An investigation was made into the recent claims of a preferential adsorption and polymerization of L-enantiomers of aspartic acid by kaolinite in dilute aqueous systems at high temperatures. After one week at 90°C no adsorption or polymerization of 0.01N solutions of L- or D-aspartic acid was effected by Na- or Al-kaolinite. Low molecular weight products were formed in 0.01N solutions of L-glutamic acid and L-aspartic acid, in the presence and in the absence of Na-kaolinite, after two months at 90°C. The feasibility of dimerization of L-lysine on clay mineral surfaces in the presence of water was investigated indirectly by carrying out cation exchange reactions. Following a review of the theory of cation exchange thermodynamics with reference to exchange on clay mineral surfaces, the method for determining the thermodynamic parameters for the cation exchange of L-lysine for L-lysyl-L-lysine on montmorillonite and kaolinite is presented. DeltaG° was calculated from equilibrium cation distribution measurements for the two clays at 30°C and 90°C and ?H° for the two clays at 30°C was determined by reaction calorimetry. These values were used to estimate the thermodynamic parameters for a hypothetical L-lysine dimerization reaction on the clay surface from assumed DeltaG° and DeltaH° values for the dimerization in aqueous solution. It was found that the clay surface dipeptide formation is as unfavourable as the reaction in aqueous solution at both temperatures. The different types of force accounting for adsorption of organic molecules on clay mineral surfaces were examined and the behaviour of other clay-amino acid systems postulated.
|Date of Award||1974|