Surface potentials and ionization equilibrium in Y-type deposition of multiple Langmuir-Blodgett films I. Effect of pH and counterions.
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.
Diaz Martin
M Elena
ME
Department of Chemical and Materials Engineering, University of Alabama in Huntsville, Huntsville, AL 35899, USA.
Cerro
Ramon L
RL