Investigations of pH-dependent domain structure of lead arachidate Langmuir-Blodgett films by means of x-ray specular and diffuse scattering and atomic force microscopy

Abstract

Langmuir-Blodgett (LB) multilayer systems are found to be inhomogeneous in morphology and structure. The most characteristic feature of these multilayer films is the appearance of three-dimensional domains in which the molecules arrange themselves as a result of the film transfer onto the substrate. A series of lead arachidate (PbA2) LB multilayers have been prepared as model systems to study the dependence of film morphology on the pH value in the subphase. Atomic force microscopy (AFM) and two different x-ray scattering methods [specular (XSR) and diffuse (XDS) x-ray reflectometry] have been used to investigate the intrinsic interface properties, such as the vertical electron density profile and lateral distribution of head groups and chains, as well as the microscopic description of the interface structure, thus providing an overall picture of the investigated multilayers. With AFM, discrete height variations of domains with minimum step widths of one double layer independent of the salt concentration in the films were observed. The lateral domain size shows a dependence on pH. It was found to be maximum at pH=4.2 (pure acid) but minimum at pH 7.0 (maximum salt content). The AFM pictures were treated by a statistical analysis to extract quantities that can be compared with the x-ray results. A considerable number of Bragg maxima were observed in XSR. The vertical correlation length LZ was calculated from the angular width of Bragg maxima along 2θ and was found to vary with pH value. It follows, in general, the tendency of the domain sizes, being maximum at pH=4.8 and minimum at pH=7.0, respectively. The lateral correlation length LX has been evaluated via XDS from the half widths Δω measured by rocking the sample across a fixed 2θ. It decreases for increasing pH. LX was compared with the respective quantity of the AFM analysis. Estimated by XDS, the correlation lengths for inner interfaces and domains complement one another with the lateral length scales resulting from AFM analysis of the surface.