title: 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 pK(A) of the system. The pK(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 pK(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 pK(A) of the system; the lower the pK(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.
authors.affiliation: Department of Chemical and Materials Engineering, University of Alabama in Huntsville, Huntsville, AL 35899, USA.
authors.forename: M Elena
authors.initials: ME
authors.lastname: Diaz Martin
authors.forename: Ramon L
authors.initials: RL
authors.lastname: Cerro