Effect of pressure on the solution behavior of nonionic surfactants in water

The phase diagrams, i.e., the pressure—temperature—composition diagrams of each system tetra-, penta-, and hexaoxyethylene dodecyl ether (C 12 E 4 , C 12 E 5 , and C 12 E 6 ,respectively) vs water, have been determined by the use of the observation that the transmittance of the solution changes sharply with the change of the dissolved state of the solute. The temperature—pressure phase diagrams involve the regions of the micellar solution, the liquid surfactant phase + water phase, the hydrated solid + water phase, and the liquid crystal + water phase. The cloud temperature increased and its increasing rate became smaller with increasing pressure. By considering that the cloud temperature—composition curve is a liquid—liquid mutual solubility curve and the idea of “associated solution” of Prigogine and Defay, we derived the thermodynamic quantities (enthalpy, entropy, and volume) accompanied by the separation of the liquid surfactant phase from the water phase above the cloud temperature for the C 12 E 5 —water system. The enthalpy and entropy changes were positive and depended little on pressure, while the volume change was positive and decreased, tending to zero, with the pressure up to 300 MPa. This suggests that hydrophobic bonding as well as the partial dehydration of oxyethylene groups of surfactant contribute to the separation of the liquid surfactant phase from the water phase. In the composition—pressure diagram close to the regions of the micellar solution and the hydrated solid + water phase for the C 12 E 5 —water system, a sudden increase in the solubility of surfactant at a certain pressure was found with decreasing pressure and at constant temperature, which corresponds to the Krafft phenomenon and can be attributed to the effective dissolution of surfactant as a micellar form in water.