Adsorption of cationic surfactants to silver halide—Gelatin dispersions

Abstract

In homodisperse and crystallographically defined AgBr-gelatin systems of known surface areas, reversible adsorption of organic cations increased with length of their alkyl chain. In neutral or alkaline 0.001 M KBr, the silver halide surface could be saturated by singly charged alkyl pyridinium or quinolinium cations at equilibrium concentrations below about 10 −3 M, provided the alkyl chains contained at least 12 methylene groups. Longer alkyl chains enhanced adsorbability without influencing the saturation coverage. Doubly charged α,ω-alkyl quaternaries gave similar adsorption characteristics but, compared with their singly charged long-chain analogs, the amounts adsorbed at saturation coverage were low. At equilibrium concentrations between 10 −4 and 10 −3 M, adsorption of both types of cations fitted the Langmuir equation for which adsorption coefficients were evaluated; these yielded free energies of adsorption ranging between 6 and 11 kcal./mole, depending on structure and substrate variables. The highest apparent free-energy values were obtained with a cationic cyanine dye in an iodide-containing substrate. It was demonstrated that the limiting area occupied by this dye or other quaternary salts was the same for cubic and octahedral substrates.