Surface potential measurements of Langmuir films of azo dye/liquid crystal mixtures Krzysztof Inglot, Tomasz Martyn ´ ski, Danuta Bauman * Faculty of Technical Physics, Poznan ´ University of Technology, Nieszawska 13A, 60-965 Poznan ´, Poland article info Article history: Received 7 January 2008 Received in revised form 29 May 2008 Accepted 30 May 2008 Available online 13 June 2008 Keywords: Langmuir film Azo dye Liquid crystal Surface potential Surface pressure BAM image abstract Two-component films of a non-amphiphilic azo dye and a liquid crystal with strongly polar –CN group were studied at an air–water interface on the basis of the surface potential–mean molecular area de- pendence recorded simultaneously with the surface pressure–mean molecular area isotherm. Addi- tionally, the morphology of the films was monitored with the aid of Brewster angle microscope. A conventional Langmuir technique was used to form dye/liquid crystal films during reduction and ex- pansion of an area occupied by the molecules. From the surface potential value, the effective dipole moment in the first monolayer formed on the water was calculated. Moreover, the number of molecules with dipole moments directed from the water to the air with respect to those directed towards water was estimated. The influence of the two components’ miscibilities upon the surface potential after addition of the dye to the liquid crystal was determined. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Azo dyes have found many applications because of providing the whole range of colours from which the most popular are red, orange, brown and yellow. Additionally, they have very high extinction coefficient and give bright, intensive colour by using small concentrations. Thus, more than half the commer- cial dyes belong to the class of azo dyes. The azo dye molecules exhibit large change in the dipole moment at p / p * electronic transition from the ground to the excited state (charge transfer transition), the direction of which is mostly parallel to the molecular plane [1]. As a result, they are characterized by high value of the second-order hyperpolarizability and therefore, incorporated into highly oriented matrices, give possibility to obtain the system with nonlinear coefficient (at optical frequencies) much larger than that found in inorganic single crystals [2]. One of the methods used to obtain highly oriented systems is the Langmuir–Blodgett technique [3]. This technique is a unique method which enables to fabricate two-dimensional ordered layers from certain molecules or particles at air–liquid (Langmuir films) and air–solid substrate (Langmuir–Blodgett films) interfaces. From our previous investigations [4–8] it is, however, known that most of the azo dyes are not able to form stable Langmuir and Langmuir–Blodgett films. In such a case it is necessary to use the admixture of a compound that makes stable monolayers at the interfaces and can act as a supporting matrix. Such compounds can be some liquid crystals which are able to form homogeneous Langmuir and Langmuir–Blodgett films [9–12]. On the other hand, it is known that azo dyes dissolve and orient very well in liquid crystals [13,14]. Thus, azo dye/liquid crystal mixtures can be used in guest–host liquid crystal display devices [13,15,16]. In this paper we study Langmuir films formed of azo dyes with strongly polar –NO 2 group mixed with two liquid crystals: 4-octyl-4 0 -cyanobiphenyl (8CB) and trans-4-octyl(4 0 - cyanophenyl)-cyclohexane (8PCH) using surface potential measurements (SPMs), in addition to surface pressure isotherms and Brewster angle microscopy. SPM is a very convenient method which can be used to deduce information about the molecular organization of polar or charged molecules at the air–water interface [17,18]. * Corresponding author. Tel.: þ48 616653172; fax: þ48 616653164. E-mail address: bauman@phys.put.poznan.pl (D. Bauman). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig 0143-7208/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2008.05.013 Dyes and Pigments 80 (2009) 106–114