Turbidity studies on mixed surfactant systems in hard water: A new method for estimation of water hardness

Abstract

Water hardness tolerance of anionic surfactants, <i style="">viz.</i> sodium dodecyl sulphate (SDS) and linear alkyl benzene sulphonate – sodium salt (LABS) has been investigated with the help of a digital Nephlo-turbidity meter. Effect of incorporation of small amount of a nonionic surfactant, Triton X-100 (TX-100), on the water hardness tolerance <i style="">i.e.</i> the lime soap dispersing ability (LSDA), of LABS has also been studied. It was observed that LABS generally exhibited superior LSDA than SDS and that incorporation of a small amount of TX-100 led to significant improvement of the LSDA of the anionic surfactants studied. In fact, incorporation of TX-100 beyond 10% concentration level, no turbidity was observed up to 300ppm hardness of water. The small angle X-ray scattering profile of the systems revealed that, at the maximum turbidity point, the anionic micelles were destroyed by the hardness causing ions. It was also observed that at certain range of the surfactant concentration in the pre-micellar region, the turbidity was found to vary linearly with the degree of hardness (ppm) of water. This linear relationship between the turbidity and the hardness can be effectively used for the estimation of the hardness causing ion with good reproducibility.