Surface potentials and ionization equilibrium in Y-type deposition of multiple Langmuir–Blodgett films: I. Effect of pH and counterions

Abstract

Conditions for depositions of multiple LB films for different divalent cations were determined experimentally at the p K A of the system. The p K A of the fatty acid–subphase salt system is the pH at which half of the fatty acid heads are dissociated. Stable Y-type depositions take place in full immersion/removal cycles. Under certain conditions, full depositions occur only during the downstroke (X-type) or during the upstroke (Z-type). Experiments show a clear trend between p K A of the system and loss of stability of Y-depositions toward full depositions taking place during immersion only, i.e., YX transitions. Stable Y-type depositions can be achieved for most divalent cations at large dipping speeds. Dipping speed necessary to maintain stable Y-depositions depend on the p K A of the system; the lower the p K A the larger the dipping speed. The accumulated time spent by the solid substrate under the water subphase is not a good indicator of YX transition because the number of dipping cycles is also a relevant variable. For a given number of cycles, the lower regions of the solid substrate are most affected by transition because they spent a larger time under the water subphase.