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Physico-chemical aspects of some biological membranes. II
Colloids and Surfaces B: Biointerfaces 27 (2002) 95 /101 www.elsevier.com/locate/colsurfb
Physico-chemical aspects of some biological membranes. II Studies on the conductances of the inner membrane of chicken egg in presence of alkali chlorides and the application of absolute reaction rate theory T. Samanta Department of Chemistry, Syamsundar College, Syamsundar, Burdwan 713 424, W.B., India Received 19 November 2001; accepted 4 March 2002
Abstract Membrane conductance of the inner thin membrane of a chicken egg has been studied with aqueous solution of some common alkali chloride, KCl, NaCl, LiCl and NH4Cl. Conductance values were measured for each concentration at different temperatures after bathing the membranes properly in the respective electrolyte solution. The normal behaviour of these conductances shows that the values increase smoothly with increase in concentrations, tending towards limiting values at higher concentrations. The magnitude follow the order NH4 /K /Na /Li which is the reverse order of the hydrated sizes of these ions. The temperature range studied is between 298 and 323 K at 5 K intervals. The temperature variation of the conductance values has been utilised to calculate the activation energies according to Arrhenius and to Eyring. Assuming the applicability of the absolute reaction rate theory in the membrane conduction process, other thermodynamic activation parameters viz. DG#, DH# and DS# have been calculated. The energy values for any particular electrolyte, however, decrease with increase in concentration of the bathing electrolyte. Very small negative DS# values indicate, some kind of immobilisation of the ions occurred during the conduction process. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Membrane conductances; Egg membrane; Alkali chlorides; Energetics of conduction process
1. Introduction Transport processes through biological membranes, as well as in many synthetic membranes, are important because of their potential use in different separation processes. Studies on the conductances of simple salts through such poly-
E-mail address: [email protected] (T. Samanta).
meric networks are related to the ionic transport through the pores. Extensive research work related to the conductance and diffusion of simple salts through ion-exchange or porous membrane impregnated with inorganic precipitates has been reported by different workers. Studies of the biological membranes are necessary to interpret the mechanism of transport occurring in the biological systems in-vivo [1 /8].
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T. Samanta / Colloids and Surfaces B: Biointerfaces 27 (2002) 95 /101
Uncharged porous membranes are not always easily available in suitable forms for the studies of different transport phenomena. Even available forms of porous glass, having a well-defined structure and pore size, also acts as a form of ionic membrane [9]. Recently Samanta and Basu have studied the conductance behaviour of some uncharged microporous membrane [10 /12] in presence of aqueous electrolyte solutions. These uncharged microporous membranes were prepared by Mizutani et al. on treatment of their ionexchange membranes in suitable ionic form with H2O2, producing microporous membranes with excellent stability and flexibility [13,14]. However, a similar type of studies involved directly with natural biological membranes [15,16] are not so found because of the difficulties of experimental studies of the electrochemical properties of such membranes for their undefined pore size and complex surface characteristics. The purpose of the present paper is to report the conductance studies of the inner thin layer membrane of an egg with aqueous solution of common alkali chlorides over a range of concentrations and at different temperatures. The energetics and the activation parameters for the permeation of electrolytes through this membrane have been calculated by applying the absolute reaction rate theory.
2. Experimental The inner thin layer membrane of a chicken egg was separated manually from the outer hard calcium carbonate shell with the help of finetipped surgical tongs. The membrane was thoroughly washed with deionised water to remove any adsorbed chemicals. It was always kept in the wet condition to avoid any disturbance arises due to the entrapped air within the pores and also to prevent the crack in dry condition. The membranes were cut into small discs and attached with an adhesive like ‘Quickfix’ between the flanged ends of the two half cells of a U-tube. Membrane conductances were measured by the method of Subramanyan and Lakshminarayanaiah [17], which was also used by many other workers in recent studies. For proper bathing of the mem-
brane pores, removing entrapped air, it was equilibrated with an electrolyte solution of desired concentration by maintaining a hydrostatic pressure difference between the solutions on the two sides of the membrane at a particular temperature of the bath. After proper bathing, the solution was replaced by mercury having the same temperature of the bath, without complete removal of the adhering surface liquid. Membrane thus remained sandwiched between mercury on both sides by eliminating the possible presence of entrapped air at the mercury/membrane interface. Solutions of different concentrations were prepared with analytical grade KCl, NaCl, LiCl and NH4Cl by using triply distilled water. The electrical connections to the conductivity bridge were made through the platinum electrodes inserted in the mercury. Conductance measurements were carried out with a Direct Reading Conductivity Meter 303 (Systronics) at a frequency of 1 kHz.
3. Results and discussion Physical characteristics relevant to this study may be briefly mentioned here as follows. The membrane is basically composed of keratin protein while some authors believe that it contains both keratin and mucin [18]. The inner thin membrane basically consists of collagen, hydroxyproline and hydroxylysine and the rest is composed of some uncharacterised protein and glycoprotein [19]. The average thickness is 0.31 mm. The average length of the pore canal is 0.20 mm and the pores are looking like funnel having initial and final diameters, which are 0.013 mm and 0.006 mm respectively. Scanning electron micrograph of the membrane has been shown elsewhere [16]. The specific conductances (k ) were determined with the solutions of KCl, NaCl, LiCl and NH4Cl in the temperature range 298 /323 K, with intervals of 5 K at an accuracy of 9/0.1 K. The range of concentrations used was 0.1 /10 mol m 3 with suitable intervals. At higher concentrations ( /10 mol m 3) membrane shrinkage occurs due to the exclusion of water within the pores, showing nonsteady conductance values.
T. Samanta / Colloids and Surfaces B: Biointerfaces 27 (2002) 95 /101
The nature of variations of k values with concentrations for different electrolytes at a particular temperature, 298 K are shown in Fig. 1. The common trend found for all the electrolytes is that the membrane conductance increases almost linearly with increase in concentration, tending towards a limiting value at high concentration (/10 mol 3). It is obvious that, at high dilution, the membrane pores are mainly occupied by more water and less electrolyte, showing small conductance values, but it increases with the gradual increase in external bathing electrolyte concentration due to the accumulation of more and more electrolyte within the pores. As regards the conductance, the sequence has been found to be NH4 ]/K /Na /Li in this membrane, in conformity with an earlier observation in a different synthetic membrane [10,11]. The same sequence as observed in aqueous solution at any temperature, indicating that the hydrated sizes retained more or less within the membrane pores. The limiting tendency at higher concentration suggests the possibility of two opposing factors: the exclusion of free water due to the osmotic effects as the external concentration increases, thereby decreasing the ionic mobilities within the pores [1,20] and at the same time, increase in conductance due to the higher salt uptake that
Fig. 1. Plots of specific conductance, k (S m 1) of different electrolytes through the egg membrane against bathing concentration (mol m 3) at 298 K.
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Fig. 2. Plots of specific conductance, k (S m 1) of KCl through the egg membrane against bathing concentration (mol m 3) at different temperatures (298 /323 K).
increases the effective concentration within the membrane pores. After certain concentrations, these two opposing factors may have a tendency to balance each other. Samanta et al. [21] have shown that the idea of large water content of the pores at lower concentrations and the gradual exclusion of water with increase in external electrolyte concentration by measuring the changes of specific weight of the membrane. The limiting tendency of membrane conductances has also been observed by Iijima et al. [22], Beg et al. [23], Jolota et al. [24] and Shahi et al. [25] with different types of charged membrane. They have tried to explain this limiting phenomena in the light of competition between the greater salt uptake, increasing the conductance values and the membrane shrinkage, producing the higher resistance to the ionic path may partially occurs in the present case. The effect of temperature on the membrane conductance is shown in Fig. 2 represents the specific conductance (k ) values for KCl solutions of different concentrations in the temperature range 298/323 K at 5 K intervals. The general trend at any temperature is the same, showing a smooth increase in concentration, tending towards a limiting value. The temperature effect of all the electrolytes studied here has also main-
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Fig. 3. Arrhenius plots of log values k at one of the concentrations (1 mol m 3) against 1/T for the calculation of activation energies of alkali metal ions from the slope, /E / 2.303R.
tained the same sequence of conductance variations NH4 /K /Na /Li indicating that the hydrated sizes remains virtually undisturbed in the studied temperature range. The smooth
increase of membrane conductance with any electrolyte from one temperature to another at any concentration may be attributed mainly to an increase in mobility of the ions compared with the swelling and de-swelling effect shown by the charged membranes [1,26]. By applying the Arrhenius equation in its basic form [27], log k values of an electrolyte at each concentration are plotted against 1/T to calculate the activation energies from the slopes. Fig. 3 shows the plots of the different electrolytes at the concentration of 1 mol m 3. The smooth linear plots suggest that there may be no abrupt irreversible change in the membrane structure within the concentration and temperature range studied. The activation energies of different electrolytes are calculated from the slopes of the Arrhenius plots shown in the column 3 of the Table 1. The general common feature is that the activation energies at low concentration are very high compared with the values at higher concentrations. The very high values at extremely low concentration may be due
Table 1 Calculated values of the activation parameters: Arrhenius (Ea) and Eyring (EE) activation energies, enthalpy of activation, DH#, free energy of activation, DG# and entropy of activation, DS#, for the conduction of KCl, NaCl, LiCl and NH4Cl solutions through the egg membrane at 2989/0.1 K. Salt KCl
Concentration (mol m 3)
0.1 1 2 5 10 NaCl 0.1 1 2 5 10 LiCl 0.1 1 2 5 10 NH4Cl 0.1 1 2 5 10
Ea (KJ mol 1) Ee (KJ mol 1) DH# (KJ mol 1) DG# (KJ mol 1) DS# (KJ mol 1 K 1) 45.05 22.34 17.64 13.44 11.44 38.29 18.76 18.46 14.48 12.16 37.13 15.64 19.54 18.93 15.61 39.46 18.42 17.57 13.61 12.76
43.96 22.72 17.24 13.53 11.47 38.86 20.39 18.55 14.59 13.10 36.74 15.26 19.46 19.33 15.19 39.30 17.68 16.96 13.02 11.88
41.88 20.24 14.76 11.05 8.99 36.38 17.91 16.07 12.11 10.62 34.26 12.78 16.98 16.85 12.71 36.82 15.20 14.48 10.54 9.40
72.10 74.45 74.15 74.55 74.64 72.77 74.53 74.48 75.10 / 72.88 74.67 74.85 75.56 75.20 71.36 73.67 73.63 74.25 74.33
/0.10 /0.18 /0.20 /0.21 /0.22 /0.12 /0.19 /0.20 /0.21 / /0.13 /0.21 /0.19 /0.20 /0.21 /0.12 /0.20 /0.20 /0.21 /0.22
T. Samanta / Colloids and Surfaces B: Biointerfaces 27 (2002) 95 /101
to the immobilisation of ions because of some obstructions to ionic paths within the pores of the membrane matrix. The binding may either be due to the small fixed charges present within the membrane or to the adsorption on the pore walls of the membrane when it initially comes into contact with the electrolyte. Samanta et al. have shown elsewhere [16] that very low negative potential values for such membrane in presence of alkali chlorides solution is due to the gradual accumulation of Cl ions produces a negative charge density. It is expected that with increasing concentrations, the conductance increases because of greater accumulation of ions, which may overcome the opposing factors, so that the smaller activation energies are required. Several workers have also reported the gradual decrease in activation energy of conduction with increasing the concentrations in the presence of different types of membranes [10,11,21 /23] with some exceptional values [22]. The most noteworthy feature revealed from Table 1 is that the activation energy for conduction of alkali chlorides through this membrane follows the order EK :/ENH4 / ENa /ELi at low concentration range and changes the order ELi /ENa /EK :/ENH4 at high concentration range. The concept of ‘activation theories’ regarding ionic movements is that the ion occupies an average equilibrium configuration within a cage of solvent molecules and it is continuously moving from one equilibrium position to another [27,28]. Thus, smaller activation energy should be required for the smaller ion. As the conductance values of the ions is controlled by the mobility of the ions which depends upon the hydrated size of the ions, required greater activation energy for the higher hydrated size and thus the energy values should follow the reverse order of conductance. Such results are quite consistent with the aqueous solutions [29] and also with these alkali chloride solutions in the presence of a synthetic microporous membrane studied earlier [11]. Whereas in some cases the mobility is controlled by the nonhydrated crystallographic radius [1,26] and obviously the activation energy should follow the decreasing order from the ions having largest crystallographic radii to those with the smallest,
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for very highly cross-linked resins, in zeolites or in any network where there is no room for hydration shell. Iijima et al. [22] have found this pattern in their studies with nylon membranes. Beg et al. [23], however, have reported some interesting results with their parchment supported cupric orthophosphate membrane, in which the conductance of alkali chlorides follows a decreasing sequence with the increase in the hydrated size. Activation energy values also follow the same sequence, indicating the higher the conductance the greater is the activation energy. In the present study with the egg-shell membrane in the presence of some common alkali chlorides the activation energy follows the same order of conductance in the low concentration range, indicating that some kinds of secondary phenomena take place which require extra energy along with the normal activation energy for jumping. At very low concentration the viscosity Bcoefficient represents the ion-solvent interaction and measures the order or disorder introduced by the ions into the solvent structure [30]. The potassium and ammonium ion have a small negative viscosity B-coefficient at 25 8C and a positive temperature coefficient, suggesting that with rise in temperature it gradually becomes a water structure promoter [31] and increases the viscosity of the aqueous medium. This structure making property will be enhanced in the presence of the structure breaking Cl ions. It is expected that the increase in viscosity will be higher within the narrow pores of the membrane. Whereas in the case of sodium and lithium ion have a positive viscosity B-coefficient at 25 8C and a negative temperature coefficient, giving rise to water structure breaker with temperature and decreases the viscosity of the medium within the membrane pores. Therefore, it now plausible to estimate that some additional energy is consumed for the ‘depolymerisation’ of the enhanced water structure made by the K and NH4 ions than that of the Na and Li ions resulting the activation energy sequence EK /ENH4 /ENa /ELi . But at higher concentrations the number of ions and its mobility dominating the membrane conductance values and hence the activation energy. As the membrane conductance of egg shell membrane
T. Samanta / Colloids and Surfaces B: Biointerfaces 27 (2002) 95 /101
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follows the sequence NH4 /K /Na /Li , so logically the least conducting ion will receive the greater activation energy for conduction. It is also known from the ion conductance values [30] that, the increment of ion conductances per degree rise in temperature follow the sequence K (1.58) / NH4 (1.47)/Na (1.29) /Li (0.99) corroborate the necessity of higher activation energy for conduction of Li ion than of K ion. Thus the activation energies required for different electrolyte follow the order ELi /ENa /EK / ENH4 showing the reverse order of conductance but a parallel order to their hydrated sizes that control the mobility and hence conductance. In order to calculate the necessary thermodynamic parameters of activation it is relevant to apply the absolute reaction rate theory of Eyring et al. [27,32]. Several authors [21 /23] have already applied this theory for the membrane conductance in the following form: L
kT # # exp DH =RT expDS =R h
(1)
where L is the equivalent conductance within the membrane, h and k are the Planck constant and Boltzmann constant, respectively, for the various ionic processes taking place within the membrane during conduction. Eq. (1) can be changed to its logarithmic form for the evaluation of DH# and DS# as: log
Lh kT
DH # 2:303RT
DS # 2:303R
(2)
Eq. (2) suggests that a plot of log(Lh /kT ) against 1/T from experimental data should yield a straight line with slope /DH#/2.303R , which enables the calculation of DH#. The smooth linear plots shown in Fig. 4 justify the applicability of the Eyring equation. DS# may also be calculated from Eq. (2) for any particular temperature. The other important thermodynamic activation parameters for the conduction process through this membrane, may be calculated with the help of usual thermodynamic relations, Ee DH # RT and
(3)
Fig. 4. Eyring plots of log(Lh /kT ) values at one of the concentrations (1 mol m 3) of the alkali metal ions against 1/ T to calculate the magnitude of DH# from the slope, equal to / DH#/2.303R.
DG # DH # TDS #
(4)
The calculated values of all these activation parameters at 298 K are tabulated in Table 1. The values of Arrhenius (Ea) and Eyring (Ee) activation energies are recorded in columns 3 and 4 of Table 1. It is expected that the Eyring activation energy values are slightly different from those obtained directly from the Arrhenius plots. Other characteristic features, i.e. the general trend of gradual decrease with further increase in concentration and the sudden drop in activation energy at very low concentration. The calculated values of DH# and DG# are also recorded in columns 5 and 6 of Table 1 shows the same increasing trend from K to Li . According to Eyring et al. [27] the values of entropy of activation, DS#, may be positive, negative or close to zero, which indicates the mechanism of flow through the membrane. A large negative DS# values indicate that some kind of bond formation occurs within the membrane pores. Whereas a large positive DS# values indicate the breakage of bonds and low values indicate that ionic movement takes place without making any disturbance. Large or small DS# values may also be interpreted by the Barrer’s concept of ‘zone of activation’ [33,34], which states that a large DS# corresponds to a large zone of activation, or reversible loosening of
T. Samanta / Colloids and Surfaces B: Biointerfaces 27 (2002) 95 /101
membrane segments, whereas small values explained in the light of small ‘zone of activation’ or no loosening of membrane structure. The very small negative DS# values are recorded in the last column of Table 1, indicating that at least some kind of immobilisation takes place during the ionic movement for conduction. Despite the small magnitude, there is an indication that some disorder introduced either by partial immobilisation or by disruption of solvent structure, for which some extra energy is needed and it is reflected in the activation parameters sequence.
Acknowledgements The author expresses his sincere thanks to the Head of the Department of Chemistry, the University of Burdwan for laboratory facilities and to the Principal, Syamsundar College for constant inspiration and other assistance. Special thanks are due to Mrs T. Samanta for correcting the English language.
References [1] N. Lakshminarayanaiah, Transport Phenomena in Membranes, Academic Press, New York, 1969. [2] A. Ayalon, G. Bahr, P. Hirsch-Ayalon, J. Membr. Biol. 51 (1979) 7. [3] A. Ayalon, J. Membr. Sci. 29 (1984) 93. [4] W.U. Malik, H. Arif, F.A. Siddiqi, Bull. Chem. Soc. Jpn. 40 (1967) 1746. [5] H.T. Tien, H.P. Ting, J. Colloid Interface Sci. 27 (1968) 702. [6] Y. Toyoshima, M. Yuasa, Y. Kobatake, H. Fujita, Trans. Faraday Soc. 21 (1956) 141. [7] S. Soubier, P. Sistort, E. Dejean, J. Sandeaux, R. Sandeaux, C. Gavach, J. Membr. Sci. 141 (1998) 111. [8] V.K. Shahi, S.K. Thampy, R. Rangarajan, J. Membr. Sci. 158 (1999) 77. [9] I. Altug, M.L. Hair, J. Phys. Chem. 72 (1968) 599.
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[10] T. Samanta, A.S. Basu, J. Membr. Sci. 45 (1989) 247. [11] T. Samanta, A.S. Basu, J. Electroanal. Chem. 274 (1989) 235. [12] T. Samanta, A.S. Basu, J. Indian Chem. Soc. 73 (1996) 119. [13] Y. Mizutani, Bull. Chem. Soc. Jpn. 43 (1970) 595. [14] Y. Mizutani, M. Nishimura, J. Appl. Polym. Sci. 14 (1970) 1847. [15] H. Arif, M. Abiriad, M. Hasan, S.Q. Mahdi, H. Rashid Khan, T.D. Seth, N. Shoaib, J. Membr. Sci. 46 (1989) 203. [16] T. Samanta, A.S. Basu, Colloids Surf. A: Physicochem. Eng. Aspects 137 (1998) 171. [17] V. Subramanyan, N. Lakshminarayanaiah, J. Phys. Chem. 72 (1968) 4314. [18] A.L. Romanoff, A.J. Romanoff, The Avian Egg, Wiley, New York, 1949, pp. 169, 328, 329 [19] A.L. Johnson, in: P.D. Sturkie (Ed.), Avian Physiology, 4th edition, Springer, New York, 1986, pp. 421 /425. [20] N. Lakshminarayanaiah, Chem. Rev. 65 (1965) 501. [21] T. Samanta, A.S. Basu, J. Membr. Sci. 31 (1987) 89. [22] T. Iijima, T. Obara, M. Tsshiki, T. Seki, K. Adachi, J. Colloid Interface Sci. 63 (1978) 421. [23] M.N. Beg, K. Ahmad, I. Altag, M. Arshad, J. Membr. Sci. 9 (1981) 303. [24] S.K. Jolota, R. Paterson, J. Chem. Soc. Faraday Trans. II 69 (1973) 1510. [25] V.K. Shahi, A.P. Murugesh, B.S. Makawana, S.K. Thampy, R. Rangarajan, Ind. J. Chem. 39A (2000) 1264. [26] F. Helfferich, Ion Exchange, McGraw-Hill, New York, NY, 1962. [27] S. Glasstone, K.J. Laidler, H. Eyring, The Theory of Rate Processes, McGraw-Hill, New York, NY, 1941, pp. 525 / 544. [28] G.J. Hills, Kinetics studies of ionic migration, In: B.E. Conway. [29] R.A. Robinson, R.H. Stokes, Electrolyte Solutions, 2nd edition, Butterworth, London, 1968, pp. 463 /465. [30] A.L. Horvath, Handbook of Aqueous Electrolyte Solutions, Ellis Horwood Limited, West Sussex, UK, 1985, pp. 335 /364. [31] M. Kaminsky, Disc. Faraday Soc. 24 (1957) 171. [32] B.J. Zwolinski, H. Eyring, C.E. Reese, J. Phys. Chem. 53 (1949) 1426. [33] R.M. Barrer, Trans. Faraday Soc. 38 (1942) 322. [34] R.M. Barrer, H.T. Chio, in: C.A. Kumina (Ed.), Transport Phenomena in Polymeric Films, Interscience, New York, NY, 1965, p. 111.
Physico-chemical aspects of some biological membranes. II Studies on the conductances of the inner membrane of chicken egg in presence of alkali chlorides and the application of absolute reaction rate theory T. Samanta Department of Chemistry, Syamsundar College, Syamsundar, Burdwan 713 424, W.B., India Received 19 November 2001; accepted 4 March 2002
Abstract Membrane conductance of the inner thin membrane of a chicken egg has been studied with aqueous solution of some common alkali chloride, KCl, NaCl, LiCl and NH4Cl. Conductance values were measured for each concentration at different temperatures after bathing the membranes properly in the respective electrolyte solution. The normal behaviour of these conductances shows that the values increase smoothly with increase in concentrations, tending towards limiting values at higher concentrations. The magnitude follow the order NH4 /K /Na /Li which is the reverse order of the hydrated sizes of these ions. The temperature range studied is between 298 and 323 K at 5 K intervals. The temperature variation of the conductance values has been utilised to calculate the activation energies according to Arrhenius and to Eyring. Assuming the applicability of the absolute reaction rate theory in the membrane conduction process, other thermodynamic activation parameters viz. DG#, DH# and DS# have been calculated. The energy values for any particular electrolyte, however, decrease with increase in concentration of the bathing electrolyte. Very small negative DS# values indicate, some kind of immobilisation of the ions occurred during the conduction process. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Membrane conductances; Egg membrane; Alkali chlorides; Energetics of conduction process
1. Introduction Transport processes through biological membranes, as well as in many synthetic membranes, are important because of their potential use in different separation processes. Studies on the conductances of simple salts through such poly-
E-mail address: [email protected] (T. Samanta).
meric networks are related to the ionic transport through the pores. Extensive research work related to the conductance and diffusion of simple salts through ion-exchange or porous membrane impregnated with inorganic precipitates has been reported by different workers. Studies of the biological membranes are necessary to interpret the mechanism of transport occurring in the biological systems in-vivo [1 /8].
0927-7765/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 6 5 ( 0 2 ) 0 0 0 4 6 - 2
96
T. Samanta / Colloids and Surfaces B: Biointerfaces 27 (2002) 95 /101
Uncharged porous membranes are not always easily available in suitable forms for the studies of different transport phenomena. Even available forms of porous glass, having a well-defined structure and pore size, also acts as a form of ionic membrane [9]. Recently Samanta and Basu have studied the conductance behaviour of some uncharged microporous membrane [10 /12] in presence of aqueous electrolyte solutions. These uncharged microporous membranes were prepared by Mizutani et al. on treatment of their ionexchange membranes in suitable ionic form with H2O2, producing microporous membranes with excellent stability and flexibility [13,14]. However, a similar type of studies involved directly with natural biological membranes [15,16] are not so found because of the difficulties of experimental studies of the electrochemical properties of such membranes for their undefined pore size and complex surface characteristics. The purpose of the present paper is to report the conductance studies of the inner thin layer membrane of an egg with aqueous solution of common alkali chlorides over a range of concentrations and at different temperatures. The energetics and the activation parameters for the permeation of electrolytes through this membrane have been calculated by applying the absolute reaction rate theory.
2. Experimental The inner thin layer membrane of a chicken egg was separated manually from the outer hard calcium carbonate shell with the help of finetipped surgical tongs. The membrane was thoroughly washed with deionised water to remove any adsorbed chemicals. It was always kept in the wet condition to avoid any disturbance arises due to the entrapped air within the pores and also to prevent the crack in dry condition. The membranes were cut into small discs and attached with an adhesive like ‘Quickfix’ between the flanged ends of the two half cells of a U-tube. Membrane conductances were measured by the method of Subramanyan and Lakshminarayanaiah [17], which was also used by many other workers in recent studies. For proper bathing of the mem-
brane pores, removing entrapped air, it was equilibrated with an electrolyte solution of desired concentration by maintaining a hydrostatic pressure difference between the solutions on the two sides of the membrane at a particular temperature of the bath. After proper bathing, the solution was replaced by mercury having the same temperature of the bath, without complete removal of the adhering surface liquid. Membrane thus remained sandwiched between mercury on both sides by eliminating the possible presence of entrapped air at the mercury/membrane interface. Solutions of different concentrations were prepared with analytical grade KCl, NaCl, LiCl and NH4Cl by using triply distilled water. The electrical connections to the conductivity bridge were made through the platinum electrodes inserted in the mercury. Conductance measurements were carried out with a Direct Reading Conductivity Meter 303 (Systronics) at a frequency of 1 kHz.
3. Results and discussion Physical characteristics relevant to this study may be briefly mentioned here as follows. The membrane is basically composed of keratin protein while some authors believe that it contains both keratin and mucin [18]. The inner thin membrane basically consists of collagen, hydroxyproline and hydroxylysine and the rest is composed of some uncharacterised protein and glycoprotein [19]. The average thickness is 0.31 mm. The average length of the pore canal is 0.20 mm and the pores are looking like funnel having initial and final diameters, which are 0.013 mm and 0.006 mm respectively. Scanning electron micrograph of the membrane has been shown elsewhere [16]. The specific conductances (k ) were determined with the solutions of KCl, NaCl, LiCl and NH4Cl in the temperature range 298 /323 K, with intervals of 5 K at an accuracy of 9/0.1 K. The range of concentrations used was 0.1 /10 mol m 3 with suitable intervals. At higher concentrations ( /10 mol m 3) membrane shrinkage occurs due to the exclusion of water within the pores, showing nonsteady conductance values.
T. Samanta / Colloids and Surfaces B: Biointerfaces 27 (2002) 95 /101
The nature of variations of k values with concentrations for different electrolytes at a particular temperature, 298 K are shown in Fig. 1. The common trend found for all the electrolytes is that the membrane conductance increases almost linearly with increase in concentration, tending towards a limiting value at high concentration (/10 mol 3). It is obvious that, at high dilution, the membrane pores are mainly occupied by more water and less electrolyte, showing small conductance values, but it increases with the gradual increase in external bathing electrolyte concentration due to the accumulation of more and more electrolyte within the pores. As regards the conductance, the sequence has been found to be NH4 ]/K /Na /Li in this membrane, in conformity with an earlier observation in a different synthetic membrane [10,11]. The same sequence as observed in aqueous solution at any temperature, indicating that the hydrated sizes retained more or less within the membrane pores. The limiting tendency at higher concentration suggests the possibility of two opposing factors: the exclusion of free water due to the osmotic effects as the external concentration increases, thereby decreasing the ionic mobilities within the pores [1,20] and at the same time, increase in conductance due to the higher salt uptake that
Fig. 1. Plots of specific conductance, k (S m 1) of different electrolytes through the egg membrane against bathing concentration (mol m 3) at 298 K.
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Fig. 2. Plots of specific conductance, k (S m 1) of KCl through the egg membrane against bathing concentration (mol m 3) at different temperatures (298 /323 K).
increases the effective concentration within the membrane pores. After certain concentrations, these two opposing factors may have a tendency to balance each other. Samanta et al. [21] have shown that the idea of large water content of the pores at lower concentrations and the gradual exclusion of water with increase in external electrolyte concentration by measuring the changes of specific weight of the membrane. The limiting tendency of membrane conductances has also been observed by Iijima et al. [22], Beg et al. [23], Jolota et al. [24] and Shahi et al. [25] with different types of charged membrane. They have tried to explain this limiting phenomena in the light of competition between the greater salt uptake, increasing the conductance values and the membrane shrinkage, producing the higher resistance to the ionic path may partially occurs in the present case. The effect of temperature on the membrane conductance is shown in Fig. 2 represents the specific conductance (k ) values for KCl solutions of different concentrations in the temperature range 298/323 K at 5 K intervals. The general trend at any temperature is the same, showing a smooth increase in concentration, tending towards a limiting value. The temperature effect of all the electrolytes studied here has also main-
T. Samanta / Colloids and Surfaces B: Biointerfaces 27 (2002) 95 /101
98
Fig. 3. Arrhenius plots of log values k at one of the concentrations (1 mol m 3) against 1/T for the calculation of activation energies of alkali metal ions from the slope, /E / 2.303R.
tained the same sequence of conductance variations NH4 /K /Na /Li indicating that the hydrated sizes remains virtually undisturbed in the studied temperature range. The smooth
increase of membrane conductance with any electrolyte from one temperature to another at any concentration may be attributed mainly to an increase in mobility of the ions compared with the swelling and de-swelling effect shown by the charged membranes [1,26]. By applying the Arrhenius equation in its basic form [27], log k values of an electrolyte at each concentration are plotted against 1/T to calculate the activation energies from the slopes. Fig. 3 shows the plots of the different electrolytes at the concentration of 1 mol m 3. The smooth linear plots suggest that there may be no abrupt irreversible change in the membrane structure within the concentration and temperature range studied. The activation energies of different electrolytes are calculated from the slopes of the Arrhenius plots shown in the column 3 of the Table 1. The general common feature is that the activation energies at low concentration are very high compared with the values at higher concentrations. The very high values at extremely low concentration may be due
Table 1 Calculated values of the activation parameters: Arrhenius (Ea) and Eyring (EE) activation energies, enthalpy of activation, DH#, free energy of activation, DG# and entropy of activation, DS#, for the conduction of KCl, NaCl, LiCl and NH4Cl solutions through the egg membrane at 2989/0.1 K. Salt KCl
Concentration (mol m 3)
0.1 1 2 5 10 NaCl 0.1 1 2 5 10 LiCl 0.1 1 2 5 10 NH4Cl 0.1 1 2 5 10
Ea (KJ mol 1) Ee (KJ mol 1) DH# (KJ mol 1) DG# (KJ mol 1) DS# (KJ mol 1 K 1) 45.05 22.34 17.64 13.44 11.44 38.29 18.76 18.46 14.48 12.16 37.13 15.64 19.54 18.93 15.61 39.46 18.42 17.57 13.61 12.76
43.96 22.72 17.24 13.53 11.47 38.86 20.39 18.55 14.59 13.10 36.74 15.26 19.46 19.33 15.19 39.30 17.68 16.96 13.02 11.88
41.88 20.24 14.76 11.05 8.99 36.38 17.91 16.07 12.11 10.62 34.26 12.78 16.98 16.85 12.71 36.82 15.20 14.48 10.54 9.40
72.10 74.45 74.15 74.55 74.64 72.77 74.53 74.48 75.10 / 72.88 74.67 74.85 75.56 75.20 71.36 73.67 73.63 74.25 74.33
/0.10 /0.18 /0.20 /0.21 /0.22 /0.12 /0.19 /0.20 /0.21 / /0.13 /0.21 /0.19 /0.20 /0.21 /0.12 /0.20 /0.20 /0.21 /0.22
T. Samanta / Colloids and Surfaces B: Biointerfaces 27 (2002) 95 /101
to the immobilisation of ions because of some obstructions to ionic paths within the pores of the membrane matrix. The binding may either be due to the small fixed charges present within the membrane or to the adsorption on the pore walls of the membrane when it initially comes into contact with the electrolyte. Samanta et al. have shown elsewhere [16] that very low negative potential values for such membrane in presence of alkali chlorides solution is due to the gradual accumulation of Cl ions produces a negative charge density. It is expected that with increasing concentrations, the conductance increases because of greater accumulation of ions, which may overcome the opposing factors, so that the smaller activation energies are required. Several workers have also reported the gradual decrease in activation energy of conduction with increasing the concentrations in the presence of different types of membranes [10,11,21 /23] with some exceptional values [22]. The most noteworthy feature revealed from Table 1 is that the activation energy for conduction of alkali chlorides through this membrane follows the order EK :/ENH4 / ENa /ELi at low concentration range and changes the order ELi /ENa /EK :/ENH4 at high concentration range. The concept of ‘activation theories’ regarding ionic movements is that the ion occupies an average equilibrium configuration within a cage of solvent molecules and it is continuously moving from one equilibrium position to another [27,28]. Thus, smaller activation energy should be required for the smaller ion. As the conductance values of the ions is controlled by the mobility of the ions which depends upon the hydrated size of the ions, required greater activation energy for the higher hydrated size and thus the energy values should follow the reverse order of conductance. Such results are quite consistent with the aqueous solutions [29] and also with these alkali chloride solutions in the presence of a synthetic microporous membrane studied earlier [11]. Whereas in some cases the mobility is controlled by the nonhydrated crystallographic radius [1,26] and obviously the activation energy should follow the decreasing order from the ions having largest crystallographic radii to those with the smallest,
99
for very highly cross-linked resins, in zeolites or in any network where there is no room for hydration shell. Iijima et al. [22] have found this pattern in their studies with nylon membranes. Beg et al. [23], however, have reported some interesting results with their parchment supported cupric orthophosphate membrane, in which the conductance of alkali chlorides follows a decreasing sequence with the increase in the hydrated size. Activation energy values also follow the same sequence, indicating the higher the conductance the greater is the activation energy. In the present study with the egg-shell membrane in the presence of some common alkali chlorides the activation energy follows the same order of conductance in the low concentration range, indicating that some kinds of secondary phenomena take place which require extra energy along with the normal activation energy for jumping. At very low concentration the viscosity Bcoefficient represents the ion-solvent interaction and measures the order or disorder introduced by the ions into the solvent structure [30]. The potassium and ammonium ion have a small negative viscosity B-coefficient at 25 8C and a positive temperature coefficient, suggesting that with rise in temperature it gradually becomes a water structure promoter [31] and increases the viscosity of the aqueous medium. This structure making property will be enhanced in the presence of the structure breaking Cl ions. It is expected that the increase in viscosity will be higher within the narrow pores of the membrane. Whereas in the case of sodium and lithium ion have a positive viscosity B-coefficient at 25 8C and a negative temperature coefficient, giving rise to water structure breaker with temperature and decreases the viscosity of the medium within the membrane pores. Therefore, it now plausible to estimate that some additional energy is consumed for the ‘depolymerisation’ of the enhanced water structure made by the K and NH4 ions than that of the Na and Li ions resulting the activation energy sequence EK /ENH4 /ENa /ELi . But at higher concentrations the number of ions and its mobility dominating the membrane conductance values and hence the activation energy. As the membrane conductance of egg shell membrane
T. Samanta / Colloids and Surfaces B: Biointerfaces 27 (2002) 95 /101
100
follows the sequence NH4 /K /Na /Li , so logically the least conducting ion will receive the greater activation energy for conduction. It is also known from the ion conductance values [30] that, the increment of ion conductances per degree rise in temperature follow the sequence K (1.58) / NH4 (1.47)/Na (1.29) /Li (0.99) corroborate the necessity of higher activation energy for conduction of Li ion than of K ion. Thus the activation energies required for different electrolyte follow the order ELi /ENa /EK / ENH4 showing the reverse order of conductance but a parallel order to their hydrated sizes that control the mobility and hence conductance. In order to calculate the necessary thermodynamic parameters of activation it is relevant to apply the absolute reaction rate theory of Eyring et al. [27,32]. Several authors [21 /23] have already applied this theory for the membrane conductance in the following form: L
kT # # exp DH =RT expDS =R h
(1)
where L is the equivalent conductance within the membrane, h and k are the Planck constant and Boltzmann constant, respectively, for the various ionic processes taking place within the membrane during conduction. Eq. (1) can be changed to its logarithmic form for the evaluation of DH# and DS# as: log
Lh kT
DH # 2:303RT
DS # 2:303R
(2)
Eq. (2) suggests that a plot of log(Lh /kT ) against 1/T from experimental data should yield a straight line with slope /DH#/2.303R , which enables the calculation of DH#. The smooth linear plots shown in Fig. 4 justify the applicability of the Eyring equation. DS# may also be calculated from Eq. (2) for any particular temperature. The other important thermodynamic activation parameters for the conduction process through this membrane, may be calculated with the help of usual thermodynamic relations, Ee DH # RT and
(3)
Fig. 4. Eyring plots of log(Lh /kT ) values at one of the concentrations (1 mol m 3) of the alkali metal ions against 1/ T to calculate the magnitude of DH# from the slope, equal to / DH#/2.303R.
DG # DH # TDS #
(4)
The calculated values of all these activation parameters at 298 K are tabulated in Table 1. The values of Arrhenius (Ea) and Eyring (Ee) activation energies are recorded in columns 3 and 4 of Table 1. It is expected that the Eyring activation energy values are slightly different from those obtained directly from the Arrhenius plots. Other characteristic features, i.e. the general trend of gradual decrease with further increase in concentration and the sudden drop in activation energy at very low concentration. The calculated values of DH# and DG# are also recorded in columns 5 and 6 of Table 1 shows the same increasing trend from K to Li . According to Eyring et al. [27] the values of entropy of activation, DS#, may be positive, negative or close to zero, which indicates the mechanism of flow through the membrane. A large negative DS# values indicate that some kind of bond formation occurs within the membrane pores. Whereas a large positive DS# values indicate the breakage of bonds and low values indicate that ionic movement takes place without making any disturbance. Large or small DS# values may also be interpreted by the Barrer’s concept of ‘zone of activation’ [33,34], which states that a large DS# corresponds to a large zone of activation, or reversible loosening of
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membrane segments, whereas small values explained in the light of small ‘zone of activation’ or no loosening of membrane structure. The very small negative DS# values are recorded in the last column of Table 1, indicating that at least some kind of immobilisation takes place during the ionic movement for conduction. Despite the small magnitude, there is an indication that some disorder introduced either by partial immobilisation or by disruption of solvent structure, for which some extra energy is needed and it is reflected in the activation parameters sequence.
Acknowledgements The author expresses his sincere thanks to the Head of the Department of Chemistry, the University of Burdwan for laboratory facilities and to the Principal, Syamsundar College for constant inspiration and other assistance. Special thanks are due to Mrs T. Samanta for correcting the English language.
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