Adsorption and Calorimetric Investigations on Carbon Black Surfaces. III. Immersion Heats in Model Elastomers and Isosteric Heats of Adsorption of C-4 Hydrocarbons

Abstract

The heats of immersion of U-1 channel and SAF, SPF, HAF-HS, and HAF furnace blacks have been measured at 25° C in three isoprenoid olefins and four liquid elastomers of the polybutadiene, polyisoprene, and SBR types. An extensive microcalorimeter redesign was necessary because of the high viscosity of these latter fluids. The immersion heats observed with the liquid elastomers were similar to n-decane immersion heats and there was little evidence for specific interactions between these liquids and any of the blacks. The possibility that the very high structure SPF black gives slightly enhanced immersion heats should be further investigated. Since only integral immersion heats could be measured with the non-volatile liquid elastomers, the possibility is not ruled out that specific, relatively high-energy interactions may occur between the adsorbate and a very small fraction of the black surface. The similarities between the immersion heats with n-decane and the liquid elastomers indicate that almost all of the carbon black surface is accessible to the polymer segments. It appears that the rate of heat evolution may be considered a measure of the rate of attainment of intimate molecular contact between the liquid and carbon substrate. The enhancement in heat of wetting with increasing chain length over the C6 to C16 n-alkane series reported by others with a highly uniform, graphitized black was not observed with HAF. Isosteric heats of adsorption of n-butane and butene-1 at low surface coverages on several carbon blacks were calculated from adsorption isotherms obtained with a precision microgravimetric system. The isosteric heats as a function of coverage curves were typical of those expected for heterogeneous surfaces and reflected the presence of high energy physical adsorption sites at low coverages (θ<0.1).