Partially Miscible Liquids: Although three types of liquid/liquid systems are commonly encountered liquid-liquid systems are mainly divided into two categories depending on the solubility of one substance in the other. The categories are complete miscibility and partial miscibility. Miscibility is the common solubilities of the components in liquid-liquid systems. Partial miscibility is when the substances only mix partially. When mixed, there are two layers formed each layer containing some of both liquids. Of these two mixed layers, each layer contains some of both the liquids, for example, phenol and water. Some liquids are practically immiscible (for example, water, and mercury), whilst others (for example, water and ethyl alcohol or acetone) mix with one another in all proportions.
The mutual solubility or miscibility of two liquids is a function of temperature and composition. When two liquids (liquid A and liquid B) are partially soluble in each other, two liquid phases can be observed. At equilibrium, each phase contains liquid A and liquid B in amounts that reflect their mutual solubility. Some systems are miscible (i.e. they form a one-phase liquid) at high temperatures, but separate into two liquid phases at lower temperatures. These systems have an upper consolute temperature, TUCT, in a plot of temperature versus mole fraction. Other systems are miscible at low temperatures but separate into two phases at higher temperatures giving rise to a lower consolute temperature, TLCT.
Oil and water don’t mix. Pouring 10 mL of olive oil into 10 mL of water results in two distinct layers, clearly separated by a curved meniscus. Each layer has the same volume and essentially the same composition as the original liquids. Because very little mixing occurs apparently, the liquids are called “immiscible”. For example, pouring grain alcohol into the water results in a single liquid phase. No meniscus forms between the alcohol and the water and the two liquids are considered “miscible”. Nearly any pair of liquids are miscible if only a trace amount of one of the liquids is present.
Many liquid mixtures fall between these two extremes. Two liquids are “partially miscible” if shaking equal volumes of the liquids together results in a meniscus visible between two layers of liquid, but the volumes of the layers are not identical to the volumes of the liquids originally mixed. For example, shaking water with certain organic acids results in two separate layers, but each layer contains water and acid (with one layer mostly water and the other, rich in acid.) Liquids tend to be immiscible when attractions between like molecules are much stronger than attractions between mixed pairs. Many examples are known, however, in which the liquids are partially miscible with one another. If, for example, water be added to ether or if ether be added to water and the mixture was shaken, the solution will form up to a certain point; beyond this point, further addition of water on the one hand, or of ether on the other, will result in the formation of two liquid layers, one consisting of a saturated solution of water in ether and the other a saturated solution of ether in water. Two such mutually saturated solutions in equilibrium at a temperature are called conjugate solutions. A conjugate system has two partially miscible liquids in contact with each other. The proportionate quantities of these liquids are responsible for their existence as two liquids in contact with. Under this condition, a saturated solution of one liquid in other or vice-versa is formed. The miscibility of such solution mixture can be increased by increasing temperature. For example, phenol – water, nicotine – water, triethanolamine – water, etc.
The phenol-water solution is characterized by increasing mutual solubility with a temperature rise. Thus, when phenol is added to water at the ordinary temperature, a homogeneous liquid is produced. When the concentration of the phenol in the solution has risen to about 8 %, the addition of more phenol results in the formation of a second liquid phase, which may be regarded as a solution of water in phenol. If now the temperature is raised, the second liquid phase will disappear and more phenol must be added to produce a separation of the liquid into two layers. By increasing the amount of phenol in this way and observing the temperature at which the two layers disappear, the so-called solubility curve of phenol in water may be determined. Similarly, the solubility curve of water in liquid phenol may be obtained, and it is found that the solubility also increases with the rise of temperature. Since with rising of temperature the concentration of water in the phenol layer and of phenol in the water layer increases. The compositions of the two conjugate solutions become more and more nearly the same and at a certain temperature, the two solutions become identical in composition. The temperature at which the two layers become identical in composition and are one layer is known as the critical solution temperature or the consolute temperature of the system. Above this temperature, the two liquids are miscible in all proportions. If the resulting mixture is represented by a point in the area enclosed by the solubility curve, separation into two layers will take place, whereas if the total composition of the mixture and the temperature are expressed by a point lying outside the solubility curve a clear homogeneous solution will result.
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