Phase behaviour inversion

Phase behaviour describes the phase or phases in which a mass of fluid exists at given conditions of pressure, volume (the inverse of the density) and temperature (PVT). The simplest way to start to understand this relationship is by considering a single component, say water, and looking at just two of the variables, say pressure and temperature. 

We start with some elementary information about anisotropic intermolec-ular interactions in liquid crystals and molecular factors that influence the smectic behaviour. The various types of molecular models and commonly accepted concepts reproducing the smectic behaviour are evaluated. Then we discuss in more detail the breaking of head-to-tail inversion symmetry in smectic layers formed by polar and (or) sterically asymmetric molecules and formation of particular phases with one and two dimensional periodicity. We then proceed with the description of the structure and phase behaviour of terminally fluorinated and polyphilic mesogens and specific polar properties of the achiral chevron structures. Finally, different possibilities for bridging the gap between smectic and columnar phases are considered. 

Salinity Effects in the Inversion Process It has been shewn for anionics that the Salager (11) equation could relate salt and alcohol effects to phase behaviour according to -... 

The phase behaviour established for concentrated aqueous solutions of PEO-PPO-PEO copolymers has its counterpart in PEO/PBO copolymer solutions. A phase diagram for PE058PB0i7PE0M based on tube inversion experiments is shown in Fig. 4.14 (Luo et al. 1992). The hard gel is isotropic under the polarizing microscope and can be characterized as a cubic phase formed from spherical micelles of a similar size to those in the dilute micellar solution. 

In the case of polystyrene blends with poly(vinyl methyl ether) two phase behaviour was found for blends from various chlorinated solvents whereas single phase behaviour was found for blends from toluene The phase separation of mixtures of these polymers in various solvents has been studied and the interaction parameters of the two polymers with the solvents measured by inverse gas chromatography It was found that those solvents which induced phase separation were those for which a large difference existed between the two separate polymer-solvent interaction parameters. This has been called the A% effect (where A% = X 2 Xi 3)-A two phase region exists within the polymer/polymer/solvent three component phase diagram as shown in Fig. 2. When a dilute solution at composition A is evaporated, phase separation takes place at B and when the system leaves the two phase region, at overall... 

At the oil-rich side, the phase behaviour is inverted temperature-wise as can be seen in the T( wA)-section provided in Fig. 1.7(c). Thus, the near-critical phase boundary 2 —1 starts at low temperatures from the lower n-octane-QoEs miscibility gap (below <0°C) and ascends steeply upon the addition of water. With increasing wA, this boundary runs through a maximum and then decreases down to the upper critical endpoint temperature Tu. The emulsification failure boundary 1 —r 2 starts at high temperatures and low values of wA, which means that only small amounts of water can be solubilised in a water-in-oil (w/o) microemulsion at temperatures far above the phase inversion. Increasing amounts of water can be solubilised by decreasing the temperature, i.e. by approaching the phase inversion. At Tu the efb intersects the near-critical phase boundary and the funnel-shaped one-phase region closes. 

Figure 1.7 Vertical sections T(wb) and 7 (wA) through the phase prism which start at the binary water-surfactant (wb = 0) and the binary oil-surfactant (wA = 0) corner, respectively. These sections have been proven useful to study the phase behaviour of water- and oil-rich microemulsions, (a) Schematic view of the sections T wg) and T(wA) performed at a constant surfactant/fwater + surfactant) mass fraction ya and at a constant surfactant/(oil + surfactant) mass fraction 7b, respectively, (b) T(wb) section through the phase prism of the system FhO-n-octane-CioEs at ya = 0.10. Starting from the binary system with increasing mass fraction of oil wg, the oil emulsification boundary (2- 1) ascends, while the near-critical phase boundary (1 - 2) descends, (c) T(wA) section through the phase prism of the system EbO-n-octane-QoEs at 7b = 0.10. The inverse temperature behaviour is found on the oil-rich side With increasing fraction of water wA the water emulsification boundary (1 - 2) descends, whereas the near-critical phase boundary (2 —> 1) ascends.

In the preceding sections, the phase behaviour of rather simple ternary and quaternary non-ionic microemulsions have been discussed. However, the first microemulsion found by Schulman more than 50 years ago was made of water, benzene, hexanol and the ionic-surfactant potassium oleate [1, 3]. Winsor also used the ionic-surfactant sodium decylsulphate and the co-surfactant octanol to micro-emulsify water/sodium sulphate and petrol ether [2], In the last 30 years, in-depth studies on ionic microemulsions have been carried out [7, 8, 65, 66]. It toned out that nearly all ionic surfactants which contain one single hydrocarbon chain are too hydrophilic to build up microemulsions. Such systems can only be driven through the phase inversion if an electrolyte and a co-surfactant is added to the mixture (see below and Fig. 1.11). 

The variation of the phase behaviour as a function of the salinity is shown in Fig. 1.10(b) in the form of an (y)-section through the phase tetrahedron of the quaternary H20/NaCl-n-decane-A0T system at a = 0.50 and a constant temperature of T = 40° C. In order to compare the variation of the phase behaviour with temperature and salinity a rectangular representation is used also for the (y)-section through the phase tetrahedron. As can be seen, the phase boundaries also resemble the shape of a fish in this isothermal (y)-section. However, with increasing mass fraction of salt the phase sequence 2, 3, 2 is found which is inverse to the 2, 3, 2 sequence observed with increasing temperature. 

Ionic surfactants with only one alkyl chain are generally extremely hydrophilic so that strongly curved and thus almost empty micelles are formed in ternary water-oil-ionic surfactant mixtures. The addition of an electrolyte to these mixtures results in a decrease of the mean curvature of the amphiphilic film. However, this electrolyte addition does not suffice to drive the system through the phase inversion. Thus, a rather hydrophobic cosurfactant has to be added to invert the structure from oil-in-water to water-in-oil [7, 66]. In order to study these complex quinary mixtures of water/electrolyte (brine)-oil-ionic surfactant-non-ionic co-surfactant, brine is considered as one component. As was the case for the quaternary sugar surfactant microemulsions (see Fig. 1.9(a)) the phase behaviour of the pseudo-quaternary ionic system can now be represented in a phase tetrahedron if one keeps temperature and pressure constant. 

In the previous section a quinary ionic microemulsion was timed through the phase inversion by adding a short-chain alcohol as a non-ionic co-surfactant to a single-tailed ionic surfactant. In the following the short-chain alcohol is replaced by an ordinary long-chain non-ionic surfactant. It was discussed above that the temperature dependence of the phase behaviour of ionic (see Section 1.2.4) and non-ionic microemulsions (see Section 1.2.1) is inverse. Thus, one can expect that at a certain ratio 8 of non-ionic and ionic surfactants the inverse temperature trends compensate so that a temperature-insensitive microemulsion forms. It goes without saying that this property is extremely relevant in technical applications, where often mixtures of non-ionic and ionic surfactants are used. 

From the above, it is clear that a pre-requisite of low water/oil interfacial tensions is the complete saturation of the water-rich and oil-rich phases as well as the water/oil interface by surfactant molecules. Of course, this pre-requisite is fulfilled if one of the phases considered is a microemulsion. Furthermore, since the pioneering work of Lang and Widom [81] it is known that if a system is driven through phase inversion the interfacial tensions may become ultra-low. However, about 20 years ago, a number of experimental investigations were devoted to clarifying the origin of the ultra-low interfacial tensions [15, 17, 39, 71, 81-85]. In order to understand this correlation between phase behaviour and interfacial... 

As can be seen independently of the parameter used to drive the system through the phase inversion the shape of the interfacial tension curves is similar. Because of the fundamental link of the interfacial tension and phase behaviour discussed above, both systems show... 

New biocompatible oils from renewable resources have also been investigated. Acharya et al. investigated the impact of the addition of ricebran oil on the phase behaviour of microemulsions [28]. In combination with isopropylmyristate as second oil a large microemulsion domain is formed in the phase diagram, which makes ricebran oil a potential oil base for microemulsions. Another approach to improve skin friendliness is a reduction of the surfactant content of microemulsions. Diec et al. report on optimised surfactant-co-surfactant systems in combination with a phase inversion process to reach this goal [29]. The resulting formulations are clear, stable over the long term and contain less than 10% of surfactants. 

Schwarz and Knoetze [24] found that for their VLE data an approximately linear relationship exists between temperature and the phase transition pressure at constant composition. This relationship has a positive gradient and indicates a higher solubility at lower temperatures, converse to that of the solid-vapour equilibrium (SVE) phase behaviour. This positive gradient was also found through the entire mass fraction range studied and the authors did not find any indications of temperature inversions in this system. 

In the paramagnetic phase, the inverse susceptibility follows Curie-Weiss behaviour with an effective moment of 7.61 ju,B/atom, 0 = 41 K and 0i = —17 K. 

Izquierdo P, Esquena J, Tadros TF, Dederen JC, Feng J, Garcia-Cehna MJ, et al. Phase behaviour and nano-emulsion formation by the phase inversion temperature method. Langmuir 2004 20 6594-8. 

A high-resolution FTIR study of " NH3 gave the following band centres for V4 1626.276(1) cm 1627.375(2) cm for the s and a inversion upper states respect-ively. CARS data were reported to give an assignment of Q-branched for the Vi modes of " NH3 and The phase behaviour of solid NH3 was followed by... 

The phase inversion that occurs on heating an emulsion is dearly demonstrated in a study of the phase behaviour of emulsions as a function of temperature. This is illustrated schematically in Figure 9.2 by what happens when the temperature is increased [13, 14]. 

The influence of the hydrophile-lipophile balance (HLB) of the non-ionic surfactant on the phase behaviour has recently been studied [34-37] with the aim of better understanding the mechanisms of emulsion and suspension stabilization. Ternary phase diagrams for dodecane, water and mixtures of Brij 92, and Brij 96 (polyoxyethylated oleyl alcohol derivatives with oxyethylene chain lengths of 2 and 10, respectively), with a range of HLB values, are shown in Fig. 2.19. The areas of the L2 and the inverse middle phase M2 (an interesting feature of these systems not usually observed in polyoxyethylene non-ionic systems) increase as the HLB increases, reaching a maximum at HLB 8. Maximum water uptake in the L2 phase in 25 %, 40 % and 50 % surfactant solutions in oil as a function of HLB is... 

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