Dimerization of molecules of methylene blue in microstructured polymer-micelle solutions

Abstract

We used spectroscopic methods to study association processes of methylene blue molecules in polymer-micelle solutions. We determined the efficiency of dimer formation of dye molecules and their structure as a function of the polymer-micelle complex composition. Introduction.Research on photophysical processes in organized molecular structures is recently gaining more and more significance. Investigations of intermolecular interactions in mixed polymer-micelle systems with positively charged components in which complexes of highly organized structures are formed due to interaction between polyelectrolyte (PE) and surfactant (SA) molecules are especially interesting [1][2][3]. The structure of such complexes changes as the component concentrations and the length of the polymer chains change [4,5]. The active molecules have a different environment when dye molecules are dissolved in these systems. This affects their spectral and luminescent characteristics and the efficiency of photophysical processes [6]. Varying the structure of these solutions changes the position in space of the dye molecules, thereby changing the efficiency of processes such as association and transfer of electronic excitation energy that are controlled by the distance between the interacting particles.Experimental. Association processes of molecules of the cationic dye methylene blue (MB) in polymermicelle solutions (PMS) were studied. The PMS were complexes of an anionic PE, polyacrylic acid (PAA), and a cationic SA, cetylpyridinium chloride (CPC). The CPC concentration in the solutions was 5⋅10 -3 M, greater than the critical micelle-formation concentration. Solutions with PAA concentrations from 10 -6 to 10 -3 M with different polymer chain lengths of 627, 831, 1510, and 3900 segments (PAA fractions with different molecular weights) were studied. The MB concentration varied from 10 -5 to 10 -2 M. PMS absorption spectra were recorded using a Perkin-Elmer Lambda 35 spectrophotometer.Results and Discussion. Absorption spectra of MB in PMS showed that spectral characteristics due to association processes of the molecules change in these systems. A short-wavelength absorption band appears [7]. Figure 1 (curve 1) shows the absorption spectrum of MB in an aqueous solution of PE-SA (C MB = 10 -4 M) with a PAA polymer chain length L = 831 segm. Our measurements show that the solutions are a binary mixture of MB monomers and dimers (absorption spectra had one isosbestic point) for the studied MB concentration range. The degree of association (1 -X) of the dye molecules was used to characterize the efficiency of the association processes in the studied solutions. Here X is the fraction of monomers in solution that was determined by a published method using the formula [7]: