Study of mixed system in monolayers and multilayers transferred by Langmuir±Blodgett technique Vishakha R. Shembekar, A. Dhanabalan, S.S. Talwar * , A.Q. Contractor Department of Chemistry, Indian Institute of Technology, Bombay, Mumbai 400 076, India Abstract Mixtures in different molar proportions of azo acid, (6Az10COOH, 6A10) and arachidic acid (AA) when spread on an aqueous subphase containing CdCl 2 , were found to form inhomogeneous monolayers on the water surface. This inhomogeneity in the monolayers at the air± water interface as well as in the transferred ®lms was studied by using p ±A isotherms, XRD and UV±visible spectroscopy. p ±A isotherm studies of the monolayers and XRD, UV±visible spectroscopy of Langmuir±Blodgett (LB) multilayers essentially indicated that there was microphase separation. Domains of azo acid and arachidic acid were found. H-aggregate formation of azo acid was observed in pure azo acid as well as in the azo acid composite ®lms. Composite LB ®lms consist of three different phases of molecular organizates and one of these involves molecular arrangement where domains of azo acid and arachidic acid straddle each other. Studies showed that strong molecular interaction in the successive layers in the mixed system led to this characteristic molecular packing in the system. The mean molecular area, DG ex p and collapse pressure also showed that the acids were immiscible, thus indirectly suggesting the presence of domains in the system. Keywords: Azobenzenes; H-aggregates; Langmuir±Blodgett (LB) ®lms 1. Introduction Langmuir monolayers of amphiphilic molecules at the air±water interface and the related Langmuir±Blodgett (LB) ®lms on solid substrates have been studied for almost a century [1]. Recently organic ultrathin ®lms fabricated using this technique have attracted much attention in the area of molecular electronics [2]. Formation of functional LB ®lms often involves transfer of monolayers formed by mixing two different compounds. Thus mixed monolayers are adopted in many cases such as for improving monolayer stability [3,4], increasing the second harmonic generation (SHG) effect [5,6], optimization of pyroelectric [7,8] and photochromic [9] properties. Mixed monolayers have also been useful for constructing LB ®lms with interlocking structures [10] and improving the orientational order of chromophores in LB ®lms [11]. However, it is often dif®cult to obtain homogeneous composite LB ®lms. The main problem with mixed systems is the phase separation of the components in the monolayers. The mixed monolayers and transferred multilayers gener- ally give heterogeneous structures with small domains of each component [12]. Several studies have shown that domain formation occurs in various mixed systems. Mixed fatty acids with different alkyl chain lengths exhibit domain formation. XRD of these composites shows two systems of re¯ections [13]. Fatty acids mixed with poly(ethyleneimine) also show domain formation. AFM studies of this system reveal the structure of these domains [14,15]. Azobenzenes mixed with arachidic acid are reported to show domain formation [16]. Micro phase separation was observed for amphiphilic heterocyclic chromophores mixed with other amphiphiles [17]. However, it is found that true molecular level mixing is attained on mixing two different molecules having similar structure [18], e.g. two azobenzene linked amphiphiles 0A10 and 10A0 when mixed together form a homogeneous monolayer [19]. It becomes important therefore to study the structure of the composite LB ®lms as well as the effect of monolayer composition, structure and transfer conditions on the preparation and structure of composite LB ®lms. In this paper we report investigations on monolayers and transferred multilayers prepared with a mixture of azo acid (6A10) and arachidic acid (AA) in different molar propor- tions. Studies carried out using p ±A isotherms for mono- layers at the air±water interface and for multilayered ®lm by XRD and UV spectroscopy indicate domain formation in which the azo acid domains show H-aggregation.