Solubility of butylated hydroxytoluene (BHT) in aqueous and alcohol solutions from 293.15 to 313.15 K

The solubility of Butylated hydroxyl toluene in aqueous and alcohol solutions were determined at different temperatures (293.15 to 313.15) K. Using Van’t Hoff and Gibb’s equations, some thermodynamic functions such as Gibbs energy, enthalpy and entropy of dissolution, and of mixing of Butylated hydroxyl toluene in aqueous and alcohol solutions, were evaluated from solubility data. The solubility was greater in butanol and minimum in water. The enthalpies, entropy and Gibb’s energy of dissolution were positive for all solvents.

These applications prompted us to study the solubility of BHT in different solvents such as methanol, ethanol, propanol, butanol and water. The study is done at different temperatures (293.15 to 313.15 K).
The data may be useful for the design process of pharmaceutical dosage form. Further, the study of temperature dependence solubility data provides the explanation of molecular mechanisms involved in the respective drug dissolution process.

1. Materials
BHT, a white powder with a purity of 99.6 mass %, was purchased from Himedia Pvt. Ltd. All the solvents, methanol, ethanol, propanol, butanol and water were analytical grade reagents and were purified by fractional distillation. Their purities were checked by SHIMADZU GC-MS (Model No QP-2010) and were found to be greater than 99.85 %. Melting point of BHT was determined by DSC and was found to be in agreement with the reported value [7].

Equipments
Mettler Toledo AB204-S, electronic balance was used with an accuracy of 0.1 mg. The UV spectrophotometer of Shimadzu make was used for concentration determination.

3. Solubility measurement
An excess mass of BHT was added to a known mass of solvent in stoppered glass flasks. The solid-liquid mixtures were placed on an ultrasonic bath for about 15 minutes and were stirred in a mechanical shaker for one hour. The samples were then allowed to stand in water bath kept at appropriate temperature with ±0.05 K. All samples were maintained at least for 48 hours to reach the equilibrium. This equilibrium time was established by quantifying the BHT concentration to obtain a constant value. After this time, the supernatant solutions were filtered (at isothermal conditions) to insure ensure that they are free of particulate matter before sampling. The concentrations were determined by measuring the absorbance after appropriate dilution and interpolation from previously constructed UV-spectrophotometric calibration curves. All the measurements were repeated at least three times.

RESULTS AND DISCUSSIONS
Tables 1 and 2 summarize the physicochemical properties of Butylated hydroxytoluene (BHT) and of studied solvents respectively.

1. Ideal and experimental solubility of BHT
id is the ideal solubility of solute as mole fraction, ΔH fus is the molar enthalpy of fusion of the pure solute, T fus is the absolute melting point, T is the absolute solution temperature, R is the gas constant (8.314 J mol -1 K -1 ) and ΔC p is the difference between the molar heat capacity of the crystalline form and the molar heat capacity of the hypothetical super cooled liquid form, both at solution temperature [8]. ΔC p can't be easily determined so it is assumed that ΔC p is approximately equal to entropy of fusion ΔS fus [9].
The ideal solubilities of BHT at different temperatures are given in Table 3 whereas experimental solubilities of BHT in water and alcohol solutions at different temperatures (293.15 to 313.15 K) are summarized in Table 4. The variation of solubility with temperature is also shown in Figure 2. It is observed that solubility increases linearly with increase in temperature. Further, solubility is higher in butanol and minimum in water. This suggests that solubility increases with increase in -CH 2 group. Further, the minimum solubility of water suggests that BHT structure leads to water association which is not favorable for solubility of BHT in water whereas butanol could form maximum hydrogen bonding with BHT which causes increase in solubility.

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ILCPA Volume 28  The mole fraction solubility x of BHT was also correlated as a function of temperature by the modified Apelblat equation [10][11] ln x = A + B (T/K) (2) where x is the mass fraction solubility of BHT; T is the absolute temperature and A and B are the coefficients in equation (2). The values of these coefficients are given in Table 5. The calculated solubilities x ci are also reported in Table 4. Further, absolute average deviations (AAD) and root-mean-square deviations (rmsd), calculated by equations (3) and (4) are listed in Table 5. International Letters of Chemistry, Physics and Astronomy Vol. 28 51 x ci = Calculated solubility of solute (BHT).

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where N is the number of experimental points and x ci is the solubility calculated by equation (2). The relative deviations (RD) between the experimental and calculated values of solubilities are also calculated by equation (5) and are given in Table 4.

Relative Deviation
Further, the activity coefficient γ for BHT in different solvents were also calculated as x 2 id /x and is reported in Table 6. It is observed that activity coefficient is minimum for butanol and maximum for water. This trend is similar to solubility of BHT in the studied solvents. International Letters of Chemistry, Physics and Astronomy Vol. 28

2. Thermodynamic functions of dissolution
From the experimental solubility data, some thermodynamic parameters such as heat of solution (ΔH sol ), Gibbs energies of dissolution (G sol ) and enthalpy of dissolution (ΔH sol ) were also evaluated. The heat of solution (ΔH sol ) can be calculated using Von't Hoff equation [12]. i.e. from the slope of the plot of ln x verses 1/T, if the solubility is low as in case of water. However, in more recent treatments, the mean harmonic temperature, T hm has been introduced in Van't Hoff equation. The mean harmonic temperature, T hm is calculated as [13][14] where n is the number of tested temperatures. In the present case, the T hm value obtained is 302.79 K. Thus, the modified van't Hoff equation can be written as: From the slopes of the plots of lnx against [(1/T)-(1/T hm )], ΔH sol values for all the solvents were evaluated. Further, the standard Gibbs energies of the dissolution process (ΔG sol ) were also calculated using following relation: ΔC p is replaced by ΔS fus and value of ΔH fus 302.79 was found to be 17.4820 KJ/mole which is quite different from enthalpy value calculated for the ideal dissolution process (Table 5). Similarly, entropy of fusion ΔS fus 302.79 was found to be 57.7365 J/mole which is quite different from that calculated for the ideal dissolution process (Table 7).
So, ΔH mix values were evaluated from equation (12) using both ΔH fus 302.79 = 17.4820 KJ/mole as well as ΔH sol id (given in Table 7). Similarly, ΔS mix values were also evaluated from equation (13) using both ΔS fus 302.79 = 57.7365 J/mole as well as ΔS sol id (given in Table 7). All these values are given in Table 8. The net variation in ΔH mix values is due to contribution of various kinds of interactions. The enthalpy of cavity formation is endothermic because energy must be supplied to overcome the cohesive forces of the solvent which is generally hydrogen bonding as in case of water. This causes a decrease of solubility. The enthalpy of solute-solvent interaction is exothermic due to various types of interactions. The structure of solvent molecules around non polar groups of solute contributes to a decrease in the net enthalpy of mixing as observed for alcohols.
Further, the minimum solubility of water suggests that BHT structure leads to water association which is not favorable for solubility of BHT in water whereas butanol could form maximum hydrogen bonding with BHT which causes increase in solubility. Thus, it is concluded that solubility of BHT is maximum in butanol and minimum in water. The

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ILCPA Volume 28 dissolution process of BHT in the studied solvents is endothermic in nature and is not spontaneous.

CONCLUSION
The solubility of BHT increases with increasing CH 2 group in alcohols and increases with increase in dielectric constant in selected primary alcohols. Further, solubility is higher in butanol and minimum in water. Further, the minimum solubility of water suggests that BHT structure leads to water association which is not favorable for solubility of BHT in water whereas butanol could form maximum hydrogen bonding with BHT which causes increase in solubility. The dissolution process of BHT in the studied solvents is endothermic in nature and is not spontaneous.