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Solubility of monobenzone in aqueous co-solvent mixtures of several alcohols: Determination, modelling and thermodynamic aspects analysis
J. Chem. Thermodynamics 142 (2020) 106023
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Solubility of monobenzone in aqueous co-solvent mixtures of several alcohols: Determination, modelling and thermodynamic aspects analysis Yanyan Zhou a, Jiaxin Wu a, Ali Farajtabar b, Jian Wang a, Hongkun Zhao a,⇑ a b
College of Chemistry & Chemical Engineering, YangZhou University, YangZhou, Jiangsu 225002, People’s Republic of China Department of Chemistry, Jouybar Branch, Islamic Azad University, 4776186131 Jouybar, Iran
a r t i c l e
i n f o
Article history: Received 1 December 2019 Accepted 4 December 2019 Available online 7 December 2019 Keywords: Monobenzone Solubility Jouyban-Acree model Preferential solvation Solvent effect
a b s t r a c t Investigation upon the solubility of monobenzone in aqueous co-solvent mixtures of isopropanol, n-propanol, ethylene glycol (EG) and ethanol was performed via a saturation shake-flask technique at temperatures from 283.15 K to 328.15 K under pressure of p = 101.2 kPa. Experimental solubility magnitude presented positive dependence upon the mass fraction of each co-solvent and temperature. The greatest solubility value on the mole fraction scale was observed in the neat co-solvents. The equilibrated solids with corresponding co-solvent mixtures were characterized through an X-ray power diffraction (XRD) technique, demonstrating the absence of polymorphic transformation or solvate formation. The Jouyban-Acree model was adopted to mathematically describe the monobenzone solubility data. The maximum magnitudes of RAD and RMSD were 3.17 102 and 7.64 104, respectively. The local mole fractions of isopropanol (n-propanol, EG or ethanol) and water adjacent monobenzone were quantitatively studied by the Inverse Kirkwood–Buff integrals method. The parameters of preferential solvation for the isopropanol (n-propanol, EG or ethanol) were positive in the isopropanol (n-propanol, EG or ethanol) mixtures in intermediate and alcohol-rich compositions, indicating that monobenzone was preferentially solvated by the isopropanol (n-propanol, EG or ethanol). Monobenzone mainly acted as a Lewis acid which interacted with proton-acceptor functional group of the alcohols. Furthermore, the analysis of linear solvation energy relationships was made with a suitable combination of the solvent polarity descriptors to reveal the nature of intermolecular interactions bringing about the solubility variation in the cosolvent mixtures. Ó 2019 Elsevier Ltd.
1. Introduction In recent times, the investigation on the solubility of drugs as well as pharmaceutical intermediates becomes an extensive topic in the pharmaceutical fields. The drugs’ solubility in solvent mixtures as a function of solvent composition and temperature is of great importance in the design of liquid dosage forms, raw material purification and acquiring the mechanisms of the drug dissolutions about physical and chemical stability [1–3]. In addition, the information upon solubility data of the active ingredients is a significant physico-chemical property in the pharmaceutical design process, because it has crucial effect on the pharmacokinetic and biopharmaceutical properties and efficacy of the drugs [4,5]. Furthermore, the solubility dependence upon temperature and co-solvent compositions in co-solvent mixtures is generally employed to make a thermodynamic analysis to insight in-depth into the intermolecu⇑ Corresponding author. E-mail address: [email protected] (H. Zhao). https://doi.org/10.1016/j.jct.2019.106023 0021-9614/Ó 2019 Elsevier Ltd.
lar mechanisms concerning the drug dissolution process and to estimate the preferential solvation of a solute by co-solvent components [6–10]. Monobenzone (CAS No, 103-16-2; IUPAC name, 4phenylmethoxyphen; molecule structure, Fig. 1) is a monobenzyl ether of hydroquinone medically used for depigmentation. It occurs as a nearly tasteless crystalline powder, soluble in alcohol and essentially insoluble in water. It exerts a depigmenting influence on the skin of mammals by increasing the excretion of melanin from the melanocytes. It may also cause destruction of the permanent and melanocytes depigmentation [11–15]. Despite the fact this drug has been recognized for many years, the physico-chemical properties relevant to monobenzone solubility in neat solvents and solvent mixtures have not yet been reported in the publications. So this paper tries to deliver an idea about the relative stabilization of monobenzone in aqua-organic mixtures with respect to water and the comprehensive solutesolvent and solvent-solvent interactions therein.
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Y. Zhou et al. / J. Chem. Thermodynamics 142 (2020) 106023
solvent-dependent properties. The distinguished success of the KAT-LSER method has been discovered in clarifying extensive chemical phenomena including the solid solubility in pure and mixed solvents [25,26]. So, the other objective of this paper is to examine the solvent effect upon the variation of monobenzone solubility in aqueous mixtures of isopropanol, n-propanol, EG and ethanol by using the KAT-LSER model to demonstrate the nature and relative importance of intermolecular interactions leading to the solvent effect. Fig. 1. Molecular structure of monobenzone.
Several theoretical and semi-empirical solubility models have been put forward to predict the solubility of drug in the mixtures [16], however the availability of solubility data through experiment is still vital for a lot of pharmaceutical scientists [1,17]. It is evident that the monobenzone solubility in water is low, which may cause low formulation or bioavailability difficulty in clinical development [4,5]. Co-solvency is a powerful solubilization method to improve the solid solubility in the pharmaceutical and chemical industries [1,2,18]. Although this method has been extensively employed in pharmacies several years ago, only recently the mechanisms relating to the increase or decrease in drug solubility start to be approached from a deep thermodynamic point of view, along with the preferential solvation analysis of a solute by the components in mixtures [6–10]. The solvent of isopropanol presents a strong odour. A number of non-polar substances and isopropanol are soluble with each other with any composition. Generally, it is employed in solvent mixtures with other solvents or solely for diverse purposes relevant to transepidermal uses and penetration-improving pharmaceutical compositions for the percutaneous [19]. Ethanol is a commonly cosolvent employed in the pharmaceutical liquid formulations with the concentrations less than 50%. As well, ethanol has significant effect on distribution, excretion, absorption and metabolism of the drugs [20]. Although n-Propanol is not generally used as a co-solvent in the design of liquid medicines, it is a solvent commonly employed in the pharmaceutical arenas for cellulose esters and resins [21]. Ethylene glycol (EG) is a safe and acceptable solvent for pharmaceutical usage [10,22]. In terms of the aspects discussed above, the chief objects of this contribution are to report the solubility of monobenzone in aqueous co-solvent solutions of isopropanol, n-propanol, ethylene glycol (EG) and ethanol and evaluate the preferential solvation of monobenzone by the co-solvents of isopropanol, n-propanol, EG and ethanol. As a generalized approach, the linear solvation energy relationships (abbreviated as LSER) treat the solvent effect through separating the interactions of solvent-solute molecules into two types, which are named as non-specific (dipole-dipole dispersion and dipole-induced dipole) and specific (hydrogen bonding) interactions. In addition, each interaction term presents a linear contribution to the Gibbs free energy of the solvent-dependent properties [23]. In this way, these empirical scales provide a suitable way to characterize the solvent capability to interact with the solutes, well-known as the solvent polarity, at a molecular level. Kamlet, Abboud and Taft (KAT) [23,24] provide extensively employed solvent scales through the solvatochromic investigation of pairs of probing molecules in solvents with a set of interacting properties. The solvent scales involve the dipolarity/polarizability (p*), hydrogen bond basicity (b) and hydrogen bond acidity (a), which may be directly achieved through determining the energy changes caused by corresponding intermolecular interactions between the solute and solvent molecules. Accordingly, the examination of solvent effect in terms of the LSER model reveals the nature and extent of solute-solvent interactions that influence the
2. Theoretical considerations The Jouyban–Acree model [10,16,27,28] is employed here to describe mathematically the monobenzone solubility in the aqueous mixtures of isopropanol, n-propanol, EG and ethanol. As well, the solvent effect upon the monobenzone solubility was examined through the KAT-LSER model [10,23]. 2.1. Jouyban-Acree model The expression of the Jouyban-Acree model is given in Eq. (1) [10,16,27,28], which has been successfully employed to describe the dependence of solute solubility on both temperature and solvent composition in solvent mixtures.
lnxw;T ¼ w1 lnx1;T þ w2 lnx2;T þ
2 w1 w2 X J ðw1 w2 Þi T=K i ¼ 0 i
ð1Þ
In Eq. (1), xw,T refers to the mole fraction solubility of monobenzone in the mixtures at absolute temperature T; w1 and w2 refer to the mass fraction of solvent 1 (isopropanol, n-propanol, EG or ethanol) and solvent 2 (water) in the monobenzone-free mixtures, respectively; xi,T denote the mole fraction solubility of monobenzone in neat solvent i at T; and Ji are the Jouyban-Acree model parameters. 2.2. KAT-LSER model The KAT-LSER model is practiced to justify the solvency effect on improvement of monobenzone solubility in mixtures. Eq. (2) is the KAT-LSER model in its universal form [23,25,29].
lnxw ¼ c0 þ c1 p þc2 b þ c3 a þ c4
V s d2H 100RT
! ð2Þ
In general, the solubility free energy is presented by lnxw; c2 b, c3 a and c1 p* refer to the energy term relevant to the specific and non-specific solute-solvent interactions, wherein the coefficients ci=1-3 disclose sensitivity of the solute solubility to their corresponding energy terms; the term relevant to c4 in Eq. (2) signifies the cavity term, which defines the energy term for solvent-solvent interactions. This term calculates the energy of solute accommodation as a product of solute molar volume (Vs) and squared Hildebrand solubility parameter (dH) of the solvent. The temperature T and universal gas constant R are considered herein to deliver a dimensionless value for this term. As a result, the coefficient c4 reveals susceptibility of the solute solubility variation to the solvent-solvent interactions. c0 is intercept of the KAT-LSER model at a = b = p*=dH = 0. In practice, the mole fraction solubility is firstly converted into lnxw, and then examined by using the Eq. (2) by means of a multiple linear regression analysis. The objective function is given as Eq. (3) in the correlation.
F¼
X
lnxew;T lnxcw;T
2
ð3Þ
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Y. Zhou et al. / J. Chem. Thermodynamics 142 (2020) 106023
Additionally, the relative average deviation (RAD) and rootmean-square deviation (RMSD), which are expressed as Eqs. (4) and (5), respectively, are also used herein so as to evaluate the Jouyban-Acree model.
1 0 c e 1 X @xw;T xw;T A RAD ¼ N xew;T sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PN 2 c e i¼1 ðxw;T xw;T Þ RMSD ¼ N
ð4Þ
ð5Þ
wherein, N refers to the number of experimental data points. xew;T is the solubility of monobenzone in mole fraction scale attained through experiment; and xcw;T , computed through the studied model.
nents in aqueous co-solvent solutions; and V 3 , the partial molar volumes of monobenzone in mixtures. The function D (expressed as Eq. (12)) presents the derivative of standard molar Gibbs energy of transfer of monobenzone from neat water (2) to aqueous mixtures of co-solvent (1) with respect to the co-solvent proportions. The function Q (expressed as Eq. (13)) is the second derivative of excess molar Gibbs energy of mixing of the two solvents (GExc 1 þ 2) with respect to water composition in the solutions. Vcor denotes the correlation volume. r3 is the radius of monobenzone molecule attained by means of Eq. (14), where NAv is the Avogadro’s number.
D¼
@ Dtr Goð3;2!1þ2Þ
!
@x1
ð12Þ T;P
" # @ 2 GExc 1þ2 Q ¼ RT þ x1 x2 @x22
ð13Þ
T;p
2.3. Preferential solvation The way of Inverse Kirkwood–Buff integrals (IKBIs), Eq. (6), is a powerful technique to investigate the preferential solvation of a non-electrolyte or a weak electrolyte in the aqueous mixtures of co-solvent. It expresses the local solvent composition near a nonelectrolyte or a weak electrolyte compared to the global mixture proportions [6–10].
Z Gi;3 ¼ 0
r cor
ðg i;3 1Þ4pr 2 dr
ð6Þ
here, gi,3 signifies the pair correlation function for the co-solvent molecules in the mixtures of co-solvent (1) + water (2) neighbouring the monobenzone (3); r refers to the distance between the molecule centres of solute monobenzone (3) and that of water (2) or co-solvent (1); rcor presents the correlation distance, for which gi,3 (r > rcor) is approximately equal to 1. Accordingly, the integral value is essentially zero under the condition of r > rcor. The parameter of preferential solvation (dx1,3) of monobenzone (3) by the co-solvent (1) in mixtures of co-solvent (1) + water (2) is described as [6–10].
dx1;3 ¼ xL1;3 x1 ¼ dx2;3
ð7Þ
Here x1 signifies bulk mole fraction of the co-solvent (1) in initial aqueous mixtures of co-solvent; and xL1;3 , local mole fraction of the co-solvent (1) near the solute monobenzone (3). If the magnitude of dx1,3 is greater than zero, the monobenzone is preferentially solvated by co-solvent (1); while if it is smaller than zero, the monobenzone is preferentially solvated by water (2). The values of dx1,3 may be attained from the IKBIs method for the individual solvent compositions [6–10].
dx1;3 ¼
x1 x2 ðG1;3 G2;3 Þ x1 G1;3 þ x2 G2;3 þ V cor
ð8Þ
With
G1;3 ¼ RT jT
G2;3 ¼ RT jT V 3 þ
ð9Þ
x1 V 1 D Q
3 1=3 V cor ¼ 2522:5 r3 þ 0:1363 xL1;3 V 1 þ xL2;3 V 2 0:085
ð10Þ
ð11Þ
wherein, jT refers to the isothermal compressibility of the solu
ð14Þ
Because the values of jT depend on the compositions of the mixtures, it is not known for the studied solutions examined. On the other hand, the contribution of RTjT term to the IKBIs is fairly slight. For that reason, the jT values in solutions are approximately assessed via considering an additive property according to solution compositions and available values for pure solvents by [6–10]:
jT ¼ x1 joT;1 þ x2 joT;2
ð15Þ
where xi is mole fraction of the component i in solutions; joT;i is the isothermal compressibility of the neat composition i. The standard molar Gibbs energy of transfer of monobenzone from neat water (2) to the isopropanol (1) + water (2), npropanol (1) + water (2), ethanol (1) + water (2) and EG (1) + water (2) mixtures can be computed by means of Eq. (16) from the solubility data and mathematically correlated by using an empirical equation {Eq. (17)}, where A0, A1, A2, t1 and t2 are the equation parameters.
x3;2 Dtr G03;2!1þ2 ¼ RTln x3;1þ2 x
t 1
Dtr Go3;2!1þ2 ¼ A0 þ A1 e
1
ð16Þ x
t 1
þ A2 e
2
ð17Þ
The definitive correlation volume requires iteration because it depends upon the local mole fractions neighbouring the solute. The iteration process is made by means of substituting Vcor and dx1,3 into the Eqs. (7), (8)–(11) to re-calculate xL1;3 until a nonvariant value of Vcor is gained. 3. Experimental 3.1. Materials
x2 V 2 D V3 þ Q
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 21 3 3 10 V3 r3 ¼ 4pNAV
tions. V 1 and V 2 denote the partial molar volumes of the compo-
The monobenzone provided by the Shanghai Energy Chemical Co., Ltd., China had a mass fraction of 0.984. It was recrystallized several times before experiment in solvent of ethanol. The final purity of monobenzone used in experiments was 0.996 in mass fraction scale, which was tested by way of an Agilent-1260 highperformance liquid phase chromatograph (HPLC). The solvents such as isopropanol, n-propanol, EG and ethanol were purchased from the Aladdin Industrial (Shanghai) Co., Ltd., China with the mass fraction of no less than 0.994, which was checked using a FULI 9790 gas chromatography (GC). These solvents were directly used in experiments without any additional treatment. The dis-
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Y. Zhou et al. / J. Chem. Thermodynamics 142 (2020) 106023
tilled deionized water (conductivity < 1 lS∙cm1) achieved by means of distillation was used in this work. The details of the monobenzone and selected solvents were presented in Table 1. 3.2. Preparation of aqueous co-solvent mixtures The aqueous mixtures of isopropanol, n-propanol, EG and ethanol were prepared by means of an analytical balance with a type of BSA224S. The aqueous co-solvent mixtures used in each experiment for solubility determination were about 15 mL, which absolute error was 0.0001 g. The mass fraction proportion of isopropanol, propanol, EG and ethanol in the mixtures covered the entire range from 0 to 1. The flask was covered with a rubber stopper so as to avoid the organic solvent from escaping in the preparation process under approximately 101.2 kPa. 3.3. Solubility determination In this work, the determination of monobenzone solubility in these co-solvent mixtures was performed by means of a saturation shake-flask method as used in the previous publications [30–32], which reliability was confirmed through the solubility of benzoic acid in pure toluene [32]. An Agilent-1260 HPLC was employed in this work to quantitatively determine the monobenzone solubility in equilibrated liquid phase. Excess monobenzone was added into a 25 mL flask filled with 15 mL of the co-solvent mixtures. Then the flask containing liquid-solid solutions was placed into a thermostatic mechanical shaker obtained from the Tianjin Ounuo Instrument Co. Ltd., China, which had a standard uncertainty of 0.02 K. The systems were continually shaken by means of the mechanical shaker. With the aim of acquiring the equilibration time of the studied solutions, about 0.5 mL of liquid phase was withdrawn by means of a 2 mL syringe connected with a pore PTFE filter (0.2 lm) at intervals of 1 h and tested using the Agilent-1260 HPLC. Analysis exhibited that it took about 10 h to reach equilibrium for all the solutions. As soon as the solutions arrived at equilibrium, they were retained at static at the desired temperature to make sure that any un-dissolved solid precipitated from the solutions. The 2 mL of upper liquid phase was withdrawn via the 2 mL of precooled or preheated syringe, and then speedily transferred into a 25 mL volumetric flask that was pre-weighed by using the BSA224S analytical balance. The total amount of flask comprising the sample was weighed again through the analytical balance. The sample was diluted (if necessary) with neat methanol to 25 mL, and analysed by means of the Agilent1260 HPLC. 3.4. Analysis method The composition of monobenzone in saturated liquid was tested by the Agilent-1260 high-performance liquid chromatography. The chromatographic column employed herein was a reverse phase
column having a type of LP-C18 (250 mm 4.6 mm). The temperature the column was approximately 303 K. The wavelength of UV–vis detector was set to 290 nm [33]. The mobile phase in analysis was neat methanol, which flow rate was set to 0.8 mLmin1. Each sample was analysed three times in order to achieve the average value. The relative standard uncertainty of the determined solubility in mole fraction scale was assessed to be no more than 0.053. 3.5. X-ray powder diffraction With the aim of checking the absence of polymorph transformation or solvate formation of monobenzone in the whole experimental process, the solid phases equilibrated with their corresponding solutions were identified by the use of X-ray powder diffraction (XRD) at room temperature and local atmospheric pressure. All tests were carried out upon a HaoYuan DX-2700B (HaoYuan, China) apparatus by means of Cu Ka radiation (k = 0.1 54184 nm) at a scan speed of 6 degmin1. The tube current and tube voltage used in determination were 30 mA and 40 kV, respectively. The data of the determined XRD scans were collected from 10° to 80° (2-Theta). 4. Results and discussion 4.1. XRD analysis The determined XRD scans of raw monobenzone as well as the solids in equilibrium with their corresponding saturated liquids are given in Fig. S1 of Supporting material. It may be found that all the XRD patterns of equilibrated solids present characteristic peaks similar with the raw monobenzone. Accordingly, no solvate formation or polymorph transformation takes place in the entire determination procedure. 4.2. Solubility data The obtained solubility (x) of monobenzone on the mole fraction scale in the isopropanol, n-propanol, EG, water and ethanol within the temperature range from 283.15 K to 328.15 K is presented in Tables 2–5, and graphically shown in Fig. 2. It can be observed from this figure that the solubility of monobenzone rises with increasing experiment temperature for the pure solvents. The highest value is found in the pure solvent of ethanol, and lowest in the pure solvent of water. The dissolution capacity of monobenzone in different solvents ranks as ethanol > propanol > isopropa nol > EG > water. The mole fraction solubility of drug monobenzone shows up to approximately 104-fold improvement in going from pure water to pure ethanol. For example, if the temperature increases from 283.15 K to 328.15 K, the mole fraction solubility of monobenzone in pure ethanol rises from 483.0 104 to
Table 1 Detailed aspects upon the materials used in this work.
a b
Chemicals
Molar mass/ gmol1
Source
Initial mass fraction purity
Final mass fraction purity
Purification method
Analytical method
monobenzone ethanol isopropanol n-propanol EG Water
200.23 46.07 60.06 60.06 62.07 18.02
Shanghai Energy Chem. Co., Ltd. Aladdin Industrial (Shanghai) Co., Ltd.
0.984 0.994 0.995 0.0.995 0.996
0.996 0.994 0.995 0.0.995 0.996 Conductivity <1 mScm1
recrystallization None None None None Distillation
HPLCa GCb GC GC GC Conductivity meter
Our lab
High-performance liquid phase chromatography; Gas chromatography.
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Y. Zhou et al. / J. Chem. Thermodynamics 142 (2020) 106023
Table 2 Equilibrium mole fraction solubility xeT;W 104 of monobenzone in isopropanol (w) + water (1-w) mixture with different mass fractions ranging from T = (283.15 to 328.15) K 4 a e under 101.2 kPa. xT;W 10 . T/K
W 0
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
0.7000
08,000
0.9000
1
283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15
0.03569 0.04488 0.05644 0.07042 0.08707 0.1067 0.1300 0.1574 0.1907 0.2292
0.2197 0.2633 0.3349 0.4016 0.4879 0.5883 0.7070 0.8404 0.9969 1.180
1.078 1.304 1.571 1.880 2.269 2.694 3.168 3.715 4.347 5.048
3.863 4.691 5.670 6.762 8.077 9.577 11.16 13.17 15.23 17.64
11.04 13.36 16.04 19.32 22.81 27.25 31.78 36.98 43.01 50.09
25.64 31.29 37.57 44.81 53.20 62.84 73.91 86.30 99.74 115.9
49.67 59.53 71.80 85.36 100.9 119.6 139.4 163.5 189.9 221.1
81.38 98.28 118.0 140.0 165.9 195.8 229.1 266.7 309.5 357.7
120.5 145.1 172.4 206.1 241.2 285.2 332.6 385.6 447.6 513.6
163.1 195.9 233.2 277.1 324.5 381.0 441.1 510.5 592.2 680.7
216.8 260.9 307.8 364.9 424.9 502.8 583.0 670.9 778.0 895.5
a Standard uncertainties u are u(T) = 0.02 K, u(p) = 0.45 kPa; Relative standard uncertainty ur is ur (x) = 0.053. Solvent mixtures were prepared by mixing various masses of the solvents with relative standard uncertainty ur(w) = 0.0002. w represents the mass fraction of isopropanol in solvent mixtures of isopropanol + water mixture free of monobenzone.
Table 3 Equilibrium mole fraction solubility xeT;W 104 of monobenzone in n-propanol (w) + water (1-w) mixture with different mass fractions ranging from T = (283.15 to 328.15) K 4 under 101.2 kPa.a xeT;W 10 . T/K
W 0
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
0.7000
08,000
0.9000
1
283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15
0.03569 0.04488 0.05644 0.07042 0.08707 0.1067 0.1300 0.1574 0.1907 0.2292
0.3930 0.4920 0.5931 0.7174 0.8690 1.042 1.242 1.442 1.745 2.154
1.956 2.381 2.918 3.526 4.242 5.086 6.005 7.134 8.388 9.785
6.851 8.404 10.11 12.24 14.65 17.45 20.54 24.31 28.22 33.39
17.15 20.34 24.30 29.15 34.19 40.01 47.08 54.36 62.53 72.85
37.69 44.46 52.93 62.25 73.64 84.74 97.09 111.6 130.0 149.7
64.98 77.60 91.02 107.0 124.2 143.6 166.5 190.4 216.8 248.9
105.8 125.4 147.4 172.3 200.9 232.6 267.7 306.1 348.8 399.1
164.3 194.7 230.4 267.4 312.5 362.3 417.1 467.5 533.0 609.9
260.8 297.8 344.0 403.7 473.2 543.1 627.9 714.0 811.2 914.5
380.7 445.7 519.5 605.4 702.1 804.4 930.9 1057 1197 1369
a Standard uncertainties u are u(T) = 0.02 K, u(p) = 0.45 kPa; Relative standard uncertainty ur is ur (x) = 0.053. Solvent mixtures were prepared by mixing various masses of the solvents with relative standard uncertainty ur(w) = 0.0002. w represents the mass fraction of n-propanol in solvent mixtures of n-propanol + water mixture free of monobenzone.
Table 4 Equilibrium mole fraction solubility xeT;W 104 of monobenzone in EG (w) + water (1-w) mixture with different mass fractions ranging from T = (283.15 to 328.15) under 4 a e 101.2 kPa. xT;W 10 . T/K
283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15
W 0
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
0.7000
08,000
0.9000
1
0.03569 0.04488 0.05644 0.07042 0.08707 0.1067 0.1300 0.1574 0.1907 0.2292
0.1229 0.1540 0.1912 0.2356 0.2898 0.3523 0.4265 0.5125 0.6143 0.7295
0.3462 0.4317 0.5321 0.6533 0.7960 0.9639 1.160 1.388 1.661 1.958
0.8647 1.060 1.307 1.590 1.935 2.336 2.812 3.323 3.975 4.676
2.190 2.697 3.278 4.010 4.837 5.866 6.950 8.335 9.856 11.77
4.531 5.580 6.751 8.610 10.45 12.69 14.90 18.24 21.35 24.77
8.734 10.56 13.05 15.74 18.68 22.74 27.59 34.01 40.35 47.58
15.87 19.09 22.66 28.05 34.26 42.21 50.55 60.34 70.69 80.62
25.00 31.23 37.47 46.23 55.66 67.40 80.35 97.04 115.1 130.2
38.10 47.26 58.08 70.81 86.00 103.8 124.1 147.0 175.7 203.4
53.01 66.44 82.15 100.6 124.7 149.3 179.8 216.5 257.0 301.7
a Standard uncertainties u are u(T) = 0.02 K, u(p) = 0.45 kPa; Relative standard uncertainty ur is ur (x) = 0.053. Solvent mixtures were prepared by mixing various masses of the solvents with relative standard uncertainty ur(w) = 0.0002. w represents the mass fraction of EG in solvent mixtures of EG + water mixture free of monobenzone.
1546 104; while in pure water, it increases from 0.03569 104 to 0.2292 104. The solubility of monobenzone in mole fraction scale in {isopropanol (1) + water (2)}, {n-propanol (1) + water (2)}, {EG (1) + water (2)} and {ethanol (1) + water (2)} mixtures is also tabulated in the Tables 2–5. Moreover, the relationship between the mole fraction solubility and co-solvent compositions and temperature is given in Fig. 3. It can also be observed from the Tables 2–5 and Fig. 3 that the values of monobenzone solubility show a function of solvent compositions and temperature for all the studied mixtures. The mole fraction solubility of monobenzone rises with
increasing temperature and mass fraction compositions of isopropanol, n-propanol, EG and ethanol. It is larger in the (ethanol + water) mixture than in the (isopropanol + water), (npropanol + water) and (EG + water) mixtures at the same temperature and co-solvent composition. 4.3. Solvent effect The chief type and degree of the solvent effect is examined by the use of KAT-LSER model in the four mixtures at 298.15 K. In an attempt to use this technique, the molar volume of monoben-
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Y. Zhou et al. / J. Chem. Thermodynamics 142 (2020) 106023
Table 5 Equilibriummole fraction solubility xeT;W 104 of monobenzone in ethanol (w) + water (1-w) mixture with different mass fractions ranging from T = (293.15 to 328.15) K under 4 a e 101.2 kPa. xT;W 10 . T/K
283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15
W 0
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
0.7000
08,000
0.9000
1
0.03569 0.04488 0.05644 0.07042 0.08707 0.1067 0.1300 0.1574 0.1907 0.2292
0.3788 0.4544 0.5472 0.6774 0.8068 0.9560 1.159 1.367 1.580 1.862
2.014 2.365 2.758 3.307 3.844 4.595 5.315 6.147 7.050 8.226
6.480 7.555 8.894 10.58 12.12 13.95 16.22 18.68 21.48 24.55
15.26 17.56 20.95 24.53 28.00 32.06 37.16 42.22 48.57 55.29
31.53 35.90 43.84 51.33 62.71 68.90 78.96 90.90 104.7 126.7
62.85 73.40 88.00 105.1 123.1 135.0 155.1 175.6 204.9 244.6
116.9 135.5 160.5 189.9 217.7 245.9 279.3 317.0 360.0 414.7
208.5 241.6 281.1 326.3 377.8 425.8 478.7 540.5 607.0 692.2
341.7 391.7 453.5 521.8 594.0 677.8 764.5 853.8 955.2 1081
483.0 559.8 643.6 736.9 846.0 956.1 1088 1227 1367 1546
a Standard uncertainties u are u(T) = 0.02 K, u(p) = 0.45 kPa; Relative standard uncertainty ur is ur (x) = 0.053. Solvent mixtures were prepared by mixing various masses of the solvents with relative standard uncertainty ur(w) = 0.0002. w represents the mass fraction of ethanol in solvent mixtures of ethanol + water mixture free of monobenzone.
Fig. 2. Mole fraction solubility of monobenzone in pure solvents at different temperatures: ., ethanol; d, propanol; w, isopropanol; ▲, EG; ◆, water.
zone (Vs), Hildebrand solubility parameter (dH) and KAT parameters should be known. The desired parameters a, b, p* and dH for the isopropanol + water, EG + water, n-propanol + water and ethanol + water mixtures can be available from the publications [10,34]. These data are collected and presented in Table S1 of Supporting material. The molar volume of monobenzone, Vs = 158.9 cm3mol1, is calculated by the use of molar mass divided by density (1.26 gcm3) [35]. For a comprehensive investigation, in addition to the general expression of the KAT-LSER model described in Eq. (2), 14 supplementary expressions for the KAT-LSER model are performed through different combinations of the four interactional energy terms. The data presented in Table S1 along with the experimental solubility data are applied to the 15 expressions by means of the multiple linear regression analysis. The results via regression are tabulated in Tables S2–S5 of Supporting material. An appropriate expression for the KAT-LSER model must comprise ci=1-3 greater than 0 and ci=4 < 0. The reason is that the solute–solvent interactions favour the drug solubility and solvent–solvent interactions disfavor the drug solubility. Amongst the remaining expressions that meet these criteria, the best model for the solvent effect is the expression with maximum F-statistic. In this way, the best KAT-LSER model is designated and bolded in the Tables S2–S5. Tables S2 and S3 illustrate that the dipolarity-polarizability and cavity term are chiefly responsible for the variation in the monobenzone solubility in isopropanol (1) + water (2), n-
propanol (1) + water (2), EG (1) + water (2) and ethanol (1) + water (2) mixtures. The best models explain 99%, 98%, 100% and 100% of variation in the four co-solvent mixtures, respectively. The contributions to the solvent effect from p* term and cavity term are 31.4% and 68.6% in isoropanol (1) + water (2) mixture, 37.3% and 62.7% in n-propanol (1) + water (2) mixture, 35.4% and 64.6% in EG (1) + water (2) mixture and 39.3% and 60.7% in ethanol (1) + water (2) mixture, respectively. Examination of c4/c1 ratio discloses that the monobenzone solubility is nearly 1.5 to 2 times more sensitive to the cavity term than to the dipolarity-polarizability term in the four aqueous co-solvent mixtures. The solvent–solvent interactions mainly control the co-solvency effect upon solubility. In other words, weakening of the cohesive force between solvent molecules with increasing the mole fraction composition of organic solvents of isopropanol, n-propanol, EG and ethanol is responsible for improvement of the monobenzone solubility in these aqueous mixtures. The reasons of this outcome are discussed as below. Monobenzone is a Lewis acid and may make hydrogenbonding interactions by the use of its hydroxyl group, Fig. 1, with both of water and protic solvents (isopropanol, n-propanol, EG and ethanol), even though the interactions is expected to be strong with isopropanol, n-propanol, EG and ethanol that have larger b than water, Table S1. Additionally, monobenzone can also be act as a Lewis base owing to its oxygen atoms in -OH and -O- groups, and may form further hydrogen-bonding interactions with water. Nevertheless, for the creation of these types of interactions appropriately, it is mandatory the solute to be well suited and oriented within the solvent structure. Monobenzone molecule has relatively large size, so it cannot readily enter into the voids in solvent network structure, especially for the water that presents high association. It means that for solubility of monobenzone, cavities with large size for solute accommodation should be formed within the solvent. Consequently, it is not surprising that the cavity term has the most noteworthy effect upon the variation of monobenzone solubility in these aqueous mixtures. So, as the mole fraction compositions of alcohols increase in mixtures, the Hildebrand solubility parameter decreases, which decreases the work required to disrupt the solvent-solvent interactions and to create cavity and results in an increase in the monobenzone solubility. The results discussed above indicate that the nature of solvent effect upon the variations of monobenzone solubility is dependent upon the complexity of intermolecular interactions in the co-solvent mixtures that may cause difference in the behavior of solvents in solvation shell of the solute and the bulk owing to occurrence of preferential solvation. For that reason, a comprehensive analysis of preferential solvation is required to clarify how solvent influences the chemical properties in co-solvent solutions, as made in next section.
Y. Zhou et al. / J. Chem. Thermodynamics 142 (2020) 106023
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Fig. 3. Mole fraction solubility (x) of monobenzone in (a) isopropanol (w) + water (1-w), (b) n-propanol (w) + water (1-w), (c) EG (w) + water (1-w) and (d) ethanol (w) + water (1-w) mixed solutions with various mass fractions at different temperatures: w, mass fraction; j, w = 0 [10]; d, w = 0.1000; ▲, w = 0.2000; ., w = 0.3000; ◆, w = 0.4000; w, w = 0.5000; 4, w = 0.6000; s, w = 0.7000; q, w = 0.8000; 5, w = 0.9000; h, w = 1 [10]. —, calculated curves by the Jouyban Acree model.
4.4. Preferential solvation analysis The transfer Gibbs energy (Dtr Go3;2!1þ2 ) computed by using Eq. (16) at 298.15 K are presented in Table S6 of Supplementary material. As well, the values of Dtr G03;2!1þ2 are graphically shown in Fig. S2 of Supplementary material. The regressed curve-fitting coefficients of Eq. (17) are presented in Table S7. Thus, the D values are obtainable from first derivative of the Eq. (12) solved based on the co-solvent compositions varying by 0.05 in mole fraction scale of isopropanol (1), n-propanol (1), EG (1) or ethanol (1) and tabulated in Tables S8–S11 of Supplementary material. In terms of the available values of RTjT and Q in conjunction with the partial molar volumes of co-solvents and water in the aqueous co-solvent mixtures at 298.15 K [6,9], the G1,3 and G2,3 values in the four mixtures can be acquired via Eqs. (9) and (10) and also presented in the Tables S8–S11. Because the partial molar volumes of monobenzone (3) in these mixtures are not available from publications, herein they are considered as similar with that of the neat monobenzone [6,9,10,34]. As a result, the radius (r3) of monobenzone molecule is computed to be 0.398 nm by using Eq. (14). The values of Vcor and dx1,3 achieved through regression are also presented in the Tables S8–S11 for the isopropanol (1) + water (2), n-propanol (1) + water (2), EG (1) + water (2) and ethanol (1) + water (2) mixtures, respectively. What is more, the dx1,3 dependence on the co-solvent compositions is shown graphically in Fig. 4. It can be found from Fig. 4 that the values of dx1,3 non-linearly change with the co-solvent (1) proportions in the four aqueous solutions.
Fig. 4. dx1,3 values of monobenzone (3) from neat water (2) to isopropanol (1) + water (2), n-propanol (1) + water (2), EG (1) + water (2) and ethanol (1) + water (2) mixtures at 298.15 K. j, isopropanol (1) + water (2); d, n-propanol (1) + water (2); ▲, EG (1) + water (2); ., ethanol (1) + water (2).
Addition of the co-solvent (1) makes negative the values of dx1,3 of monobenzone (3) from neat water up to x1 = 0.20 mol fraction of n-propanol and x1 = 0.25 mol fraction of isopropanol/EG/ethanol. Within the above regions, the local mole fractions of isopropanol (n-propanol, EG or ethanol) (1) are samller than that of the mix-
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tures and consequently the values of dx1,3 are negative, which indicates that the monobenzone is preferentially solvated by the water. Perhaps the structuring of the water molecules around the nonpolar aromatic group of monobenzone (i.e. hydrophobic hydration of benzene ring group) contributes to decreasing the net values of dx1,3 to negative in the isopropanol (n-propanol, EG or ethanol) mixtures. The minimum negative values are acquired with the mole fraction compositions x1 = 0.10 with dx1,3 = 6.6 41 102 for the isopropanol (1) + water (2) mixture, x1 = 0.05 with dx1,3 = 7.109 102 for the n-propanol (1) + water (2) mixture, x1 = 0.05 with dx1,3 = 3.496 102 for the EG (1) + water (2) mixture and x1 = 0.05with dx1,3 = 6.666 102 for the ethanol (1) + water (2) mixture. In the n-propanol (1) + water (2) mixture with compositions 0.20 < x1 < 1 and isopropanol (EG or ethanol) (1) + water (2) mixtures with compositions 0.25 < x1 < 1, the local mole fractions of isopropanol (n-propanol, EG or ethanol) are greater than those of the mixtures and therefore the dx1,3 values are positive, which indicates the preferential solvation of monobenzone by the isopropanol (n-propanol, EG or ethanol). The co-solvent action to improve the solubility of monobenzone may be related to the breaking of ordered structure of water nearby the polar moiety of monobenzone which increases the solvation having maximum values near to x1 = 0.60 with dx1,3 = 11.25 102 for the isopropanol (1) + water (2), x1 = 0.45 with dx1,3 = 2.705 102 for the n-propanol (1) + water (2), x1 = 0.50 with dx1,3 = 1.669 102 for the EG (1) + water (2) and x1 = 0.65 with dx1,3 = 6.144 102 for the ethanol (1) + water (2) mixtures. According to the analysis of group structure and function of the monobenzone molecule, monobenzone may act as a Lewis acid in the mixtures attributable to the ability of acidic hydrogen atom in its –OH group (Fig. 1) to form hydrogen bonds with protonacceptor functional groups of the co-solvents (oxygen atoms in – OH group). As well, monobenzone may also act as a Lewis base thanks to the free electron pairs in hydrogen atom of –OH and oxygen atom of -O- (Fig. 1), which can interact with acidic hydrogen atoms of water. Based on the analysis results of preferential solvation, it is conjecturable that in the ranges of 0.20 < x1 < 1 for npropanol and 0.25 < x1 < 1 for isopropanol/EG/ethanol, monobenzone is acting as a Lewis acid with n-propanol, isopropanol, EG or ethanol molecules, because the co-solvents show more basic than the water, as defined by the Kamlet–Taft hydrogen bond acceptor parameters, i.e. b = 0.75 for ethanol, b = 0.84 for isopropanol, b = 0.90 for n-propanol, b = 0.52 for EG and b = 0.47 for water [36]. While in the other ranges, where the monobenzone is preferentially solvated by the water, monobenzone is mainly acting as a Lewis base in front to water because the water is more acidic than n-propanol, isopropanol, EG and ethanol as described by the Kamlet–Taft hydrogen bond donor parameters, i.e. a = 1.17 for water, a = 0.76 for isopropanol, a = 0.84 for npropanol, a = 0.90 for EG and a = 0.86 for ethanol, respectively [36]. 4.5. Solubility modelling In terms of the solubility values determined, the equation parameters of the Jouyban–Acree model are regressed by using the Mathcad software and tabulated in Table S12 of the Supporting material in consort with the values of RAD and RMSD. The backcalculated solubility values of monobenzone in the (isopropanol + water), (n-propanol + water), (EG + water) and (ethanol + water) solutions are graphically shown in Fig. 3. Analysis of the Table S12 shows that the RAD values are 2.58% for isopropanol + water mixture, 3.17% for n-propanol + water mixture, 2.02% for EG + water mixture and 2.14% for ethanol + water mixture. Besides, the maximum RMSD value is 7.64 104, which is achieved for (ethanol + water) mixture. Accordingly, the Jouyban-
Acree model may present accepted correlation results for these mixtures. 5. Conclusion The solubility of monobenzone in four mixtures of isopropanol (1) + water (2), n-propanol (1) + water (2), EG (1) + water (2) and ethanol (1) + water (2) was experimentally studied via the saturation shake-flask technique within temperatures from 283.15 to 328.15 K under local atmospheric pressure of 101.2 kPa. The maximum solubility value was observed in the neat solvents of isopropanol, n-propanol, EG or ethanol. The monobenzone solubility in mole fraction scale was mathematically expressed via the Jouyban-Acree model attaining the RAD values smaller than 3.17%. The dx1,3 values for isopropanol, n-propanol, EG or ethanol were positive in the intermediate and isopropanol (n-propanol, EG or ethanol)-rich compositions, which indicated that the monobenzone was preferentially solvated by the isopropanol (npropanol, EG or ethanol). The higher solvation by the co-solvent could be explained upon higher basic behavior of the alcohols which interacted with the Lewis acidic groups of monobenzone. The KAT-LSER model successfully described the solvent effect upon the monobenzone solubility variation. Analysis results revealed that the cavity term and dipolarity-polarizability were the major factors which leaded to the solubility variation of monobenzone in the isopropanol/n-propanol/EG/ethanol + water mixtures. CRediT authorship contribution statement Yanyan Zhou: Methodology, Project administration, Writing original draft. Jiaxin Wu: Investigation. Ali Farajtabar: Formal analysis, Data curation. Jian Wang: Data curation, Resources. Hongkun Zhao: Conceptualization, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jct.2019.106023. References [1] A. Jouyban, Handbook of Solubility Data for Pharmaceuticals, CRC Press, BocaRaton, FL, 2010. [2] J.T. Rubino, Cosolvents and Cosolvency, in: J. Swarbrick, J.C. Boylan (Eds.), Encyclopedia of Pharmaceutical Technology, 3, Marcel Dekker, New York, NY, 1988. [3] P. Kolárˇ, J.W. Shen, A. Tsuboi, T. Ishikawa, Solvent selection for pharmaceuticals, Fluid Phase Equilibr. 194–197 (2002) 771–782. [4] A. Avdeef, Absorption and Drug Development, Solubility, in: Permeability and Charge State, Wiley-Interscience, Hoboken, NJ, 2003. [5] M.E. Aulton, Pharmaceutics. The Science of Dosage Forms Design, second ed., Churchill Livingstone, London, 2002. [6] A. Jouyban, W.E. Acree Jr, F. Martínez, Modelling the solubility and preferential solvation of gallic acid in cosolvent + water mixtures, J. Mol. Liq. 224 (2016) 502–506. [7] Y. Marcus, Solvent Mixtures: Properties and Selective Solvation, Marcel Dekker Inc, New York, NY, 2002. [8] Y. Marcus, Preferential Solvation in Mixed Solvents, in: P.E. Smith, E. Matteoli, J.P. O’Connell (Eds.), Fluctuation Theory of Solutions: Applications in Chemistry, Chemical Engineering, and Biophysics, CRC, Press, Taylor & Francis Group, BocaRaton, FL, 2013. [9] X.B. Li, C. Cheng, Y. Cong, C.B. Du, H.K. Zhao, Preferential solvation of pioglitazone hydrochloride in some binary co-solvent mixtures according to the inverse Kirkwood-Buff integrals method, J. Chem. Thermodyn. 110 (2017) 218–226.
Y. Zhou et al. / J. Chem. Thermodynamics 142 (2020) 106023 [10] Y. Yuan, A. Farajtabar, L.M. Kong, H.K. Zhao, Thermodynamic solubility modelling, solvent effect and preferential solvation of p-nitrobenzamide in aqueous co-solvent mixtures of dimethyl sulfoxide, ethanol, isopropanol and ethylene glycol, J. Chem. Thermodyn. 136 (2019) 123–131. [11] M.S. Mohamed, T. Maki, M.M. Shah, Y. Ichinose, Synthesis and antimicrobial activity of nitrobenzyl-oxy-phenol derivatives, Biol. Pharm. Bull. 39 (2016) 1888–1892. [12] O.M. Rordam, E.W. Lenouvel, M. Maalo, Successful treatment of extensive vitiligo with monobenzone, J. Clin. Aesthet Dermatol. 5 (2012) 36–39. [13] J.J. Nordlund, B. Forget, J. Kirkwood, A.B. Lerner, Dermatitis produced by applications of monobenzone in patients with active vitiligo, Arch. Dermatol. 121 (1985) 1141–1144. [14] J.G. van den Boorn, D.I. Picavet, P.F. van Swieten, H.A. van Veen, D. Konijnenberg, P.A. van Veelen, T. van Capel, E.C. Jong, E.A. Reits, J.W. Drijfhout, J.D. Bos, C.J.M. Melief, R.M. Luiten, Skin-depigmenting agent monobenzone induces potent T-cell autoimmunity toward pigmented cells by tyrosinase haptenation and melanosome autophagy, J. Invest. Dermatol. 131 (2011) 1240–1251. [15] C.S. Lee, Y.H. Joo, H.S. Baek, M. Park, J.H. Kim, H.J. Shin, N.H. Park, J.H. Lee, Y.H. Park, S.S. Shin, H.K. Lee, Different effects of five depigmentary compounds, rhododendrol, raspberry ketone, monobenzone, rucinol and AP736 on melanogenesis and viability of human epidermal melanocytes, Exp. Dermatol. 25 (2016) 44–49. [16] A. Jouyban, Review of the cosolvency models for predicting solubility of drugs in water-cosolvent mixtures, J. Pharm. Pharmaceut. Sci. 11 (2008) 32–58. [17] F. Martínez, A. Jouyban, W.E. Acree Jr, Pharmaceuticals solubility is still nowadays widely studied everywhere, Pharmaceut. Sci. 23 (2017) 1–2. [18] S.H. Yalkowsky, Solubility and Solubilization in Aqueous Media, American Chemical Society and Oxford University Press, New York, 1999, pp. 180–235. [19] M. Mohammadzade, M. Barzegar-Jalali, A. Jouyban, Solubility of naproxen in 2propanol + water mixtures at various temperatures, J. Mol. Liq. 206 (2015) 110–113. [20] A. Jouyban, J. Shokri, M. Barzegar-Jalali, D. Hassanzadeh, W.E. Acree Jr., T. Ghafourian, A. Nokhodchi, Solubility of chlordiazepoxide, diazepam, and lorazepam in ethanol + water mixtures at 303.2 K, J. Chem. Eng. Data 54 (2009) 2142–2145. [21] D.R. Delgado, F. Martínez, Solubility and solution thermodynamics of some sulfonamides in 1-propanol + water mixtures, J. Sol. Chem. 43 (2014) 836–852. [22] F. Shakeel, M.F. Alajmi, N. Haq, N.A. Siddiqui, P. Alam, A.J. Al-Rehaily, Solubility and thermodynamic function of a bioactive compound bergenin in various pharmaceutically acceptable neat solvents at different temperatures, J. Chem. Thermodyn. 101 (2016) 19–24. [23] R.W. Taft, J.L.M. Abboud, M.J. Kamlet, M.H. Abraham, Linear solvation energy relations, J. Sol. Chem. 14 (1985) 153–186.
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[24] R.W. Taft, M.J. Kamlet, The solvatochromic comparison method. 2. The alpha.scale of solvent hydrogen-bond donor (HBD) acidities, J. Am. Chem. Soc. 98 (1976) 2886–2894. [25] M.J. Kamlet, R.M. Doherty, J.L.M. Abboud, M.H. Abraham, R.W. Taft, Linear solvation energy relationships: 36. Molecular properties governing solubilities of organic nonelectrolytes in water, J. Pharm. Sci. 75 (1986) 338–349. [26] M.J. Kamlet, J.L.M. Abboud, M.H. Abraham, R.W. Taft, Linear solvation energy relationships. 23. A comprehensive collection of the solvatochromic parameters, pi.*,alpha., and.beta., and some methods for simplifying the generalized solvatochromic equation, J. Org. Chem. 48 (1983) 2877–2887. [27] G.B. Yao, Q.C. Yao, Z.X. Xia, Z.H. Li, Solubility determination and correlation for o-phenylenediamine in (methanol, ethanol, acetonitrile and water) and their binary solvents from T = (283.15–318.15) K, J. Chem. Thermodyn. 105 (2017) 179–186. [28] J. Wang, A.L. Xu, R.J. Xu, Determination and correlation of terephthaldialdehyde solubility in (ethanol, isopropanol, ethyl acetate, isopentanol) + N, N-dimethylformamide mixed solvents at temperatures from 273.15 K to 318.15 K, J. Chem. Thermodyn. 105 (2017) 327–336. [29] M.J. Kamlet, R.M. Doherty, J.L. Abboud, M.H. Abraham, R.W. Taft, Solubility: a new look, Chemtech 16 (1986) 566–576. [30] A. Jouyban, M.A.A. Fakhree, Experimental, Computational Methods Pertaining to Drug Solubility, in: W.E. Acree (Ed.), Toxicity and Drug Testing, InTech, Rijeka, Croatia, 2012, pp. 187–218. [31] J.J. Ma, J.H. Liang, J.C. Han, M. Zheng, H.K. Zhao, Solubility modelling and solvent effect for flubendazole in 12 neat solvents, J. Chem. Eng. Data 64 (2019) 1237–1243. [32] G.Q. Chen, J.H. Liang, J.C. Han, H.K. Zhao, Solubility modelling, solute-solvent interactions and thermodynamic dissolution properties of pnitrophenylacetonitrile in sixteen mono-solvents at temperatures ranging from 278.15 K to 333.15 K, J. Chem. Eng. Data 64 (2019) 315–323. [33] H. Barange, S. Asghar, Development of analytical method for simultaneous estimation of hydroquinone and Monobenzone in topical formulation by RPHPLC, World J. Pharm. Res. 6 (2017) 742–753. [34] Q. He, M. Zheng, H.K. Zhao, Baicalin solubility in aqueous co-solvent mixtures of methanol, ethanol, isopropanol and n-propanol revisited: solvent-solvent and solvent-solute interactions and IKBI preferential solvation analysis, Phy. Chem. Liq. (2019), https://doi.org/10.1080/00319104.2019.1660981. [35] SciFinder: https://scifinder.cas.org/scifinder/view/scifinder/scifinderExplore. jsf, Nov 15, 2019. [36] Y. Marcus, The Properties of Solvents, John Wiley & Sons, Chichester, 1998.
JCT 2019-921
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Solubility of monobenzone in aqueous co-solvent mixtures of several alcohols: Determination, modelling and thermodynamic aspects analysis Yanyan Zhou a, Jiaxin Wu a, Ali Farajtabar b, Jian Wang a, Hongkun Zhao a,⇑ a b
College of Chemistry & Chemical Engineering, YangZhou University, YangZhou, Jiangsu 225002, People’s Republic of China Department of Chemistry, Jouybar Branch, Islamic Azad University, 4776186131 Jouybar, Iran
a r t i c l e
i n f o
Article history: Received 1 December 2019 Accepted 4 December 2019 Available online 7 December 2019 Keywords: Monobenzone Solubility Jouyban-Acree model Preferential solvation Solvent effect
a b s t r a c t Investigation upon the solubility of monobenzone in aqueous co-solvent mixtures of isopropanol, n-propanol, ethylene glycol (EG) and ethanol was performed via a saturation shake-flask technique at temperatures from 283.15 K to 328.15 K under pressure of p = 101.2 kPa. Experimental solubility magnitude presented positive dependence upon the mass fraction of each co-solvent and temperature. The greatest solubility value on the mole fraction scale was observed in the neat co-solvents. The equilibrated solids with corresponding co-solvent mixtures were characterized through an X-ray power diffraction (XRD) technique, demonstrating the absence of polymorphic transformation or solvate formation. The Jouyban-Acree model was adopted to mathematically describe the monobenzone solubility data. The maximum magnitudes of RAD and RMSD were 3.17 102 and 7.64 104, respectively. The local mole fractions of isopropanol (n-propanol, EG or ethanol) and water adjacent monobenzone were quantitatively studied by the Inverse Kirkwood–Buff integrals method. The parameters of preferential solvation for the isopropanol (n-propanol, EG or ethanol) were positive in the isopropanol (n-propanol, EG or ethanol) mixtures in intermediate and alcohol-rich compositions, indicating that monobenzone was preferentially solvated by the isopropanol (n-propanol, EG or ethanol). Monobenzone mainly acted as a Lewis acid which interacted with proton-acceptor functional group of the alcohols. Furthermore, the analysis of linear solvation energy relationships was made with a suitable combination of the solvent polarity descriptors to reveal the nature of intermolecular interactions bringing about the solubility variation in the cosolvent mixtures. Ó 2019 Elsevier Ltd.
1. Introduction In recent times, the investigation on the solubility of drugs as well as pharmaceutical intermediates becomes an extensive topic in the pharmaceutical fields. The drugs’ solubility in solvent mixtures as a function of solvent composition and temperature is of great importance in the design of liquid dosage forms, raw material purification and acquiring the mechanisms of the drug dissolutions about physical and chemical stability [1–3]. In addition, the information upon solubility data of the active ingredients is a significant physico-chemical property in the pharmaceutical design process, because it has crucial effect on the pharmacokinetic and biopharmaceutical properties and efficacy of the drugs [4,5]. Furthermore, the solubility dependence upon temperature and co-solvent compositions in co-solvent mixtures is generally employed to make a thermodynamic analysis to insight in-depth into the intermolecu⇑ Corresponding author. E-mail address: [email protected] (H. Zhao). https://doi.org/10.1016/j.jct.2019.106023 0021-9614/Ó 2019 Elsevier Ltd.
lar mechanisms concerning the drug dissolution process and to estimate the preferential solvation of a solute by co-solvent components [6–10]. Monobenzone (CAS No, 103-16-2; IUPAC name, 4phenylmethoxyphen; molecule structure, Fig. 1) is a monobenzyl ether of hydroquinone medically used for depigmentation. It occurs as a nearly tasteless crystalline powder, soluble in alcohol and essentially insoluble in water. It exerts a depigmenting influence on the skin of mammals by increasing the excretion of melanin from the melanocytes. It may also cause destruction of the permanent and melanocytes depigmentation [11–15]. Despite the fact this drug has been recognized for many years, the physico-chemical properties relevant to monobenzone solubility in neat solvents and solvent mixtures have not yet been reported in the publications. So this paper tries to deliver an idea about the relative stabilization of monobenzone in aqua-organic mixtures with respect to water and the comprehensive solutesolvent and solvent-solvent interactions therein.
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Y. Zhou et al. / J. Chem. Thermodynamics 142 (2020) 106023
solvent-dependent properties. The distinguished success of the KAT-LSER method has been discovered in clarifying extensive chemical phenomena including the solid solubility in pure and mixed solvents [25,26]. So, the other objective of this paper is to examine the solvent effect upon the variation of monobenzone solubility in aqueous mixtures of isopropanol, n-propanol, EG and ethanol by using the KAT-LSER model to demonstrate the nature and relative importance of intermolecular interactions leading to the solvent effect. Fig. 1. Molecular structure of monobenzone.
Several theoretical and semi-empirical solubility models have been put forward to predict the solubility of drug in the mixtures [16], however the availability of solubility data through experiment is still vital for a lot of pharmaceutical scientists [1,17]. It is evident that the monobenzone solubility in water is low, which may cause low formulation or bioavailability difficulty in clinical development [4,5]. Co-solvency is a powerful solubilization method to improve the solid solubility in the pharmaceutical and chemical industries [1,2,18]. Although this method has been extensively employed in pharmacies several years ago, only recently the mechanisms relating to the increase or decrease in drug solubility start to be approached from a deep thermodynamic point of view, along with the preferential solvation analysis of a solute by the components in mixtures [6–10]. The solvent of isopropanol presents a strong odour. A number of non-polar substances and isopropanol are soluble with each other with any composition. Generally, it is employed in solvent mixtures with other solvents or solely for diverse purposes relevant to transepidermal uses and penetration-improving pharmaceutical compositions for the percutaneous [19]. Ethanol is a commonly cosolvent employed in the pharmaceutical liquid formulations with the concentrations less than 50%. As well, ethanol has significant effect on distribution, excretion, absorption and metabolism of the drugs [20]. Although n-Propanol is not generally used as a co-solvent in the design of liquid medicines, it is a solvent commonly employed in the pharmaceutical arenas for cellulose esters and resins [21]. Ethylene glycol (EG) is a safe and acceptable solvent for pharmaceutical usage [10,22]. In terms of the aspects discussed above, the chief objects of this contribution are to report the solubility of monobenzone in aqueous co-solvent solutions of isopropanol, n-propanol, ethylene glycol (EG) and ethanol and evaluate the preferential solvation of monobenzone by the co-solvents of isopropanol, n-propanol, EG and ethanol. As a generalized approach, the linear solvation energy relationships (abbreviated as LSER) treat the solvent effect through separating the interactions of solvent-solute molecules into two types, which are named as non-specific (dipole-dipole dispersion and dipole-induced dipole) and specific (hydrogen bonding) interactions. In addition, each interaction term presents a linear contribution to the Gibbs free energy of the solvent-dependent properties [23]. In this way, these empirical scales provide a suitable way to characterize the solvent capability to interact with the solutes, well-known as the solvent polarity, at a molecular level. Kamlet, Abboud and Taft (KAT) [23,24] provide extensively employed solvent scales through the solvatochromic investigation of pairs of probing molecules in solvents with a set of interacting properties. The solvent scales involve the dipolarity/polarizability (p*), hydrogen bond basicity (b) and hydrogen bond acidity (a), which may be directly achieved through determining the energy changes caused by corresponding intermolecular interactions between the solute and solvent molecules. Accordingly, the examination of solvent effect in terms of the LSER model reveals the nature and extent of solute-solvent interactions that influence the
2. Theoretical considerations The Jouyban–Acree model [10,16,27,28] is employed here to describe mathematically the monobenzone solubility in the aqueous mixtures of isopropanol, n-propanol, EG and ethanol. As well, the solvent effect upon the monobenzone solubility was examined through the KAT-LSER model [10,23]. 2.1. Jouyban-Acree model The expression of the Jouyban-Acree model is given in Eq. (1) [10,16,27,28], which has been successfully employed to describe the dependence of solute solubility on both temperature and solvent composition in solvent mixtures.
lnxw;T ¼ w1 lnx1;T þ w2 lnx2;T þ
2 w1 w2 X J ðw1 w2 Þi T=K i ¼ 0 i
ð1Þ
In Eq. (1), xw,T refers to the mole fraction solubility of monobenzone in the mixtures at absolute temperature T; w1 and w2 refer to the mass fraction of solvent 1 (isopropanol, n-propanol, EG or ethanol) and solvent 2 (water) in the monobenzone-free mixtures, respectively; xi,T denote the mole fraction solubility of monobenzone in neat solvent i at T; and Ji are the Jouyban-Acree model parameters. 2.2. KAT-LSER model The KAT-LSER model is practiced to justify the solvency effect on improvement of monobenzone solubility in mixtures. Eq. (2) is the KAT-LSER model in its universal form [23,25,29].
lnxw ¼ c0 þ c1 p þc2 b þ c3 a þ c4
V s d2H 100RT
! ð2Þ
In general, the solubility free energy is presented by lnxw; c2 b, c3 a and c1 p* refer to the energy term relevant to the specific and non-specific solute-solvent interactions, wherein the coefficients ci=1-3 disclose sensitivity of the solute solubility to their corresponding energy terms; the term relevant to c4 in Eq. (2) signifies the cavity term, which defines the energy term for solvent-solvent interactions. This term calculates the energy of solute accommodation as a product of solute molar volume (Vs) and squared Hildebrand solubility parameter (dH) of the solvent. The temperature T and universal gas constant R are considered herein to deliver a dimensionless value for this term. As a result, the coefficient c4 reveals susceptibility of the solute solubility variation to the solvent-solvent interactions. c0 is intercept of the KAT-LSER model at a = b = p*=dH = 0. In practice, the mole fraction solubility is firstly converted into lnxw, and then examined by using the Eq. (2) by means of a multiple linear regression analysis. The objective function is given as Eq. (3) in the correlation.
F¼
X
lnxew;T lnxcw;T
2
ð3Þ
3
Y. Zhou et al. / J. Chem. Thermodynamics 142 (2020) 106023
Additionally, the relative average deviation (RAD) and rootmean-square deviation (RMSD), which are expressed as Eqs. (4) and (5), respectively, are also used herein so as to evaluate the Jouyban-Acree model.
1 0 c e 1 X @xw;T xw;T A RAD ¼ N xew;T sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PN 2 c e i¼1 ðxw;T xw;T Þ RMSD ¼ N
ð4Þ
ð5Þ
wherein, N refers to the number of experimental data points. xew;T is the solubility of monobenzone in mole fraction scale attained through experiment; and xcw;T , computed through the studied model.
nents in aqueous co-solvent solutions; and V 3 , the partial molar volumes of monobenzone in mixtures. The function D (expressed as Eq. (12)) presents the derivative of standard molar Gibbs energy of transfer of monobenzone from neat water (2) to aqueous mixtures of co-solvent (1) with respect to the co-solvent proportions. The function Q (expressed as Eq. (13)) is the second derivative of excess molar Gibbs energy of mixing of the two solvents (GExc 1 þ 2) with respect to water composition in the solutions. Vcor denotes the correlation volume. r3 is the radius of monobenzone molecule attained by means of Eq. (14), where NAv is the Avogadro’s number.
D¼
@ Dtr Goð3;2!1þ2Þ
!
@x1
ð12Þ T;P
" # @ 2 GExc 1þ2 Q ¼ RT þ x1 x2 @x22
ð13Þ
T;p
2.3. Preferential solvation The way of Inverse Kirkwood–Buff integrals (IKBIs), Eq. (6), is a powerful technique to investigate the preferential solvation of a non-electrolyte or a weak electrolyte in the aqueous mixtures of co-solvent. It expresses the local solvent composition near a nonelectrolyte or a weak electrolyte compared to the global mixture proportions [6–10].
Z Gi;3 ¼ 0
r cor
ðg i;3 1Þ4pr 2 dr
ð6Þ
here, gi,3 signifies the pair correlation function for the co-solvent molecules in the mixtures of co-solvent (1) + water (2) neighbouring the monobenzone (3); r refers to the distance between the molecule centres of solute monobenzone (3) and that of water (2) or co-solvent (1); rcor presents the correlation distance, for which gi,3 (r > rcor) is approximately equal to 1. Accordingly, the integral value is essentially zero under the condition of r > rcor. The parameter of preferential solvation (dx1,3) of monobenzone (3) by the co-solvent (1) in mixtures of co-solvent (1) + water (2) is described as [6–10].
dx1;3 ¼ xL1;3 x1 ¼ dx2;3
ð7Þ
Here x1 signifies bulk mole fraction of the co-solvent (1) in initial aqueous mixtures of co-solvent; and xL1;3 , local mole fraction of the co-solvent (1) near the solute monobenzone (3). If the magnitude of dx1,3 is greater than zero, the monobenzone is preferentially solvated by co-solvent (1); while if it is smaller than zero, the monobenzone is preferentially solvated by water (2). The values of dx1,3 may be attained from the IKBIs method for the individual solvent compositions [6–10].
dx1;3 ¼
x1 x2 ðG1;3 G2;3 Þ x1 G1;3 þ x2 G2;3 þ V cor
ð8Þ
With
G1;3 ¼ RT jT
G2;3 ¼ RT jT V 3 þ
ð9Þ
x1 V 1 D Q
3 1=3 V cor ¼ 2522:5 r3 þ 0:1363 xL1;3 V 1 þ xL2;3 V 2 0:085
ð10Þ
ð11Þ
wherein, jT refers to the isothermal compressibility of the solu
ð14Þ
Because the values of jT depend on the compositions of the mixtures, it is not known for the studied solutions examined. On the other hand, the contribution of RTjT term to the IKBIs is fairly slight. For that reason, the jT values in solutions are approximately assessed via considering an additive property according to solution compositions and available values for pure solvents by [6–10]:
jT ¼ x1 joT;1 þ x2 joT;2
ð15Þ
where xi is mole fraction of the component i in solutions; joT;i is the isothermal compressibility of the neat composition i. The standard molar Gibbs energy of transfer of monobenzone from neat water (2) to the isopropanol (1) + water (2), npropanol (1) + water (2), ethanol (1) + water (2) and EG (1) + water (2) mixtures can be computed by means of Eq. (16) from the solubility data and mathematically correlated by using an empirical equation {Eq. (17)}, where A0, A1, A2, t1 and t2 are the equation parameters.
x3;2 Dtr G03;2!1þ2 ¼ RTln x3;1þ2 x
t 1
Dtr Go3;2!1þ2 ¼ A0 þ A1 e
1
ð16Þ x
t 1
þ A2 e
2
ð17Þ
The definitive correlation volume requires iteration because it depends upon the local mole fractions neighbouring the solute. The iteration process is made by means of substituting Vcor and dx1,3 into the Eqs. (7), (8)–(11) to re-calculate xL1;3 until a nonvariant value of Vcor is gained. 3. Experimental 3.1. Materials
x2 V 2 D V3 þ Q
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 21 3 3 10 V3 r3 ¼ 4pNAV
tions. V 1 and V 2 denote the partial molar volumes of the compo-
The monobenzone provided by the Shanghai Energy Chemical Co., Ltd., China had a mass fraction of 0.984. It was recrystallized several times before experiment in solvent of ethanol. The final purity of monobenzone used in experiments was 0.996 in mass fraction scale, which was tested by way of an Agilent-1260 highperformance liquid phase chromatograph (HPLC). The solvents such as isopropanol, n-propanol, EG and ethanol were purchased from the Aladdin Industrial (Shanghai) Co., Ltd., China with the mass fraction of no less than 0.994, which was checked using a FULI 9790 gas chromatography (GC). These solvents were directly used in experiments without any additional treatment. The dis-
4
Y. Zhou et al. / J. Chem. Thermodynamics 142 (2020) 106023
tilled deionized water (conductivity < 1 lS∙cm1) achieved by means of distillation was used in this work. The details of the monobenzone and selected solvents were presented in Table 1. 3.2. Preparation of aqueous co-solvent mixtures The aqueous mixtures of isopropanol, n-propanol, EG and ethanol were prepared by means of an analytical balance with a type of BSA224S. The aqueous co-solvent mixtures used in each experiment for solubility determination were about 15 mL, which absolute error was 0.0001 g. The mass fraction proportion of isopropanol, propanol, EG and ethanol in the mixtures covered the entire range from 0 to 1. The flask was covered with a rubber stopper so as to avoid the organic solvent from escaping in the preparation process under approximately 101.2 kPa. 3.3. Solubility determination In this work, the determination of monobenzone solubility in these co-solvent mixtures was performed by means of a saturation shake-flask method as used in the previous publications [30–32], which reliability was confirmed through the solubility of benzoic acid in pure toluene [32]. An Agilent-1260 HPLC was employed in this work to quantitatively determine the monobenzone solubility in equilibrated liquid phase. Excess monobenzone was added into a 25 mL flask filled with 15 mL of the co-solvent mixtures. Then the flask containing liquid-solid solutions was placed into a thermostatic mechanical shaker obtained from the Tianjin Ounuo Instrument Co. Ltd., China, which had a standard uncertainty of 0.02 K. The systems were continually shaken by means of the mechanical shaker. With the aim of acquiring the equilibration time of the studied solutions, about 0.5 mL of liquid phase was withdrawn by means of a 2 mL syringe connected with a pore PTFE filter (0.2 lm) at intervals of 1 h and tested using the Agilent-1260 HPLC. Analysis exhibited that it took about 10 h to reach equilibrium for all the solutions. As soon as the solutions arrived at equilibrium, they were retained at static at the desired temperature to make sure that any un-dissolved solid precipitated from the solutions. The 2 mL of upper liquid phase was withdrawn via the 2 mL of precooled or preheated syringe, and then speedily transferred into a 25 mL volumetric flask that was pre-weighed by using the BSA224S analytical balance. The total amount of flask comprising the sample was weighed again through the analytical balance. The sample was diluted (if necessary) with neat methanol to 25 mL, and analysed by means of the Agilent1260 HPLC. 3.4. Analysis method The composition of monobenzone in saturated liquid was tested by the Agilent-1260 high-performance liquid chromatography. The chromatographic column employed herein was a reverse phase
column having a type of LP-C18 (250 mm 4.6 mm). The temperature the column was approximately 303 K. The wavelength of UV–vis detector was set to 290 nm [33]. The mobile phase in analysis was neat methanol, which flow rate was set to 0.8 mLmin1. Each sample was analysed three times in order to achieve the average value. The relative standard uncertainty of the determined solubility in mole fraction scale was assessed to be no more than 0.053. 3.5. X-ray powder diffraction With the aim of checking the absence of polymorph transformation or solvate formation of monobenzone in the whole experimental process, the solid phases equilibrated with their corresponding solutions were identified by the use of X-ray powder diffraction (XRD) at room temperature and local atmospheric pressure. All tests were carried out upon a HaoYuan DX-2700B (HaoYuan, China) apparatus by means of Cu Ka radiation (k = 0.1 54184 nm) at a scan speed of 6 degmin1. The tube current and tube voltage used in determination were 30 mA and 40 kV, respectively. The data of the determined XRD scans were collected from 10° to 80° (2-Theta). 4. Results and discussion 4.1. XRD analysis The determined XRD scans of raw monobenzone as well as the solids in equilibrium with their corresponding saturated liquids are given in Fig. S1 of Supporting material. It may be found that all the XRD patterns of equilibrated solids present characteristic peaks similar with the raw monobenzone. Accordingly, no solvate formation or polymorph transformation takes place in the entire determination procedure. 4.2. Solubility data The obtained solubility (x) of monobenzone on the mole fraction scale in the isopropanol, n-propanol, EG, water and ethanol within the temperature range from 283.15 K to 328.15 K is presented in Tables 2–5, and graphically shown in Fig. 2. It can be observed from this figure that the solubility of monobenzone rises with increasing experiment temperature for the pure solvents. The highest value is found in the pure solvent of ethanol, and lowest in the pure solvent of water. The dissolution capacity of monobenzone in different solvents ranks as ethanol > propanol > isopropa nol > EG > water. The mole fraction solubility of drug monobenzone shows up to approximately 104-fold improvement in going from pure water to pure ethanol. For example, if the temperature increases from 283.15 K to 328.15 K, the mole fraction solubility of monobenzone in pure ethanol rises from 483.0 104 to
Table 1 Detailed aspects upon the materials used in this work.
a b
Chemicals
Molar mass/ gmol1
Source
Initial mass fraction purity
Final mass fraction purity
Purification method
Analytical method
monobenzone ethanol isopropanol n-propanol EG Water
200.23 46.07 60.06 60.06 62.07 18.02
Shanghai Energy Chem. Co., Ltd. Aladdin Industrial (Shanghai) Co., Ltd.
0.984 0.994 0.995 0.0.995 0.996
0.996 0.994 0.995 0.0.995 0.996 Conductivity <1 mScm1
recrystallization None None None None Distillation
HPLCa GCb GC GC GC Conductivity meter
Our lab
High-performance liquid phase chromatography; Gas chromatography.
5
Y. Zhou et al. / J. Chem. Thermodynamics 142 (2020) 106023
Table 2 Equilibrium mole fraction solubility xeT;W 104 of monobenzone in isopropanol (w) + water (1-w) mixture with different mass fractions ranging from T = (283.15 to 328.15) K 4 a e under 101.2 kPa. xT;W 10 . T/K
W 0
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
0.7000
08,000
0.9000
1
283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15
0.03569 0.04488 0.05644 0.07042 0.08707 0.1067 0.1300 0.1574 0.1907 0.2292
0.2197 0.2633 0.3349 0.4016 0.4879 0.5883 0.7070 0.8404 0.9969 1.180
1.078 1.304 1.571 1.880 2.269 2.694 3.168 3.715 4.347 5.048
3.863 4.691 5.670 6.762 8.077 9.577 11.16 13.17 15.23 17.64
11.04 13.36 16.04 19.32 22.81 27.25 31.78 36.98 43.01 50.09
25.64 31.29 37.57 44.81 53.20 62.84 73.91 86.30 99.74 115.9
49.67 59.53 71.80 85.36 100.9 119.6 139.4 163.5 189.9 221.1
81.38 98.28 118.0 140.0 165.9 195.8 229.1 266.7 309.5 357.7
120.5 145.1 172.4 206.1 241.2 285.2 332.6 385.6 447.6 513.6
163.1 195.9 233.2 277.1 324.5 381.0 441.1 510.5 592.2 680.7
216.8 260.9 307.8 364.9 424.9 502.8 583.0 670.9 778.0 895.5
a Standard uncertainties u are u(T) = 0.02 K, u(p) = 0.45 kPa; Relative standard uncertainty ur is ur (x) = 0.053. Solvent mixtures were prepared by mixing various masses of the solvents with relative standard uncertainty ur(w) = 0.0002. w represents the mass fraction of isopropanol in solvent mixtures of isopropanol + water mixture free of monobenzone.
Table 3 Equilibrium mole fraction solubility xeT;W 104 of monobenzone in n-propanol (w) + water (1-w) mixture with different mass fractions ranging from T = (283.15 to 328.15) K 4 under 101.2 kPa.a xeT;W 10 . T/K
W 0
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
0.7000
08,000
0.9000
1
283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15
0.03569 0.04488 0.05644 0.07042 0.08707 0.1067 0.1300 0.1574 0.1907 0.2292
0.3930 0.4920 0.5931 0.7174 0.8690 1.042 1.242 1.442 1.745 2.154
1.956 2.381 2.918 3.526 4.242 5.086 6.005 7.134 8.388 9.785
6.851 8.404 10.11 12.24 14.65 17.45 20.54 24.31 28.22 33.39
17.15 20.34 24.30 29.15 34.19 40.01 47.08 54.36 62.53 72.85
37.69 44.46 52.93 62.25 73.64 84.74 97.09 111.6 130.0 149.7
64.98 77.60 91.02 107.0 124.2 143.6 166.5 190.4 216.8 248.9
105.8 125.4 147.4 172.3 200.9 232.6 267.7 306.1 348.8 399.1
164.3 194.7 230.4 267.4 312.5 362.3 417.1 467.5 533.0 609.9
260.8 297.8 344.0 403.7 473.2 543.1 627.9 714.0 811.2 914.5
380.7 445.7 519.5 605.4 702.1 804.4 930.9 1057 1197 1369
a Standard uncertainties u are u(T) = 0.02 K, u(p) = 0.45 kPa; Relative standard uncertainty ur is ur (x) = 0.053. Solvent mixtures were prepared by mixing various masses of the solvents with relative standard uncertainty ur(w) = 0.0002. w represents the mass fraction of n-propanol in solvent mixtures of n-propanol + water mixture free of monobenzone.
Table 4 Equilibrium mole fraction solubility xeT;W 104 of monobenzone in EG (w) + water (1-w) mixture with different mass fractions ranging from T = (283.15 to 328.15) under 4 a e 101.2 kPa. xT;W 10 . T/K
283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15
W 0
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
0.7000
08,000
0.9000
1
0.03569 0.04488 0.05644 0.07042 0.08707 0.1067 0.1300 0.1574 0.1907 0.2292
0.1229 0.1540 0.1912 0.2356 0.2898 0.3523 0.4265 0.5125 0.6143 0.7295
0.3462 0.4317 0.5321 0.6533 0.7960 0.9639 1.160 1.388 1.661 1.958
0.8647 1.060 1.307 1.590 1.935 2.336 2.812 3.323 3.975 4.676
2.190 2.697 3.278 4.010 4.837 5.866 6.950 8.335 9.856 11.77
4.531 5.580 6.751 8.610 10.45 12.69 14.90 18.24 21.35 24.77
8.734 10.56 13.05 15.74 18.68 22.74 27.59 34.01 40.35 47.58
15.87 19.09 22.66 28.05 34.26 42.21 50.55 60.34 70.69 80.62
25.00 31.23 37.47 46.23 55.66 67.40 80.35 97.04 115.1 130.2
38.10 47.26 58.08 70.81 86.00 103.8 124.1 147.0 175.7 203.4
53.01 66.44 82.15 100.6 124.7 149.3 179.8 216.5 257.0 301.7
a Standard uncertainties u are u(T) = 0.02 K, u(p) = 0.45 kPa; Relative standard uncertainty ur is ur (x) = 0.053. Solvent mixtures were prepared by mixing various masses of the solvents with relative standard uncertainty ur(w) = 0.0002. w represents the mass fraction of EG in solvent mixtures of EG + water mixture free of monobenzone.
1546 104; while in pure water, it increases from 0.03569 104 to 0.2292 104. The solubility of monobenzone in mole fraction scale in {isopropanol (1) + water (2)}, {n-propanol (1) + water (2)}, {EG (1) + water (2)} and {ethanol (1) + water (2)} mixtures is also tabulated in the Tables 2–5. Moreover, the relationship between the mole fraction solubility and co-solvent compositions and temperature is given in Fig. 3. It can also be observed from the Tables 2–5 and Fig. 3 that the values of monobenzone solubility show a function of solvent compositions and temperature for all the studied mixtures. The mole fraction solubility of monobenzone rises with
increasing temperature and mass fraction compositions of isopropanol, n-propanol, EG and ethanol. It is larger in the (ethanol + water) mixture than in the (isopropanol + water), (npropanol + water) and (EG + water) mixtures at the same temperature and co-solvent composition. 4.3. Solvent effect The chief type and degree of the solvent effect is examined by the use of KAT-LSER model in the four mixtures at 298.15 K. In an attempt to use this technique, the molar volume of monoben-
6
Y. Zhou et al. / J. Chem. Thermodynamics 142 (2020) 106023
Table 5 Equilibriummole fraction solubility xeT;W 104 of monobenzone in ethanol (w) + water (1-w) mixture with different mass fractions ranging from T = (293.15 to 328.15) K under 4 a e 101.2 kPa. xT;W 10 . T/K
283.15 288.15 293.15 298.15 303.15 308.15 313.15 318.15 323.15 328.15
W 0
0.1000
0.2000
0.3000
0.4000
0.5000
0.6000
0.7000
08,000
0.9000
1
0.03569 0.04488 0.05644 0.07042 0.08707 0.1067 0.1300 0.1574 0.1907 0.2292
0.3788 0.4544 0.5472 0.6774 0.8068 0.9560 1.159 1.367 1.580 1.862
2.014 2.365 2.758 3.307 3.844 4.595 5.315 6.147 7.050 8.226
6.480 7.555 8.894 10.58 12.12 13.95 16.22 18.68 21.48 24.55
15.26 17.56 20.95 24.53 28.00 32.06 37.16 42.22 48.57 55.29
31.53 35.90 43.84 51.33 62.71 68.90 78.96 90.90 104.7 126.7
62.85 73.40 88.00 105.1 123.1 135.0 155.1 175.6 204.9 244.6
116.9 135.5 160.5 189.9 217.7 245.9 279.3 317.0 360.0 414.7
208.5 241.6 281.1 326.3 377.8 425.8 478.7 540.5 607.0 692.2
341.7 391.7 453.5 521.8 594.0 677.8 764.5 853.8 955.2 1081
483.0 559.8 643.6 736.9 846.0 956.1 1088 1227 1367 1546
a Standard uncertainties u are u(T) = 0.02 K, u(p) = 0.45 kPa; Relative standard uncertainty ur is ur (x) = 0.053. Solvent mixtures were prepared by mixing various masses of the solvents with relative standard uncertainty ur(w) = 0.0002. w represents the mass fraction of ethanol in solvent mixtures of ethanol + water mixture free of monobenzone.
Fig. 2. Mole fraction solubility of monobenzone in pure solvents at different temperatures: ., ethanol; d, propanol; w, isopropanol; ▲, EG; ◆, water.
zone (Vs), Hildebrand solubility parameter (dH) and KAT parameters should be known. The desired parameters a, b, p* and dH for the isopropanol + water, EG + water, n-propanol + water and ethanol + water mixtures can be available from the publications [10,34]. These data are collected and presented in Table S1 of Supporting material. The molar volume of monobenzone, Vs = 158.9 cm3mol1, is calculated by the use of molar mass divided by density (1.26 gcm3) [35]. For a comprehensive investigation, in addition to the general expression of the KAT-LSER model described in Eq. (2), 14 supplementary expressions for the KAT-LSER model are performed through different combinations of the four interactional energy terms. The data presented in Table S1 along with the experimental solubility data are applied to the 15 expressions by means of the multiple linear regression analysis. The results via regression are tabulated in Tables S2–S5 of Supporting material. An appropriate expression for the KAT-LSER model must comprise ci=1-3 greater than 0 and ci=4 < 0. The reason is that the solute–solvent interactions favour the drug solubility and solvent–solvent interactions disfavor the drug solubility. Amongst the remaining expressions that meet these criteria, the best model for the solvent effect is the expression with maximum F-statistic. In this way, the best KAT-LSER model is designated and bolded in the Tables S2–S5. Tables S2 and S3 illustrate that the dipolarity-polarizability and cavity term are chiefly responsible for the variation in the monobenzone solubility in isopropanol (1) + water (2), n-
propanol (1) + water (2), EG (1) + water (2) and ethanol (1) + water (2) mixtures. The best models explain 99%, 98%, 100% and 100% of variation in the four co-solvent mixtures, respectively. The contributions to the solvent effect from p* term and cavity term are 31.4% and 68.6% in isoropanol (1) + water (2) mixture, 37.3% and 62.7% in n-propanol (1) + water (2) mixture, 35.4% and 64.6% in EG (1) + water (2) mixture and 39.3% and 60.7% in ethanol (1) + water (2) mixture, respectively. Examination of c4/c1 ratio discloses that the monobenzone solubility is nearly 1.5 to 2 times more sensitive to the cavity term than to the dipolarity-polarizability term in the four aqueous co-solvent mixtures. The solvent–solvent interactions mainly control the co-solvency effect upon solubility. In other words, weakening of the cohesive force between solvent molecules with increasing the mole fraction composition of organic solvents of isopropanol, n-propanol, EG and ethanol is responsible for improvement of the monobenzone solubility in these aqueous mixtures. The reasons of this outcome are discussed as below. Monobenzone is a Lewis acid and may make hydrogenbonding interactions by the use of its hydroxyl group, Fig. 1, with both of water and protic solvents (isopropanol, n-propanol, EG and ethanol), even though the interactions is expected to be strong with isopropanol, n-propanol, EG and ethanol that have larger b than water, Table S1. Additionally, monobenzone can also be act as a Lewis base owing to its oxygen atoms in -OH and -O- groups, and may form further hydrogen-bonding interactions with water. Nevertheless, for the creation of these types of interactions appropriately, it is mandatory the solute to be well suited and oriented within the solvent structure. Monobenzone molecule has relatively large size, so it cannot readily enter into the voids in solvent network structure, especially for the water that presents high association. It means that for solubility of monobenzone, cavities with large size for solute accommodation should be formed within the solvent. Consequently, it is not surprising that the cavity term has the most noteworthy effect upon the variation of monobenzone solubility in these aqueous mixtures. So, as the mole fraction compositions of alcohols increase in mixtures, the Hildebrand solubility parameter decreases, which decreases the work required to disrupt the solvent-solvent interactions and to create cavity and results in an increase in the monobenzone solubility. The results discussed above indicate that the nature of solvent effect upon the variations of monobenzone solubility is dependent upon the complexity of intermolecular interactions in the co-solvent mixtures that may cause difference in the behavior of solvents in solvation shell of the solute and the bulk owing to occurrence of preferential solvation. For that reason, a comprehensive analysis of preferential solvation is required to clarify how solvent influences the chemical properties in co-solvent solutions, as made in next section.
Y. Zhou et al. / J. Chem. Thermodynamics 142 (2020) 106023
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Fig. 3. Mole fraction solubility (x) of monobenzone in (a) isopropanol (w) + water (1-w), (b) n-propanol (w) + water (1-w), (c) EG (w) + water (1-w) and (d) ethanol (w) + water (1-w) mixed solutions with various mass fractions at different temperatures: w, mass fraction; j, w = 0 [10]; d, w = 0.1000; ▲, w = 0.2000; ., w = 0.3000; ◆, w = 0.4000; w, w = 0.5000; 4, w = 0.6000; s, w = 0.7000; q, w = 0.8000; 5, w = 0.9000; h, w = 1 [10]. —, calculated curves by the Jouyban Acree model.
4.4. Preferential solvation analysis The transfer Gibbs energy (Dtr Go3;2!1þ2 ) computed by using Eq. (16) at 298.15 K are presented in Table S6 of Supplementary material. As well, the values of Dtr G03;2!1þ2 are graphically shown in Fig. S2 of Supplementary material. The regressed curve-fitting coefficients of Eq. (17) are presented in Table S7. Thus, the D values are obtainable from first derivative of the Eq. (12) solved based on the co-solvent compositions varying by 0.05 in mole fraction scale of isopropanol (1), n-propanol (1), EG (1) or ethanol (1) and tabulated in Tables S8–S11 of Supplementary material. In terms of the available values of RTjT and Q in conjunction with the partial molar volumes of co-solvents and water in the aqueous co-solvent mixtures at 298.15 K [6,9], the G1,3 and G2,3 values in the four mixtures can be acquired via Eqs. (9) and (10) and also presented in the Tables S8–S11. Because the partial molar volumes of monobenzone (3) in these mixtures are not available from publications, herein they are considered as similar with that of the neat monobenzone [6,9,10,34]. As a result, the radius (r3) of monobenzone molecule is computed to be 0.398 nm by using Eq. (14). The values of Vcor and dx1,3 achieved through regression are also presented in the Tables S8–S11 for the isopropanol (1) + water (2), n-propanol (1) + water (2), EG (1) + water (2) and ethanol (1) + water (2) mixtures, respectively. What is more, the dx1,3 dependence on the co-solvent compositions is shown graphically in Fig. 4. It can be found from Fig. 4 that the values of dx1,3 non-linearly change with the co-solvent (1) proportions in the four aqueous solutions.
Fig. 4. dx1,3 values of monobenzone (3) from neat water (2) to isopropanol (1) + water (2), n-propanol (1) + water (2), EG (1) + water (2) and ethanol (1) + water (2) mixtures at 298.15 K. j, isopropanol (1) + water (2); d, n-propanol (1) + water (2); ▲, EG (1) + water (2); ., ethanol (1) + water (2).
Addition of the co-solvent (1) makes negative the values of dx1,3 of monobenzone (3) from neat water up to x1 = 0.20 mol fraction of n-propanol and x1 = 0.25 mol fraction of isopropanol/EG/ethanol. Within the above regions, the local mole fractions of isopropanol (n-propanol, EG or ethanol) (1) are samller than that of the mix-
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Y. Zhou et al. / J. Chem. Thermodynamics 142 (2020) 106023
tures and consequently the values of dx1,3 are negative, which indicates that the monobenzone is preferentially solvated by the water. Perhaps the structuring of the water molecules around the nonpolar aromatic group of monobenzone (i.e. hydrophobic hydration of benzene ring group) contributes to decreasing the net values of dx1,3 to negative in the isopropanol (n-propanol, EG or ethanol) mixtures. The minimum negative values are acquired with the mole fraction compositions x1 = 0.10 with dx1,3 = 6.6 41 102 for the isopropanol (1) + water (2) mixture, x1 = 0.05 with dx1,3 = 7.109 102 for the n-propanol (1) + water (2) mixture, x1 = 0.05 with dx1,3 = 3.496 102 for the EG (1) + water (2) mixture and x1 = 0.05with dx1,3 = 6.666 102 for the ethanol (1) + water (2) mixture. In the n-propanol (1) + water (2) mixture with compositions 0.20 < x1 < 1 and isopropanol (EG or ethanol) (1) + water (2) mixtures with compositions 0.25 < x1 < 1, the local mole fractions of isopropanol (n-propanol, EG or ethanol) are greater than those of the mixtures and therefore the dx1,3 values are positive, which indicates the preferential solvation of monobenzone by the isopropanol (n-propanol, EG or ethanol). The co-solvent action to improve the solubility of monobenzone may be related to the breaking of ordered structure of water nearby the polar moiety of monobenzone which increases the solvation having maximum values near to x1 = 0.60 with dx1,3 = 11.25 102 for the isopropanol (1) + water (2), x1 = 0.45 with dx1,3 = 2.705 102 for the n-propanol (1) + water (2), x1 = 0.50 with dx1,3 = 1.669 102 for the EG (1) + water (2) and x1 = 0.65 with dx1,3 = 6.144 102 for the ethanol (1) + water (2) mixtures. According to the analysis of group structure and function of the monobenzone molecule, monobenzone may act as a Lewis acid in the mixtures attributable to the ability of acidic hydrogen atom in its –OH group (Fig. 1) to form hydrogen bonds with protonacceptor functional groups of the co-solvents (oxygen atoms in – OH group). As well, monobenzone may also act as a Lewis base thanks to the free electron pairs in hydrogen atom of –OH and oxygen atom of -O- (Fig. 1), which can interact with acidic hydrogen atoms of water. Based on the analysis results of preferential solvation, it is conjecturable that in the ranges of 0.20 < x1 < 1 for npropanol and 0.25 < x1 < 1 for isopropanol/EG/ethanol, monobenzone is acting as a Lewis acid with n-propanol, isopropanol, EG or ethanol molecules, because the co-solvents show more basic than the water, as defined by the Kamlet–Taft hydrogen bond acceptor parameters, i.e. b = 0.75 for ethanol, b = 0.84 for isopropanol, b = 0.90 for n-propanol, b = 0.52 for EG and b = 0.47 for water [36]. While in the other ranges, where the monobenzone is preferentially solvated by the water, monobenzone is mainly acting as a Lewis base in front to water because the water is more acidic than n-propanol, isopropanol, EG and ethanol as described by the Kamlet–Taft hydrogen bond donor parameters, i.e. a = 1.17 for water, a = 0.76 for isopropanol, a = 0.84 for npropanol, a = 0.90 for EG and a = 0.86 for ethanol, respectively [36]. 4.5. Solubility modelling In terms of the solubility values determined, the equation parameters of the Jouyban–Acree model are regressed by using the Mathcad software and tabulated in Table S12 of the Supporting material in consort with the values of RAD and RMSD. The backcalculated solubility values of monobenzone in the (isopropanol + water), (n-propanol + water), (EG + water) and (ethanol + water) solutions are graphically shown in Fig. 3. Analysis of the Table S12 shows that the RAD values are 2.58% for isopropanol + water mixture, 3.17% for n-propanol + water mixture, 2.02% for EG + water mixture and 2.14% for ethanol + water mixture. Besides, the maximum RMSD value is 7.64 104, which is achieved for (ethanol + water) mixture. Accordingly, the Jouyban-
Acree model may present accepted correlation results for these mixtures. 5. Conclusion The solubility of monobenzone in four mixtures of isopropanol (1) + water (2), n-propanol (1) + water (2), EG (1) + water (2) and ethanol (1) + water (2) was experimentally studied via the saturation shake-flask technique within temperatures from 283.15 to 328.15 K under local atmospheric pressure of 101.2 kPa. The maximum solubility value was observed in the neat solvents of isopropanol, n-propanol, EG or ethanol. The monobenzone solubility in mole fraction scale was mathematically expressed via the Jouyban-Acree model attaining the RAD values smaller than 3.17%. The dx1,3 values for isopropanol, n-propanol, EG or ethanol were positive in the intermediate and isopropanol (n-propanol, EG or ethanol)-rich compositions, which indicated that the monobenzone was preferentially solvated by the isopropanol (npropanol, EG or ethanol). The higher solvation by the co-solvent could be explained upon higher basic behavior of the alcohols which interacted with the Lewis acidic groups of monobenzone. The KAT-LSER model successfully described the solvent effect upon the monobenzone solubility variation. Analysis results revealed that the cavity term and dipolarity-polarizability were the major factors which leaded to the solubility variation of monobenzone in the isopropanol/n-propanol/EG/ethanol + water mixtures. CRediT authorship contribution statement Yanyan Zhou: Methodology, Project administration, Writing original draft. Jiaxin Wu: Investigation. Ali Farajtabar: Formal analysis, Data curation. Jian Wang: Data curation, Resources. Hongkun Zhao: Conceptualization, Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jct.2019.106023. References [1] A. Jouyban, Handbook of Solubility Data for Pharmaceuticals, CRC Press, BocaRaton, FL, 2010. [2] J.T. Rubino, Cosolvents and Cosolvency, in: J. Swarbrick, J.C. Boylan (Eds.), Encyclopedia of Pharmaceutical Technology, 3, Marcel Dekker, New York, NY, 1988. [3] P. Kolárˇ, J.W. Shen, A. Tsuboi, T. Ishikawa, Solvent selection for pharmaceuticals, Fluid Phase Equilibr. 194–197 (2002) 771–782. [4] A. Avdeef, Absorption and Drug Development, Solubility, in: Permeability and Charge State, Wiley-Interscience, Hoboken, NJ, 2003. [5] M.E. Aulton, Pharmaceutics. The Science of Dosage Forms Design, second ed., Churchill Livingstone, London, 2002. [6] A. Jouyban, W.E. Acree Jr, F. Martínez, Modelling the solubility and preferential solvation of gallic acid in cosolvent + water mixtures, J. Mol. Liq. 224 (2016) 502–506. [7] Y. Marcus, Solvent Mixtures: Properties and Selective Solvation, Marcel Dekker Inc, New York, NY, 2002. [8] Y. Marcus, Preferential Solvation in Mixed Solvents, in: P.E. Smith, E. Matteoli, J.P. O’Connell (Eds.), Fluctuation Theory of Solutions: Applications in Chemistry, Chemical Engineering, and Biophysics, CRC, Press, Taylor & Francis Group, BocaRaton, FL, 2013. [9] X.B. Li, C. Cheng, Y. Cong, C.B. Du, H.K. Zhao, Preferential solvation of pioglitazone hydrochloride in some binary co-solvent mixtures according to the inverse Kirkwood-Buff integrals method, J. Chem. Thermodyn. 110 (2017) 218–226.
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JCT 2019-921