The role of dielectric constant in sodium chloride solution chemistry: Magnitude of super saturation

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1 International Journal o the Physical Sciences Vol. 7(4), pp , 23 January, 212 Available online at DOI: /IJPS ISSN Academic Journals Full Length Research Paper The role o dielectric constant in sodium chloride solution chemistry: agnitude o super saturation utasim I. Khalil* and Saud I. Al-Resayes Department o Chemistry, Faculty o Science, King Saud University, P. O. Box 2455, Riyadh 11451, Saudi Arabia. Accepted 4 January, 212 The eect o ethanol addition to aqueous semi saturated NaCl solutions on the super saturation was studied. The amount o precipitated salt was ound to increase as the percent o added ethanol increased. The rates o ormation o precipitate were calculated and ound to increase with decrease o the weighted average o the solvents mixture dielectric constant,, but levels out at 5. The solubility o salt was then connected with the dielectric constant and a zero solubility was observed at = 35. A magnitude o super saturation was deined or the measurement o solubility. Key words: Dielectric constant, precipitate, activation energy, magnitude o super saturation. INTRODUCTION Crystallization o sodium chloride was investigated in several works (Birchall and Davey, 1981; ohameed et al., 1997; Kubota et al., 2; Al-Jubbouri and Ulrich, 21, 22; Doki et al., 22; Linnikov, 24; 26a, b; 27). It was reported that the overall growth, dissolution rates and the surace integration step are linear with respect to supersaturation o solution (ohameed et al., 1997; Al-Jubbouri and Ulrich, 21, 22). Linnikov (27) investigated the spontaneous crystallization o sodium chloride rom aqueous ethanol solutions, reported that the use o ethanol or inducing super saturation in solutions allowed varying strongly the crystallization process conditions. Doki et al. (22) crystallized sodium chloride rom aqueous solution by adding pure ethanol (99 vol.% ethanol) or an ethanol-water (75- vol.% ethanol mixture as diluents). The kinetics o spontaneous crystallization o potassium and sodium chlorides have been extensively studied by Linnikov (24, 27), who determined the growth rates and activations energies and ound that the earlier was proportional to supersaturation. He described the kinetics o the observed crystals aggregation by the amous Smoluchowski equation or coagulation o colloid particles. Smoluchowski (1917) and Linnikov (26a, b) *Corresponding author. mkhalil@ksu.edu.sa. indicated that the coalescence o crystals at the spontaneous crystallization o sodium chloride rom supersaturated aqueous ethanol solutions is due to ormation o contacts o coalescence in the orm o nucleus-bridges according to the theory o Polak (1966). However, a decrease in aggregation constant was observed as super saturation is lowered. A model o spontaneous crystallization process was then proposed (Linnikov, 26b). The crystal size distribution and their weighed raction in a crystal deposit o the salt were determined approximately. Such inding could be argued in terms o the solvent dielectric constant which is a measure o orientational correlation between molecules. In act, it is one characteristic o great importance in causing ion ormation by a solute. It could be deined in terms o the medium s ability to regulate the orce between two ions. Hence, the degree o solute ionization, precipitation and coagulation could be varied by varying the dielectric constant o the medium. This has a great impact on experimental applications to chemical problems in the sense that the procedures o materials building (rom bottom to top) and separation (or ionization) (rom top to bottom) could be manipulated, attained and controlled by varying the medium dielectric constant. In this paper, we present the relation between the average dielectric constant o the aqueous-ethanol mixture

2 Khalil and Al-Resayes 579 Table 1. Comparison between calculated and experimental dielectric constants. Solvent (%) H 2O C 2H 5OH ( ) Calculated Experimental Pure Pure and the solubility o NaCl and velocity o crystal ormulation. A measurement o induced super saturation, that is, the magnitude o super saturation is suggested and calculated to account or the induced supersaturation. EXPERIENTAL PROCEDURE A 23.6% sodium chloride aqueous stock solution was prepared and stored in a 5 cm 3 volumetric lask. The density o this stock solution was estimated by averaging the mass o dierent volumes, that is, 1, 5 and 1 cm 3 and by calculation. A set o six experiments was then carried out as ollows: 5. cm 3 stock solution was transerred into each o six 5. cm 3 beakers to which 1., 2. 5., 1., 15. and 2. cm 3 o 96% ethanol were then added successively stirred and allowed to reach equilibrium at ambient temperature (27 C), then iltered. Caution was exercised to transer the precipitate. The precipitates, dried at 12 C, were then weighed and the amount o sodium chloride let in solution was calculated by dierence. The average dielectric constant o the aqueous-ethanol solvent mixture was estimated by the relation: Vi i T (1) (1) V T where V i and i are the volume and dielectric constant o solvent i, respectively; and V T is the total volume o solution. The validity o Equation (1) was tested by comparing the calculated dielectric constant with the experimental one using WYNE KERR precision component analyser-644 B. The relative dielectric constant ( r) was calculated by Equation (2): C x r (2) C where C x is the capacitance o dielectric and C is the capacitance o test capacitor when vacuum is between its plates (C = 8.85*1F). RESULTS AND DISCUSSION The capacitance (C x ) o dielectrics was measured applying a wide range o requencies (2 Hz 3 Hz). The reported values in Table 1 are or.3 khz at 32 C. The trend o dielectric lowering as percent o ethanol increases is in agreement with recent report or benzene and nitrobenzene mixture Ramana and alakondaiah (21). However, detailed study will be published. In dealing with the solubility o the alkali metal salts, or example, NaCl, it is the ion-dipole interactions that are the main source o the solvation energy o the alkali metal ions. With negative ion, dipole orces are the major solvent interaction taking place. Such a act is rationalized by applying coulomb s law in the orm: q1q2 Force (3) 2 (3) r where r is the distance between the two charges q 1 and q 2 and is the dielectric constant. It then goes without saying that the orce o interaction between the charges, is inversely proportional to the dielectric constant,. One may assume that the solubility o salts is connected with dielectric constant o the medium. This is the same as saying addition o ethanol ( =24.6`) to an aqueous solution ( = 8) create supersaturation allowing to vary strongly the crystallization process conditions (Doki et al., 22). The results obtained show that the solubility o sodium chloride is directly proportional to T or inversely proportional to ethanol percent (volume o ethanol added /total volume o solution)*1] in mixture in accordance with the earlier argument (Table 2 and Figures 1a and b). The initial and inal concentrations o solutions are calculated rom the number o dissolved NaCl moles. The average growth rate o crystal V could be calculated using Linnikov (26a,b) proposed equation: 3 E a 1 V = (i-) exp( ) (4) RT where E a is the activation energy o growth process (kj mol -1 ), i and are the initial and inal concentrations shown in Table 2. The activation energy o growth process o NaCl used in this calculation is 49.1 kj mol -1 (Linnikov, 26a). Table 3 and Figures 2a and b show the values calculated. Here we assume that Equation (4) is valid or all percentages o alcohol. It is evident then that an inverse correlation between the velocity growth rate and the weighted dielectric constant is only valid at high values o the dielectric constant, that is, above 52., or, expressed in other words, directly proportional to ethanol percent in the solution mixture. This is in near agreement with the linear dependence o the growth rate with respect to supersaturation o solution reported (Linnikov, 26b). However it is evident that such a velocity remains almost constant at above 5% ethanol, that is, below a dielectric constant value o 52.. The magnitude o supersaturation, which is a measure

3 NaCl Solubitlity Solubility o NaCl g/1g 58 Int. J. Phys. Sci. Table 2. Calculated data o dielectric constant by Equation 1 and solubility o NaCl results. No. Volume o 23.6% NaCl solution (ml)* Volume o 96% ethanol added (ml)* Vol. % o ethanol in solution Dielectric constant o solvent mixture* Initial concentration o solution (, mol l -1 ) Final concentration o solution (, mol l -1) NaCl solubility (g/1 g) *Density o 23.6% NaCl = g ml -1 ; density o 96% ethanol =.81 g ml -1 dielectric constant o: H 2O = 8.; ethanol = Dielectric Constant Figure 1a. Solubility o NaCl versus dielectric constant o the aqueous-ethanol solvent mixture Ethanol Percent Figure 1b. Ethanol percent in aqueous-methanol solvent mixture versus NaCl solubility.

4 Khalil and Al-Resayes 581 Table 3. Calculated velocity o NaCl growth by Equation (4) versus ethanol percent and dielectric constant o aqueous-ethanol solvent mixture. No. Ethanol (%) Dielectric constant Velocity 1 11 (m s -1 ) V V Dielectric constant Figure 2a. Dielectric constant o the aqueous-ethanol solvent mixture versus velocity o NaCl precipitation V 1 11 V % o ethanol Figure 2b. Percent o ethanol in aqueous-ethanol solvent mixture versus velocity o NaCl precipitate.

5 agnitude o supersaturation agnitude o Supersaturation 582 Int. J. Phys. Sci. Table 4. Calculated magnitude o super saturation by Equation (4). No. % Ethanol Dielectric constant i Ethanol Percent Figure 3a. Percent o ethanol versus magnitude o super saturation Dielectric Constant Figure 3b. Dielectric constant versus magnitude o super saturation. o induced supersaturation, is calculated by Equation (5): i (5) (5) where, i and are the initial and inal (solubility) solution concentration, respectively or each experiment. Table 4 and Figure 3a and b represent the results obtained.

6 Khalil and Al-Resayes 583 Conclusion The induced super saturation o a semi saturated NaCl aqueous solutions was studied upon addition o 96% ethanol and correlated to the average dielectric constant o the aqueous-ethanol mixture. It was evident that the solubility o the salt is connected with the dielectric constant o the medium. The velocity o crystal growth was ound to increase with the lowering o the dielectric constant up to a value o 52. and remains almost constant below that value. A magnitude o supersaturation was introduced as a measure o precipitation. Linnikov OD (27). Spontaneous Crystallization o Sodium chloride rom aqueous and aqueous ethanol solutions Part 3. ibid, 42(8): Linnikov OD (24). Spontaneous crystallization o potassium chloride rom aqueous and aqueous-ethanol solutions Part 2. ech. o aggregation coalescence crystals. Ibid, 39(6): ohameed UH, Zhang SB, Yuan JJ (1997). Inluence o the ph value on the growth dissolution rate o potassium chloride. Bull. Soc. Sea Water Sci. Jpn., 51: 973. Polak AF (1966). Hardening o ono-mineral Astringent Substances. Publishers literature constructions, pp Ramana VV, alakondaiah K (21). PC Based Instruments or the easurement o Dielectric Constant o Liquids. Sensors Transducer J., 112: Smoluchowski V (1917). Trying a athematical Theory or the Kinetics o Solution co-agulation. Z. Phys. Chem., 92: 129. ACKNOWLEDGEENT This project was supported by King Saud University, Deanship o Scientiic Research, College o Science Research Center. REFERENCES Al-Jibbouri S, Ulrich J (21). The Inluence o Impurities on the Crystallization Kinetics o sodium chloride. Cryst. Res. Technol., 3: Al-Jibbouri S, Ulrich J (22). Kinetics and echanism o the Crystallization Process. Cryst. Growth, pp Birchall JD, Davey RJ (1981). The crystallization o sodium chloride rom aqueous Solution in the presence o polysaccharides. J. Cryst. Growth, 54(2): Doki N, Kubota N, Yokota, Kimura S, Sasaki S (22). Production o Sodium chloride crystals o uni-modal size distribution by Batch Dilution crystallization. J. Chem. Eng. Jpn., 35: 199. Kubota N, Otosaka H, Doki N, Yokota, Sato A (2). Eect o lead(ii) impurity on the growth o sodium chloride. J. Cryst. Growth, 22: 135. Linnikov OD (26a). Spontaneous crystallization o potassium chloride rom aqueous and aqueous ethanol solutions. Cryst. Res. Technol., 41(1): Linnikov OD (26b). Spontaneous crystallization o sodium chloride rom aqueous and aqueous ethanol solutions Part 2. ibid, (2):