since γc values were nearly identical for each surface at each extreme of the RH, the surface energy of these hydrophilic solids must be independent of the constitution of the underlying solid substrate and dependent only upon the surface concentration of the adsorbed water.

The spreading of liquids on low-energy surfaces. VI. Branched-chain monolayers, aromatic surfaces, and thin liquid films

Abstract A study has been made of the wettability of adsorbed monolayers of branched-chain and cyclic molecules. It is shown that such monolayers are nonwetted by organic liquids with surface tensions higher than a critical value characteristic of the monolayer. Oleophobicity is therefore not restricted to monolayers whose surfaces comprise close-packed methyl groups, the latter merely representing an extreme case. It is shown, further, that adsorbed layers of nonpolar liquids behave, as regards wettability, like low-energy solids with the same atomic groups exposed in the surface. It is concluded that the free surface energy of adsorbed liquid or solid films depends only on the atomic groups in the surface and their packing.

Hydrophobic monolayers adsorbed from aqueous solutions

Abstract Experimental methods are described for the preparation and examination of hydrophobic monomolecular films adsorbed, and isolated from aqueous solutions of certain types of polar-nonpolar organic compounds. The use of contact angle measurements for the quantitative characterizations of the hydrophobic and oleophobic properties of such films permits the evaluation of the various factors governing their formation, structure, and repellency characteristics. The present paper reports the results of studies on several members of a homologous family of primary amines of the normal alkyl type adsorbed onto platinum surfaces. The critical limits to hydrophobic film formation and the degree of hydrophobicity are found to be functions of the amine concentration and the pH of the generating solution. A definite correlation is established for the relative effectiveness of these factors and the length of the hydrocarbon chain in the aliphatic portion of the amine molecule. The marked response of hydrophobic film formation to pH and concentration is interpreted as an adsorption-desorption phenomenon in which the equilibrium conditions are determined by the solubility and degree of ionization of the individual amines. The short chain amines (up through dodecyl) are found to be most adsorptive in the alkaline region, whereas film formation of the hexadecylamine is limited to the highly acid regions. The intermediate tetradecylamine evinces a transitional behavior, having two separate pH regions favorable to film formation. The identity of adsorbed monomolecular films isolated hydrophobically from aqueous solution and oleophobically from the molten amine or from solution in hexadecane is established. The use of water as solvent increases the number of molecular types available for study as reproducible, repellent films. The method is proposed as a possible approach to the examination of films important in the fields of selective adsorption, mineral flotation, corrosion inhibition, and “pickling” of metals.