Phase behaviour ionic surfactants

As a practical example for the phase behaviour of surfactants, Figure 3.18 shows the phase diagram of a pure non-ionic surfactant of the alkyl polyglycol ether type C Em. n denotes the length of the hydrocarbon chain and m the degree of ethoxylation [20]. 

The choice of surfactant, which is mostly constrained by the choice of the oil and the resulting phase behaviour of the microemulsion, can have different effects on the enzyme stability and activity. In general we have to differentiate between ionic and nonionic surfactant types ... 

In contrast to nonionic surfactants, ionic surfactants build up a high zeta-po-tential at the water-oil interface which can also can influence the enzyme activity. Most investigated systems used AOT as the surfactant because its phase behaviour is well understood. However, AOT is often not very suitable, because it can totally inhibit enzymes (e.g. the formate dehydrogenase from Candida bodinii). The usage of lipases in AOT-based microemulsions is generally unfavourable as AOT is an ester that is hydrolysed itself. 

In case of non-ionic surfactants in water, the behaviour of the water structure outlines three main concentration regions, which closely coincide with the three phases intersected by the experimental isotherms. In the micellar solution phase, no significant changes in the water structure are indicated, while, in the lamellar phase, rapid destruction of the tetrahedral hydrogen bond network occurs due to the confinement of the water between the hydrophilic surfaces of the lamellae. The dehydration of the surfactant head groups was found to start near the border between the lamellar and the reverse micellar solution phases. At higher concentrations, water demonstrates its trend to form clusters of tetrahedrally bonded molecules even at the very low content in the system. The results with surfactant solutions have been obtained by Raman spectroscopy (Marinov el al., 2001). 

Forgoes, E. Cserhati, T. Determination of retention behaviour of some non-ionic surfactants on reversed-phase high-performance liquid chromatography supports by spectral mapping in combination with cluster analysis or nonlinear mapping. J. Chromatogr., A 1996, 722, 281-287. 

The extensive research on microemulsions was prompted by two oil crises in 1973 and 1979, respectively. To optimise oil recovery, the oil reservoirs were flooded with a water-surfactant mixture. Oil entrapped in the rock pores can thus be removed easily as a microemulsion with an ultra-low interfacial tension is formed in the pores (see Section 10.2 in Chapter 10). Obviously, this method of tertiary oil recovery requires some understanding of the phase behaviour and interfacial tensions of mixtures of water/salt, crude oil and surfactant [4]. These in-depth studies were carried out in the 1970s and 1980s, yielding very precise insights into the phase behaviour of microemulsions stabilised by non-ionic [5, 6] and ionic surfactants [7-9] and mixtures thereof [10]. The influence of additives, like hydro- and lyotropic salts [11], short- and medium-chain alcohols (co-surfactant) [12] on both non-ionic [13] and ionic microemulsions [14] was also studied in detail. The most striking and relevant property of micro emulsions in technical applications is the low or even ultra-low interfacial tension between the water excess phase and the oil excess phase in the presence of a microemulsion phase. The dependence of the interfacial tension on salt [15], the alcohol concentration [16] and temperature [17] as well as its interrelation with the phase behaviour [18, 19] can be regarded as well understood. 

The primary aim of microemulsion research is to find the conditions under which the surfactant solubilises the maximum amounts of water and oil, i.e. the phase behaviour has to be studied. As the effect of pressure on the phase behaviour is (in general) rather weak [30 ], it is sufficient to consider the effect of the temperature. Furthermore, it hasbeen shown that simple ternary systems consisting of water, oil and non-ionic n-alkyl polyglycol ethers (QEj) exhibit all properties of complex and technically relevant systems [6]. Therefore, we will first describe the phase behaviour of ternary non-ionic microemulsions. 

Figure 1.1 Schematic view of the phase behaviour of the three binary systems water (A) oiI (B), oil (B)-non-ionic surfactant (C), water (A)-non-ionic surfactant (C) presented as an unfolded phase prism [6]. The most important features are the upper critical point cpc, of the B-C miscibility gap and the lower critical point cpp of the binary A-C diagram. Thus, at low temperatures water is a good solvent for the non-ionic surfactant, whereas at high temperatures the surfactant becomes increasingly soluble in the oil. The thick lines represent the phase boundaries, while the thin lines represent the tie lines.

Figure 1.2 Isothermal Gibbs triangles of the system water (A)-oil (B)-non-ionic surfactant (C) at different temperatures. Increasing the temperature leads to the phase sequence 2-3-2. A large miscibility gap can be found both at low and high temperatures. While at low temperatures a surfactant-rich water phase (a) coexists with an oil-excess phase (b), a coexistence of a surfactant-rich oil phase (b) with a water-excess phase (a) is found at high temperatures. At intermediate temperatures the phase behaviour is dominated by an extended three-phase triangle with its adjacent three two-phase regions. The test tubes illustrate the relative change in phase volumes.

Stacking the isothermal Gibbs triangles on top of each other results in a phase prism (see Fig. 1.3(a)), which represents the temperature-dependent phase behaviour of ternary water-oil-non-ionic surfactant systems. As discussed above, non-ionic surfactants mainly dissolve in the aqueous phase at low temperatures (2). Increasing the temperature one observes that this surfactant-rich water phase splits into two phases (a) and (c) at the temperature T of the lower critical endpoint cepp, i.e. the three-phase body appears. Subsequently, the lower water-rich phase (a) moves towards the water corner, while the surfactant-rich middle phase (c) moves towards the oil corner of the phase prism. At the temperature Tu of the upper critical endpoint cepa a surfactant-rich oil phase is formed by the combination of the two phases (c) and (b) and the three-phase body disappears. Each point in such a phase prism is unambiguously defined by the temperature T and two composition variables. It has proved useful [6] to choose the mass fraction of the oil in the... 

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 phase behaviour of the pseudo-ternary ionic mixture can again be explained with the interplay of the three binary base systems. At low temperatures the ionic surfactant is... 

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. 

Figure 1.14 Schematic phase prism (a) and interfacial tensions (b) as function of temperature for the system water-oil-non-ionic surfactant. The minimum of the water/oil interfacial tension crab at T is a consequence of the phase behaviour. Increasing the temperature the aqueous phases separates into the phases (a) and (c) at the critical endpoints cepp whereas the phases (b) and (c) merge into a single oil-rich phase at cepa. Thus, the interfacial tensions

The formulation has been related with the type and properties of emulsions since Bancroft s rule of thumb (1913) and Langmuirs wedge theory (1917). The hydrophilic-lipophilic balance (HLB) was introduced by Griffin 60 years ago, probably as a selling argument for the (by the time) new non-ionic surfactants. It accounts for the relative importance of the hydrophilic and lipophilic parts of an amphiphilic molecule on a weight basis [19]. For decades there was no other numerical yardstick. The simplicity of the HLB concept was its main advantage in spite of very serious limitations, such as an inaccuracy sometimes over two units, and the fact that it does not take into account several variables which are known to alter the phase behaviour, independently of the surfactant. 

The last technique, which is the best one when there is very little information on the unknown component, is based on the mixing of a pair of known components with the unknown one, and the use of a linear mixing rule. For instance, if the characteristic parameter ((3 = a - EON) of an unknown non-ionic surfactant is to be determined, the correlation to be used for the mixture of the two base products, such as two ethoxylated nonylphenols with different EONs, e.g. EONi and EON2, so that the mixture that results in three-phase behaviour is EONm is as follows ... 

---