1. Introduction

The solubility of an electrolyte is influenced by a wide range of factors , including ion association ,variation in ionic activity coefficients , complexation and temperature. Solubility is an equilibrium property enable to thermodynamic parameters through the standard state free energy. Ion pairing can occur in dilute solutions for many electrolytes, particularly these with multivalent ions and for all electrolytes in concentrated solutions. Ion pairing is generally more pronounced in non-aqueous solvents which have lower dielectric constants than water. In effect, the ion pairs represent a reservoir of electrolyte in the solution and increase the solubility.The complexityof the system increases for unsymmetrical electrolytes or in mixed electrolyte systems[1].

Bjerrum[2] proposed that the motion of ions would be coupled when the energy of attraction between them exceeded the thermal energy. For solely columbic interactions his theory predicts a distance within which the electrostatic attraction between ions is greater than 2kT. Which will be sufficient to couple the motions of the ions. The treatment takes account of only electrostatic interactions and neglects the molecularity of solvent, Nevertheless in low concentration interactions between ions and solvent molecules resulting in ion pair formation. The three commonly assumed structures are the first in which the ion retains their Individual solvation shells, and so is separated by two solvent molecules. The second in which the ions share some part of their solvation shells so are separated by one molecule and the third where the ions are in contact and share a common solvation shell.

The presence of species such creates an experimental difficulty; the different techniques will have different sensitivities to the species present. Thus the conductance will see on the dissociated ions and the presence of ion pairs is determined by difference from experimental molar conductance and that expected for strong electrolyte[3].

The formation of complexes (complexation) provides a route to increased solubility. Several equivalent representations of the speciation in these systems have been used[4].

Many publications have appeared on the behaviour of weak acids in anhydrous solvents. Interesting work has been done by Kolthoff et al.[5,6]. Aleksandrov et al.[7,8] studied the dissociation of salicylic acid in butane-2-one. Kreshkov et al.[9] studied the dissociation of amino acids (as weak) acids) in mixtures of formic and ethylmethylketone and in mixtures of acetic acid-ethylmethylketone.

Gomaa et al.[10] studied association, dissociation and hydrogen bonding of salicylic acid in water-N,N- dimethylformamide mixtures from solubility measurements.

The aim of this work is to evaluate the solubilities of O-chlorobenzoic acid in different solvents and discuss in detail the solvation parameters for the solubility process for O-chloro benzoic acid which is one of the most widely used preservation materials .O-chloro benzoic acid has the advantage of low cost, ease of in- corporation into products. Lack of colour and relatively low toxicity.

Also knowing the other factors affecting the solubility is very important here. Is the electrostatic energy play important role in the solubility or not.

Many authors like Bjerrum and other reported that the electrostatic energy plays important role in the solvation energy. In this work more effort was done to explain the main factors affecting the solubility of orthochlorobenzoic acid in different solvents[11]

2. Experimental

The O-chlorobenzoic acid 98 % m.p type Cambarian Chemicals was used. The solvents, ethanol (ET), dimethylsulphoxide (DMSO), acetonitrile (AN), methanol (Me), 1-4 dioxane (Di) and dimethylformamide (DMF) were supplied from Merck. The saturated solutions of O-chlorobenzoic acid were prepared by dissolving it in the solvents used. The solutions were saturated with N₂ gas in closed test tubes. The tubes were placed in a shaking water bath of the type assistant for a period of four days, followed by another two days without shaking to reach the necessary equilibrium.

The solubility of O-chlorobenzoic acid in each solution was determined gravimetrically by taking 1 ml of the saturated solution and subjecting it to complete evaporation using small aluminium disks heated by an infrared lamp.

The pH readings of the saturated solutions were measured using a pH-meter of the type Tacussel/Minis 5000. The densities were measured by using weighing bottle 1 ml and analytical balance (4 digits) of the type Mettler Toledo DA.

3. Results and Discussion

Determination of the dissociation constants of organic acids in aqueous and non aqueous solutions by different methods like condutometric method during a period of 100 years was discussed. From time of pioneering works of Arrhenius and Ostwald the conductometric method provide simple method for the determination of the dissociation constants of organic solvents in water. This method is based on the assumption that strong electrolytes are completely dissociated while weak electrolytes attain this state only at infinite dilution. At fixed temperature, an equilibrium exists in solutions of weak electrolytes between ions and undissocited molecules to which the law of mass action and the Ostwald dilution law can be applied. Therefore complete study need more work for this field which is the target of this work.

The calculated molal solubilities (m) for O-chlorobenzoic acid saturated solutions were given in Table (1) from at least three average measurements. Also the measured densities and pH values of the acid used are also listed in Table (1).

The molar volumes (V_M) of benzoic acid were obtained by dividing the molar mass by the densities and their values are listed in Table (2). The packing density as reported by Kim et al.[11] and Gomaa et al.[12], i.e. the relation between Van der Waals volume (V_W) and the molar volume (V_M) of relatively large molecules (above 40) was found to be a constant value and equal to 0.661.

(1)

The electrostriction volume (V_e) which is the volume compressed by the solvent, was calculated by using equation (2) after Gomaa[13].

(2)

The molar, Van der Waals and electrostriction volumes of O-chloro- benzoic acid in various solvents at 298.15 K were tabulated in Tables (2).

The apparent molar volumes V[14,15] were calculated using equation (3)[16].

(3)

Where M is the molar mass of benzoic acid, m is the concentration, d and d_o are the densities of saturated solution and pure solvents, respectively.

The values of V for O-chlorobenzoic acid in various solvents at 298.15 K are presented in Table (2).

Table 1. Molal solubilities (m), densities (d) and pH values for saturated O-chlorobenzoic acid solution in different solvents at 298.15 K Solvent m/(g.mol/kg solvent) d pH H2O 1.3319x10-2 1.00527 6.24 Ethanol (Et) 1.9168 0.95137 2.12 DMSO (Dimethyl sulphoxide) 1.1721 1.2527 3.25 AN (Acetonitrile) 0.6035 0.82307 3.14 Me (Methanol) 1.7169 0.91787 1.89 Di (1,4 dioxane) 3.1219 1.12743 9.01 DMF (dimethylformamide) 3.1431 1.16610 9.01

Table 2. Molar volumes (VM), Van der Waals volumes (VW), electrostriction volumes (Ve) and apparent molar volumes (V) for saturated solutions of O-chlorobenzoic acid in different solvents at 298.15 K (in cm3/mole) Solvent VM VW -Ve V H2O 155.780 102.972 52.808 -454.940 Et 164.625 108.817 55.8087 83.445 DMSO 152.075 82.674 42.401 46.077 AN 190.287 125.779 64.508 29.793 Me 170.534 112.789 57.845 93.156 Di 138.917 91.824 47.093 123.715 DMF 134.310 88.778 45.5329 101.718

The activity coefficient was calculated by using the relation log γ± = -0.5062 [17].

K_ass values were calculated[18] from the ratios of association constant to dissociation constant (i.e., K₁/K₂) for the dimers of O-chlorobenzoic acid which form a complex ion (and hydrogen ion (H⁺), and the values of K` (where K<sup>`</sup> is the dissociation constant of the associated acid complex, H₂A₂) are given by the following equations

(4)

(5)

(6)

Where a is the activity and γ± is the activity coefficient .The values obtained K₁/K₂, K` and K_ass are reported in Table (3).

Prediction of electrolyte activity coefficients is one of the classical problems in physical chemistry and is outlined in classical works[17]. The defining characteristic of ions is that they carry a net charge and so the principle interaction between ions are largest contribution to the activity coefficients are columbic. Debye and Hückel solved the problem for a system purely electrostatic interactions between point charges surrounded by a dielectric contnium.Therefore the extended Debye- Hückel equation was applied taking account of the ion size[18]

From the activity coefficients ɣ± , calculated using Debye Hückel equation as explained in ref. 16 and from the molal solubility data ,values of K`, K₁/K₂ and K_ass were evaluated following equations 4 – 6[11,12].

The maximum value of K_ass was found to be by using Di where water is the least association parameters.

Table 3. Log activity coefficients (log γ±), dissociation constants (K`) and association constants for O-chlorobenzoic acid saturated solutions in different solvents at 298.15 K Solvent log γ± Pa K` Kass H2O -0.0584 6.2984 2.2361X105 7.0106X108 3.1352X103 Et -0.7008 2.8208 4.1510 1.6097X106 3.8777X105 DMSO -0.5480 3.7980 3.7980 5.4189X107 5.1574X106 AN -0.3932 3.5332 34.276 2.0600X103 60.099 Me -0.6633 2.5533 2.2116 6.1380X102 277.536 Di -0.8944 9.9044 10.065 2.5054X1010 2.4889X109 DMF -0.8974 10.1774 10.485 4.7288X1010 4.5100X109

The free energies of dissociation (ΔG_d), free energies of association (ΔG_A), difference free energies (ΔΔG) and free energies of solvation (ΔG_s) for O-chlorobenzoic acid saturated solutions in various solvents were calculated by using the following equations and collected in Table (4).

(7)

(8)

(9)

(10)

(11)

Dissociation of electrolytes introduces two solutes, the anion and the cation into solution. In principle, these solutes have individual activity coeffients but, since there is no experiment that allows the measurement of thermodynamic properties of individual ions .The formation of complexes becomes more important at high concentrations of the complexing ion and is likely to be more extensive in non-aqueous solvents, partially in dipolar aprotic solvents whereas the solvation of anion is weaker leading to stronger complexation[18].

The solvation volumes for O-chlorobenzaoic acid were evaluated from the difference between Van der Waals of O-chlorobenzoic in various solvents. The Van der Waals volume of O-chlorobenzoic acid in solid state, was calculated from the Bondi method[19,20] and found to be 167.537 cm³/mole. Subtracting this value from V_w in solvents and dividing the results by the molar volumes of the solvents taken from refs.[19-21], n (the solvation numbers) were obtained and is given in Table (5).

It was concluded that the solute-solvent interaction increased by increasing ΔΔG and ΔG_s due mainly to the increase of the association parameters in the corresponding solvents.Also it was observed that the association ,dissection and microscopic interactions between solute and solvent is important for any solvent individually, whether it is polar ,aprotic or amphiprotic .

Increasing of the solvation numbers favor more solute- solvent interactions between orthocholrobenzic acid and solvents. Also small solvation numbers favour more solute solute interaction or ion pair formation resulting in the decrease of the solute – solvent interactions in the solvent under discussion. Big positive values for ΔΔG and big negative values for ΔG_s indicate also more solute –solvent interactions_.

Table 4. Free energies of dissociation (ΔGd), free energies of association (ΔGA), difference free energies (ΔΔG) and free energies of solvation (ΔGs) for O-chlorobenzoic acid saturated solutions in different solvents at 298.15 K (in k Joule/mole) Solvent ΔGd ΔGA ΔΔG ΔGs H2O -30.538 -19.9590 10.9480 -9.582 Et -3.5288 -31.9034 -28.3746 -2.0728 DMSO -5.8312 -38.3192 -32.4880 -2.3740 AN -8.7627 -10.1549 -1.3922 -3.0355 Me -1.9677 -13.9481 -11.9804 -4.4507 Di -5.7247 -53.6389 -47.9142 -6.8814 DMF -5.8261 -55.1127 -49.2866 -6.9123

Table 5. Molar volumes of solvents (Vs), difference in different Van der Waals volumes (ΔVw) and solvation numbers (ns) for O-chlorobenzoic acid saturated solutions in different solvents at 298.15 K Solvent Vs ΔVw ns H2O 18.0724 64.5649 3.5725 Et 58.6804 58.7199 1.0006 DMSO 71.2995 84.8629 1.1902 AN 52.8450 41.7669 0.7904 Me 40.7322 54.7479 1.3441 Di 86.3552 75.7129 0.8767 DMF 77.4118 78.7589 1.0174