Density Measurements on C₁₂E_j Nonionic Micellar Solutions as a Function of the Head Group Degree of Polymerization (j = 5−8)

Abstract

Density measurements have been performed on water solutions of nonionic surfactants oligooxyethylene glycol-monoether (C 12 E j with j ) 5, 6, 7, 8) in a wide range of temperatures and concentrations. The densities of the pure surfactants in their liquid state were measured too. The observed values are almost a linear combination of the densities of an oil and of an oxyethylene (EO) bulk liquid phases. The deviation from ideality reduces as j increases and may be reflects an entropic contribution due to a partial mixing of the oil and EO termination. A temperature (T cross ), at which the C 12 E j aqueous solution density coincides with the solvent one up to 50 wt % has been found in all the E j species. The T cross of the different E j results to be scaling temperatures for the excess density of these surfactants. The sphere-to-rod transition temperatures are scaling temperatures too; thus it is expected that the differences between these two temperatures be constant for each of the E j species. In the case of j ) 6, 7, and 8 the values of the sphere-to-rod transition temperatures are known and these differences are all about 20 °C. For C 12 E 5 only rods were experimentally observed and, coherently, the previous consideration led to a sphere-to-rod transition temperature of -7 °C. The difference between the critical and the crossing temperatures is constant too, at about 15 °C for j < 8, whereas for j ) 8 it is 7 °C. In this case the densities of the separated phases, two degrees above the critical temperature, result to be the same, within 10 -6 g/cm 3 . It implies that, when slightly above the critical temperature, C 12 E 8water solution undergoes phase separate as it was in microgravitational condition. In our analysis, the C 12 E j solution is considered a three-component system: the oil core, the bulk water, and the region containing the aggregate interface, a mixture of water molecules, and EO units. Fixing the density of the oil core and of the bulk water phase respectively equal to those of pure dodecane oil and water at the same temperature, we obtain the density and the molar volume of the micellar interface. These data indicate that the amount of water molecules per EO segment decreases with the number of hydrophilic unit and with temperature. The same conclusion follows looking at the temperatures at which the maximum densities as a function of surfactant species and concentrations occur .

Phase behavior of the Pluronic P103/water system in the dilute and semi-dilute regimes

“…Using a quadratic fit for density data at temperatures lower than CMT, the temperature where the density reaches its maximum value for each concentration was evaluated. This temperature shifts to lower values as surfactant concentrations augment, indicating that P103 is a structure former [39]. Fig.…”

Solubilization of Polyelectrolytic Hairy-Rod Polyfluorene in Aqueous Solutions of Nonionic Surfactant

“…14,15,17 This technique can also be used to induce liquid crystallinity as shown for PPP 18,19 and poly-(p-pyridine). 20 Oxyethylene glycol-monoethers (C i E j ) 21,22 are archetypical surfactants widely used in fundamental phase studies of microemulsions, [23][24][25][26][27] block-co-polymers, 28 and micelles. 29,30 The phase behavior of pentaethylene glycol monododecyl ether (ndodecyl pentaoxyethylene glycol ether) (C 12 E 5 ) 31,32 is particularly well-known: Pure C 12 E 5 forms cylindrical micelles but the oil-swollen C 12 E 5 micelles are wormlike with a rather high flexibility and cannot be described as rigid rods.…”

Formulating Nonionic Detergents via the Integrated Free Energy Model

“…c illustrate the change in each of the interactions, due to the change in temperature from 6.5 to 45 °C. The four van der Waals (vdW) interactions (Woo, Wss, Wso, and Wot) decrease with temperature because the density of the liquid decreases with temperature (Maccarini and Briganti, ), meaning that the molecule‐molecule distance is larger, producing weaker vdW interactions. For the interactions that oppose solubilization, Woo and Wot, the decrease in these interactions means less resistance to solubilization, showing negative deltas.…”