Surface Tension of Aqueous Solutions of Disodium Monoalkyl Phosphates and Their Mixtures

Disodium mono-dodecyl, mono-tetradecyl, and mono-hexadecyl phosphates were synthesized and the surface tension of aqueous solutions of each disodium monoalkyl phosphate (SAP) and of their mixtures were measured. On the basis of the surface tension values thus obtained, the influence of the hydrocarbon chain length of SAP and the effect of the mixing of SAP on the adsorption and the micelle formation were discussed. A modified Szyszkowski equation was derived from the Gibbs adsorption equation and the modified Langmuir adsorption equation by using the activity instead of the concentration. The modified Szyszkowski equation gave calculated values which were in good agreement with the experimental values of the surface tension. It was also found that the energy of adsorption per methylene group, ω, is 682 cal/mol. In mixed solutions, the modified Szyszkowski equation was used to obtain results in good agreement with the measured values of the surface tension, assuming an ideal mixing of SAP in the adsorption layer. Further, the change in the standard free energy was calculated for the micelle formation for each SAP from the hypothetical standard state of the hydrated monomer in solution; it gave the energy of micelle formation per methylene group, ωm, is 613 cal/mol. The theoretical equation to give the cmc of SAP, was obtained also, assuming the ideal mixing of SAP in the mixed micelle phase; it was proved to be in good agreement with the experimental values of the cmc.