Abstract
An investigation was made of the effects of sodium ions for retarding the precipitation of calcium salts of α-sulfohexadecanoic acid methyl ester (SFM) under specified washing conditions. Sodium ions were noted to bind to SFM competitively with calcium ions and effectively retarded the precipitation of SFM calcium salts. At 0.49mM SFM, 0.54mM CaCl2 and 1mM NaCl in ammonium buffer, bound counterions of SFM aggregates were almost entirely calcium and the aggregates appeared electrically nearly neutral, each consisting of one calcium ion with two SFMs. With increase in sodium ion concentration, the sodium ions partially replaced bound calcium, with consequent enhancement of the negative charge of the aggregates. The aggregates then appeared to take on greater stability in solution from which virtually no precipitation of calcium salts was observed at least for a few hours.
Significant change in aggregate stability was observed with increase in sodium chloride concentration from 30mM to 100mM. At 100mM, bound calcium ions were far fewer than the number determined at less than 30mM. Discontinuous change in aggregate structure to result in greater stability thus appears to occur as a function of sodium chloride concentration.
