Quarter-Salt Formation Defining the Anomalous Temperature Dependence of the Aqueous Solubility of Sodium Monododecyl Phosphate

The salts of monoalkyl phosphates (MAPs) have been identified as a class of inherently mild surfactants for use in household and personal products. They represent an anionic species intermediate in terms of pK_a between the sulfates and the carboxylates and are analogous to the carboxylates in that they form acid-salts (which are here termed quarter-salts)hydrogen-bonded dimers consisting of an undissociated MAP acid and an MAP monosalt. These complexes precipitate from solutions of the monosalt over a range of lower MAP concentrations giving rise to an unusual solubility/temperature relationship. The solubility of monosodium monododecyl phosphate (NaC₁₂MAP) increases with temperature up to 0.01 M at ∼60 °C, which corresponds to the conventional Krafft point as shown by the appearance of micelles in solution. The solubility then increases further to ∼0.04 M as the solubility temperature declines from 60 to 38 °C. The transition between these two trends is characterized by a rather sharp temperature maximum in the solubility curve. In a third stage, the solubility then rises rapidly with very small change of temperature. This unusual overall behavior is shown to correspond with three distinct solid-phase compositions for the precipitates at temperatures below the solubility curve. At the lowest concentrations and up through the Krafft Point, the solid phase has been identified as the stoichiometric quarter-salt. Over the declining temperature portion of the solubility curve, the supernatant solution coexists with a macroscopic mixture of separate quarter-salt and monosalt solids. In the high-concentration third region the solid phase is exclusively the MAP monosalt. The coprecipitation of quarter-salt and monosalt from the monosalt solution occurs reversibly in the declining portion of the solubility curve and is accompanied by an increase in pH. The four phase system (solution, vapor, and two pure solid phases) retains one degree of freedom according to the phase rule since the system is in effect three component in that region.