Phase behaviour of surfactants

Although much of the published work on surfactant solutions has been concerned with the properties of dilute aqueous solutions, many surfactants also form liquid crystalline phases at high concentration. The dilute isotropic surfactant solutions represent only a part of the overall picture of surfactant-solvent interaction. In this chapter we examine the structure and properties of the various lyotropic mesophases which may be present in surfactant solutions both in the presence and absence of solubilizate. 

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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]. 

Undoubtly, phase diagrams are the most convenient. The above discussed is in fact the first attempt to construct such a diagram for foam bilayers. Phase diagrams of surfactant solutions indicating the system state can be found in the monograph of Laughlin The Aqueous Phase behaviour of Surfactants [455] and are of major practical importance. 

R.G. Laughlin, The Aqueous Phase Behaviour of Surfactants, Academic Press, London,... 

The phase behaviour at equilibrium turned out to be the main property reported in Win-sor s work in the late 1940s. Winsor interpreted the phase behaviour through the so-called R ratio of molecular interaction energies at interface. The R ratio was a handy theoretical concept to understand the variations of the phase behaviour of surfactant-oil-water systems and somehow of the emulsion properties. It is essentially qualitative, but for the first time the phase behaviour was linked with a condition that depended on all formulation variables, but could be expressed as a single generalised variable, i.e. the R ratio [1]. The original R ratio was... 

Due to the rapid development of computer performance in the last few years, the molecular dynamics simulation method can now be used in order to get more detailed information on the phase behaviour of surfactant molecules at the air/water interface, provided that the number of molecules does not exceed a certain threshold value. With these computer simulation methods, it is possible to directly calculate molecular parameters such as the average tilt angle of the surfactant molecules adsorbed in the monolayer. 

The phase behaviour of surfactants is best illustrated using nonionic surfactants of the poly(ethylene oxide) type. Figure 3.18 illustrates this with the phase diagram for the binary system, dodecyl hexaoxyethylene glycol monoether-water [14]. 

As discussed in the chapter on phase behaviour of surfactants, the latter produce liquid crystalline phases at high concentrations. Three main types of liquid crystals can be identified Hexagonal phase (sometimes referred to as middle phase), cubic phase and lamellar (neat phase). All these structures are highly viscous and they... 

The various phases produced by a surfactant can be related to its structure. An important parameter that can be used to predict the phase behaviour of surfactants is the critical packing parameter (CPP) described above. (CPP = v/la, where v is the volume of the hydrocarbon chain with a length I and a is the cross sectional area of the hydrophilic head group.) For spherical micelles, CPP for cylindrical micelles 1 > CPP > and for lamellar micelles CPP 1. 

Study of the phase behaviour of surfactants (which can be performed using polarizing microscopy) is crucial in the selection of adjuvants. Interaction of the above units with the agrochemical is crucial in determining performance (e.g. solubilization). Similar interactions may also occur between the above structural units and the leaf surface (wax solubilization). 

In the first part of this review, the general rheological behaviour of condensed matter will be introduced. In the second part, the aggregation and phase behaviour of surfactants in solution will be described briefly, because the rheological properties of such systems are determined by the surfactant aggregates and the interactions which exist in the solutions. Then, in the main part of this chapter, the rheology of various surfactant phases will be discussed in detail. 

Laughlin, R.G. The Aqueous Phase Behaviour of Surfactants , Academic Press, London (1994). Fontell, K., Mol. Cryst. Liquid Cryst., 63,59 (1981). 

The phase behaviour of surfactants at high concentration is described by two types of model. The first is based on the curvature of a surfactant film at an interface. The second is based on the shape of the surfactant molecules themselves. We now consider each of these approaches in turn. 

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