1. Intoduction

In recent years, macrocyclic ligands have been used in ion chromatography as components of both the stationary and mobile phases. The interest in these molecules derives from their selectivity in binding specificcations. Some labs focused on the use of macro-cycles in ion chromatography in the separation of both cations and anions .Nakagawa et al. investigated the use of crown ethers in the mobile phase for the separation of amine cations, peptides, and sulfonic acids. They showed that the increase in capacity factors is related to the stability of the complex for those compounds that form complexes with the crown ether. They also showed that the concentration of crown ether influences the capacity factors for some compounds. Peak resolution between mono- and divalent cations using an unmodified silica gel column . The improved resolution is due to adsorption of 18-crown-6 onto the stationary phase and the formation ofmacrocycle-cation complexes with selected metal ions . The logK values indicate those metal ions that bind 18-crown-6 preferentially .Recently, Dionex developed the Ion Pac CS15 analyticalcolumn, which like its precursor the CS12A,contains phosphonate and carboxylate functionalgroups, but in addition contains 18-crown-6 groups[1-5].

Macrocyclic polyethers (crown ethers) are a family of compounds which possess the ability to transport ionic species across natural and artificial membranes. Because of this characteristic, they have wide applications in industry and are being investigated for their potential as pharmacologic agents.However, these compounds are highly cytotoxic in both prokaryotes and eukaryotes. Because of the cytotoxicity of crown ethers, an investigation of the potential genotoxicity of these compounds in Salmonella typhimurium was used[6].

Crown ethers are very important, especially in application in thermodynamic studies[7], solvent extraction of divalent and monovalent ions[8], selective synergistic solvent extraction[9] and competitive complexation of some alkaline earths and transition metal ions[10]. Synthesis, characterization, crystal structure if some salts with dibenzo–18–crown– 6 and dibenzo–24–crown–8 macromolecules[11] are valuable ones. Therefore, studying the physical properties of the crown ethers, especially dibenzo–18–crown–6 are wanted to follow their behaviour in solutions like, apparent molar volumes and adiabatic compressibility[12].

The aim of this work is to evaluate the solvation, thermodynamic and polarization parameters of dibenzo–18– crown–6 in mixed methanol–water solvents at different temperatures.

2. Experimental

Dibenzo–18–crown–6 (DB18C6) was supplied from Merck. Methanol of BDH was used. Saturated solutions of DB18C6 were prepared by shaking 10 ml of different percentages of methanol and water in 15 ml glass tubes containing excess of the solid substance in a thermostatic water bath of the type Assistant 3193 at time intervals for a period of 2–3 weeks at 25, 30, 35 and 40℃. The molal solubility's of DB18C6 were measured (gravimetrically) by drying 1 ml of the saturated solutions in small aluminium dishes or smallbeakers under a tungsten lamp to avoid scattering of the solutions as explained by Gomaa[13,14] .

The molal solubilities of DB18C6 in mixed MeOH–H₂O solvents were also confirmed by spectrophotometric measurements using (Unicom UV2–100 UV/ visible spectrometer V 3.32 ) by dissolving the above residues after evaporation in pure methanol and measuring the absorption at wave length (λ) of 275 nm done using different concentrations of DB18C6 in methanol mixture at the same wave length, small differences were observed in both measurements, there for the mean values were listed in Table 1.

The solubilities were read by the use from the standard curve. The refractive indices of the saturated solutions of DB18C6 were measured at 25, 30, 35 and 40℃ using a refractometer ATAGO 3T No.52507, 1 ml weighing bottle was used for measuring the densities of the saturated DB18C6 in mixed MeOH–H₂O solvents at different temperatures which used for calculating the molar volume by dividing the molecular weight by density values. The heat of evaporation for solid DB18C6 was done by using DSC (Differential Scanning Calorimeter) apparatus of the type Mettler TA 3000, DSC25, made in Swizzerland.

3. Results and Discussion

The molal solubilities of DB18C6 were determined as reported in the experimental part at 25, 30, 35 and 40℃ in MeOH–H₂O mixtures with energy of ±0.001 g/L. Their solubility are shown in Table(1) which recorded from the average of minimum experiments with experimental error of ± 0.01

The solvation free energies for neutral compounds like tetraphenyl methane (Ph₄C), tetra phenyl germanium (Ph₄Ge)[8,9] DB18C6 in mixed MeOH–H₂O solvents were calculated from the molal solubility[10] by using the following equation (1):

(1)

Where C is the molal solubility of the crown ether used.

The transfer free energies for DB18C6 from water to mixed MeOH–H₂O solvents were calculated by applying equation (2) without correcting the activity coefficient[15].

(2)

Table 1. The solubility products of dibenzo –18 –crown 6 (DB18C6) in mixed methanol – water mixtures at different temperatures 40℃ 35℃ 30℃ 25℃ Vol % of MeOH 0.0302 0.0236 0.0205 0.0167 0.000 0.0367 0.0292 0.0239 0.0206 0.047 0.0395 0.0318 0.0268 0.0239 0.10 0.0419 0.0342 0.0315 0.0275 0.151 0.0429 0.0366 0.0345 0.0303 0.228 0.0483 0.0408 0.0389 0.0325 0.323 0.0558 0.0467 0.0439 0.0352 0.400 0.0580 0.0489 0.0470 0.0397 0.509 0.0609 0.0529 0.0509 0.0459 0.640 0.0651 0.0584 0.0550 0.0518 0.800 0.0759 0.0652 0.0594 0.0562 1.000

From the relation of (-log C) versus 1/T, the enthalpy of solvation (ΔH_s) are calculated where the slope equals ΔH_s /2.303 R. From the values of ΔG_s.

(3)

Where ΔG_s and ΔG_w are the solvation free energies in mixed solvent and water as reference solvent and ΔH_s the values of TΔS_s of solvation at 25℃ can be calculated using Gibbs equation (4):

(4)

The calculated values of the different thermodynamic salvation parameters (ΔG_s, ΔH_s and TΔS_s ) for DB18C6 in mixed MeOH–H₂O solvents at 25℃ were tabulated in Table (2) together with their transfer values ΔG_t , ΔH_t & TΔS_t from mixed solvent to water as standard solvent[16].

Table 2. The solvation free energies (ΔGs), enthalpy of solvation (ΔHs), entropy of solvation (TΔSs) and their transfer values ΔGt & ΔHt & TΔSt for DB18C6 in mixed MeOH-H2O solvents 25℃ (in KJ / mole) TΔSt TΔSs ΔHt ΔHs ΔGt ΔGs Vol % of MeOH 0 41.241 0 31.112 0 10.129 0 -1.762 34.479 -1.242 29.870 0.520 9.609 10 -7.108 34.133 -6.221 24.891 0.887 9.242 20 -8.453 32.788 -7.218 23.894 1.235 8.894 30 -10.047 31.194 -8.572 22.540 1.475 8.654 40 -11.260 29.981 9.612 21.500 1.648 8.481 50 -11.148 30.093 -9.302 21.810 1.846 8.283 60 -12.072 29.169 -9.930 21.182 2.144 7.985 70 -12.589 28.652 -10.086 21.026 2.503 7.626 80 -14.888 26.353 -12.086 19.026 2.802 7.327 90 -19.264 21.477 -16.260 14.852 3.004 7.125 100

It was observed from Table (2) that by more increasing the percentage of MeOH in the mixed solvents the transfer free energies, enthalpies and entropies of solvation of DB18C6 increase in their negative values favouring more solvation behaviour.

The measured refractive indices for the saturated DB18C6 solutions in MeOH–H₂O solvents are shown in Table (3). The plot of refractive index versus absolute temperature follows a linear law

(5)

Table 3. Refractive indices (nd) for DB18C6 in mixed MeOH–H2O solvents at different temperatures and C, D factors for equation (5) Dx104 C nd Vol% of MeOH 40℃ 35℃ 30℃ 25℃ -5 1.3337 1.3312 1.3319 1.3326 1.3332 0 -5.4 1.3374 1.3348 1.3355 1.3364 1.3368 20 -6.4 1.3417 1.3376 1.3388 1.3398 1.3408 40 -6.8 1.3434 1.3368 1.3388 1.3404 1.3420 60 -7.8 1.3402 1.3227 1.3365 1.3372 1.3390 80 -8.8 1.3313 1.3222 1.3255 1.3276 1.3304 100

By means of least–squares method it was found C and D values are given in Table 3 also with error of C and D ± 0.003 and error for n_d is ±0.02%. The observed D values increase in their values whereas C values are almost constant indicating more solvent effect by increasing percentage of MeOH.

From Clausius – Mosotti and Maxwell derivation[17] a relationship connecting the dielectric constant of a substance with its refractive index for low frequency radiation namely n²_d as in case of DB18C6, so that, the molar refraction (R) equal the molar polarization (P) which may be equated by:

(6)

(7)

Where R, molar refraction, P molar polarization, n (n_d) refractive index, ε, dielectric constant, V_m molar volume ( M/d ), α the polarizability and N is Avogadro's number. The molar volumes V_m, P and α for saturated solutions of DB18C6 in mixed MeOH–H₂O solvents are presented in Table (4) at different temperatures. This results indicates the increase in their values with more increasing MeOH content in the mixtures favouring more solvaton behavior.

The molar refraction for DB18C6 was calculated in its solid state from the summation of bond refraction[18] and was found to be 57.626. Also the standard polarizability for solid DB18C6 was calculated by adding the bond polarizabilities and found to be 2.12x 10^-23 C.C/mole. In comparing these values with values in Table (4), it was found that increase in molar polarization and polarizabilies for DB18C6 on mixed MeOH–H₂O solvents.

On drawing the relation between P versus 1/T straight lines were obtained (Fig.1) with slope B and intercept equal to the sum of atomic polarization (P_A) and electronic polarization (P_E)[17]. The dipole moment can be calculated by using equation 8:

(8)

Table 4. Molar volumes (vm), molar polarization P and Polarizabilities (α) for DB18C6 in mixed MeOH–H2O mixtures at different temperatures

Figure 1. Polarization of saturaled solutions of DB18C6 in mixed MeOH-H2O solvents

and their values are listed in Table (5). Also, from the intercepts the values of P_A + P_E are calculated and tabulated in Table (5). Also, small effect of solvent on the dipole moment of DB18C6 was observed while very influenced effect on P_A + P_E can be understood.

The difference between the molar polarization and the sum of P_A + P_E gives the orientation polarization P_o, which will be small for DB18C6 due to its lower dipole moment values in mixed MeOH–H₂O solvents.

Table 5. Induced Dipole moment μi, PA + PE (atomic and electronic polarizations ), Polarization permittivity (ε ∞) and solubility parameters (δ) for DB18C6 in mixed MeOH–H2O Solvents at 25℃ . J.cm-3 δ ε ∞ PA + PE (c.c/mole) μi (x 10-18 esu ) Vol%of MeOH 7.029 1.866 74.6 1.451 0 6.939 1.876 76.4 1.551 20 6.820 1.888 80.4 1.645 40 6.755 1.891 85.5 1.939 60 6.587 1.883 89.4 2.228 80 6.233 1.858 91.7 0.949 100

The decrease in entropy and entropy of transfer values for DB18C6 by more increasing the percentage of MeOH shown in Table (2) is also an evidence that the orientation polarization and the dipole moments decrease in the same order because they are dependent on each other.

The refractive index gives the contribution of the electronic polarization to permittivity ε ∞. The contribution of the atomic polarization can by estimated for organic molecules approximately by[18,19]:

(9)

The calculated ε _∞ values for DB18C6 are presented also in Table (5) at 25℃ in mixed MeOH–H₂O solvents indicating its small effect. The differential scanning calorimeter (DSC) was used to evaluate the heat of vaporization (ΔH_v) of DB18C6 in solid state and was found to be 75.7 J/mole with one endothermic peak at 273.2℃ (Fig. 2), having the order of 0.46. This verify the heat of solvation values given in Table (2) with also an endothermic behavior of solvation.

Figure 2. DSC diagram for solid DB18C6

From the heat of evaporization (ΔH_v), the solubility parameters (equation 10) for the ether used were calculated by the use of Hildebrand equation[19]:

(10)

The calculated δ values are given also in Table (5) with its small effect on illustrating the solvent mole in solvation processes. Therefore, increasing the mole fraction % of methanol (organic solvent)[20-22] increases the solubility of DB18C6 in mixed MeOH–H₂O solvents due to the increase of molecule- solvent interactions. These molecules – solvent interactions were explained by increase in molar volumes, total polarization, polarizabilites, electronic and atomic polarizations.

4. Conclusions

All the solubility and thermodynamic parameters for dibenzo-18-crown-6 are increased by increasing methanol percentage in the mixed methanol-water solvents. This work give a lot of data about this wide range polycompound for helping in its uses and applications.