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Thermodynamics of the solubility of ciprofloxacin in methanol, ethanol, 1-propanol, acetone, and chloroform from 293.15 to 333.15 K
Journal of Molecular Liquids 156 (2010) 191–193
Contents lists available at ScienceDirect
Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m o l l i q
Short Communication
Thermodynamics of the solubility of ciprofloxacin in methanol, ethanol, 1-propanol, acetone, and chloroform from 293.15 to 333.15 K Cong-Liang Zhang ⁎, Fang Zhao, Yan Wang College of Chemical Engineering, Zhengzhou University, Zhengzhou, Henan 450002, PR China
a r t i c l e
i n f o
Article history: Received 30 May 2010 Accepted 23 June 2010 Available online 30 June 2010
a b s t r a c t The solubilities of ciprofloxacin in methanol, ethanol, 1-propanol, acetone, and chloroform have been determined with temperatures from 293.15 to 333.15 K by a static equilibrium method. The experimental data were correlated with the modified Apelblat equation. © 2010 Elsevier B.V. All rights reserved.
Keywords: Solubility Ciprofloxacin Thermodynamics Physicochemical properties
1. Introduction
2.2. Apparatus and procedure
Ciprofloxacin is a member of the quinolones that are widely used in agriculture to prevent diseases in livestock and to treat illness; therefore, soil and groundwater body have been seriously contaminated. Its solubility plays a prominent role in the prediction of the environmental fate of chemicals and can characterize transportation through membranes and the topical activity of drugs [1]. In determining the transport of ciprofloxacin in the environment and assessing its risk to terrestrial and aquatic ecosystems, it is necessary to know its solubilities in various solvents. However, only a limited amount of solubility data for ciprofloxacin has been reported with temperatures from 298.15 to 313.15 K [2–5]. In this study, the solubilities of ciprofloxacin in methanol, ethanol, 1-propanol, acetone, and chloroform have been measured from 293.15 to 333.15 K. The experimental data were correlated with the modified Apelblat equation [6,7].
The solubility was measured by a static equilibrium method [8]. Nearly 100 mg of ciprofloxacin was added separately to 50 mL of each solvent in glass flasks. The mixtures were then stirred in a mechanical shaker for 1 h. Samples were then allowed to stand in water baths (type 501, Shanghai Laboratory Instrument Works) kept at the appropriate temperature (±0.02 K). The equilibrium of other quinolones has been reported to be achieved after 30 h. Therefore, in this work, the initial equilibrium time of the saturated solution was 72 h; then, it was analyzed once every 5 h until the results were replicated three consecutive times. After this time, the supernatant solutions were filtered to ensure that they were free of particulate matter before sampling. We determined the concentrations by measuring UV absorbance after appropriate dilution and interpolation from previously constructed calibration curves for each system. To permit conversion between concentration and mole fraction, the density of the saturated solutions was determined with a digital density meter. All of the solubility experiments were repeated at least three times, and the mean values were considered to be the measured results. As the solubilities are sensitive to temperature, it was controlled to ±0.05 K. The reproducibility of mole-fraction measurements was ±1 · 10−8, and uncertainties of these were assumed to be less than 5 · 10−8. The results showed that the deviation of the measured solubility from the literature values [3] was less than 1.0%. Therefore, the reliability of the experimental apparatus was verified.
2. Experimental 2.1. Materials Ciprofloxacin obtained from Daming Biotech was further purified by recrystallization from aqueous solutions. After filtration and drying, its mass fraction was determined by UV spectrometry (type UV-2401PC, Shimadzu), to be 0.996. All of the solvents selected for the present study were analytical grade reagents, which were obtained from Tianjin Kermel Chemical Reagent (China) and used without any further purification. ⁎ Corresponding author. E-mail address: [email protected] (C.-L. Zhang). 0167-7322/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2010.06.004
3. Results and discussions The solubilities of norfloxacin in water listed in Table 1 are measured to complete the data reported in literature [3].
192
C.-L. Zhang et al. / Journal of Molecular Liquids 156 (2010) 191–193
Table 1 Mole fraction solubility (x) of norfloxacin in water compared with literature data at 298.15 K. System
105xexptl
105xref
100RD
Norfloxacin + water
2.270 [5]
2.258 [3]
0.53
The temperature dependence of ciprofloxacin solubility in methanol, ethanol, 1-propanol, 2-propanol, acetone, and chloroform has been described by the modified Apelblat equation [6,7] ln x = A +
B + C lnðT = KÞ T=K
ð1Þ
where x is the mole fraction of ciprofloxacin, T is the absolute temperature, and A, B, and C are parameters determined by least-squares analysis. The values of these parameters are listed in Table 3. The relative deviations (RD values) between the experimental and the calculated values of solubilities are also calculated by Eq. (2) and are listed in Table 2. RD =
x−x c
ð2Þ
x
where x and xc are respectively the experimental and the calculated mole fraction of ciprofloxacin. The average relative deviations (ARD values) for each system in this study are also calculated by Eq. (3) and are given in Table 3. ARD =
1 N xi −xci ∑i = 1 N xi
ð3Þ
where xi and xci are respectively the experimental and the calculated mole fraction of ciprofloxacin at each experimental point. The data in Table 2 Solubility data of ciprofloxacin in different solvents and the regression results obtained using the modified Apelblat equation. T/K
105x
105x
100RD
T/K
Ciprofloxacin + methanol 293.15 0.8142 298.15 1.126 303.15 1.488 308.15 1.946 313.15 2.468
−0.52 0.94 −0.024 −0.017 −1.2
318.15 323.15 328.15 333.15
3.174 3.908 4.745 5.719
0.83 0.18 −0.26 0.065
Ciprofloxacin + ethanol 293.15 0.8046 298.15 1.022 303.15 1.259 308.15 1.537 313.15 1.854
−1.1 1.0 0.79 0.35 −0.62
318.15 323.15 328.15 333.15
2.243 2.712 3.242 3.886
−0.66 −0.15 −0.20 0.57
Ciprofloxacin + 1-propanol 293.15 0.2057 298.15 0.3394 303.15 0.5282 308.15 0.7712 313.15 1.089
−0.93 0.62 1.2 0.013 −0.20
318.15 323.15 328.15 333.15
1.468 1.928 2.461 2.986
−0.99 −0.56 0.65 0.19
Ciprofloxacin + acetone 293.15 0.9285 298.15 1.265 303.15 1.662 308.15 2.156 313.15 2.690
−0.79 0.63 0.45 0.92 −0.70
318.15 323.15 328.15 333.15
3.349 4.148 4.982 5.990
−0.82 0.16 −0.41 0.58
Ciprofloxacin + chloroform 293.15 7.856 298.15 9.421 303.15 11.08 308.15 12.83 313.15 14.76
−0.46 0.35 0.43 0.078 0.15
318.15 323.15 328.15 333.15
16.68 18.83 21.18 23.45
100RD
−0.56 −0.42 0.25 0.20
Table 3 Parameters in the modified Apelblat equation for different systems. System
A
B
C
100ARD
Ciprofloxacin + methanol Ciprofloxacin + ethanol Ciprofloxacin + 1-propanol Ciprofloxacin + acetone Ciprofloxacin + chloroform
274.86 5.4482 809.82 263.31 152.82
−17262 −3998.9 −43573 −16531 −9746.6
−40.081 −0.62084 −118.69 −38.463 −22.711
0.45 0.60 0.60 0.60 0.32
Tables 2 and 3 indicate that the calculated solubilities show good agreement with the experimental data, which demonstrates that the modified Apelblat equation can be used to correlate the solubility data of ciprofloxacin in different solvents. RD values among the 45 data points for the studied systems do not exceed 1.2%, and the total average relative deviation is 0.51%. By using the data shown in Table 2, we plotted the solubility curves for the studied systems in Fig. 1. It is evident that the solubility of each system is low. The solubility data of ciprofloxacin in methanol, ethanol, 1-propanol, acetone, and chloroform showed a flat uptrend when the temperature increased. Moreover, the structure of ciprofloxacin indicates that the molecule is highly aromatic and the functional groups may not contribute much to the aqueous solubility. So, the solubility is at a minimum in 1-propanol and a maximum in chloroform. According to a pseudochemical reaction process [9], the dissolution process of solid, S, in liquid, W, can be expressed as S + W = SW; the relationship of its dissolution equilibrium constants and activities can be expressed as Ki =
ai as aw
ð4Þ
where ai is the activity of ciprofloxacin in solution, as and aw are the activities of pure solid, S, and pure liquid, W, respectively. Because of the relatively small solubility of ciprofloxacin in each solvent, it is believed that as and aw almost remain constant in the experimental range, and each is considered to be a constant. Therefore, Eq. (4) can be written as Ki =
γi xi as aw
ð5Þ
where γi is the activity coefficient of ciprofloxacin, i, in the solution and xi is the mole fraction of ciprofloxacin, i, in the solution.
Fig. 1. Solubilities of ciprofloxacin in studied solvents: ▲, ciprofloxacin + methanol; Δ, ciprofloxacin + ethanol; ■, ciprofloxacin + 1-propanol; □, ciprofloxacin + acetone; ●, ciprofloxacin + chloroform; , calculated from Eq. (1).
C.-L. Zhang et al. / Journal of Molecular Liquids 156 (2010) 191–193
193
Table 4 ΔsolH and ΔsolS for different systems at different temperatures. T/K Ciprofloxacin + methanol Ciprofloxacin + ethanol Ciprofloxacin + 1-propanol Ciprofloxacin + acetone Ciprofloxacin + chloroform
ΔsolH/kJ mol−1 ΔsolS/J mol−1 K−1 ΔsolH/kJ mol−1 ΔsolS/J mol−1 K−1 ΔsolH/kJ mol−1 ΔsolS/J mol−1 K−1 ΔsolH/kJ mol−1 ΔsolS/J mol−1 K−1 ΔsolH/kJ mol−1 ΔsolS/J mol−1 K−1
293.15
303.15
313.15
323.15
333.15
45.83 156.3 31.73 108.3 72.99 249.0 43.69 149.1 25.68 87.60
42.50 140.2 31.68 104.5 63.12 208.2 40.50 133.6 23.79 78.48
39.16 125.1 31.63 101.0 53.25 170.1 37.30 119.1 21.90 69.95
35.83 110.9 31.58 97.92 43.39 143.3 34.10 105.5 20.02 61.94
32.50 97.55 31.53 94.63 33.52 100.6 30.90 92.76 18.13 54.41
On the basis of the assumption used in the inferential process for the modified Apelblat equation that the activity coefficient is invariable during a certain temperature range [10], γi in Eq. (5) can be merged into asaw. Eq. (6) can be obtained from Eq. (5) by logarithmic treatment ln Ki = ln xi + J
ð6Þ
where J = lnγi − ln asaw is a temperature-independent constant. On the basis of the Gibbs equation and the modified Van't Hoff method [11], the equation for calculating the molar enthalpies of dissolution ΔsolH can be obtained Δsol H = −R
d lnKi dT −1
ð7Þ
4. Conclusions
Substituting the differential of Eq. (6) into Eq. (7) yields Δsol H = −R
d lnxi dT −1
ð8Þ
Using Eq. (1) to obtain the derivative and substituting it into Eq. (8) gives Δsol H = RTðC−B = ðT = KÞÞ
complicated groups with different characteristics such as –CH3, –F, ciprofloxacin perhaps involve various forces such as electrostatic force, hydrogen bond, hydrophobic interaction, and stereoscopic effect in the dissolving process [12]. The reason for the entropy increase during the dissolution process is that ciprofloxacin disrupted the alignment of solvent molecules and therefore reduced the degree of order of the system while they were dissolved in various solvent. The endothermic effect in the dissolving process (ΔsolH N 0) is perhaps because the interactions between ciprofloxacin molecules and solvent molecules are more powerful than those between the solvent molecules; the newly formed bond energy between ciprofloxacin molecule and solvent molecule is not powerful enough to compensate the energy needed for breaking the original association bond in various solvents.
Using a static equilibrium method, the solubilities of ciprofloxacin in methanol, ethanol, 1-propanol, acetone, and chloroform with temperatures from 293.15 to 333.15 K were determined. The experimental data were correlated with the modified Apelblat equation. The calculated results show good agreement with the experimental data.
ð9Þ Acknowledgment
According to the fundamental thermodynamic relation [12], the equation for calculating the molar entropies of dissolution ΔsolS can be obtained accordingly Δsol S = RðC−B = ðT = KÞÞ
ð10Þ
According to parameters of the modified Apelblat equation listed in Table 3, ΔsolH and ΔsolS listed in Table 4 can be calculated from Eqs. (9) and (10), respectively. From Table 4, it is found that the course of ciprofloxacin dissolving in each solvent in the experimental temperature range was endothermic, ΔsolH N 0, and ΔsolS for ciprofloxacin dissolving in each solvent was relatively large. The positive ΔsolH and ΔsolS for ciprofloxacin revealed that ciprofloxacin being dissolved in each solvent was an entropy-driving process. This phenomenon likely resulted from the different molecular structures and space conformations between solute and solvent. Solvent molecules selected for the present study are strong association complexes with small molecular dimension [13]. Owing to the solute ciprofloxacin molecules containing basic groups such as NNH, NN–, acidic groups such as –COOH, and
This research was supported by the Henan Province Natural Science Foundation of China (Project NO. 0611033400). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13]
Y. Picó, V. Andreu, Anal. Bioanal. Chem. 387 (2007) 1287. B. Faller, P. Ertl, Adv. Drug Deliv. Rev. 59 (2007) 533. D.L. Ross, C. Riley, Int. J. Pharm. 63 (1990) 237. X. Yu, G.L. Zipp, G.W.R. Davidson, Pharm. Res. 11 (1994) 522. C.L. Zhang, Y. Wang, J. Chem. Eng. Data 53 (2008) 1295. A. Apelblat, E. Manzurola, J. Chem. Thermodyn. 19 (1987) 317. Q. Jia, P. Ma, S. Yi, Q. Wang, C. Wang, G. Li, J. Chem. Eng. Data 53 (2008) 1278. F. Martinez, C.M. Avila, A. Gomez, J. Braz. Chem. Soc. 14 (2003) 803. F.A. Wang, Molecular Thermodynamics and Chromatographic Retention, Meteorology Press, Beijing, 2001. F.A. Wang, L.C. Wang, J.C. Song, L. Wang, H.S. Chen, J. Chem. Eng. Data 49 (2004) 1539. D. Bourgois, D. Thomas, J.L. Fanlo, J. Vanderschuren, J. Chem. Eng. Data 51 (2006) 1212. J.M. Prausnitz, R.N. Lichtenthaler, E.G. Azevedo, Molecular Thermodynamics of Fluid-Phase Equilibria, 3rd ed. Prentice Hall PTR, Upper Saddle River, New Jersey, 1999. I. Nagata, K. Gotoh, K. Tamura, Fluid Phase Equilib. 124 (1996) 31.
Contents lists available at ScienceDirect
Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m o l l i q
Short Communication
Thermodynamics of the solubility of ciprofloxacin in methanol, ethanol, 1-propanol, acetone, and chloroform from 293.15 to 333.15 K Cong-Liang Zhang ⁎, Fang Zhao, Yan Wang College of Chemical Engineering, Zhengzhou University, Zhengzhou, Henan 450002, PR China
a r t i c l e
i n f o
Article history: Received 30 May 2010 Accepted 23 June 2010 Available online 30 June 2010
a b s t r a c t The solubilities of ciprofloxacin in methanol, ethanol, 1-propanol, acetone, and chloroform have been determined with temperatures from 293.15 to 333.15 K by a static equilibrium method. The experimental data were correlated with the modified Apelblat equation. © 2010 Elsevier B.V. All rights reserved.
Keywords: Solubility Ciprofloxacin Thermodynamics Physicochemical properties
1. Introduction
2.2. Apparatus and procedure
Ciprofloxacin is a member of the quinolones that are widely used in agriculture to prevent diseases in livestock and to treat illness; therefore, soil and groundwater body have been seriously contaminated. Its solubility plays a prominent role in the prediction of the environmental fate of chemicals and can characterize transportation through membranes and the topical activity of drugs [1]. In determining the transport of ciprofloxacin in the environment and assessing its risk to terrestrial and aquatic ecosystems, it is necessary to know its solubilities in various solvents. However, only a limited amount of solubility data for ciprofloxacin has been reported with temperatures from 298.15 to 313.15 K [2–5]. In this study, the solubilities of ciprofloxacin in methanol, ethanol, 1-propanol, acetone, and chloroform have been measured from 293.15 to 333.15 K. The experimental data were correlated with the modified Apelblat equation [6,7].
The solubility was measured by a static equilibrium method [8]. Nearly 100 mg of ciprofloxacin was added separately to 50 mL of each solvent in glass flasks. The mixtures were then stirred in a mechanical shaker for 1 h. Samples were then allowed to stand in water baths (type 501, Shanghai Laboratory Instrument Works) kept at the appropriate temperature (±0.02 K). The equilibrium of other quinolones has been reported to be achieved after 30 h. Therefore, in this work, the initial equilibrium time of the saturated solution was 72 h; then, it was analyzed once every 5 h until the results were replicated three consecutive times. After this time, the supernatant solutions were filtered to ensure that they were free of particulate matter before sampling. We determined the concentrations by measuring UV absorbance after appropriate dilution and interpolation from previously constructed calibration curves for each system. To permit conversion between concentration and mole fraction, the density of the saturated solutions was determined with a digital density meter. All of the solubility experiments were repeated at least three times, and the mean values were considered to be the measured results. As the solubilities are sensitive to temperature, it was controlled to ±0.05 K. The reproducibility of mole-fraction measurements was ±1 · 10−8, and uncertainties of these were assumed to be less than 5 · 10−8. The results showed that the deviation of the measured solubility from the literature values [3] was less than 1.0%. Therefore, the reliability of the experimental apparatus was verified.
2. Experimental 2.1. Materials Ciprofloxacin obtained from Daming Biotech was further purified by recrystallization from aqueous solutions. After filtration and drying, its mass fraction was determined by UV spectrometry (type UV-2401PC, Shimadzu), to be 0.996. All of the solvents selected for the present study were analytical grade reagents, which were obtained from Tianjin Kermel Chemical Reagent (China) and used without any further purification. ⁎ Corresponding author. E-mail address: [email protected] (C.-L. Zhang). 0167-7322/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2010.06.004
3. Results and discussions The solubilities of norfloxacin in water listed in Table 1 are measured to complete the data reported in literature [3].
192
C.-L. Zhang et al. / Journal of Molecular Liquids 156 (2010) 191–193
Table 1 Mole fraction solubility (x) of norfloxacin in water compared with literature data at 298.15 K. System
105xexptl
105xref
100RD
Norfloxacin + water
2.270 [5]
2.258 [3]
0.53
The temperature dependence of ciprofloxacin solubility in methanol, ethanol, 1-propanol, 2-propanol, acetone, and chloroform has been described by the modified Apelblat equation [6,7] ln x = A +
B + C lnðT = KÞ T=K
ð1Þ
where x is the mole fraction of ciprofloxacin, T is the absolute temperature, and A, B, and C are parameters determined by least-squares analysis. The values of these parameters are listed in Table 3. The relative deviations (RD values) between the experimental and the calculated values of solubilities are also calculated by Eq. (2) and are listed in Table 2. RD =
x−x c
ð2Þ
x
where x and xc are respectively the experimental and the calculated mole fraction of ciprofloxacin. The average relative deviations (ARD values) for each system in this study are also calculated by Eq. (3) and are given in Table 3. ARD =
1 N xi −xci ∑i = 1 N xi
ð3Þ
where xi and xci are respectively the experimental and the calculated mole fraction of ciprofloxacin at each experimental point. The data in Table 2 Solubility data of ciprofloxacin in different solvents and the regression results obtained using the modified Apelblat equation. T/K
105x
105x
100RD
T/K
Ciprofloxacin + methanol 293.15 0.8142 298.15 1.126 303.15 1.488 308.15 1.946 313.15 2.468
−0.52 0.94 −0.024 −0.017 −1.2
318.15 323.15 328.15 333.15
3.174 3.908 4.745 5.719
0.83 0.18 −0.26 0.065
Ciprofloxacin + ethanol 293.15 0.8046 298.15 1.022 303.15 1.259 308.15 1.537 313.15 1.854
−1.1 1.0 0.79 0.35 −0.62
318.15 323.15 328.15 333.15
2.243 2.712 3.242 3.886
−0.66 −0.15 −0.20 0.57
Ciprofloxacin + 1-propanol 293.15 0.2057 298.15 0.3394 303.15 0.5282 308.15 0.7712 313.15 1.089
−0.93 0.62 1.2 0.013 −0.20
318.15 323.15 328.15 333.15
1.468 1.928 2.461 2.986
−0.99 −0.56 0.65 0.19
Ciprofloxacin + acetone 293.15 0.9285 298.15 1.265 303.15 1.662 308.15 2.156 313.15 2.690
−0.79 0.63 0.45 0.92 −0.70
318.15 323.15 328.15 333.15
3.349 4.148 4.982 5.990
−0.82 0.16 −0.41 0.58
Ciprofloxacin + chloroform 293.15 7.856 298.15 9.421 303.15 11.08 308.15 12.83 313.15 14.76
−0.46 0.35 0.43 0.078 0.15
318.15 323.15 328.15 333.15
16.68 18.83 21.18 23.45
100RD
−0.56 −0.42 0.25 0.20
Table 3 Parameters in the modified Apelblat equation for different systems. System
A
B
C
100ARD
Ciprofloxacin + methanol Ciprofloxacin + ethanol Ciprofloxacin + 1-propanol Ciprofloxacin + acetone Ciprofloxacin + chloroform
274.86 5.4482 809.82 263.31 152.82
−17262 −3998.9 −43573 −16531 −9746.6
−40.081 −0.62084 −118.69 −38.463 −22.711
0.45 0.60 0.60 0.60 0.32
Tables 2 and 3 indicate that the calculated solubilities show good agreement with the experimental data, which demonstrates that the modified Apelblat equation can be used to correlate the solubility data of ciprofloxacin in different solvents. RD values among the 45 data points for the studied systems do not exceed 1.2%, and the total average relative deviation is 0.51%. By using the data shown in Table 2, we plotted the solubility curves for the studied systems in Fig. 1. It is evident that the solubility of each system is low. The solubility data of ciprofloxacin in methanol, ethanol, 1-propanol, acetone, and chloroform showed a flat uptrend when the temperature increased. Moreover, the structure of ciprofloxacin indicates that the molecule is highly aromatic and the functional groups may not contribute much to the aqueous solubility. So, the solubility is at a minimum in 1-propanol and a maximum in chloroform. According to a pseudochemical reaction process [9], the dissolution process of solid, S, in liquid, W, can be expressed as S + W = SW; the relationship of its dissolution equilibrium constants and activities can be expressed as Ki =
ai as aw
ð4Þ
where ai is the activity of ciprofloxacin in solution, as and aw are the activities of pure solid, S, and pure liquid, W, respectively. Because of the relatively small solubility of ciprofloxacin in each solvent, it is believed that as and aw almost remain constant in the experimental range, and each is considered to be a constant. Therefore, Eq. (4) can be written as Ki =
γi xi as aw
ð5Þ
where γi is the activity coefficient of ciprofloxacin, i, in the solution and xi is the mole fraction of ciprofloxacin, i, in the solution.
Fig. 1. Solubilities of ciprofloxacin in studied solvents: ▲, ciprofloxacin + methanol; Δ, ciprofloxacin + ethanol; ■, ciprofloxacin + 1-propanol; □, ciprofloxacin + acetone; ●, ciprofloxacin + chloroform; , calculated from Eq. (1).
C.-L. Zhang et al. / Journal of Molecular Liquids 156 (2010) 191–193
193
Table 4 ΔsolH and ΔsolS for different systems at different temperatures. T/K Ciprofloxacin + methanol Ciprofloxacin + ethanol Ciprofloxacin + 1-propanol Ciprofloxacin + acetone Ciprofloxacin + chloroform
ΔsolH/kJ mol−1 ΔsolS/J mol−1 K−1 ΔsolH/kJ mol−1 ΔsolS/J mol−1 K−1 ΔsolH/kJ mol−1 ΔsolS/J mol−1 K−1 ΔsolH/kJ mol−1 ΔsolS/J mol−1 K−1 ΔsolH/kJ mol−1 ΔsolS/J mol−1 K−1
293.15
303.15
313.15
323.15
333.15
45.83 156.3 31.73 108.3 72.99 249.0 43.69 149.1 25.68 87.60
42.50 140.2 31.68 104.5 63.12 208.2 40.50 133.6 23.79 78.48
39.16 125.1 31.63 101.0 53.25 170.1 37.30 119.1 21.90 69.95
35.83 110.9 31.58 97.92 43.39 143.3 34.10 105.5 20.02 61.94
32.50 97.55 31.53 94.63 33.52 100.6 30.90 92.76 18.13 54.41
On the basis of the assumption used in the inferential process for the modified Apelblat equation that the activity coefficient is invariable during a certain temperature range [10], γi in Eq. (5) can be merged into asaw. Eq. (6) can be obtained from Eq. (5) by logarithmic treatment ln Ki = ln xi + J
ð6Þ
where J = lnγi − ln asaw is a temperature-independent constant. On the basis of the Gibbs equation and the modified Van't Hoff method [11], the equation for calculating the molar enthalpies of dissolution ΔsolH can be obtained Δsol H = −R
d lnKi dT −1
ð7Þ
4. Conclusions
Substituting the differential of Eq. (6) into Eq. (7) yields Δsol H = −R
d lnxi dT −1
ð8Þ
Using Eq. (1) to obtain the derivative and substituting it into Eq. (8) gives Δsol H = RTðC−B = ðT = KÞÞ
complicated groups with different characteristics such as –CH3, –F, ciprofloxacin perhaps involve various forces such as electrostatic force, hydrogen bond, hydrophobic interaction, and stereoscopic effect in the dissolving process [12]. The reason for the entropy increase during the dissolution process is that ciprofloxacin disrupted the alignment of solvent molecules and therefore reduced the degree of order of the system while they were dissolved in various solvent. The endothermic effect in the dissolving process (ΔsolH N 0) is perhaps because the interactions between ciprofloxacin molecules and solvent molecules are more powerful than those between the solvent molecules; the newly formed bond energy between ciprofloxacin molecule and solvent molecule is not powerful enough to compensate the energy needed for breaking the original association bond in various solvents.
Using a static equilibrium method, the solubilities of ciprofloxacin in methanol, ethanol, 1-propanol, acetone, and chloroform with temperatures from 293.15 to 333.15 K were determined. The experimental data were correlated with the modified Apelblat equation. The calculated results show good agreement with the experimental data.
ð9Þ Acknowledgment
According to the fundamental thermodynamic relation [12], the equation for calculating the molar entropies of dissolution ΔsolS can be obtained accordingly Δsol S = RðC−B = ðT = KÞÞ
ð10Þ
According to parameters of the modified Apelblat equation listed in Table 3, ΔsolH and ΔsolS listed in Table 4 can be calculated from Eqs. (9) and (10), respectively. From Table 4, it is found that the course of ciprofloxacin dissolving in each solvent in the experimental temperature range was endothermic, ΔsolH N 0, and ΔsolS for ciprofloxacin dissolving in each solvent was relatively large. The positive ΔsolH and ΔsolS for ciprofloxacin revealed that ciprofloxacin being dissolved in each solvent was an entropy-driving process. This phenomenon likely resulted from the different molecular structures and space conformations between solute and solvent. Solvent molecules selected for the present study are strong association complexes with small molecular dimension [13]. Owing to the solute ciprofloxacin molecules containing basic groups such as NNH, NN–, acidic groups such as –COOH, and
This research was supported by the Henan Province Natural Science Foundation of China (Project NO. 0611033400). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
[13]
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