Activity coefficients alcohol water mixtures

A modified local composition (LC) expression is suggested, which accounts for the recent finding that the LC in an ideal binary mixture should be equal to the bulk composition only when the molar volumes of the two pure components are equal. However, the expressions available in the literature for the LCs in binary mixtures do not satisfy this requirement. Some LCs are examined including the popular LC-based NRTL model, to show how the above inconsistency can be eliminated. Further, the emphasis is on the modified NRTL model. The newly derived activity coefficient expressions have three adjustable parameters as the NRTL equations do, but contain, in addition, the ratio of the molar volumes of the pure components, a quantity that is usually available. The correlation capability of the modified activity coefficients was compared to the traditional NRTL equations for 42 vapor—liquid equilibrium data sets from two different kinds of binary mixtures (i) highly nonideal alcohol/water mixtures (33 sets), and (ii) mixtures formed of weakly interacting components, such as benzene, hexafiuorobenzene, toluene, and cyclohexane (9 sets). The new equations provided better performances in correlating the vapor pressure than the NRTL for 36 data sets, less well for 4 data sets, and equal performances for 2 data sets. Similar modifications can be applied to any phase equilibrium model based on the LC concept. 

In this Article, a modified LC expression is suggested. This modification is a result of the observation that the traditional expressions for the LCs " are inconsistent with the expressions for the excesses around molecules in ideal binary mixtures (see the next section). The new LCs will be used to obtain expressions for the activity coefficients of binary mixtures using the NRTL equations for illustration. The traditional and corrected NRTL equations wUl be used to correlate the vapor—liquid equilibria (VLE) for alcohol + water binary mixtures and binary mixtures containing benzene, hexafiuorobenzene, toluene, and cyclohexane. It is shown that the modified LCs provide a moderate improvement of the NRTL results. 

Figure 5.3. Activity coefficients of water-normal alcohol mixtures at 25°C. (a) Alcohols in water, (h) Water in alcohols.

Some organic compounds can be in solution with water and the mixture may still be a flammable mixture. The vapors above these mixtures such as ethanol, methanol, or acetone can form flammable mixtures with air. Bodurtha [39] and Albaugh and Pratt [47] discuss the use of Raoult s law (activity coefficients) in evaluating the effects. Figures 7-52A and B illustrate the vapor-liquid data for ethyl alcohol and the flash point of various concentrations, the shaded area of flammability limits, and the UEL. Note that some of the plots are calculated and bear experimental data verification. 

Using the Wilson equation, calculate the activity coefficients for isopropyl alcohol (IPA) and water in a mixture of IPA, methanol, water, and ethanol composition, all mol fraction ... 

If, for example, a mixture of ethanol and water is distilled, the concentration of the alcohol steadily increases until it reaches 96 per cent by mass, when the composition of the vapour equals that of the liquid, and no further enrichment occurs. This mixture is called an azeotrope, and it cannot be separated by straightforward distillation. Such a condition is shown in the y — x curves of Fig. 11.4 where it is seen that the equilibrium curve crosses the diagonal, indicating the existence of an azeotrope. A large number of azeotropic mixtures have been found, some of which are of great industrial importance, such as water-nitric acid, water-hydrochloric acid, and water-alcohols. The problem of non-ideality is discussed in Section 11.2.4 where the determination of the equilibrium data is considered. When the activity coefficient is greater than unity, giving a positive deviation from Raoult s law, the molecules of the components in the system repel each... 

The treatment has been extended to dioxan/water and dioxan/alcohol mixtures, where the concentration of self-associated alcohol has to be calculated from activity coefficient data. It was found that alcoholysis of 4-nitro-benzoyl chloride in ether and dioxan can be accounted for solely on the grounds of specific solvation, but in the case of acetone some of the reaction proceeds by a mechanism without specific solvation, possibly due to dielectric solvation of the transition state. Table 24 shows the relative reactivities of associated alcohol in several solvents. Hudson et al.l72b propose that in carbon tetrachloride the smallest associate is probably the trimer whereas in the ethers the corresponding associate has an open structure, viz. 

Electromotive force measurements of the cell Pt, H2 HBr(m), X% alcohol, Y% water AgBr-Ag were made at 25°, 35°, and 45°C in the following solvent systems (1) water, (2) water-ethanol (30%, 60%, 90%, 99% ethanol), (3) anhydrous ethanol, (4) water-tert-butanol (30%, 60%, 91% and 99% tert-butanol), and (5) anhydrous tert-butanol. Calculations of standard cell potential were made using the Debye-Huckel theory as extended by Gronwall, LaMer, and Sandved. Gibbs free energy, enthalpy, entropy changes, and mean ionic activity coefficients were calculated for each solvent mixture and temperature. Relationships of the stand-ard potentials and thermodynamic functons with respect to solvent compositions in the two mixed-solvent systems and the pure solvents were discussed. 

Gutbezahl and Grunwald considered liquid-junction potentials between a solution of aqueous potassium chloride and solutions of acids in ethanol-water mixtures both theoretically and experimentally. They concluded that for mixtures containing up to 33% ethanol the liquid-junction potential should be 6 mV or less. For solvents containing higher percentages of alcohol, the liquid-junction potential increases rapidly—25 mV for 50%, 44 mV for 65%, and 75 mV for 80% ethanol. These numerical values should not be interpreted too literally, particularly as the composition approaches 100% ethanol. Calculated liquid-junction potentials contain an indeterminate term that involves all quantities other than those arising from unequal transfer activity coefficients (such as dipole orientation effects). 

As already mentioned, the Krichevsky equation (eq 1) is valid when the binary mixtures 1—2 and 2—3 (gas solute/pure solvents) and the ternary mixture 1—2—3 are ideal. However, these conditions are often far from reality. Let us consider, for example, the solubility of a hydrocarbon in a water—alcohol solvent (for instance, water—methanol, water—ethanol, etc.). The activity coefficient of propane in water at infinite dilution is 4 X 10 , whereas the activity coefficients of alcohols and water in aqueous solutions of simple alcohols seldom exceed 10. It is therefore clear that the main contribution to the nonideality of the ternary gas-binary solvent mixture comes from the nonidealities of the gas solute in the individual solvents, which are neglected in the Krichevsky equation. 

The problem of solvation of dextrins is still unsolved in general. It was documented that even such commonly used solvents as ethanol exhibit an unusually strong interaction with dextrin, as determined by H-n.m.r. spectroscopy. The interaction of dextrin with normal alcohols from methanol to 1 -hexanol was studied by determination of their infinite-dilution activity-coefficients in dextrin - water mixtures. " Studies carried out on the adsorption of dextrin on oxidized coal revealed that hydrophobic moieties in dextrin are involved in that process. The possibility of hydrophobic bonding has made dextrin an interesting component of the media used for flotation. Dextrin acts as a flocculant (depressant). When combined with a proper additive providing action by means of hydrogen bonding, reverse flotation... 

Banneijee (h) determined the solubilities of mixtures of several chlorobenzenes with one another and with toluene and benzyl alcohol. The results agreed satisfactorily with predictions from equation (8). The water phase activity coefficients were obtained from the pure component solubilities via equation (6). The organic phase activity coefficients for mixtures of chlorobenzenes with hydrocarbons were predicted by the UNIFAC group contribution method. Mixtures containing only chlorobenzenes were essentially ideal, Yi = When benzyl alcohol was used as a cosolute, the agreement with equation (8) was improved by using UNIFAC to predict the activity coefficient in the aqueous phase. 

Diedrichs, A. and J. Gmehling. 2011. Solubility calculation of active pharmaceutical ingredients in alkanes, alcohols, water and their mixtures using various activity coefficient models. Industrial and Engineering Chemistry Research. 50, 1757. 

V. V. Aleksandrov and Y. V. Sych, Zh. Fiz. Khim., 46, 812 (1972). Thermodynamic properties of alkali metal iodides in iso propyl alcohol and its mixtures with water. Standard electromotive forces and activity coefficients. 

Scott and Simpson studied the adsorption of ahphatic alcohols, aldehydes and carboxylic acids in binary mixtures with water by ODS-2 sUica [9]. They found that the distribution coefficient increases exponentially with the carbon number of the moderator. When using an aliphatic moderator having a chain length of four or five carbon atoms, the surface of a bonded phase could be completely covered with a monolayer. They stated further that the chromatographic characteristics of the surface could be changed by choosing appropriately active groups. 

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