COMPLEXES BETWEEN LAMELLAR STRUCTURES AND BROMINE, IODINE CHLORIDE, AND CHROMYL CHLORIDE

Abstract

Isotherms of Br₂ on four sizes of natural graphite crystals were measured with a recording vacuum balance. For 0.02- to 2-mm crystals, capillary condensation near saturation was small but significant, surface adsorption was negligible, and the composition C₁₆Br₂ was independent of pressure from 0.9 to 0.5. For the 0.002- to 0.01-mm crystals capillary condensation near saturation and surface adsorption were much greater, there was less intercalation than in the larger crystals, and there were no pressure-independent compositions. These effects of crystal size on the isotherm are believed to account for many of the variations in the reported compositions of lamellates in general and show that the complete isotherm on natural crystals at least 0.02 mm across should be determined. When this was done, there were pressure-independent compositions of C₁₆Br₂, C₁₆ICl, C₈ICl, and about C₄₀CrO₂Cl₂. In the latter case the longer the intercalated material remained in the structure the less of it diffused out on reducing the pressure to zero, until finally, none left. For these three adducts there were threshold partial pressures below which no intercalation occurred of 0.05 for Br₂, 0.03 for ICl, and 0.25 for CrO₂Cl₂ at 20 °C. The adsorption curves above these pressures gave isosteric heats of 11, 12, and 13 kcal mole⁻¹. For a mixture of 0.02- to 2-mm crystals the rate of absorption of Br₂ or ICl at saturation was inversely proportional to composition for the first third of the reaction and was reproduced in subsequent runs on the same sample. The diffusion coefficient for this initial portion for ICl was three times the value for Br₂.Isotherms of the three adducts on BN and MoS₂showed no conclusive evidence for intercalation.