Elaine G. Shafrin

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.

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.

Auger Compositional Analysis of Ball Bearing Steels Reacted with Tricresyl Phosphate

The low-speed lifetime of steel ball bearings under boundary lubrication can be significantly extended by prior soaking in hot tricresyl phosphate (TCP). Auger electron spectroscopy (AES) and depth-profiling techniques were combined to monitor changes in the chemistry of 52100 bearing steel surfaces as a function of cleaning history and conditions of TCP exposure. Phosphorus was detected in the outer layer of all samples exposed to TCP; the length of TCP exposure beyond three days had minimal effect on the surface concentration of phosphorus but did increase its depth of penetration and alter its local chemical environment. When presoaked with conditions (15 days in TCP at 110°C) which lead to optimum bearing lifetimes (I), the steel developed a surface film comprising three distinct regions of different composition. The outermost region had 4–6 percent phosphorus where the phosphorus Auger signal shapes were consistent with those of a phosphorus-oxygen moiety. The innermost region resembled the oxide/met...

The Detection, Wettability, and Durability of Fluorescent Barrier Films

The storage and operational life of instrument ball bearings are greatly improved by barrier films—thin films of fluorinated organic polymers applied to specific metallic areas to prevent lubricant migration. Current difficulties in determining barrier-film location, a factor critical to effective barrier performance, are shown to be alleviated by incorporating long-lived fluorescent indicators in the solutions used to prepare the barriers. The resulting films exhibit useful levels of fluorescence under ultraviolet radiation after solidification in situ and prolonged storage at relative humidities below 50 percent. Fluorescence decreases with storage at high humidities, but moisture sensitivity of the indicator provides no greater a limitation than does corrosion sensitivity of the bearings. Retention experiments with chlorinated silicones and contact angle measurements with additional liquids show barrier performance to be unaffected by the fluorescent indicator. Therefore, incorporation of long-lived fl...