Solubilities of Calix [6] arene and 4-tert-Butylcalix [4] arene in

Article pubs.acs.org/jced

Solubilities of Calix[6]arene and 4-tert-Butylcalix[4]arene in Pressurized Hot Water Pavel Karásek, Josef Planeta, and Michal Roth* Institute of Analytical Chemistry of the ASCR, v. v. i., Veveří 97, 60200 Brno, Czech Republic ABSTRACT: Aqueous solubilities of calix[6]arene and 4-tertbutylcalix[4]arene were measured at temperatures of (433 to 533) K and (433 to 513) K, respectively, along the 5 MPa isobar. The relative rates of increase in the aqueous solubility with temperature, (∂ ln x2/∂T)P,σ, for the two solutes differ markedly, with values of 0.042 K−1 for calix[6]arene and 0.080 K−1 for 4-tert-butylcalix[4]arene. The solubility (equilibrium mole fraction) of 4-tert-butylcalix[4]arene is always lower than that of calix[6]arene at the same temperature, possibly reflecting the presence of four bulky 4-tert-butyl groups in the former molecule and also the presence of only four hydroxyl groups in the former molecule compared with six in the latter. Best-fit coefficients of the modified Apelblat equation are also reported for the two calixarenes.

1. INTRODUCTION

2. EXPERIMENTAL SECTION Materials. Calix[6]arene (99 %, CAS no. 96107-95-8) and 4-tert-butylcalix[4]arene (99 %, CAS no. 60705-62-6) were purchased from Alfa Aesar (Karlsruhe, Germany) and used as received. The structures of the two solutes are shown in Figure 1. Acetonitrile (HPLC gradient grade, > 99.9 %) was purchased from Chem-Lab NV (Zedelgem, Belgium), and chloroform (HPLC grade, > 99.8 %) was supplied by Chromservis s.r.o. (Prague, Czech Republic). Caffeine and theobromine, for use as internal standards, were purchased from Sigma-Aldrich (Prague, Czech Republic). Water was purified with an Ultra Clear UV reverse-osmosis system (SG Wasseraufbereitung and Regenerierstation, Barsbüttel, Germany). Immediately before use, water was degassed with a gentle stream of helium (99.995 %, SIAD, Braňany u Mostu, Czech Republic). The pH of water entering the process ranged from 6.95 to 7.01. Apparatus and Procedure. The aqueous solubilities were measured by the dynamic method employing the apparatus described15 and used16−18 before. The dynamic method was employed to prepare the saturated aqueous solution of the solute at the particular temperature and pressure, and a known mass of the solution was collected in a sampling vial and allowed to cool to room temperature. The procedure for generating the hot aqueous solutions was the same as in our previous studies of solid solubilities in pressurized hot water.15−18 In the present work, the dimensions of fused-silica tubing used as the flow restrictor were 1.25 m length and 75 μm i.d. The mass of individual samples of the aqueous solution ranged from (1.65 to 40) g, and the mass flow rate of water

Among all single-component solvents, water has been known to display the largest extent of tunability of the solvent properties through changes in the operating temperature and pressure.1,2 As a result, for example, the aqueous solubilities (mole fractions) of sodium chloride3 and benzene4 at 298 K and 0.1 MPa are 0.1 and 4.07 × 10−4, respectively, whereas at 773.15 K and 30 MPa, the solubility of sodium chloride5 drops by about three decadic orders of magnitude to 1.08 × 10−4 and benzene becomes miscible with water.6 Interactions of hightemperature water with diverse solutes have been the subject of numerous investigations, including a large number of aqueous solubility measurements of organic nonelectrolyte solids.7−11 If necessary, the solvating ability of high-temperature water toward polar or ionic substances can be modified through the use of cosolvents or suitable additives. A possible mechanism of action of the additive can make use of host−guest interactions with the target molecule or ion. Naturally, the additive would have to be sufficiently stable in the aqueous environment under the operating temperature and pressure. Calixarenes (macrocyclic phenol−formaldehyde polycondensates) are an important class of macrocyclic molecules that may serve as hosts to form supramolecular structures through host− guest interactions, resulting in trapping of the guest ion or molecule in the calix-like cavity of the calixarene molecule.12−14 An assessment of the potential prospects of calixarenes as cavitand additives to high-temperature water requires information on their aqueous solubilities at high temperatures. On purpose, we have focused here on calixarenes without hydrophilic substituents. In this contribution, high-temperature aqueous solubilities of calix[6]arene and 4-tert-butylcalix[4]arene are reported. © 2014 American Chemical Society

Received: February 13, 2014 Accepted: June 15, 2014 Published: July 2, 2014 2433

dx.doi.org/10.1021/je500149k | J. Chem. Eng. Data 2014, 59, 2433−2436

Journal of Chemical & Engineering Data

Article

calix[6]arene, whereas theobromine was used with 4-tertbutylcalix[4]arene. HPLC Operating Conditions. An Agilent 1200 series HPLC system (Agilent Technologies, Santa Clara, CA) with an autosampler and a diode array UV/vis detector was equipped with a Merck SeQuant ZIC-HILIC column (150 mm × 2.1 mm i.d., 5 μm particle size, 20 nm pore size; Merck KGaA, Darmstadt, Germany). The injected volume of the sample solution was 10 μL, and the chromatographic runs were carried out in isocratic mode employing an acetonitrile/water mixture (9:1 v/v) as the mobile phase. The detection wavelengths were 220 nm for the calixarene solutes and 270 nm for the internal standards.

3. RESULTS AND DISCUSSION Solubility Data. The aqueous solubilities (equilibrium mole fractions, x2) of the two solutes are listed in Table 1. In order to Table 1. Aqueous Solubilities of the Solutes (x2) and Their Standard Deviations (SD) as Functions of Temperature T and Pressure P solute calix[6]arene

4-tert-butylcalix[4]arene

T/Ka

P/MPab

109·x2

109·SDc

433.2 453.2 473.2 493.2 513.2 533.2 433.2 453.2 473.2 493.2 513.2

4.9 5.0 4.9 5.1 5.4 5.5 5.1 5.1 5.4 5.1 5.1

893 2170 4780 12500 28500 59500 4.43 16.8 73.4 457 2530

33.9 43.1 195 174 555 671 0.530 0.560 4.50 21.0 180

a

Standard uncertainty u(T) = 0.1 K. bStandard uncertainty u(P) = 0.1 MPa. cThe SDs are based on five fractions collected under each set of conditions and are assumed to be equivalent to the standard uncertainties for the solubilities.

disclose the effects of individual structural features on the aqueous solubilities, it was our original intention also to include calix[4]arene and 4-tert-butylcalix[6]arene in the present study. However, with the apparatus and procedure described above, we did not obtain reliable data for either calix[4]arene or 4-tertbutylcalix[6]arene, presumably because of very low solubilities of the two solutes. Variation of Solubility with Temperature. As indicated by simple linear fits of the data from Table 1, the mean values of the relative rates of the increase in the solubility with temperature, (∂ ln x2/∂T)σ, for calix[6]arene and 4-tertbutylcalix[4]arene are 0.042 K−1 and 0.080 K−1, respectively. However, a more detailed insight into the effect of temperature on solubility results from fitting the data in Table 1 with the modified Apelblat equation,19

Figure 1. Structures of (a) calix[6]arene and (b) 4-tert-butylcalix[4]arene.

through the system did not exceed 0.015 g·s−1. Five samples of the hot aqueous solution were collected at each temperature and pressure. Because of very slow phase separation, a period of 1 month was allowed for the solute to aggregate and separate properly from the aqueous medium. Subsequently, water was allowed to evaporate from the sampling vials during another (1 to 8) week period at room temperature, depending on the mass of the collected solution. Then, a pre-estimated amount of chloroform [(1 to 10) mL] was added to the vial, and the closed vial was immersed in an ultrasonic bath for 120 s and then allowed to stand for 2 weeks for complete dissolution of the solute. A known amount [(50 to 800) μL] of the solution was then transferred to a 2 mL vial, and the chloroform was allowed to evaporate because of its incompatibility with the subsequent analysis by high-performance liquid chromatography (HPLC). Finally, 1 mL of acetonitrile containing a suitable amount of the internal standard was added to the vial, and the resultant solution was analyzed by HPLC. Caffeine served as the internal standard in the determination of

ln x 2 = a1 + a 2(T0/T ) + a3 ln(T /T0)

(1)

where T0 = 298.15 K. Figure 2 shows a comparison of the fits of the data from Table 1 with previous results for several aromatic hydrocarbons.15,16 Figure 2 illustrates that the slope (∂ ln x2/ ∂T)σ for 4-tert-butylcalix[4]arene is comparable to those for aromatic hydrocarbons, whereas the slope for calix[6]arene is less steep. The least-squares estimates of the coefficients a1, a2, and a3 for the two calixarenes are listed in Table 2. For all of the 2434



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physical meaning to the coefficients a1, a2, and a3 in eq 1. The values of RT2(∂ ln x2/∂T)P,σ can be estimated by differentiating eq 1 and using the coefficients from Table 2: ⎛ ∂ ln x 2 ⎞ ⎟ = R(a3T − a 2T0) RT 2⎜ ⎝ ∂T ⎠ P , σ

(3)

The results are listed in Table 3. Table 3. Values of RT2(∂ ln x2/∂T)P,σ Calculated from Equation 3 RT2(∂ ln x2/∂T)P,σ/kJ·mol−1

Figure 2. Aqueous solubilities measured in this work: ▽, calix[6]arene; ▼, 4-tert-butylcalix[4]arene. Solubilities of the following compounds are shown for comparison: ○, naphthalene;15 ●, anthracene;15 □, triphenylene;15 ■, p-terphenyl;15 △, 9-phenylanthracene;16 ▲, triptycene.16 The lines show the best fits to eq 1.

solute

calix[6]arene


433.2 533.2 −34.02 16.0 13.53 17.4 28.83 10.8

433.2 513.2 −218.8 31.6 200.6 34.2 164.7 21.7


433.2 453.2 473.2 493.2 513.2 533.2

70.3 75.1 79.9 84.7 89.4 94.2

95.9 123.2 150.6 178.0 205 −

4. CONCLUSION Aqueous solubilities of calix[6]arene and 4-tert-butylcalix[4]arene were measured at temperatures of (433 to 533) K and (433 to 513) K, respectively, along the 5 MPa isobar. To put the resultant solubilities into perspective, they may be compared with those of anthracene at the same temperature.15 Over the temperature range (433 to 473) K, the aqueous solubility of calix[6]arene is 18 to 27 times lower than that of anthracene, whereas the aqueous solubility of 4-tert-butylcalix[4]arene is 1800 to 3600 times lower than that of anthracene. Unfortunately, the measured solubilities are probably too low for the two calixarenes to serve as effective additives to modify the solvent properties of pressurized hot water, at least within the temperature range of the present study.

coefficients, the ratio of the coefficient estimate to the standard deviation of the coefficient estimate can be compared to the pertinent critical values of Student’s t distribution20 to test the hypothesis “the coefficient equals zero”. For all of the coefficients, the hypothesis is rejected at a confidence level of 95 %, indicating that the coefficients are statistically significant. The temperature dependence of the solid solubility is related to the enthalpy of solution as follows:15 ⎡ ⎛ ∂ ln γ2 ⎞ ⎤ ⎛ ∂ ln x 2 ⎞ ⎟ = (H̅ 2 − H2s0)/⎢1 + ⎜ RT 2⎜ ⎟ ⎥ ⎢ ⎥ ⎝ ∂T ⎠ P , σ ∂ ln x ⎝ 2 ⎠T , P ⎦ ⎣

calix[6]arene

It follows from eq 2 that the relationship between the transfer enthalpy H̅ 2 − Hs0 2 and the slope (∂ ln x2/∂T)P,σ is complicated by the quotient (∂ ln γ2/∂ ln x2)T,P. However, eq 2 may still provide some clue to explain the different values of (∂ ln x2/∂T)P,σ for the two calixarenes. Possibly, the higher value of (∂ ln x2/∂T)P,σ for 4-tert-butylcalix[4]arene compared with calix[6]arene reflects a higher value of H̅ 2 − Hs0 2 , which may arise from the presence of the four bulky, hydrophobic tertbutyl groups in the former molecule and also from the presence of only four hydrophilic hydroxyl groups in the former molecule compared with six in the latter.

Table 2. Least-Squares Estimates of the Coefficients a1, a2, and a3 of Equation 1 and the Standard Deviations (SD) of the Estimates; Tmin and Tmax Indicate the Minimum and the Maximum Temperatures of the Solubility Measurements, Respectively Tmin/K Tmax/K a1 SD a1 a2 SD a2 a3 SD a3

T/K

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +420 532 290 171. Fax: +420 541 212 113. Funding

(2)

This work was supported by the Czech Science Foundation (Project P206/11/0138) and the Academy of Sciences of the Czech Republic (Institutional Support RVO:68081715). Financial support granted by the European Social Fund (Project CZ.1.07/2.3.00/20.0182 administered by the Ministry of Education, Youth and Sports of the Czech Republic) is gratefully acknowledged.

where H̅ 2 is the partial molar enthalpy of the solute in the solution, Hs0 2 is the molar enthalpy of the pure solid solute, R is the molar gas constant, P is the pressure, γ2 is the Raoult’s law activity coefficient of the solute referred to the pure subcooled liquid solute at the particular T and P, and the subscript σ denotes saturation. The presence of the isothermal, isobaric composition derivative of the solute activity coefficient in eq 2 makes it difficult to assign any straightforward and rigorous

Notes

The authors declare no competing financial interest. 2435



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REFERENCES

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Solubilities of Calix [6] arene and 4-tert-Butylcalix [4] arene in

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