A new approach for the selective and sensitive colorimetric detection of ionic surfactants in water

A novel two-step protocol for the colorimetric detection of ionic surfactants was developed using simple silica supports functionalised with suitable binding groups. Solid S1 was synthesised by reaction of silica with N-methyl-N'-propyltrimethoxysilylimidazolium chloride to obtain a final solid containing imidazolium coordinating moieties. In the first step, solid S1 in contact with anionic surfactant solutions provides the superficial self-assembly organization of the surfactants on the cationic functionalised surface. This layer would be remarkably hydrophobic with the long alkyl chains most likely pointing toward the bulk solution, which in the second step is able to extract a suitable dye (Methylene Blue) from water. The detection principle depends on the fact that the self-assembly process of the anionic surfactant is selective as a consequence of unique strong hydrophobic interactions between surfactant molecules in the monolayer formed. The final behaviour signalling the presence of anionic surfactants would be depletion of the dye solution and coloration of the solid, both visible to the naked eye. A similar protocol was used for the detection of cationic surfactants. In this case solid S2 was prepared containing sulfonate groups. S2 was obtained by reaction of silica with 3-mercaptopropyltriethoxysilane (MPTS) following the oxidation of the mercapto group to sulfonate by adding H2O2. In this case the interaction of cationic surfactants with the S2 surface results in the self-assembly of the cationic surfactants on the anionic surface. In the second step, the addition of a suitable anionic dye (i.e. Patent Blue) then leads to the signalling of the presence of cationic surfactants. S1 and S2 solids were characterised using standard solid-state techniques. A study of the response of these solids was carried out on different concentrations of anionic, cationic and neutral surfactants and in the presence of compulsory water-present anions and cations. The partial decolouration of the dye solution or the corresponding colouration of the solid was related to the concentration of the surfactants in the aqueous solution. The method comprises a novel organic-solvent-free approach for the colorimetric detection of anionic or cationic surfactants. A model was also developed able to interpret the colorimetric behaviour of the solids based on (i) surfactant adsorption on the charged surface and (ii) further extraction of the corresponding dye.