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

• Published in: Journal of colloid science 1952-04, Vol.7 (2), p.166-177
• Main Authors: Shafrin, Elaine G, Zisman, William A
• Format: Article
• Language: English
• ISSN: 0095-8522
• DOI: 10.1016/0095-8522(52)90059-7

The equilibrium contact angle ( θ E ) 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 θ E and the liquid surface tension ( gg LV ∘ ) for every homologous series. When homology was disregarded, the cos θ E - υs.- γ LV ∘ 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 ( W A ) and γ LV ∘ and between the final spreading coefficient ( S LV ∘/ SV ∘ ) and γ LV ∘ , 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, γ SL , and to the relative solvent power of the liquids for the adsorbed octadecylamine. The same correlation obtained for the critical surface tension ( γ C ), 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 ( f SL ) were observed for the homologous series of n-alkanes and n-alkyl ethers, while the alkylbenzene series showed a linear decrease with increasing γ LV ∘ . From the small range and low experimental values of f SL 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.