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

Abstract

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 pKa 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 (NaC(12)MAP) increases with temperature up to 0.01 M at approximately 60 degrees C, which corresponds to the conventional Krafft point as shown by the appearance of micelles in solution. The solubility then increases further to approximately 0.04 M as the solubility temperature declines from 60 to 38 degrees 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.

Sodium Monododecylphosphate Vesicles Formed in Alcohol/Water Mixtures

“…In additon, the FT‐IR spectra of SDP and freeze‐dried SDP/NPT/H 2 O vesicle samples were determined, as shown in Figure 4 (b and c). The adsorption peak at ∼2406 cm −1 for SDP arises from the H−O stretching and bending vibration in P(=O)O−H [39] . The location of this peak shifts obviously from 2406 to 2390 cm −1 for the SDP/NPT/H 2 O sample, suggesting the formation of the hydrogen bond O−H … O between alkyl phosphate species [39] .…”

“…The adsorption peak at ∼2406 cm −1 for SDP arises from the H−O stretching and bending vibration in P(=O)O−H [39] . The location of this peak shifts obviously from 2406 to 2390 cm −1 for the SDP/NPT/H 2 O sample, suggesting the formation of the hydrogen bond O−H … O between alkyl phosphate species [39] . Furthermore, the DSC trace for the SDP/NPT/H 2 O vesicle solution was found to exhibit one endothermic peak at 76.8 °C (Figure S7), corresponding to the disruption of the hydrogen bond in the SDP dimers [39,40] .…”

Highly-organized stacked multilayers via layer-by-layer assembly of lipid-like surfactants and polyelectrolytes. Stratified supramolecular structures for (bio)electrochemical nanoarchitectonics

“…The bands at about 950 cm -1 are due to P-OH stretching modes whereas the bands at about 1075 cm -1 has been assigned to P-O stretching modes in POC groups. 71 the bands at about 1150-1250 cm -1 are assigned to P=O stretching. 72 There are also other bands at 1100-1190 cm -1 , assigned to C-C and C-O stretching, and the intense band at 1470 cm -1 has been assigned to the HCH scissoring modes.…”

Kemp elimination in cationic micelles: designed enzyme‐like rates achieved through the addition of long‐chain bases

“…In order to determine whether the monoanionic or the dianionic form of C12P were the reactive species, we varied the pH while keeping the base concentration fixed at 42 μM. Because the p K a of C12P is 7.0, in case the dianion was the reactive species, we expected the observed velocity to be constant above pH 7.0, as most of the phosphate would be in the productive dianionic state above such pH. On the contrary, if the monoanion were the reactive species, we expected the observed velocity to drop above pH 7.0 because the concentration of the monoanion would significantly decrease above the p K a value.…”