Effect of CTAB on the aggregation properties of Congo Red organized in layer-by-layer electrostatic self-assembled film: a spectroscopic and Atomic force microscopy study

In this communication, the interaction and association behavior of a dianionic azo dye namely Congo Red (abbreviated as CR) and a cationic surfactant Cetyltrimethylammonium Bromide (CTAB) while organized in layer-by-layer (LbL) self-assembled film has been studied via spectroscopic techniques. The effect of concentrations of CTAB solution on the aggregation and related photophysical properties of dye molecules deposited on to poly(acrylic acid) (PAA) coated quartz substrate via LbL method have been explored. Aggregation behavior of CR in CTAB/CR mixed solution are also studied for comparison and to assess the nature of dye aggregation with the change in surrounding environment. The nature of such aggregation both in solution and LbL films are highly sensitive to the CTAB concentration used to fabricate LbL film. Both H and J-type aggregates of dye molecules were formed in dye/surfactant mixed solution whereas only J-type aggregates were present in PAA/CTAB/CR LbL films. Also the degree of dye aggregations is found to be different in solution and LbL films as confirmed by UV-vis absorption and steady state fluorescence spectroscopic results. The CTAB/CR multilayer film growth in PAA coated quartz substrate is also studied in this work. Fourier transform infrared spectroscopic technique is further employed to understand the interaction of CR with CTAB in LbL film. The morphology and surface roughness of the as-prepared LbL film have been analyzed by Atomic force microscopic method. The study reveals many fundamental and useful information about the association of CR dye molecules in the restricted solid state geometry of LbL films.