Hidden Minima of the Gibbs Free Energy Revealed in a Phase Separation in Polymer/Surfactant/Water Mixture

Abstract

We observed a very unusual kinetic pathway in a separating C(12)E(6)/PEG/H(2)O ternary mixture. We let the mixture separate above the spinodal temperature (cloud point temperature) for some time and next cool it into a metastable region of a phase diagram, characterized by two minima of the Gibbs potential, one corresponding to the homogeneous mixture and one to the fully separated PEG-rich and C(12)E(6)-rich phases. Despite the fact that in the metastable region the thermodynamic equilibrium corresponds to the separated phases (global minimum of the Gibbs free energy), we observe perfect mixing of the initially separated phase. The homogeneous state, obtained in this way, does not separate, if left undisturbed. However, many cooling-heating cycles or full separation with visible meniscus above the cloud point temperature induce the phase separation in the metastable region. The metastable region can exist tens of degrees below the cloud point temperature. This effect is not observed in the binary mixture of C(12)E(6)/H(2)O.

Polymerizable Cationic Micelles Form Cylinders at Intermediate Conversions

“…We can attribute the unanticipated occurrence of two types of amphiphilic aggregates to a kinetic effect, as in emulsion polymerization, or to an equilibrium coexistence, such as in certain mixed polymer−micelle systems. − A kinetic model in which micelles are consumed and converted to rodlike particles is inconsistent with the data, which shows that rodlike micelles are longest at low conversions and disappear completely near full conversion.…”

Salting-Out Effect Induced by Temperature Cycling on a Water/Nonionic Surfactant/Oil System

“…Such an aspect involves consideration of the Gibbs free energy of the ternary system as a whole, and then following its evolution on the profile of Gibbs free energy as a function of the arbitrary concentration of the water continuous phase. Inspired by the volumic approach of Hołyst et al, 46 the interfacial behavior of nonionic surfactant, here, can be described with Figure 5a. The two configurations that exhibit a single minimum of Gibbs energy are the so-called stable regions, for lower and higher temperatures, corresponding respectively to the stabilization of O/W and W/O macroemulsions.…”

“…Indeed, instead of considering the surfactant behavior as a result of the intermolecular rearrangements of water molecules, a different and thermodynamic way can also be considered, focusing the study on the global Gibbs free energy of water/nonionic surfactant systems. This approach was reported in recent works of Hołyst et al, [46][47][48] in which homogeneous mixtures (uniform states, below the CP) and twophase systems (above the CP) are described through a single minimum of Gibbs free energy. These authors revealed the presence of hidden minima of free energy in the transitional region between the two single-minimum states, the so-called metastable region.…”

“…These authors revealed the presence of hidden minima of free energy in the transitional region between the two single-minimum states, the so-called metastable region. 46 Then, according to the thermal and kinetic pathways imposed on the system, for a given temperature in the metastable region, it is possible to observe either a metastable homogeneous system or a stable phase-separated system.…”

Polymer-induced ordering and phase separation in ionic surfactants

“…The phase separation processes induced by the addition of nonionic water-soluble polymers in solutions of nonionic surfactants have recently been investigated [19][20][21]. It has been shown [19] that the addition of poly(ethylene glycol) (PEG) to the aqueous solution of nonionic surfactant C 12 E 6 (n-dodecyl hexaoxyethylene glycol monoether) can result in the separation of the system into the polymer-rich and the surfactant-rich phases.…”