Micelles, reverse micelles and microemulsions as media for kinetic studies and electrodeposition of Co, Cu, Zn, Ni and Sn

Abstract

Series of micelles, reverse micelles and microemulsions of cationic, anionic and surfactants were prepared with varying water to surfactant molar ratios in a wide range of compositions and their physicochemical properties have been studied in detail. These have been evaluated as media and catalysts for model reactions through systematic kinetic studies and theoretical treatments. Their suitability as media for electrodeposition of different metals with controllable size, shape and morphology has also been studied in detail. Efforts have been made to correlate physicochemical properties of the micelles, reverse micelles and microemulsions with kinetic behavior and electrodeposition. Kinetics of the hydrolysis of crystal violet (CV) and bromazepam (Bz) has been studied in micelles, reverse micelles and microemulsions of cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS)/sodium dodecyl benzene sulfate (SDBS) and tritonX-100 (TX-100) by using spectrophotometric method under pseudo first order condition. The physicochemical properties of micelles, reverse micelles and microemulsions have been investigated by measuring turbidity, conductivity, density, viscosity, refractive index and surface tension/interfacial tension. The Critical micelle concentrations (CMC) of CTAB and SDBS/SDS were determined by conductivity method; while the CMC of TX-100 was determined by fluorescence method. Turbidity, conductivity, viscosity, refractive index and density increase while surface tension decreases to attain an almost constant value with increasing [CTAB] or [SDS] or [TX- 100]. Hydrolysis of CV was carried out under alkaline conditions; while that of Bz was followed under acidic conditions. The pseudo first order rate constants, k'-[CTAB] profiles for hydrolysis of CV show a maximum at ca. [CTAB] = 10 mM, after which k' gradually decreases with increasing [CTAB]; while for hydrolysis of Bz, addition of CTAB has been found to enhance the rate of hydrolysis of Bz at concentrations below the CMC. Above the CMC of CTAB, at low [H+], as [CTAB] increases the k' attains a limiting value; while at high [H+], the k' passes through a maximum and then decreases. SDBS produced an inhibitory effect on the reaction rates for both reactions. The micellar solution of TX-100 accelerates and inhibits hydrolysis of CV and Bz, respectively. The catalysis of these reactions by micelles of CTAB, SDS/SDBS and TX-100 were treated in terms of Piszkiewicz and Pseudophase Ion-Exchange (PPIE) model to fit the experimental data for obtaining the binding constant of CV or Bz with CTAB, SDBS and TX-100 and other kinetic parameters. The k's of the reactions were also greatly affected by reverse micelles and microemulsions of CTAB, SDS and TX-100 under identical experimental conditions. The k' - volume fraction of water (ϕW) profiles for hydrolysis of CV and Bz shows special pattern in reverse micelles and microemulsions, such as water in oil (w/o) to oil in water (o/w) via bicontinuous (BC) microemulsions with different microstructures, which have been determined by measuring physicochemical properties of reverse micelles and microemulsions. Conductivity, viscosity and density increase as well as refractive index decreases and the interfacial tension at first decreases and then increases with increasing ϕW. Three different microstructure regions with increasing ϕW were also established by applying percolation theory and percolation scaling law on conductivity results and the percolation thresholds (ϕC) obtained from conductivity results were in accordance with viscosity results. The phase transitions at these two percolation thresholds correspond to structural change from w/o to BC ( C1  ) and BC to o/w ( C2  ) microemulsions. Excess volume - ϕW, excess refraction - ϕW and excess molar refraction - ϕW profiles for these microemulsions also indicates structural transitions. The k' - ϕW profiles for hydrolysis of CV and Bz shows that the transition of hydrolysis rates could be observed at structural transitions from w/o to BC and from BC to o/w obtained from percolation theory and the rates of both reaction are higher in w/o microemulsions and decrease with ϕW while almost constant in BC microemulsions and decrease in o/w microemulsions. Electrochemical behavior of different metal ions: cobalt (II) (Co2+), copper (II) (Cu2+), zinc (II) (Zn2+), nickel (II) (Ni2+) and tin (II) (Sn2+) also have been studied in aqueous solution and reverse microemulsions of CTAB and SDS with different water to surfactant ratios (wo) on copper electrodes (CuE) by cyclic voltammetry. The reduction of different metal ions to metals has been found to occur with ease at CuE in aqueous solution. Electrodeposition of different metals was therefore performed from aqueous solution at a fixed potential below reduction potential using constant potential electrolysis method on CuE. In reverse microemulsions with different wo, electrodeposition of different metals on CuE has been performed at different reduction potentials. Morphology and structures of the electrodeposited metals were examined by scanning electron microscopy; while elemental characterization was carried out by energy dispersive x-ray spectroscopic method. From aqueous solution, electrodeposition of metals occur very fast and a gross deposition of bulk metal results without any definite morphology. In CTAB/1- butanol/water and SDS/1-butanol/water reverse microemulsions with fixed wo, the deposition rate varies with change in applied potentials and thickness of deposited metals only changed with change in deposition potential. The variation in wo brings about changes in size and size distribution of reverse microemulsion as revealed by dynamic light scattering measurements and consequently influences morphology of electrodeposited metals. SEM images show that deposition of metals occurs with definite size and shape even the shape changes with increasing wo of reverse microemulsions. The electrodeposits from reverse microemulsions of CTAB and SDS with different wo were compared to judge the suitability of the reverse microemulsions for electrodeposition of metals with tunable morphology. Kinetic results of those hydrolysis reactions and morphology of electrodeposited metals have been correlated with physicochemical properties such as viscosity and conductivity of reverse micelles and microemulsions. The k' of the reactions decreases with increasing conductivity and viscosity of reverse micelles and microemulsions. The reduction potentials for reduction of metal ions to metal shifted to more negative values with increasing conductivity of reverse microemulsions. An increase in conductivity of the system brings about changes in the structures of electrodeposited metals depending on the diffusion as well as deposition rate. Thus, micelles, reverse micelles and microemulsions can serve as suitable media to control the reaction rates of different reactions and the morphology and structure of different metals by electrodepostion.