Variation of monolayer behaviour and molecular packing in zinc arachidate LB films with subphase pH

Monolayer characteristics and multilayer LB films of zinc arachidate (ZnA) have been studied as a function of subphase pH in the range 4.9-8.0. The monolayer stability was found to be strongly dependent on the subphase pH. Measurement of bilayer spacings in transferred multilayers using X-ray diffraction revealed that depending on subphase pH, three types of molecular packing arrangements corresponding to alkyl chain tilt angles of 0 degrees, 19 degrees and 31 degrees are possible in ZnA multilayers. In certain ranges of subphase pH, more than one type of packing arrangements were found to coexist. These results suggest that the packing arrangement in ZnA multilayers is significantly affected by the properties of the monolayer at the air-water interface and may not be simply related to metal ion electronegativity. (C) 1998 Elsevier Science S.A. All rights reserved.