Use of a Nonionic Surfactant to Inhibit Precipitation of Anionic Surfactants by Calcium

Precipitation of anionic surfactants by calcium ions has always been a major concern in formulations using anionic surfactants in hard water. In our previous work, it was found that the precipitation reaction was delayed when a mixture of anionic surfactants (sodium dodecyl sulfate and sodium octylbenzene sulfonate) was used at a certain mole ratio that would give the minimum in the difference in the supersaturation ratios between the two precipitating surfactants. In this work, the addition of nonylphenol ethoxylate (NP(EO)(10)) as a nonionic surfactant is shown to delay the onset of precipitation for a longer time. The zeta potential of precipitates reveals that the NP(EO)(10) is able to slow down the precipitate growth by adsorbing onto the precipitate crystals and this results in larger mean particle size.