On the effect of subphase pH and counterions on transfer ratios and dynamic contact angles during deposition of multiple Langmuir-Blodgett monolayers

The effects of pH and counterions on the type of deposition of Langmuir-Blodgett (LB) arachidic acid films onto hydrophobic glass slides is revisited. Unusually large differences in contact angles and transfer ratios (TR) were observed for subphase containing 10−4 M of zinc sulfate and 2.10−4 M of cadmium chloride, respectively, for a wide range of pH. Variations in TR occur at the same pH for different subphase cations and at different pH for the same divalent cations. These large variations in dynamic contact angles and TR as a function of pH point to the crucial role of electrical double layer forces in LB deposition phenomena. Transitions from Y- to X-type deposition are reported for pH larger or smaller than the pKA of the fatty acid-subphase salt system. Experimental results are compared with data reported in literature showing Z- to Y-transitions at pH close to the pKA of docosanoic acid monolayers.