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 (C 36). The calculated value of the final spreading coefficient ( S LV ∘/ SV ∘ ) is given and from the data there can be calculated the values of the free energy of immersion ( f SL ), and the work of adhesion ( W A ). 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 C 36 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, C 36. This is attributed to the increase in the proportion of methyl to methylene groups in the surface. The C 36 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 C 36. 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 C 36 are shown to have multiple curves of cos θ vs. γ. The multiplicity of these curves is attributed to differing dependencies of γ SL on γ LV ∘ 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 C 36 surface, the latter tendency predominates; for the paraffin surface, the former tendency predominates, f SL is shown to be the upper bound of the solid surface tension for systems where γ SL ⩾ 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 γ SL 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 γ C .