Effect of aminoacids on the surface free energy of nitrofurantoin

Changes in nitrofurantoin surface free energy components, Lifshitz-van der Waals, _s ^LW , electron donor, _s ^– , and electron acceptor, _s ⁺ , due to adsorption of the aminoacids: lysine, alanine and glutamic acid, were determined by means of the thin-layer wicking technique. It was found that the aminoacids slightly increase the _s ^– component already at 10^–4 M concentration. They reduce the _s ⁺ component practically to zero, and a very sharp increase of _s ^– was observed when the nitrofurantoin surface was precovered from the solutions at concentrations larger tan 10^–4 M. It is concluded that the adsorption of the aminoacids takes place via hydrogen bonding and electrostatic interactions between nitrofurantoin surface and aminoacid molecules. The increase in the _s ^– parameter is probably caused by the presence of carboxyl groups in the aminoacid molecules.