Phase Diagrams of Nonionic Surfactants

This phase diagram shows the various phases formed when the surfactant concentration and temperature is changed. Let us first consider a dilute nonionic surfactant solution, say 1% this solution is isotropic (denoted by I) at low temperatures, but on increasing the temperature, a critical point is reached above which the solution becomes turbid. This critical temperature is defined as the doud point 

The origin of doudiness with nonionic surfactants has been the subject of considerable debate. Using light scattering, Corkill et al. [15] suggested that cloudiness is assodated with a rapid increase in the micellar aggregation number, with 

Several ideas have been put forward to explain the driving force for formation of the different liquid crystalline phases. One of the simplest methods for predicting the shape of an aggregated structure is based on the critical packing parameter concept (P) introduced by Israelachvili and his co-workers [22, 23]. This concept will be discussed in detail in the chapter on emulsions (selection of emulsifiers). Basically, P is the ratio between the cross sectional area of the alkyl chain (that is given by v/l, where v is the volume of the hydrocarbon chain and is the maximum length to which the alkyl chain can extend) and the optimum head group area ao, i.e., 

Spherical micelles require P to be less than cylindrical micelles require whereas lamellar micelles require P 1. 

Using the above concept, one may predict the shape of a micelle in a dilute solution. For a nonionic surfactant such as CnEe, the preferred shape will be a spherical micelle. As the volume fraction of the surfactant is increased, repulsion between the micelles tends to space them out, forming first a cubic array of spher- 

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Solubilisation can also be illustrated by considering the phase diagrams of nonionic surfactants containing polyfethylene oxide) (PEO) head groups. Such surfactants do not generally need a cosurfactant for microemulsion formation. 

Solubilization and formation of swollen micelles can also be illustrated by considering the phase diagrams of nonionic surfactants containing poly(ethylene oxide). Such surfactants do not generally need a cosurfactant for microemulsion formation. At low temperatures, the ethoxylated surfactant is soluble in water, and at a given concentration it can solubilize a given amount of oil. However, by adding more oil to such a solution, separation into two phases occurs O/W solubilized - - oil. If the temperature of such a two-phase system is increased the excess oil may be solubilized. This occurs at the solubilization temperature of the system. Above this temperature an isotropic O/W microemulsion is produced. 

Karlstrom, G. Carlsson, A. Lindman, B., "Phase Diagrams of Nonionic Polymer-Water Systems. Experimental and Theoretical Studies of the Effects of Surfactants and Other Cosolutes," J. Phys. Chem., 94, 5005 (1990). 

As an example of the different phases of surfactants. Figure 3.27 shows the phase diagram of a pure nonionic surfactant of the alkyl polyglycol ether type (20). In particular, the phase behaviour of nonionic surfactants with a low degree of ethoxylation is very complex. As the lower consolute boundary is shifted to lower temperatures with a decreasing EO (ethylene oxide) number of the molecule, an overlapping of this boundary... 

Karlstrom G, Carlsson A, Lindman B (1990) Phase diagrams of nonionic polymer -water systems. Experimental and theoretical studies of the effects of surfactants and other cosolutes. J Phys Chem 94 5005-5015... 

In summary, the above studies provide the equilibrium phase diagram of the Ci2MG-water system below 80°C. This work established, in addition, that the cloud point boundary is absent below 100 °C. (This is the boundary of the liquid/liquid miscibility gap commonly found in the diagrams of nonionic surfactant water systems). The absence of the cloud point boundary is significant with respect to analysis of the intrinsic hydrophilicity of this poly functional group [2]. The kinetic and nonequilibrium aspects of the phase behavior of aqueous C12MG mixtures will now be considered. 

Whereas the water-surfactant association implied by N-w/eo is usually considered to be rather weak [11], the existence of definite stable hydrates was shown for the polyoxyethylene-water system [46] and for some (nonionic) surfactant-water systems [47-49]. It was, however, argued that if the phase diagrams of these surfactant-based systems included the putative hydrates, the phase rule would be violated [50]. Clearly, this issue merits further investigation. 

The Kraft point (T ) is the temperature at which the erne of a surfactant equals the solubility. This is an important point in a temperature-solubility phase diagram. Below the surfactant cannot fonn micelles. Above the solubility increases with increasing temperature due to micelle fonnation. has been shown to follow linear empirical relationships for ionic and nonionic surfactants. One found [25] to apply for various ionic surfactants is ... 

A considerable amount of experimental work has been carried out on the so-called gel emulsions of water/nonionic surfactant/oil systems [9-14, 80, 106, 107]. These form in either the water-rich or oil-rich regions of the ternary phase diagrams, depending on the surfactant and system temperature. The latter parameter is important as a result of the property of nonionic surfactants known as the HLB temperature, or phase inversion temperature (PIT). Below the PIT, nonionic surfactants are water-soluble (hydrophilic form o/w emulsions) whereas above the PIT they are oil-soluble (hydrophobic form w/o emulsions). The systems studied were all of very high phase volume fraction, and were stabilised by nonionic polyether surfactants. 

Phase diagrams of water, hydrocarbon, and nonionic surfactants (polyoxyethylene alkyl ethers) are presented, and their general features are related to the PIT value or HLB temperature. The pronounced solubilization changes in the isotropic liquid phases which have been observed in the HLB temperature range were limited to the association of the surfactant into micelles. The solubility of water in a liquid surfactant and the regions of liquid crystals obtained from water-surfactant interaction varied only slightly in the HLB temperature range. 

The presence of a liquid crystalline phase at high surfactant concentrations has been shown by Shinoda (31), but the method of presentation renders the evaluation of the temperature dependence of necessary emulsifier concentrations to obtain the liquid crystalline phase difficult. Although several phase diagrams of the system (water, emulsifier, and nonionic surfactant) have been published (4, 45, 46, 47, 48), no results have been given on the relation between the surfactant phase and the lamellar liquid crystalline phase in these systems. 

Peters and Luthy (1993, 1994) performed a detailed analysis of the equilibrium behavior of solvent coal tar water mixtures in work that was complementary to column studies performed by Roy, et. al. (1995). Peters and Luthy successfully modeled ternary phase diagrams of coal tar/n-butylamine/water systems. In addition, Peters and Luthy identified n-butylamine as the leading solvent for coal tar extraction. Pennell and Abriola (1993) report the solubilization of residual dodecane in Ottawa sand using a nonionic surfactant, polyoxyethylene sorbitan monooleate, which achieved a 5 order of magnitude increase over the aqueous solubility, but is still 7 times less than the equilibrium batch solubility with the same surfactant system. 

A limited number of studies have considered the use of surfactant and cosolvent mixtures to enhance the recovery of NAPLs (Martel et al., 1993 Martel and Gelinas, 1996). Martel et al. (1993) and Martel and Gelinas (1996) employed ternary phase diagrams to select surfactant+cosolvent formulatons for treatment of NAPL-contaminated aquifers. The surfactant+cosolvent formulations used in their work, which included lauryl alcohol ethersulfate/n-amyl alcohol, secondary alkane sulfonate/n-butanol, and alkyl benzene sulfonate/n-butanol, were shown to be effective solubilizers of residual trichloroethene (TCE) and PCE in soil columns (Martel et al., 1993). However, very little information is available regarding the effect of cosolvents on the solubilization capacity and phase behavior of ethoxylated nonionic surfactants. 

The phase diagrams of two-component surfactant-water systems are typically quite different for nonionic and ionic compounds. As exemplified in Fig. 2.22 there are at low temperatures different liquid crystalline phases while at intermediate temperatures there may be a total mutual solubility of surfactant and water98. At higher temperatures, there is, as already noted, a separation into two phases with a very large two-phase region. One of the phases contains very little surfactant, while the other contains appreciable amounts of both components. The cloud-point curve can be described as a liquid-liquid solubility curve with a lower consolute tempera-... 

Schomacker compared the use of nonionic microemulsions with phase transfer catalysis for several different types of organic reactions and concluded that the former was more laborious since the pseudo-ternary phase diagram of the system had to be determined and the reaction temperature needed to be carefully monitored [13,29]. The main advantage of the microemulsion route for industrial use is related to the ecotoxicity of the effluent. Whereas nonionic surfactants are considered relatively harmless, quaternary ammonium compounds exhibit considerable fish toxicity. 

Fig. 10 Binary phase diagram of a a) monomeric nonionic surfactant, b) non-ionic polysurfactant in aqueous solution A heterogeneous mixed crystals B heterogeneous melt C homogeneous isotropic solution D homogeneous mesomorphous phases...

With nonionic surfactants of the efhoxylate type, an increase in the temperature of a solution at a given concentration causes dehydration of the PEO chains and, at a critical temperature, the solution will become cloudy. This is illustrated in Figure 3.6, which shows the phase diagram of Cyj E. Below the CP curve it is possible to identify the different liquid crystalline phases hexagonal-cubic-lamellar, which are shown schematically in Figure 3.7. 

Buzier, M. and Ravey, J.C. (1983) Solubilization properties of nonionic surfactants 1. Evolution of ternary phase diagrams with temperature, salinity, HLB, and ACN. /. Colloid Interface Sci., 91, 20-33. 

Fig. VI-18. Phase diagrams of water - hydrocarbon (oil) - nonionic surfactant system at three different temperatures. Winsor equilibria.

The shape of this phase diagram changes with temperature. In the case of nonionic surfactants this is related to the dehydration of their polar groups at elevated temperatures, which results in an increase in affinity of surfactant molecules towards hydrocarbons and a decrease in their affinity towards water. Due to this the elevation of temperature results in a decrease in the size of the separation region in the oil-rich area of the phase diagram and in an increase in the size of the separation region in the water-rich area (Fig. VI-18). Similar effects are caused by the addition of electrolytes. 

The presence of mixed surfactant adsorption seems to be a factor in obtaining films with very viscous surfaces (12). For example, in some cases, the addition of a small amount of nonionic surfactant to a solution of anionic surfactant can enhance foam stability because of the formation of a viscous surface layer, which is possibly a liquid-crystalline surface phase in equilibrium with a bulk isotropic solution phase (6, 8). In general, some very stable foams can be formed from systems in which a liquid-crystal phase is present at lamella surfaces and in equilibrium with an isotropic interior liquid. If only the liquid-crystal phase is present, stable foams are not produced. In this connection, foam phase diagrams... 

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