Probing solute distribution and acid-base behaviour in water-in-oil microemulsions by fluorescence techniques

Abstract

The distribution and acid-base behaviour of the four solutes harmine, chromotropic acid (4,5-dihydroxynaphthalene-2,7-disulfonate, disodium salt), 2-naphthol and 5,10,15,20-tetrakis [4-trimethylammonium)phenyl]-21H,23H-porphine tetra-p-tosylate (TTMP) have been studied in water-in-oil (w/o) microemulsions using fluorescence and absorption spectroscopy. Carbon tetrachloride is a quencher of fluorescence of these compounds, and studies using this as oil phase in microemulsions show that chromotropic acid is located in the water domain, TTMP at the surfactant-water interface, while the distribution of harmine or 2-naphthol depends on the degree of protonation. Detailed studies have been made on harmine. In water/AOT/cyclohexane microemulsions the cationic form is observed up to much higher apparent pH than in aqueous solutions. An important factor is shown to be the compartmentalisation of hydroxide ions between water pools. Similar effects are observed with the other probes, and it is suggested that compartmentalisation of hydrogen or hydroxide ions is a major effect in many acid-base reactions in microemulsions. The validity of the concept of pH in microemulsions under these conditions is questioned. Fluorescence lifetime measurements are also shown to provide information on the dynamics of the processes, and demonstrate the importance of diffusion of solutes from organic solvent to the microemulsion pool. A comparison is made of the behaviour of harmine in water/AOT/cyclohexane and water/lecithin/cyclohexane microemulsions.