Immiscible solvents distillations

Raoult s law is the basis for steam distillation. Chapter 5, and its reverse, immiscible solvents distillation, also in Chapter 5. When the sum of the individual vapor pressures reaches 760 torr at sea level, the mixture will boil. By adding steam to a liquid, the vapor pressure of the water is quite high compared to the other components, and they will distill at a much lower temperature. This can minimize destruction caused by overheating, and is usually applied to liquids immiscible with water. The reverse, using a liquid such as toluene, can be used to remove large amounts of water from something like a watermelon section prior to additional chemical analysis. 

Emulsion formation is generally never a problem in immiscible solvents distillation. Can you explain why this is the case ... 

When the distribution ratio is low, continuous methods of extraction are used. This procedure makes use of a continuous flow of immiscible solvent through the solution if the solvent is volatile, it is recycled by distillation and condensation and is dispersed in the aqueous phase by means of a sintered glass disc or equivalent device. Apparatus is available for effecting such continuous extractions with automatic return of the volatilised solvent (see the Bibliography, Section 9.10). 

Probably the most common distillation method used as a form of sample preparation for chromatographic analysis is steam distillation [31,32]. Solvent extraction and gas phase stripping methods are generally inefficient procedures for isolating polar, acidic, or basic compounds in an aqueous matrix due to the low efficiency of water immiscible solvents for the extraction of these compounds and their low volatility and high water affinity which results in a very slow transfer to the gas phase using... 

The one-phase liquid system is more frequently encountered since many organic reactions are carried out in solution. Direct fractional distillation may separate the product, if it is a liquid, from the solvent and other liquid reagents, or concentration or cooling may lead to direct crystallisation of the product if this is a solid. However, it is often more appropriate, whether the required product is a liquid or solid, to subject the solution to the acid/base extraction procedure outlined above and considered in detail on p. 162. This acid/base extraction procedure can be done directly if the product is in solution in a water-immiscible solvent. A knowledge of the acid-base nature of the product and of its water solubility is necessary to ensure that the appropriate fraction is retained for product recovery. In those cases where the reaction solvent is water miscible (e.g. methanol, ethanol, dimethylsulphoxide, etc.) it is necessary to remove all or most of the solvent by distillation and to dissolve the residue in an excess of a water-immiscible solvent before commencing the extraction procedure. The removal of solvent from fractions obtained by these extraction procedures is these days readily effected by the use of a rotary evaporator (p. 185) and this obviates the tedium of removal of large volumes of solvent by conventional distillation. 

Moisture-Distillation Method (Ca 2a-45) determines only moisture in triacylglyc-erols and emulsions by distillation with an immiscible solvent (toluene). 

Principle This method determines water by distillation of a sample with an immiscible solvent, usually toluene. 

During operation, the apparatus shown in Fig. 2 requires attention beyond occasional draining of the water trap. It is suitable for a number of preparations in which water is removed by distillation with an immiscible solvent it functions only when the condensate separates into two phases, of which water is the more dense. 

For quatemisation products that are IL starting materials rather than ILs themselves, traditional purification techniques such as, distillation, recrystallisation, immiscible solvent washing and stirring over activated carbon can be used. These traditional techniques are often so effective that the previously mentioned precautions are not necessary. 

In the laboratory, a multi-stage liquid-liquid extraction can be performed by a simultaneous distillation-extraction process according to Likens-Nickerson [29] (Fig. 2.10). Here, the liquid matrix with the solute in one flask is evaporated together with an immiscible solvent in a second flask. Extraction takes place in the vapour phase where an intensive distribution of both phases is ensured. The condensed vapours from the two phases are separated via a siphon using their different densities and their reintroduction into the original flasks. As the distillation process is continued, extraction is repeated until the solute is exhausted in the original matrix. This method is very useful when traces of non-volatile solutes are present, which are only partly miscible... 

Vacuum can be applied in order to reduce thermal exposure. The cooling funnel requires a deep-freezing mixture. This extraction method can easily be transferred onto an industrial scale. An important application is essential oils in water where steam distillation is carried out. For the distillative extraction process, different water-immiscible solvents are used. Thermal deterioration and retrieval ratio in the solvent have been studied intensively for fragrance materials [30],... 

Simultaneous Steam Distillation/Extraction An elegant apparatus was described by Nickerson and Likens ( 5) for the simultaneous steam distillation and extraction (SDE) of volatile components. This device has become one of the mainstays in the flavor field. In this apparatus, both the aqueous sample and water-immiscible solvent are simultaneously distilled. The steam which contains the aroma chemicals and the organic solvent are condensed together, and the aroma compounds are transferred from the aqueous phase to the organic phase. Typical solvents used are diethyl ether, pentane or a mixture thereof normal extraction times are one to two hours. 

One part by weight of compound II in the form of a salt, e.g. ergotamine hydrochloride, is heated with four parts by weight of anhydrous hydrazine for one hour at 90° C. The reaction mixture is then diluted, preferably with water, the excess hydrazine and water distilled off azeotropically, and the residue shaken between aqueous tartaric acid and an inert water-immiscible solvent, e.g. chloroform or ether. The aqueous phase is made alkaline and the final product taken up in a water-immiscible solvent, e.g. chloroform. 

Citrus oils contain up to 95 % monoterpene hydrocarbons (usually (-t)-limonene, but others as well e.g., lemon oil also contains y-terpinene and /l-pinene). The important aroma-determining components of citrus oils are functionalized terpenes and aliphatic compounds (predominantly carbonyl compounds and esters), present only in relatively low concentrations [358, 358a]. Thus, several methods are employed to concentrate citrus oils on an industrial scale. The monoterpene hydrocarbon content is decreased by distillation, liquid-liquid partitioning between two immiscible solvents, or absorption on a carrier such as silica gel. The deterpenized concentrates are marketed under the name Citrus oil a-fold, depending on the concentration factor [358b]. 

The types of distillation processes to be examined will be simple, fractional, steam, immiscible solvent, azeotropic, extractive, vacuum, molecular, entrainer sublimation, and freeze drying. 

In order to eliminate interferences, to provide suitable selectivity in the measurement, or to preconcentrate the analyte for more sensitive or accurate measurement, the analyst must often perform one or more separation steps. It is preferable to separate the analyte away from the sample matrix, in order to minimize losses of the analjde. Separation steps may include precipitation, extraction into an immiscible solvent, chromatography, dialysis, and distillation. 

The solvent process uses about 10 per cent of a water-immiscible solvent in the reaction mixture. This solvent promotes better control by reducing the viscosity and provides a good means of water removal by azeotropic distillation. 

Up to this point only the steam distillation of sparingly miscible solvents such as hydrocarbons or chlorinated hydrocarbons has been considered. The evaporation, using direct steam injection, of fiiUy water-miscible solvents with atmospheric boiling points below 100 °C is different in principle and is commonly practised. Whereas in the case of immiscible solvents, dry distillation was shown to require less heat and therefore would be more attractive, unless a difficult residue made it hard to carry out, there is no such advantage in this case. Any used solvent of this sort, whether or not it contains a difficult residue, can be evaporated by injecting steam into it, thus avoiding the need to have a reboiler or evaporator. This can be a useful technique if the solvent contains, for instance, halide salts that would require a heat exchanger made of exotic metals. 

The disadvantage of such a course of action is that water builds up in the residue and will be present in the vapour leaving the still. For an immiscible solvent the distillate will separate into two phases after condensing and because of the shape of the vapour-liquid equilibrium (VLB) diagram (Fig. 5.4) no fractionating column is needed. However, a water-miscible solvent will have to be freed of water by fractionation or some other means. Further, there are only two solvents in this class that do not form azeotropes with water—methanol and acetone. The latter is difficult to separate from water by fractionation below a level of about 1.5% w/w water so that only methanol can be mixed with water without a... 

The mechanism for purification of recovered solvent depends on the nature of the material collected and the purity required. Where steam is used with water-immiscible solvents (e.g. toluene), purification can be done by simple decantation, but for water-soluble solvents, distillation is needed. Where water forms an azeotrope with the solvent (e.g. ethyl acetate) further complexity and cost is added to separate the mixture. With nitrogen desorption, purification can be simpler, but with mixed solvents which form azeotropes (e.g. ethyl acetate and ethanol) separation is still very difficult and costly. In this case, the cost of equipment needed to distil and purify the solvent can be higher than that for the recovery unit. 

The residual bitumen for properties determination is extracted by distillation procedure. The azeotropic distillation (by means of a carrier vapour from a water-immiscible solvent-carrier liquid) is used only for the determination of the water content in bitumen emulsion. 

---