Transfer of protons from bulk solution to the surface of poly(L-lactide) microcapsules

Excessive proton transfer from bulk solution to the surface of poly(L-lactide) microcapsules was observed. In acidic solutions of pH less than 3.0, the zeta potential of the microcapsules became slightly positive, although poly(L-lactide) has only carbonyl groups and alcoholic hydroxyl groups which can be charged. In an NMR study, the signal of the water protons shifted towards a higher applied field with increasing interaction of water with the poly(L-lactide) microcapsule surface when the microcapsules were dispersed at low concentrations (up to 4 x 10(-3) per cent). In solutions of pH 3.2-7.6, the observed values of the zeta potential were in good accordance with theoretical values calculated assuming that the charged groups distribute uniformly in the microcapsule wall. In this pH range, the average density of charged groups in the wall was estimated to be 0.04 M. The average volume occupied by one polymer unit (one poly(L-lactide) molecule) in the microcapsule membrane was estimated to be (35 A)3, suggesting a rather tight polymer network in the poly(L-lactide) microcapsule membrane.