The changes of the surface free energy of the adsorptive gelatin films

Measurements of the contact angles for water (W), formamide (F), ethylene glycol (E) and diiodomethane (D) on polymethyl methacrylate (PMMA) covered by adsorptive gelatin films were made. Adsorption was performed from solutions in the concentration range 0–100 g/l. It was found that the biggest changes of the contact angles were up to a monolayer coverage of PMMA surface. For all liquids (besides water) the contact angle was practically constant at gelatin concentration over 2 g/l. Based on these contact angle data, it was suggested that the adsorptive gelatin film structure depended on the concentration of the aqueous gelatin solution from which the adsorption was performed. On the basis of the contact angles obtained the Lifshitz–van der Waals components and the values of the electron–acceptor and electron–donor parameters of the acid–base components of the films were calculated for three triplets of liquids (W–F–D, W–E–D and F–E–D). It was found that the water contact angle strongly influenced the Lifshitz–van der Waals component and acid–base parameters of the gelatin film surface free energy. Systems involving water gave different results than those without water. It was concluded that the gelatin film had a monopole electron–donor character and that this character was caused by the existence of carbonyl or ionised carboxyl groups on gelatin film surface.