Colloids and Surfaces A: Physicochemical and Engineering Aspects
Study of mixed Langmuir and Langmuir–Blodgett films of dissimilar components by AFM and force spectroscopy
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.
Section snippets
Langmuir and Langmuir–Blodgett film formation
Langmuir films were obtained in a NIMA 1232D1D2 Langmuir–Blodgett trough placed on an isolation platform and BAM images were obtained with a micro-BAM model (NIMA-Nanofilm) which has a lateral resolution of around 8 μm. Pure water (Millipore MilliQ grade) was used as subphase, arachidic acid (M = 312.0) was of high quality and the macrocyclic compound (M = 513.8) was synthesized as described previously [35]. A solution of mixed arachidic acid (AA) and the macrocycle (MC) in chloroform was spread
Isotherms
The surface pressure–area isotherms for the three studied compositions, together with those corresponding to pure AA and MC, are shown in Fig. 1. For the mixed films, an average area per molecule is represented in the X-axes. As was reported previously [35], the MC compound does not have a well-defined hydrophilic head group and a sort of collapse occurs at low surface pressures, reaching the isotherm a quasi-plateau at around 12 mN/m. In this zone (labelled as II in Fig. 1), the formation of
Discussion
The analysis of Π-A isotherms and LB films indicates that separated phases form in the mixed films. A quantitative calculation of the area occupied by the island-like domains in the LB films over several AFM images, gives the experimental percentages of 40 ± 3%, 25 ± 1% and 16 ± 3% for the 1:2, 1:1 and 2:1 mixed compositions of MC:AA. Using the individual area per molecule corresponding to the individual isotherms for both AA and MC at the same transfer surface pressure, the theoretically calculated
Conclusions
Mixed films of AA and MC form separated phases, with higher AA domains according to the higher length of the AA molecules. The calculated values of the excess area and of the AA coverage indicate a partial solubility of AA in the MC phase. A combined set of nanometric techniques has permitted to observe the behaviour of the system. The AA micrometric domains are more compact than the MC phase and present less friction and less adhesion.
Acknowledgements
To MCYT through project CTQ2004-08046-C02 and Generalitat de Catalunya through project PIR2002-00167. To Dr. A. Errachid and group of Prof. J. Casabó for kindly providing the macrocyclic compound.
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