Complexation of magnesium ions in sodium dodecyl benzenesulfonate-Flopaam AN125SH mixtures using sodium citrate

Abstract

Presence of divalent metal ions, especially alkaline earth cations in brine water is an important factor during different processes applying anionic surfactants and negatively charged polymers, because these cations bind to the anionic head group of surfactants and may cause the precipitation of these materials. They also interact with the polymer chains, decreasing the polymer’s viscosity, reducing the net charge of the polymer chains, which also may lead to precipitation. The present study focuses on describing the interactions between magnesium ions with anionic sodium dodecyl benzenesulfonate (SDBS)–Flopaam AN125SH mixtures and the complexation of the magnesium with sodium citrate. Description of these interactions are important to evaluate the applicability of sodium citrate to decrease the undesired effects of alkaline earth cations in polymer-surfactant mixtures. The mixtures contained the high molecular weight anionic polymer in constant 1.0 g/L concentration and the surfactant in 5.0 g/L concentration. The magnesium ion concentration was systematically increased in the samples (0.03–1.5 g/L) and sodium citrate was added to the magnesium ion containing samples in 2.0 and 3.0 M equivalent amount compared to the magnesium ion concentration. The samples were characterized by using turbidimetry and rheology measurements (measuring consistency index, yield stress, flow number and zero shear viscosity), as the change of viscosity is an important parameter during application of polymers in different processes. The formed precipitates were characterized using infrared spectroscopy. The results showed that sodium citrate successfully inhibited the magnesium ions caused precipitation up to 1.2 g/L metal ion concentration (1200 ppm), but caused a considerable decrease in the viscosity of the samples (the consistency index of the mixtures decreased from 47.5 mPa s to 7.53 mPa s) and that magnesium ions do not induce precipitation of the polymer. However, this amount of decrease in viscosity did not change the flow properties of the polymer, all the investigated samples were measured to possess pseudoplastic flow behavior. The viscosity decreasing effects of the sodium citrate were also measured and approximately 13.0 mPa s decrease was measured in zero shear viscosity.