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Surface tensions and thermodynamic parameters of surface formation of aqueous salt solutions: III. Aqueous solution of KCl, KBr and KI
Colloids and Surfaces A: Physicochem. Eng. Aspects 337 (2009) 194–199
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Surface tensions and thermodynamic parameters of surface formation of aqueous salt solutions: III. Aqueous solution of KCl, KBr and KI Khurshid Ali a , Anwar-ul-Haq Ali Shah a,∗ , Salma Bilal b , Azhar-ul-Haq Ali Shah b a b
Institute of Chemical Sciences, University of Peshawar, Pakistan Department of Chemistry, Kohat University of Science and Technology, Pakistan
a r t i c l e
i n f o
Article history: Received 14 September 2008 Received in revised form 17 December 2008 Accepted 18 December 2008 Available online 25 December 2008 Keywords: Gibbs adsorption equation Surface tension Salts Surface parameters Surface excess concentration Thermodynamics
a b s t r a c t Surface tension of aqueous electrolyte solutions has been determined experimentally. Concentration and temperature dependence of the surface tension of three different salts has been studied. These include the chloride, bromide and iodide salts of potassium. The study was conducted at five different concentrations, i.e. 0.1, 0.5, 1.0, 1.5 and 2.0 mol dm−3 , and five different temperatures ranging from 10 to 30 ◦ C with 5 ◦ C interval. The surface tension data were fitted into linear regression for estimation of surface excess concentration using Gibbs adsorption equation. The thermodynamic quantities of surface formation such as enthalpy and entropy were estimated from the slopes (d/dT)C of surface tension versus temperature plots at constant concentration. Concentration and temperature dependence of surface excess concentration and thermodynamic parameters of surface formation have been discussed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction It has been known for a long time that simple electrolytes, such as sodium chloride, raise the surface tension of water [1]. Onsager and Samaras presented preliminary explanations to this phenomenon [2]. The surface tension of aqueous electrolyte solutions is an important physicochemical property. It has an important effect on liquid–liquid extraction processes such as hydrometallurgy and liquid–liquid dispersions. It is regarded as an important parameter in the study of distillation in the presence of salt and absorption refrigeration. It may also effect considerably phase separation, coagulation of droplets and building up of interfacial areas [3]. The phenomenon of surface tension has been a subject of considerable research interest worldwide. Researchers are trying to understand structural changes taking place at the surface from the knowledge of surface tension and surface thermodynamic parameters. In this regard much work has been devoted to the studies of surface tension versus concentration relationship of simple salts solutions for elucidating the structure of the surface region [4,5]. Molecular dynamic simulations of the surface tension of aqueous electrolyte solutions as a function of concentration at constant temperature have also been presented [6]. Similarly,
∗ Corresponding author. Tel.: +92 91 9216652/70120x3033; fax: +92 91 9216652. E-mail address: [email protected] (A.-u.-H.A. Shah). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.12.023
a calculation method was developed for representing and predicting surface tension of aqueous electrolyte solutions at high concentration [7]. The studies concerning temperature effect on the surface tension of aqueous solutions are limited [8]. This is because of low accuracy and precision of the results based on surface tension measurements that often lead to negligible deviation of d/dT from that of pure water. Nevertheless, the importance of the temperature effect on the surface tension cannot be neglected. It enables one to estimate thermodynamic quantities such as entropy, enthalpy and energy of surface formation. The thermodynamic quantities might be helpful in the explanation of interaction between components of the surface region [9,10]. Matubayasi et al. have reported the importance of the temperature effect on surface tension of aqueous solution of chloride salts of sodium, magnesium, and lanthanum, and evaluated the thermodynamic quantities such as entropy and free energy of surface formation [11]. A fact, currently of large historical interest, is that aqueous inorganic salt solutions exhibit higher surface tension than the pure water. The surface tension of such salt solutions increases with the increase in concentration. This increase in surface tension has been correlated with the structure of the surface region [4,5,12–18]. Gibbs adsorption equation and some thermodynamic treatment have been used to correlate the increase in surface tension with the ion-free layer and the thickness of the surface region [14–17]. Johansson and Eriksson have evaluated the entropy change of the surface of salt solution from the slope of the surface tension ver-
K. Ali et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 337 (2009) 194–199
sus temperature curve [13]. Ikeda et al. determined the effect of temperature, pressure, and concentration on the interfacial tension of an aqueous solution of sodium chloride–hexane interface. They also showed that the change of surface tension with respect to temperature decreases slightly with increasing salt concentration [19]. Literature shows that the surface tension of aqueous inorganic salt solutions have shown marked specific ion effects that had remained an ambiguous and unexplained fact. Though the issue is under thorough investigation but still a lot work would be needed to fully understand the phenomenon of specificity of ion that contributes to the surface tension changes of ionic solutions. Recently we have reported concentration and temperature dependence of surface quantities of chloride salts of lithium, sodium, potassium and ammonium [20]. In this report we attempted for the first time a simple approach for measuring surface tension with low cost set-up and applying different set of equations for calculation of thermodynamic parameters. In a subsequent publication [21] the same approach was applied successfully to salts with polyatomic anions. It is important to mention that in both cases anions were the same but cations were different. The present paper reports on the measurements of surface tension of aqueous solutions of chloride, bromide and iodide salts of potassium, as a function of concentration and temperature. The study has been extended up to five different concentrations and temperatures. The aim of the study was to apply recently developed approach [20–22] to aqueous solutions of chloride, bromide and iodide salts of potassium. In this case anions are different but
195
cation is the same. Data of KCl has been taken from our previously published results [20]. 2. Experimental All the chemicals used in this work were of anal. grade from B.D.H. and used without further purification. Water was doubly distilled before use. The surface tension was determined by the drop method using a stalagmometer (hellige No. 1211-A8779, Germany) with a capillary having internal diameter of 2.71 mm. All the glassware was washed thoroughly with chromic acid solution before use. The temperature of the test solution was kept constant within ±0.10 ◦ C by circulating controlled-temperature water through a glass jacket around the stalagmometer made in our glass-blowing workshop. The water supply to this glass jacket was made from an external thermostat, the temperature of which was maintained constant within ±0.10 ◦ C. Surface tension was calculated by using the following formula [23]:
=
d NH2 O F dH2 O N FH2 O
H2 O
where d and dH2 O are the densities of the solution and water, respectively, N and NH2 O are the number of drops of solution and water, respectively, F and FH2 O are factors that depend on the stalagmometer capillary and the volume of solution drop, and H2 O is the surface tension of water. The procedure of the surface tension and density measurement has been described elsewhere [20].
Fig. 1. Concentration dependence of surface tension of (a) KCl, (b) KBr and (c) KI solutions at different temperatures as indicated.
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K. Ali et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 337 (2009) 194–199
3. Results and discussion The surface tension of aqueous solutions of three different salts, namely potassium chloride, potassium bromide and potassium iodide has been measured experimentally. The measurements were carried out at five different temperatures in the range of 10–30 ◦ C with an increment of 5 ◦ C and five different concentrations, i.e. 0.1, 0.5, 1.0, 1.5 and 2.0 mol dm−3 . The effect of concentration and temperature on the surface tension, surface excess concentration and enthalpy has been discussed. The entropy of surface formation has also been estimated. The surface tension data were plotted against concentrations C at constant temperatures. By fitting the data into the linear regression lines: = 0 +
d dC
T,p
C
(1)
the coefficients of the regression lines (d/dC)T,p were calculated in order to estimate the surface excess concentration by using the Gibbs adsorption equation (Eq. (2)). The standard error of the estimate of on C for aqueous solutions of potassium chloride, potassium bromide and potassium iodide was in the range of 0.04–0.08: =−
C RT
d dC
(2) T,p
The surface excess concentrations were estimated and their concentration and temperature dependence has been studied. Similarly, by fitting the surface tension versus temperature data at constant concentration into the linear regression line, the coefficients of the regression lines (d/dT)C,p were calculated. The enthalpy H and entropy s changes were estimated by using the following equations [24]: H = − T
d dT
d
s = −
dT
(3) C,p
(4) C,p
Fig. 1a–c shows concentration dependence of surface tension of aqueous solutions of the selected salts at different temperatures. Generally, the surface tension is observed to increase linearly with concentration. Apparently the surface tensions versus concentration lines seem to be parallel for all the temperatures. Discrepancies are, however, observed in the case of KCl. For KCl the surface tension versus concentration lines are parallel at 10 and 15 ◦ C. Beyond this temperature in the high concentration region the surface tension versus concentration lines tend to depart. This deviation from the parallel behaviour is not a general trend. Because beyond 20 ◦ C the lines are once again parallel. The reason of the discrepancy in the case of KCl solution is unknown. The surface tension data of KI show unusual behaviour at low temperature in the low concentration region. At 10 and 15 ◦ C the surface tension tends to
Fig. 2. Temperature dependence of surface tension of (a) KCl, (b) KBr and (c) KI solutions at different concentrations as indicated.
K. Ali et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 337 (2009) 194–199
decrease rather than increasing with concentration in the concentration region <0.1 mol dm−3 . Beyond this concentration the surface tension increases linearly with concentration. It is important to mention that this abnormal behaviour could not be seen in the data obtained at relatively higher temperature. This anomalous behaviour is called as the Jones–Ray effect [25]. In 1937, Jones and Ray [26] observed a minimum in the surface tension of KCl, CsNO3 and K2 SO4 solutions using the capillary rise method. The following year, Langmuir [27,28] theoretically refuted the surface tension decrease as an artifact of the indirect experimental method. However, Dole [29] was able to explain the effect in terms of the adsorption of ions to a fixed number of active surface sites that would saturate around 1 mM; which is essentially equivalent to the familiar Langmuir adsorption model. In 1941, Jones and Ray [30] reproduced the effect for 11 different electrolytes, all showing a minimum in the surface tension around 1 mM. The theoretical argument of Langmuir was later criticized as being inadequate to explain the entire effect [31,32]. In fact Jones–Ray effect had remained as a curiosity that could neither be proved nor completely refuted until 2004 when direct experimental confirmation of the effect was presented using resonance-enhanced femtosecond second harmonic generation [33]. In our previous paper [21] Jones–Ray effect was observed only in the surface tension data of KNO3 . In the present work only KI data is showing such behaviour. Now question arises what is special about these compounds. This needs further investigation for setting up any hypothesis about their behaviour with respect to Jones–Ray effect. In order to have a more insight into the temperature effect on the surface tension, surface tensions were plotted against temperature at constant concentration. The results are depicted in Fig. 2a–c. There is linear decrease in the surface tension with temperature. Moreover, the lines appear to be parallel within the experimental error. This indicates that within the concentration range of present study the temperature has a uniform effect on the surface tension. According to Eötvös rule the surface tension is linear function of the temperature. Generally the surface tension of liquids decreases with the rise of temperature. The mathematical form of Eötvös rule can be written as: V 2/3 = k(Tc − T )
(5)
where V and Tc are the molar volume and critical temperature of a liquid, respectively. k is known as Eötvös constant. The molar volume is given by the molar mass M and the density : V=
M
Substituting value of V into Eq. (5) and rearranging, the surface tension can be written as: =k
2/3 M
(Tc − T )
(6)
This equation is very helpful to conceive the decrease in surface tension with temperature. Actually Eötvös rule presents a relationship between the surface tension of liquids, their molar mass and temperature. It states: as the temperature increases the surface tension will go linearly to zero at a critical temperature. In case of liquids the constant is so fundamental as the universal gas constant in case of gases. Since this equation tells that varies directly with (/M)2/3 , increase in temperature will obviously decrease . This is because of the decrease in with the rise in temperature. The increase in the surface tension of water by addition of electrolytes is attributed to a combined effect of electrostatic forces, dispersion forces, ionic hydration and dissolved gas concentration. The influence, each phenomenon has, depends on the type of anion and cation in the solution and also their concentration [34]. Fig. 3
197
Fig. 3. Temperature dependence of d/dC of KCl, KBr and KI salts solutions.
shows the dependence of d/dC on temperature for the selected salts. It exhibits two interesting features that need to be discussed. It illustrates that the surface tension tends to increase in the order KI < KBr < KCl. Since cation is the same, the effect may be attributed to the contribution of anions. This observation suggests that the magnitude of d/dC is greatly influenced by the kind of anions. It is important to mention here that the magnitude of d/dC is influenced very slightly by the variation of cations as reported elsewhere [21,22]. The surface tension decreasing order KCl > KBr > KI can be explained on the basis of electrostatic attraction of the ions of the salts with water molecules. The electrostatic attraction depends on the size and charge or more precisely the charge density of the ions. In the present case the cation is the same, i.e. K+ but anions are different having the same charge. Therefore, the charge density, in the present case, is only dependent on the size of the anions which decreases in the order Cl− > Br− > I− . Theoretical estimation of solvation parameters and interfacial tension of potassium halides in aqueous solution were reported elsewhere [35]. The trend in the decrease of surface tension was reported [35] to be KCl > KBr > KI which is in good agreement with the presently observed trend. Alternatively, the phenomenon may be explained by taking into account the heat of hydration of the ions. The heat of hydration of ions is usually considered as the degree of hydration of the ion. Greater the heat of hydration of an ion, the more hydrated the ion will be and vice versa. An ion with higher heat of hydration prefers to be fully hydrated at the interface. Consequently, the surface tension of aqueous solution is greatly affected by such ions as compared to an ion with lower heat of hydration, which is partially hydrated. The heats of hydration of Cl− , Br− and I− have been determined experimentally to be 354.1, 326.9 and 285.9 kJ mol−1 , respectively [36]. It seems conceivable that Cl− will affect the surface tension of water to a great extent as compared to Br− , which in turn will affect more than I− ions do. The second interesting feature of Fig. 3 is the difference in behaviour of the curves for KCl, KBr and KI. The d/dC versus temperature curves of KBr and KI seems to be roughly parallel indicating approximately uniform affect of temperature on their surface tension. Apparently d/dC curve for KCl seems to decay exponentially with temperature. A closer look, however, indicates that the experimental data points are parallel to the corresponding points for KBr and KI and also to the temperature axis in the temperature range of 10–15 ◦ C and 20–30 ◦ C. A major drift is, however, observed in the temperature range of 15–20 ◦ C. A similar trend was also observed in Fig. 1a for the surface tension of KCl solution.
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K. Ali et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 337 (2009) 194–199
Fig. 4. Concentration dependence of surface excess concentration of KI solutions at different temperatures.
In all the cases surface excess concentration was found to be negative indicating that the solute has less concentration at the surface as compared to that in the bulk. This is known as negative adsorption, exhibited by all the ionic solutes, contrary to the positive adsorption in the case of surfactant molecules which accumulate mostly on the surface. In the case of ionic solutions the solute ions are present just beneath the surface layer and solvent molecules are dominant at the surface. Due to electrostatic attraction of these ions with the water molecules, an additional pull, other than the H-bonding already present between the water molecules, is exerted on the surface water molecules, which is responsible for the additional surface tension. There is a general decreasing trend in surface excess concentration with the increase in concentration in all the cases at specified temperature (Fig. 4). The surface excess concentration
Fig. 5. Temperature dependence of surface excess concentration of KI solutions at different concentrations.
versus concentration lines at all the temperatures converge at a single point in the lower concentration region, however, in higher concentration region slight divergence is observed indicating that at higher concentration the surface excess concentrations have different values for different temperatures. This implies that at lower concentration surface excess concentration is independent of temperature but becomes temperature dependent in the higher concentration region. This is clearly demonstrated in Fig. 5 where versus temperature lines are parallel to x-axis at lower concentrations while at higher concentration the lines show slightly downward trend towards x-axis. This implies that for higher concentration at higher temperature the surface excess concentration is less negative as compared to that of at lower temperature for
Fig. 6. (a) Concentration and (b) temperature dependence of enthalpy of KI solutions at different temperatures and concentrations, respectively.
K. Ali et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 337 (2009) 194–199
199
layer of anions which shields parts of the physical forces. The decrease in s with concentration may be partly due to this electrical double layer formation. The double layer formation will be increased with the increase in concentration of electrolyte and therefore, is responsible for more deviations of s from that of pure water. This surface phenomenon of increasing double layer formation and an increasing surface density of ions with concentration has been correlated with the decrease in s values for electrolyte and non-electrolyte solutions [22]. Acknowledgements Financial support from the fonds provided by Advanced Studies and Research Board (ASRB) of University of Peshawar is gratefully appreciated. References
Fig. 7. Concentration dependence of entropy of surface formation of KBr solution.
the same concentration. One possible reason for this phenomenon might be the high kinetic energy of ions at higher temperature. This probably decreases the holding capacity of solute ions beneath the surface layer of water thereby increasing the surface excess concentration and thus decreasing the surface tension. Enthalpy of surface formation generally increases with increase in concentration for all the temperature. However, in the case of KI enthalpy first decreases and then increases showing a minimum at 1.0 mol dm−3 (Fig. 6a). This may be due to Jones–Ray effect as discussed earlier. For all the concentration enthalpy of surface formation remains almost constant at all temperatures as shown in Fig. 6b. The entropy change (s) of surface formation of KBr and KI shows a slight decreasing trend with concentration in Fig. 7. It can be explained by taking into account the difference in the partial molal entropy of water between the surface region and the bulk solution [11,37]. The interaction between water-dipole and the charge of the ion constitute a fundamental factor for the change in s. The larger value of entropy of the solvent indicates that the partial molal entropy of water is increased by the contact of water and air in the surface region. By the addition of salt, there is a steady decrease in s over the whole concentration range studied. This might be attributed to the salting out of air molecules from the surface region, thereby causing more ordering of water molecules in the surface region. Johansson and Eriksson have interpreted the distribution of ions in surface zone by surface potential measurements [13]. They reported that the surface potential increments were strongly dependent upon the type of anions but not of cations. It was noted that the ions adsorbed into the surface region loosely form an electric double layer wherein anions form an outer side while cations are directed towards the solution [12]. Outside the layer of anions there are water molecules. They interact with cations through the
[1] N.K. Adam, The Physics and Chemistry of Surfaces, 3rd ed., Oxford University Press, London, 1941 (Chapter 3). [2] L. Onsager, N.N. Samaras, J. Chem. Phys. 2 (1934) 528. [3] Z. Li, B.C.-Y. Lu, Fluid Phase Equilibr. 200 (2002) 239. [4] J.E.B. Randles, in: P. Delahay, C.W. Tobis (Eds.), Advances in Electrochemistry, Electrochemical Engineering, vol. 3, Interscience, New York, 1963, p. 1. [5] P.K. Weissenborn, R.J. Pugh, J. Colloid Interf. Sci. 184 (1996) 550. [6] D. Bhatt, R. Chee, J. Newman, C.J. Radke, Curr. Opin. Colloid Interf. Sci. 9 (2004) 145. [7] Z. Li, B.C.-Y. Lu, Chem. Eng. Sci. 56 (2001) 2679. [8] N. Matubayasi, S. Yamaguchi, K. Yamamoto, H. Matsuo, J. Colloid Interf. Sci. 209 (1999) 403. [9] K. Motomura, N. Ando, H. Matsuki, M. Aratono, J. Colloid Interf. Sci. 139 (1990) 1887. [10] K. Motomura, J. Colloid Interf. Sci. 64 (1978) 348. [11] N. Matubayasi, H. Matsuo, K. Yamamoto, S. Yamaguchi, A. Matuzawa, J. Colloid Interf. Sci. 209 (1999) 398. [12] N.L. Jarvis, M.A. Scheiman, J. Phys. Chem. 72 (1968) 74. [13] K.L. Johansson, J.C. Eriksson, J. Colloid Interf. Sci. 49 (1974) 469. [14] R. Aveyard, S.M. Saleem, J. Chem. Soc., Faraday Trans. 172 (1976) 1609. [15] H.H. Girault, D.J. Schiffrin, J. Electroanal. Chem. 150 (1983) 43. [16] J.M. Hey, D.W. Shield, J.M. Speight, M.C. Will, J. Chem. Soc., Faraday Trans. 177 (1981) 123. [17] L. Ghosh, K.P. Das, D.K. Chattorj, J. Colloid Interf. Sci. 121 (1988) 278. [18] P.K. Weissenborn, R.J. Pugh, Langmuir 11 (1995) 1422. [19] N. Ikeda, M. Aratono, K. Motomura, J. Colloid Interf. Sci. 149 (1992) 208. [20] K. Ali, S. Anwar-ul-Haq, B.S. Siddiqi, Colloid Surf. A: Physicochem. Eng. Aspects 272 (2006) 105. [21] K. Ali, A.A. Shah, S. Bilal, Azhar-ul-Haq, Colloid Surf. A: Physicochem. Eng. Aspects 330 (2008) 28. [22] N. Matubayasi, R. Yoshikawa, J. Colloid Interf. Sci. 315 (2007) 597. [23] H. Frej, M. Jakubczyk, J. Chem. Eng. Data 43 (1998) 158. [24] M. Abraham, M.C. Abraham, I. Ziogas, J. Am. Chem. Soc. 113 (1991) 8583. [25] N.N. Kochurova, A.I. Rusanov, N.O. Myrakhmetova, Dokl. Akad. Nauk. SSSR 316 (1991) 1425. [26] G. Jones, W.A. Ray, J. Am. Chem. Soc. 59 (1937) 187. [27] I. Langmuir, Science 88 (1938) 430. [28] I. Langmuir, J. Chem. Phys. 6 (1938) 873. [29] M. Dole, J. Am. Chem. Soc. 60 (1938) 904. [30] G. Jones, W.A. Ray, J. Am. Chem. Soc. 63 (1941) 288. [31] L.A. Wood, L.B. Robinson, J. Chem. Phys. 14 (1946) 258. [32] H.M. Cassel, J. Chem. Phys. 14 (1946) 462. [33] P.B. Petersen, J.C. Johnson, K.P. Knutsen, R.J. Saykally, Chem. Phys. Lett. 397 (2004) 46. [34] K.P. Weissenborn, Encyclopedia of Surface and Colloid Science, Marcel Dekker, Inc., 2002, pp. 5127–5131. [35] W. Polak, K. Sangwal, J. Cryst. Growth 160 (1996) 154. [36] V.S. Bagotsky, Fundamentals of Electrochemistry, 2nd ed., Wiley & Sons, Inc., Hoboken, NJ, 2006 (Chapter 7). [37] N. Matubayasi, K. Tsunetomo, I. Sato, R. Akizuki, T. Morishita, A. Matuzawa, Y. Natsukari, J. Colloid Interf. Sci. 243 (2001) 444.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Surface tensions and thermodynamic parameters of surface formation of aqueous salt solutions: III. Aqueous solution of KCl, KBr and KI Khurshid Ali a , Anwar-ul-Haq Ali Shah a,∗ , Salma Bilal b , Azhar-ul-Haq Ali Shah b a b
Institute of Chemical Sciences, University of Peshawar, Pakistan Department of Chemistry, Kohat University of Science and Technology, Pakistan
a r t i c l e
i n f o
Article history: Received 14 September 2008 Received in revised form 17 December 2008 Accepted 18 December 2008 Available online 25 December 2008 Keywords: Gibbs adsorption equation Surface tension Salts Surface parameters Surface excess concentration Thermodynamics
a b s t r a c t Surface tension of aqueous electrolyte solutions has been determined experimentally. Concentration and temperature dependence of the surface tension of three different salts has been studied. These include the chloride, bromide and iodide salts of potassium. The study was conducted at five different concentrations, i.e. 0.1, 0.5, 1.0, 1.5 and 2.0 mol dm−3 , and five different temperatures ranging from 10 to 30 ◦ C with 5 ◦ C interval. The surface tension data were fitted into linear regression for estimation of surface excess concentration using Gibbs adsorption equation. The thermodynamic quantities of surface formation such as enthalpy and entropy were estimated from the slopes (d/dT)C of surface tension versus temperature plots at constant concentration. Concentration and temperature dependence of surface excess concentration and thermodynamic parameters of surface formation have been discussed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction It has been known for a long time that simple electrolytes, such as sodium chloride, raise the surface tension of water [1]. Onsager and Samaras presented preliminary explanations to this phenomenon [2]. The surface tension of aqueous electrolyte solutions is an important physicochemical property. It has an important effect on liquid–liquid extraction processes such as hydrometallurgy and liquid–liquid dispersions. It is regarded as an important parameter in the study of distillation in the presence of salt and absorption refrigeration. It may also effect considerably phase separation, coagulation of droplets and building up of interfacial areas [3]. The phenomenon of surface tension has been a subject of considerable research interest worldwide. Researchers are trying to understand structural changes taking place at the surface from the knowledge of surface tension and surface thermodynamic parameters. In this regard much work has been devoted to the studies of surface tension versus concentration relationship of simple salts solutions for elucidating the structure of the surface region [4,5]. Molecular dynamic simulations of the surface tension of aqueous electrolyte solutions as a function of concentration at constant temperature have also been presented [6]. Similarly,
∗ Corresponding author. Tel.: +92 91 9216652/70120x3033; fax: +92 91 9216652. E-mail address: [email protected] (A.-u.-H.A. Shah). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.12.023
a calculation method was developed for representing and predicting surface tension of aqueous electrolyte solutions at high concentration [7]. The studies concerning temperature effect on the surface tension of aqueous solutions are limited [8]. This is because of low accuracy and precision of the results based on surface tension measurements that often lead to negligible deviation of d/dT from that of pure water. Nevertheless, the importance of the temperature effect on the surface tension cannot be neglected. It enables one to estimate thermodynamic quantities such as entropy, enthalpy and energy of surface formation. The thermodynamic quantities might be helpful in the explanation of interaction between components of the surface region [9,10]. Matubayasi et al. have reported the importance of the temperature effect on surface tension of aqueous solution of chloride salts of sodium, magnesium, and lanthanum, and evaluated the thermodynamic quantities such as entropy and free energy of surface formation [11]. A fact, currently of large historical interest, is that aqueous inorganic salt solutions exhibit higher surface tension than the pure water. The surface tension of such salt solutions increases with the increase in concentration. This increase in surface tension has been correlated with the structure of the surface region [4,5,12–18]. Gibbs adsorption equation and some thermodynamic treatment have been used to correlate the increase in surface tension with the ion-free layer and the thickness of the surface region [14–17]. Johansson and Eriksson have evaluated the entropy change of the surface of salt solution from the slope of the surface tension ver-
K. Ali et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 337 (2009) 194–199
sus temperature curve [13]. Ikeda et al. determined the effect of temperature, pressure, and concentration on the interfacial tension of an aqueous solution of sodium chloride–hexane interface. They also showed that the change of surface tension with respect to temperature decreases slightly with increasing salt concentration [19]. Literature shows that the surface tension of aqueous inorganic salt solutions have shown marked specific ion effects that had remained an ambiguous and unexplained fact. Though the issue is under thorough investigation but still a lot work would be needed to fully understand the phenomenon of specificity of ion that contributes to the surface tension changes of ionic solutions. Recently we have reported concentration and temperature dependence of surface quantities of chloride salts of lithium, sodium, potassium and ammonium [20]. In this report we attempted for the first time a simple approach for measuring surface tension with low cost set-up and applying different set of equations for calculation of thermodynamic parameters. In a subsequent publication [21] the same approach was applied successfully to salts with polyatomic anions. It is important to mention that in both cases anions were the same but cations were different. The present paper reports on the measurements of surface tension of aqueous solutions of chloride, bromide and iodide salts of potassium, as a function of concentration and temperature. The study has been extended up to five different concentrations and temperatures. The aim of the study was to apply recently developed approach [20–22] to aqueous solutions of chloride, bromide and iodide salts of potassium. In this case anions are different but
195
cation is the same. Data of KCl has been taken from our previously published results [20]. 2. Experimental All the chemicals used in this work were of anal. grade from B.D.H. and used without further purification. Water was doubly distilled before use. The surface tension was determined by the drop method using a stalagmometer (hellige No. 1211-A8779, Germany) with a capillary having internal diameter of 2.71 mm. All the glassware was washed thoroughly with chromic acid solution before use. The temperature of the test solution was kept constant within ±0.10 ◦ C by circulating controlled-temperature water through a glass jacket around the stalagmometer made in our glass-blowing workshop. The water supply to this glass jacket was made from an external thermostat, the temperature of which was maintained constant within ±0.10 ◦ C. Surface tension was calculated by using the following formula [23]:
=
d NH2 O F dH2 O N FH2 O
H2 O
where d and dH2 O are the densities of the solution and water, respectively, N and NH2 O are the number of drops of solution and water, respectively, F and FH2 O are factors that depend on the stalagmometer capillary and the volume of solution drop, and H2 O is the surface tension of water. The procedure of the surface tension and density measurement has been described elsewhere [20].
Fig. 1. Concentration dependence of surface tension of (a) KCl, (b) KBr and (c) KI solutions at different temperatures as indicated.
196
K. Ali et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 337 (2009) 194–199
3. Results and discussion The surface tension of aqueous solutions of three different salts, namely potassium chloride, potassium bromide and potassium iodide has been measured experimentally. The measurements were carried out at five different temperatures in the range of 10–30 ◦ C with an increment of 5 ◦ C and five different concentrations, i.e. 0.1, 0.5, 1.0, 1.5 and 2.0 mol dm−3 . The effect of concentration and temperature on the surface tension, surface excess concentration and enthalpy has been discussed. The entropy of surface formation has also been estimated. The surface tension data were plotted against concentrations C at constant temperatures. By fitting the data into the linear regression lines: = 0 +
d dC
T,p
C
(1)
the coefficients of the regression lines (d/dC)T,p were calculated in order to estimate the surface excess concentration by using the Gibbs adsorption equation (Eq. (2)). The standard error of the estimate of on C for aqueous solutions of potassium chloride, potassium bromide and potassium iodide was in the range of 0.04–0.08: =−
C RT
d dC
(2) T,p
The surface excess concentrations were estimated and their concentration and temperature dependence has been studied. Similarly, by fitting the surface tension versus temperature data at constant concentration into the linear regression line, the coefficients of the regression lines (d/dT)C,p were calculated. The enthalpy H and entropy s changes were estimated by using the following equations [24]: H = − T
d dT
d
s = −
dT
(3) C,p
(4) C,p
Fig. 1a–c shows concentration dependence of surface tension of aqueous solutions of the selected salts at different temperatures. Generally, the surface tension is observed to increase linearly with concentration. Apparently the surface tensions versus concentration lines seem to be parallel for all the temperatures. Discrepancies are, however, observed in the case of KCl. For KCl the surface tension versus concentration lines are parallel at 10 and 15 ◦ C. Beyond this temperature in the high concentration region the surface tension versus concentration lines tend to depart. This deviation from the parallel behaviour is not a general trend. Because beyond 20 ◦ C the lines are once again parallel. The reason of the discrepancy in the case of KCl solution is unknown. The surface tension data of KI show unusual behaviour at low temperature in the low concentration region. At 10 and 15 ◦ C the surface tension tends to
Fig. 2. Temperature dependence of surface tension of (a) KCl, (b) KBr and (c) KI solutions at different concentrations as indicated.
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decrease rather than increasing with concentration in the concentration region <0.1 mol dm−3 . Beyond this concentration the surface tension increases linearly with concentration. It is important to mention that this abnormal behaviour could not be seen in the data obtained at relatively higher temperature. This anomalous behaviour is called as the Jones–Ray effect [25]. In 1937, Jones and Ray [26] observed a minimum in the surface tension of KCl, CsNO3 and K2 SO4 solutions using the capillary rise method. The following year, Langmuir [27,28] theoretically refuted the surface tension decrease as an artifact of the indirect experimental method. However, Dole [29] was able to explain the effect in terms of the adsorption of ions to a fixed number of active surface sites that would saturate around 1 mM; which is essentially equivalent to the familiar Langmuir adsorption model. In 1941, Jones and Ray [30] reproduced the effect for 11 different electrolytes, all showing a minimum in the surface tension around 1 mM. The theoretical argument of Langmuir was later criticized as being inadequate to explain the entire effect [31,32]. In fact Jones–Ray effect had remained as a curiosity that could neither be proved nor completely refuted until 2004 when direct experimental confirmation of the effect was presented using resonance-enhanced femtosecond second harmonic generation [33]. In our previous paper [21] Jones–Ray effect was observed only in the surface tension data of KNO3 . In the present work only KI data is showing such behaviour. Now question arises what is special about these compounds. This needs further investigation for setting up any hypothesis about their behaviour with respect to Jones–Ray effect. In order to have a more insight into the temperature effect on the surface tension, surface tensions were plotted against temperature at constant concentration. The results are depicted in Fig. 2a–c. There is linear decrease in the surface tension with temperature. Moreover, the lines appear to be parallel within the experimental error. This indicates that within the concentration range of present study the temperature has a uniform effect on the surface tension. According to Eötvös rule the surface tension is linear function of the temperature. Generally the surface tension of liquids decreases with the rise of temperature. The mathematical form of Eötvös rule can be written as: V 2/3 = k(Tc − T )
(5)
where V and Tc are the molar volume and critical temperature of a liquid, respectively. k is known as Eötvös constant. The molar volume is given by the molar mass M and the density : V=
M
Substituting value of V into Eq. (5) and rearranging, the surface tension can be written as: =k
2/3 M
(Tc − T )
(6)
This equation is very helpful to conceive the decrease in surface tension with temperature. Actually Eötvös rule presents a relationship between the surface tension of liquids, their molar mass and temperature. It states: as the temperature increases the surface tension will go linearly to zero at a critical temperature. In case of liquids the constant is so fundamental as the universal gas constant in case of gases. Since this equation tells that varies directly with (/M)2/3 , increase in temperature will obviously decrease . This is because of the decrease in with the rise in temperature. The increase in the surface tension of water by addition of electrolytes is attributed to a combined effect of electrostatic forces, dispersion forces, ionic hydration and dissolved gas concentration. The influence, each phenomenon has, depends on the type of anion and cation in the solution and also their concentration [34]. Fig. 3
197
Fig. 3. Temperature dependence of d/dC of KCl, KBr and KI salts solutions.
shows the dependence of d/dC on temperature for the selected salts. It exhibits two interesting features that need to be discussed. It illustrates that the surface tension tends to increase in the order KI < KBr < KCl. Since cation is the same, the effect may be attributed to the contribution of anions. This observation suggests that the magnitude of d/dC is greatly influenced by the kind of anions. It is important to mention here that the magnitude of d/dC is influenced very slightly by the variation of cations as reported elsewhere [21,22]. The surface tension decreasing order KCl > KBr > KI can be explained on the basis of electrostatic attraction of the ions of the salts with water molecules. The electrostatic attraction depends on the size and charge or more precisely the charge density of the ions. In the present case the cation is the same, i.e. K+ but anions are different having the same charge. Therefore, the charge density, in the present case, is only dependent on the size of the anions which decreases in the order Cl− > Br− > I− . Theoretical estimation of solvation parameters and interfacial tension of potassium halides in aqueous solution were reported elsewhere [35]. The trend in the decrease of surface tension was reported [35] to be KCl > KBr > KI which is in good agreement with the presently observed trend. Alternatively, the phenomenon may be explained by taking into account the heat of hydration of the ions. The heat of hydration of ions is usually considered as the degree of hydration of the ion. Greater the heat of hydration of an ion, the more hydrated the ion will be and vice versa. An ion with higher heat of hydration prefers to be fully hydrated at the interface. Consequently, the surface tension of aqueous solution is greatly affected by such ions as compared to an ion with lower heat of hydration, which is partially hydrated. The heats of hydration of Cl− , Br− and I− have been determined experimentally to be 354.1, 326.9 and 285.9 kJ mol−1 , respectively [36]. It seems conceivable that Cl− will affect the surface tension of water to a great extent as compared to Br− , which in turn will affect more than I− ions do. The second interesting feature of Fig. 3 is the difference in behaviour of the curves for KCl, KBr and KI. The d/dC versus temperature curves of KBr and KI seems to be roughly parallel indicating approximately uniform affect of temperature on their surface tension. Apparently d/dC curve for KCl seems to decay exponentially with temperature. A closer look, however, indicates that the experimental data points are parallel to the corresponding points for KBr and KI and also to the temperature axis in the temperature range of 10–15 ◦ C and 20–30 ◦ C. A major drift is, however, observed in the temperature range of 15–20 ◦ C. A similar trend was also observed in Fig. 1a for the surface tension of KCl solution.
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Fig. 4. Concentration dependence of surface excess concentration of KI solutions at different temperatures.
In all the cases surface excess concentration was found to be negative indicating that the solute has less concentration at the surface as compared to that in the bulk. This is known as negative adsorption, exhibited by all the ionic solutes, contrary to the positive adsorption in the case of surfactant molecules which accumulate mostly on the surface. In the case of ionic solutions the solute ions are present just beneath the surface layer and solvent molecules are dominant at the surface. Due to electrostatic attraction of these ions with the water molecules, an additional pull, other than the H-bonding already present between the water molecules, is exerted on the surface water molecules, which is responsible for the additional surface tension. There is a general decreasing trend in surface excess concentration with the increase in concentration in all the cases at specified temperature (Fig. 4). The surface excess concentration
Fig. 5. Temperature dependence of surface excess concentration of KI solutions at different concentrations.
versus concentration lines at all the temperatures converge at a single point in the lower concentration region, however, in higher concentration region slight divergence is observed indicating that at higher concentration the surface excess concentrations have different values for different temperatures. This implies that at lower concentration surface excess concentration is independent of temperature but becomes temperature dependent in the higher concentration region. This is clearly demonstrated in Fig. 5 where versus temperature lines are parallel to x-axis at lower concentrations while at higher concentration the lines show slightly downward trend towards x-axis. This implies that for higher concentration at higher temperature the surface excess concentration is less negative as compared to that of at lower temperature for
Fig. 6. (a) Concentration and (b) temperature dependence of enthalpy of KI solutions at different temperatures and concentrations, respectively.
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layer of anions which shields parts of the physical forces. The decrease in s with concentration may be partly due to this electrical double layer formation. The double layer formation will be increased with the increase in concentration of electrolyte and therefore, is responsible for more deviations of s from that of pure water. This surface phenomenon of increasing double layer formation and an increasing surface density of ions with concentration has been correlated with the decrease in s values for electrolyte and non-electrolyte solutions [22]. Acknowledgements Financial support from the fonds provided by Advanced Studies and Research Board (ASRB) of University of Peshawar is gratefully appreciated. References
Fig. 7. Concentration dependence of entropy of surface formation of KBr solution.
the same concentration. One possible reason for this phenomenon might be the high kinetic energy of ions at higher temperature. This probably decreases the holding capacity of solute ions beneath the surface layer of water thereby increasing the surface excess concentration and thus decreasing the surface tension. Enthalpy of surface formation generally increases with increase in concentration for all the temperature. However, in the case of KI enthalpy first decreases and then increases showing a minimum at 1.0 mol dm−3 (Fig. 6a). This may be due to Jones–Ray effect as discussed earlier. For all the concentration enthalpy of surface formation remains almost constant at all temperatures as shown in Fig. 6b. The entropy change (s) of surface formation of KBr and KI shows a slight decreasing trend with concentration in Fig. 7. It can be explained by taking into account the difference in the partial molal entropy of water between the surface region and the bulk solution [11,37]. The interaction between water-dipole and the charge of the ion constitute a fundamental factor for the change in s. The larger value of entropy of the solvent indicates that the partial molal entropy of water is increased by the contact of water and air in the surface region. By the addition of salt, there is a steady decrease in s over the whole concentration range studied. This might be attributed to the salting out of air molecules from the surface region, thereby causing more ordering of water molecules in the surface region. Johansson and Eriksson have interpreted the distribution of ions in surface zone by surface potential measurements [13]. They reported that the surface potential increments were strongly dependent upon the type of anions but not of cations. It was noted that the ions adsorbed into the surface region loosely form an electric double layer wherein anions form an outer side while cations are directed towards the solution [12]. Outside the layer of anions there are water molecules. They interact with cations through the
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