Critical solution temperature, phenol-water systems

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. 

As the temperature is raised, the solubility of phenol in water increases, whereas that of water in phenol also increases. Ultimately at a certain temperature, the two conjugate solutions change into one homogeneous solution. This temperature is known as critical solution temperature or consolute temperature. The value of consolute temperature for this system is 68.3°, and the composition is 33% phenol and 67% water. Above 68.3°, the two liquids are completely miscible in all proportions. The variation of mutual solubility of water and phenol with temperature is shown in fig. (16). The solubility of phenol in water increases with rise of temperature along die curve AB, while the solubility of water in phenol increases along CB. The two curves do not intersect each other, but meet at a certain point B, known as C.S.T. 

The critical solution temperature of the water + phenol system is 65 °C. Addition of 1% of naphthalene raises the critical temperature to 82 °C naphthalene is soluble in phenol, insoluble in water. 

When the third bst ce dissnlms m only one of the two liquids the mutual solubility of the latter is diminished, and the temperature at which the system becomes homogeneous is raised, in the case of liquids having an upper critical solution temperature, and lowered in the case of liquids having a lower critical solution temperature. The elevation (or the lowering) of temperature depends not only on the nature and amount of the added substance, but also on the composition of the liquid mixture. When the two liquids are present in the proportions of the critical composition, it is found that, for concentrations of the addendum (non-electrolyte) less than about 0 i molar, the elevation (or depression) of the critical solution temperature is nearly proportional to the amount added. The elevation (or depression) of the critical solution temperature for small equi-molecular quantities of different substances is, however, not constant, but depends on the nature of the substance added. In the following tables are given the values for the elevation of the critical solution temperature of phenol and water by naphthalene (soluble only in phenol) and by potassium chloride (soluble only in water). E represents the molecular elevation of the critical solution temperature —... 

B. Systems Formed of Two Liquid Phases only. Solutions of Liquids in Liquids. Partial or limited miscibility. Phenol and water. Methylethylketone and water. Triethylamine and water. General form of the concentration-temperature curve. Influence of pressure on the critical solution temperature. Influence of foreign substances on the critical solution temperature. Presence of vapour phase. 

Experiment 14.1 Demonstration of the presence of a miscibility gap with the help of the systems phenolfwater and triethylaminel water. When heated, a heterogeneous mixture of phenol and water will become a homogeneous solution when the upper critical solution temperature (approx. 339 K) is exceeded. However, even at higher temperatures, a heterogeneous mixture of triethylamine and water remains separated, but when cooled with ice to below the lower critical solution temperature (approx. 292 K), it will become a homogeneous solution. The phenol-water mixture, however, continues to consist of two phases after cooling. 

The Txx diagram shown in Figure 8.20 is typical of most binary liquid-liquid systems the two-phase curve passes through a maximum in temperature. The maximum is called a consolute point (also known as a critical mixing point or a critical solution point), and since T is a maximum, the mixture is said to have an upper critical solution temperature (UCST). A particular example is phenol and water, shown in Figure 9.13. At T > T, molecular motions are sufficient to counteract the intermolecular forces that cause separation. 

Systems with an Upper Critical Solution Temperature. In the case described in Fig. 2.1, which is typified by the system phenol-water, the solubilities of A in J5 and J5 in A increase with increase in temperature, so that at some elevated temperature the two conjugate solutions become identical and the interface between them consequently disappears. This temperature, termed the critical solution temperature (C.S.T.), or conso-lute temperature, occurs at the point M in the figure and represents the temperature above which mixtures of A and B in any proportions form but one liquid phase. Point M is the maximum on the continuous solubility curve but is not ordinarily at the midpoint of composition, nor are the solubility curves ordinarily symmetrical. The C.S.T. is the point where the two branches of the solubility curve merge, and the constant temperature ordinate is tangent to the curve at this temperature. The phase rule may be applied to this significant point ... 

Effect of Impurities on the Critical Solution Temperature. The addition of even a small amount of a third component to a two-liquid system will ordinarily alter the C.S.T. considerably. Thus, for example, the addition of 0.2 per cent of water to glacial acetic acid raises the C.S.T. with cyclohexane from 4.2 to approximately 8.2 C. Useful methods of analysis have been devised based on such observations. For example, the amount of deuterium oxide in water can be estimated by measuring the C.S.T. with phenol and the aromatic hydrocarbon content of petroleum fractions by the C.S.T. with aniline. In general, the C.S.T. will be raised if the added component is highly soluble in only one of the original components (salting out) and lowered if it is highly soluble in both. Such systems properly must be considered as three-component mixtures, however. 

The composition of phenol in water is given by point A while, that of water in phenol is given by B at that temperature. The line joining the two points A and B is called the tie line. Mth increase of temperature as seen from the curve, the composition of the two layers approach each other. At 339 K, the two layers merge into one homogeneous solution. This temperature is called the critical solution temperature (C.S.T) or the consolute temperature of the system. Above this temperature, it is seen that the two liquids are miscible with each other in all proportions and only a single layer of liquid results. 

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. 

The temperature effect on solubility may have different characters depending on the molecular structure of solute. For systems of liquid-amorphous or liquid-liquid polymers, the temperature raise can cause improvement of compatibility. Such systems are considered to have the upper critical solution temperature (UCST). If the system of two liquids becomes compatible at at r ratio at the temperature below the defined critical point, the system is considered to have the lower critical solution temperature (LCST). Examples of a system with UCST are mixtures of methyl ethyl ketone-water (150°C) and phenol-water (65.8°C). An example of a system with LCST is the mixture of water-triethylamine (18°C). There are systems with both critical points, for example, nicotine-water and glyc-erol-benzyl-ethylamine. 

---