2.1 Aniline (benzenamine) + water system

This system belongs to type II [12]. The solubilities for this system have been compiled and critically evaluated in SDS-96-3 [13] (hereafter, the abbreviation SDS-X-Y defines “Solubility Data Series, Volume X, Part Y”; if Volume X does not have separate parts, its abbreviation is contracted to SDS-X). The literature sources associated with the LLE data for this system are listed in Supplement 1A. Two additional sources [14], [15] have appeared since this SDS publication, and one publication [16] was not included in the SDS-96-3 evaluation. These additional data are in fair agreement with the earlier work, except for the solubility of aniline in water from Ref. [15], which looks erroneous (Fig. 1) and, hence, was rejected. An additional validation has been done with an NRTL equation that shows fair agreement with other available data such as VLE pressures and activity coefficients (Supplement 1B).

Fig. 1:

Experimental mole-fraction LLE data (symbols) for aniline (1) + water (2) in a composition-stretched representation [17] vs. the SDS equation (line) [13]. The rejected solubilities of aniline in water from Ref. [15] are shown as red triangles. The points, which are designated as doubtful in the SDS-96-3 evaluation, are not shown in the figure.

The equations recommended in the SDS-96-3 evaluation for smoothing LLE data for aniline + water (Eqs. (1) and (2) in reference [13]) have been adopted in this work. For the solubility of aniline (1) in water (2):

where x₁ is the mole fraction of aniline; x_c is the mole fraction of aniline at the upper consolute point; T is the temperature in K; T_c is the upper consolute temperature in K; and a₁, a₂, and a₃ are empirical parameters;

For the solubility of water (2) in aniline (1):

where x₂ is the mole fraction of water; x_c, T, and T_c have already been defined for Eq. (1); and b₁, b₂, and b₃ are empirical parameters.

The parameters for Eqs. (1) and (2) taken from SDS-96-3 are listed in Table 1. The smoothed LLE solubility values with the corresponding confidence intervals are given in Table 2.

2.2 Phenol + water system

This mixture belongs to type II [12] and has been compiled and critically evaluated in SDS-91-1 [18]. The literature sources associated with the LLE data for this system are listed in Supplement 1D, including additional sources that were not cited in the SDS-91-1 evaluation or were published after this evaluation. These additional data are in fair agreement with the earlier publications and evaluation (Fig. 2). Most of the deviant data on phenol solubility in water originate from one source [19], were reported in graphical form only, and have been denoted as doubtful in Ref. [18]. An additional validation has been attempted with an NRTL equation that shows fair agreement with other available property data such as SLE (Supplement 1E), although SLE data are to some extent ambiguous. The source reporting doubtful LLE data [19] also deviates from the SLE data sources. The formation of an inter-component compound 2C₆H₅OH·H₂O has been claimed in several reports summarized in Ref. [20] and is consistent with SLE data from Ref. [21]. Thus, LLE at T < 289 K may be supercooled and such data should be used with caution.

Fig. 2:

Experimental mole-fraction LLE data (symbols) for phenol (1) + water (2) in a composition-stretched representation [17] compared with the SDS equation (line) [18]. The data from Ref. [19] are shown as red triangles (the data points for log₁₀(x₁/x₂) < −1.3, i.e., for x₁ < 0.05, from that source were not considered reliable in Ref. [18]).

The equations recommended in the SDS-91-1 evaluation for smoothing LLE data for phenol + water (Eqs. (8) and (9) in reference [18]) have been adopted in this work: Eq. (1) for the mole-fraction solubility of phenol in water and Eq. (2) for the mole-fraction solubility of water in phenol with the parameters listed in Table 3. The smoothed LLE solubilities with the corresponding confidence intervals are given in Table 4.

2.3 Nitromethane + water system

This mixture belongs to type II [12] and has been discussed in detail in SDS-71 [22]. The literature sources associated with the LLE data for the system are listed in Supplement 1F, including additional sources that were not cited in the SDS-71 evaluation or were published after this evaluation. Most of the data are consistent, with the prominent exception of Ref. [23] (Fig. 3).

Fig. 3:

Experimental mole-fraction LLE data (symbols) for nitromethane (1) + water (2) in a composition-stretched representation [17], compared with the evaluation with parameters from Table 5 (line). The rejected data from Ref. [23] and three obviously erroneous data points from Ref. [24] are shown as red triangles.

We fitted Eq. (1) for the mole-fraction solubility of nitromethane in water and Eq. (2) for the mole-fraction solubility of water in nitromethane with the use of all LLE data (excluding three obviously erroneous data points from Ref. [24] and the data from Ref. [23], despite a part of these data being consistent with the other sources). The parameters are listed in Table 5. The reliability of the LLE data was corroborated by the fair consistency with other properties, as demonstrated with a UNIQUAC equation in Supplement 1G. The smoothed LLE solubilities with the corresponding confidence intervals are given in Table 6. As the LLE data at T < 290 K are from two sources that are not fully consistent for the nitromethane-rich phase and as we cannot reliably extrapolate the other data to this region, the recommended values for this system are restricted to temperatures above 290 K.

The system butan-1-ol + water may serve as a reserve test mixture for aqueous LLE measurements because it was thoroughly analyzed in SDS-82-1 [25] and the corresponding empirical equations were provided there. It has not been selected as primary in this work because the data scatter is larger for this system in comparison to the selected mixtures. However, butan-1-ol is readily available as a high-purity sample, which may justify using the reserve butan-1-ol + water system.

3.1 Cyclohexane + methanol system

The most-studied non-aqueous system with consistent data is cyclohexane + methanol, which belongs to type II [12] and has been discussed in SDS-56 [26]. The literature sources associated with the LLE data for this system are listed in Supplement 1H, including additional sources that were not cited in the SDS-56 evaluation or were published after this evaluation. We fitted Eq. (1) for the mole-fraction solubility of cyclohexane in methanol and Eq. (2) for the mole-fraction solubility of methanol in cyclohexane with the use of all LLE data. The parameters are listed in Table 7. Although the reported values of the upper consolute temperature range from 318 to 322 K, a relatively low value of 318.5 K from Ref. [27] has been selected because higher values from other sources may be caused by water impurity according to the studies [28], [29]. The selected data and the smoothing equation are shown in Fig. 4. The largest deviations (up to a mole fraction of 0.02, which is 30 % of the “guest component” content, or up to 4 K near the upper consolute temperature) come from three recent sources [30], [31], [32], but there are only a few deviating points, and numerous other sources are consistent. The whole data set was rejected only for Ref. [32]. The lower limit of the equation was selected to be the monotectic temperature of 275 K, determined graphically in Ref. [26]. The smoothed LLE solubilities with the corresponding confidence intervals are given in Table 8.

Fig. 4:

Experimental mole-fraction LLE data (symbols) for cyclohexane (1) + methanol (2) in a composition-stretched representation [17] compared with the evaluation with parameters from Table 7 (line). The rejected data from Ref. [32] are shown as red triangles.

A promising system with less-hygroscopic components is acetonitrile + octane, which has been discussed in SDS-78 [33]. Unfortunately, the reported solubility of acetonitrile in octane relative to that in hexane and decane (involving data from the same sources) changes inconsistently and so needs additional investigation before it can be considered as a suitable reference system.

4.1 Methylbenzene (toluene) + water and ethylbenzene + water systems

Arguably the most studied binary mixture with low solubility is benzene + water, reviewed in SDS-81-2 [34]. However, this system has been excluded from consideration in the current project due to the toxicity of benzene and restrictions on its use. Hence, we selected two other similar well-studied mixtures, toluene + water and ethylbenzene + water, for this low-solubility category. The LLE data were discussed in SDS-81-5 [35] (for toluene + water) and SDS-81-6 [36] (for ethylbenzene + water). The literature sources associated with the LLE data for these two systems are listed in Supplements 1I and 1J, respectively, including additional sources that were not cited in the SDS-81-5 and SDS-81-6 evaluations or were published after those evaluations (Figs. 5 and 6).

Fig. 5:

Experimental mole-fraction LLE data (symbols) for toluene (1) + water (2) in a composition-stretched representation [17] compared with the SDS equations (lines) [35]. A deviating data set from Ref. [37] is shown as red triangles. Other rejected data believed erroneous are shown in Fig. S12 in Supplement 1I.

Fig. 6:

Experimental mole-fraction LLE data (symbols) for ethylbenzene (1) + water (2) in a composition-stretched representation [17] compared with the SDS equations (lines) [36]. Rejected data believed erroneous are shown in Fig. S13 in Supplement 1J.

The equations recommended in the SDS-81-5 [35] and SDS-81-6 [36] evaluations for smoothing LLE data for hydrocarbon + water have been adopted in this work. For the mole-fraction solubility of hydrocarbons in water (x₁):

and for the mole-fraction solubility of water in hydrocarbons (x₂ = 1 − x₁):

where T_ms is the temperature of the minimum mole-fraction solubility of the hydrocarbon (x_min,1); T is the temperature in K; D, d₁, d₂, d₃, and d₄ are empirical parameters; and T_r = T/T_c, where T_c is an adjustable parameter, which is close to the three-phase liquid–liquid–gas critical temperature (also known as the upper critical end point or UCEP, which is the maximum temperature of L1 + L2 + G coexistence).

The parameters for Eqs. (3) and (4) for toluene + water and ethylbenzene + water taken from SDS-81-5 and SDS-81-6 are summarized in Table 9 and the evaluations are shown in Figs. 5 and 6. The toluene + water system belongs to type III [12] with an UCEP of 556.1 K (or 558 K [38]) that defines the upper temperature limit of validity of Eqs. (3) and (4). The ethylbenzene + water system shows a similar behavior, with reported UCEP values of 574 K [38] and 568 K [39]. However, the upper limit of the temperature range for this system is restricted to 450 K (below UCEP) due to a limited number of experimental points above 450 K, which the fitting procedure in SDS-81-6 was based on.

The change in the LLE behavior from benzene to toluene looks reasonable, as does the subsequent change to ethylbenzene (Fig. 7). A qualitative consistency test using other properties is given in Supplement 1K. The smoothed LLE solubilities are given in Tables 10 and 11.

Fig. 7:

Variation in the LLE behavior for the systems water (2) + benzene (1), toluene (1), and ethylbenzene (1). The data points and lines are red for water + ethylbenzene (squares), green for water + toluene (circles), and blue for water + benzene (triangles). Symbols are experimental values; lines are the SDS equations [34], [35], [36]. The experimental data for benzene + water were taken from Ref. [34].

5.1 Hexan-1-ol + 1-hexyl-3-methyl-1H-imidazolium bis[(trifluoromethyl)sulfonyl]amide system

Although LLE measurements for ionic liquids are generally less accurate and more affected by hydrolysis, impurities, and moisture, they are a popular object of research nowadays. Consequently, we have selected one of the most studied and stable mixtures, hexan-1-ol + 1-hexyl-3-methyl-1H-imidazolium bis[(trifluoromethyl)sulfonyl]amide (abbreviation [C₆mim][NTf₂] used hereafter), which belongs to type II [12] and was employed in an earlier IUPAC project [40]. The data sources are listed in Supplement 1L. A smoothing equation was proposed in Ref. [40] for the LLE for this mixture. We have updated this LLE representation by including new data [41], [42], [43], [44] and using fitting Eqs. (1) and (2). The parameters are listed in Table 12. Unfortunately, one report [44] deviates from all other investigations, which challenges the accuracy of the LLE data (Fig. 8). We considered the presence of water and hydrolysis products as possible reasons for such a disagreement. Although water would probably make the upper consolute temperature higher, as for cyclohexane + methanol above, it is not likely that all experiments except one had the same amount of water impurity. Other impurities such as hydrolysis products could lower the consolute temperature, so we propose this recommendation as provisional and suggest exploration of the effects of the third components, especially water, on the LLE for this system, as was done for some non-aqueous mixtures forming LLE [45]. The smoothed LLE solubilities with the corresponding confidence intervals are given in Table 13. Because accurate extrapolation below 288 K cannot be done on the basis of the available data, the recommended values for this system are restricted to temperatures above 288 K.

Fig. 8:

Experimental mole-fraction LLE data (symbols) for hexan-1-ol (1) + [C₆mim][NTf₂] (2) in a composition-stretched representation [17] compared with the evaluation with parameters from Table 12 (line). The data from [44] are shown as red triangles.