Phenol-water systems

If the third substance dissolves in both liquids (and the solubility in each of the liquids is of the same order), the mutual solubility of the liquids will be increased and an upper C.S.T. will be lowered, as is the case when succinic acid or sodium oleate is added to the phenol - water system. A 0 083 molar solution of sodium oleate lowers the C.S.T. by 56 -7° this large effect has been applied industrially in the preparation of the disinfectant sold under the name of Lysol. Mixtures of tar acids (phenol cresols) do not mix completely with water at the ordinary temperature, but the addition of a small amount of soap ( = sodium oleate) lowers the miscibility temperature so that Lysol exists as a clear liquid at the ordinary temperature. 

In these examples, as one would expect, the interfacial tensions are small and diminish as the critical solution temperature is approached. The differences between the surface tensions of the two phases are generally too small to decide whether the interfacial tension approaches zero asymptotically in all cases although such appears to be the case in the phenol water system we notice however that the temperature coefficient is very small indeed, as is the case for surface tensions of liquids near their critical point, but to a still greater degree. 

Taken from K. Roth, G. Schneider, and E. U. Franck, Liquid-Liquid and Liquid-Solid Phase Equilibriums in Cyclohexane-Methanol and Phenol-Water Systems up to 6000 Bars , Ber. Bunsenges. Physik. Chem., 70, 5-10 (1966). 

If we consider two liquids A and B and shake together, then some of A dissolves in B, while some of B dissolves in A. We then have two saturated solutions—one of A in B and the other of B in A. On increasing the temperature, the solubility of A increases in B and also that of B in A, in this particular case. As an example, we take the familiar phenol-water system. 

Figure 3.11 shows the partial miscibility of a phenol-water system. At a given temperature (e.g., 45°C), there is initially a homogeneous solution denoted point x, which is an unsaturated solution of phenol in water. When more phenol is added to the solution at the same temperature, the composition line (i.e., tie-line) moves to the right horizontally. When the composition of phenol in water reaches point a, a... 

Feller D, Feyereisen MW (1993) Ab initio study of hydrogen bonding in the phenol-water system. J Comput Chem 14 1027-1035... 

However, some solutes exhibit a strong attraction to the membrane material and are therefore preferentially sorbed compared to the solvent water. This can lead to positive, zero, or negative separation depending on both the magnitude of the attractive forces and the mobility of the solute at the solution-membrane interface (relative to the mobility of water in this region). Both the polar and nonpolar character of the solute may be important in the separation process. Consider the case of a polar solute. Since the cellulose acetate membrane material has a net proton acceptor nature (3 ), any polar compound is attracted to the membrane surface. The more polar the compound the stronger the attraction. When the solute is more polar than the solvent, as is the case in phenol-water systems, the solute is preferentially sorbed by the membrane. [Note the polar nature of a compound can be conveniently and quantitatively expressed by the Taft number (1,19).] Since the solute and the solvent are both polar, a strong interaction exists between the two solution components. 

Earlier we considered a system in which the components deviated positively from Raoult s Law, and had vapour pressures greater than the law would predict. In such a solution at > xh and y( > 1. For most normal liquid mixtures in which the components do not interact specifically (as in hydrogen-bond formation) this is the most common behaviour. The components show a greater tendency to escape into the vapour phase than in the corresponding ideal solution. In extreme cases the dislike of the components for each other in the solution may cause the solution to separate into two phases at sufficiently low temperatures. The phenol-water system deviates... 

This can be observed in phenol-water system. A very small amount of water is miscible in phenol and the reverse is also true. Beyond a certain concentration phenol and water do not mix at room temperature. 

Any composition at a given temperature represented by points on the left of the curve AC or the right of the curve CB consists of only one layer. All compositions between pure water and point A yield a solution of phenol in water. Within the dome shaped area ACB, the system is heterogenous and two liquid phases exist, while in the area outside the dome only a single liquid layer, i.e., a homogeneous system exists. The upper critical solution temperature may, therefore be defined as the temperature above which the two partially miscible liquids become miscible in all proportions. For phenol-water system the temperature is 339 K. 

The critical solution temperature is affected considerably by the presence of foreign substances. A foreign substance soluble in only one of the liquids decreases the mutual solubility resulting in an increase in the critical solution temperature. For example, 0.15 M KCl raises the critical solution temperature of phenol-water system by about 12 K. On the other hand, if the foreign substance dissolves in both the liquids uniformly, the mutual solubility is increased and the critical solution temperature is lowered. For example, 0.083 M sodium oleate decreases the critical solution temperature of phenol-water system by 9.3 K. 

The upper critical solution temperature (CST) is that temperature above which the two liquids become completely miscible in all proportions. Phenol-water system shows a lower CST. 

Discuss the phenol-water system. Sketch the variation of mutual solubility of this system with temperature. 

Two-phase Systems. These are best avoided since they cannot be used straight after mixing and are, moreover, extremely sensitive to temperature changes. In most cases, it suffices to use a solvent mixture in which the organic phase is nearly saturated with water. We have foimd, for instance, that the i2/-values in the phenol-water system change only slightly if water-saturated phenol, containing about 71% phenol, w/w, is replaced by 80% phenol we therefore use 76% phenol (Table 178). 

Figure 6.6 A diagrammatic representation of the cetyltrimethylammonium bromide-phenol-water system. (After Prins [44]). Horizontal arrow shows increasing phenol concentrations and passage from solutions containing spherical micelles, through solutions containing asymmetric micelles to a molecular dispersion on breakdown of the micelles.

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