Effect of Monovalent and Divalent Cations on the Biocidal Efficacy of Quaternary Cationic Biocides – A Salty Situation

Abstract Quaternary Cationic Biocides (ammonium and phosphonium based) are broad spectrum and fast acting. Due to their compatibility, low cost, and high effectiveness, they are often used as biocides in oil and gas operations alone and also as formulation components for multi-biocide packages. They have also found use as additives in corrosion inhibitor formulations, emulsifiers, nonemulsifiers, and even as scale inhibitors. While they tend to work well in fresh water where there is little to no salt present, they are shown to provide decreasing performance as the salt concentration increases. This is due to many factors including changes in hydration shells, indirect dipole interactions, electrostatic shielding, counter ion exchange, and a preference for association with anionic species. In this paper we demonstrate the effects that monovalent and divalent chloride salts have on the physical solubility and biocidal efficacy of alkyl dimethyl benzyl ammonium chloride (ADBAC), dodecyl dimethyl ammonium chloride, (DDAC) and trimethyl tetradecyl phosphonium chloride (TTPC). We will also explore the effects that salts have on other biocides such as tetrakis hydroxymethyl phosphonium sulfate (THPS) and glutaraldehyde. The results of this research should prove helpful in selecting the appropriate biocidal mitigation strategy for a variety of waters and brines and help guide the development of biocidal, corrosion inhibitor, emulsifier and nonemulsifier, and scale inhibitor formulations.