A pH-study of n-dodecyl-beta-D-maltoside foam films

The influence the pH has on the properties of foam films stabilized by the nonionic surfactant n-dodecyl-beta-d-maltoside (beta-C12G2) was studied. Foam film measurements were carried out with the thin film pressure balance (TFPB) technique using two different film holders, namely, the Scheludko-Exerowa cell and the porous plate. With the former, the equilibrium film thickness h at a given capillary pressure Pc and, with the latter, complete disjoining pressure versus thickness curves (Pi-h curves) were measured. Most of the results were obtained for 10(-4) and 10(-5) M beta-C12G2 solutions that contained 10(-3) M electrolyte. Measurements were carried out in a pH range from 3 to 9. The major results are the following: (1) For a given pH, a pronounced effect of the surfactant concentration cs is seen only if cs approximately cmc. This holds true for both low and high pH values. (2) For a given cs, at least one pronounced effect is seen if the pH is changed, namely a drop of the surface charge density down to zero when the isoelectric point (pH* and pHcr) is reached. (3) The pH of the isoelectric point increases with increasing surfactant concentration. (4) The q0-pH curve of beta-C12G2 shows two pH ranges (3-5.5 and 7-10) in which the surface charge density q0 is pH-insensitive, while a significant change of q0 was observed between pH=5.5 and 7.0. A possible explanation is given.