Study of mixed Langmuir and Langmuir–Blodgett films of dissimilar components by AFM and force spectroscopy

Abstract

In this study the structure of mixed Langmuir–Blodgett (LB) monolayers has been investigated using atomic force microscopy, lateral force microscopy and force spectroscopy, as well as the characteristics of the Langmuir monolayers by surface pressure–area isotherms and Brewster angle microscopy. Mixed films were of dissimilar compounds, a fatty acid such as arachidic acid and a macrocyclic compound. The mixture forms separated phases, but some degree of partial miscibility occurs, with domains at the micro-scale that have different nanomechanical and nanotribological properties. LB films transferred at the same surface pressure show different characteristics depending on the composition. The higher domains correspond to arachidic acid and some of these domains show the presence of two phases, which have been identified as phases with discrete molecular tilting angles.

Introduction

Mixed films have been widely studied to investigate the phase separation, especially in those systems with notable applied interest, as phospholipids or fatty acids. In these systems the mixed components are structurally similar but differing in the chain length, head group or halogen substitution. The effect of these parameters on mixing behaviour has been reported elsewhere [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. As a general conclusion, significant differences in the parameters lead to phase separation, but sometimes non-coincident results have been reported for the same system that the authors attributed to different experimental conditions [1], [3].

Recently, systems with structurally dissimilar components have been investigated, due to its interesting potential applications [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31]. In mixed films of a macrocyclic compound such as a phthalocyanine with a fatty acid such as arachidic acid (AA), an increase in the area per molecule has been observed [20], [25], [26] despite the small value of this parameter for fatty acids. Complemented with results of RAIRS, UV–vis absorption and micro-Raman imaging, this phenomenon has been explained by a change in the molecular orientation of the macrocycle induced by the presence of the fatty acid.

The topographic mode of atomic force microscopy (AFM) has become a general technique in the study of mixed Langmuir–Blodgett (LB) films, while other related techniques such as force curves or lateral force microscopy (LFM) [1], [4], [17], [32], [33], [34] have been scarcely used. These techniques provide useful information on the mechanical properties of the films and the different response of the different domains of the film attributed to different chemical composition or differences in structure or molecular orientation. The formation of separated phases is an important issue in mixed films, so these techniques can provide a powerful tool to investigate them, especially when domains present micrometric or nanometric size.

Recently we reported the behaviour of Langmuir and Langmuir–Blodgett films of a macrocyclic compound (MC), 4-phenyl-4-sulfide-11-(1-oxodecyl)-1,7-dithia-11-aza-4 phosphacyclotetradecane [35], [36] (see Scheme 1), and a fatty acid such as AA [37]. The behaviour of the MC is not similar to that of the phthalocyanines, then providing a new model system to study mixing behaviour of dissimilar components. The MC acts as a copper(II) ionophore, thus being an interesting system for sensing applications. In this work we have investigated the film behaviour of mixtures of this macrocyclic compound and AA using AFM, LFM and force spectroscopy, in addition to the surface pressure measuring technique and Brewster angle microscopy (BAM), in order to determine the characteristics of these mixed films and to determine whether phase separation occurs.