G. BARONE, V. CRESCENZI, AND V. VITAQLIANO
2588
Potassium Chloride Conductance in Aqueous Solution of a Structure-Forming Nonionic Solute, Hexamethylenetetramine1 by G. Barone, V. Crescenzi, and V. Vitagliano Laboratorio d i Chimica Fisica, Istituto Chimico, Universith di h’apoli, Naples, Italy
(Received January 83,1968)
Conductance data of KCl in aqueous solutions of hexamethylenetetramine and data on the “structural temperature” of hexamethylenetetramine aqueous solutions, evaluated by a spectroscopic technique, are reported. It is proposed that the observed reduction of the KC1 limiting equivalent conductivity in the hexamine sohtions be accounted for in terms both of a different structure of the water in these solutions with respect to pure water and of an obstruction effect of the hexamine molecules.
I n previous papers from this laboratory, it was shown that equilibrium and transport properties of hexamethylenetetramine (HMT) aqueous solutions may be explained in terms of marked solute-solvent interAs a result of these studies, it was concluded that H M T is a distinct structure-forming solute in water. Furthermore, it was thought worthwhile to investigate how the limiting equivalent conductivity of a simple electrolyte is altered in HMT solutions with respect to the limiting equivalent conductance in pure water. Such a study might, in fact, be of interest in connection with studies on the mechanism whereby nonpolar solutes which are heavily solvated and which also markedly increase the viscosity of water (such as HMT) modify ionic motion. Studies of electrolytic conductance in aqueous solutions of a number of nonionic compounds have shown that limiting equivalent conductivities, A,, of common electrolytes are higher than predictable from the macroscopic viscosity of the solutions.6 An approach in which the ions are considered as electrical carriers of negligible dimensions moving through a medium containing obstructions-the nonelectrolyte molecules (depicted as insulating spheres)-leads conversely to estimated values which are too high, as compared to experimental data.* Comparison of HC1 conductance data in aqueous sucrose solutions at 25’ with those in water for different temperatures also suggested that correlation of conductance data in terms of a “structural temperature” of water would not be feasible.’ Our data on the limiting equivalent conductivity of KC1 in H M T aqueous solutions reveal a pattern similar to that found with other nonelectrolyte solutions, in the sense that neither of the above-mentioned approaches appears to correlate satisfactorily with the observed reduction of KC1 conductance. It is suggested, however, that in the case of H M T solutions, the reductions in the KC1 limiting conductivity may be related to the structural temperature of the solutions, as evaluated by the spectroscopic procedure of Worley The Journal of Physical Chemistry
and Klotz,8 once the obstruction effect of the hexamine molecules is also taken into account.
Experimental Section Hexamethylenetetramine, a Merck product, was recrystallized from ethanol and vacuum dried. KC1 was a C. Erba (Milan, Italy) reagent grade product. D20 was a Fluka (Buchs, Switzerland) product, 99.7% DzO. H M T solutions were always prepared immediately before use in order to minimize hydrolysis of the hexamine. The conductance of HMT solutions was close to that of the redistilled water used to prepare them. It was found, however, that if cells with platinized electrodes were employed, the conductance of the HMT solutions slowly increased with time. This effect was not observed with cells having bright platinum electrodes, which, therefore, had to be employed in all measurements with KCI. For these measurements, a concentrated KC1 aqueous solution (0.5 N ) was added in small amounts, with a microsyringe, directly into the conductance cell containing ca. 60 ml of H M T solution. Resistance readings were taken after each KC1 addition, after thorough mixing of the solution and temperature equilibration. Dilution of the H M T solutions with increasing KCl concentration (from equiv/l.) was neglected. about 5 X lom4to (1) This research has been carried out with the financial support of the Italian Considio Nazionale delle Ricerche (Contract No. 116.1385.0495). (2) V. Crescenzi, F. Quadrifoglio, and V. Vitagliano, J . Phys. Chem., 71, 2313 (1967). (3) L. Costantino, V. Crescenzi, and V. Vitagliano, ibid., 72, 149 (1968). (4) V. Crescenzi, F. Quadrifoglio, and V. Vitagliano, Ric. Sci., 37, 529 (1967). (5) R.H.Stokes and R. Mills, “Viscosity of Electrolytes and Related Substances,” Pergamon Press Ltd., Oxford, 1965. (6) B. J. Steel, S. M. Stokes, and R. H. Stokes, J . Phys. Chem., 62, 1514 (1958). (7) R. A. Robinson and R. H. Stokes, “Electrolyte Solutions,” Butterworth and Co. Ltd., London, 1959, p 307. (8) J. D.Worley and I. M. Klotz, J . Chem. Phys., 45, 2868 (1966).
KCl CONDUCTANCE IN AQUEOUS SOLUTIONS OF HEXAMETHYLENETETRAMINE
2589
Table I : Experimental Values of KCl Equivalent Conductivity in Aqueous Hexamethylenetetramine Solutions (t = 24.93')"
4.401 5.822 7.154 7.916 8.602 9.397
127.12 126.81 124,58 123.86 123.16 122.70
1,847 2.768 3.327 4.018 4,696 5.347 6.234 7.434 8.551 9.462 10.31 11.49
131.08 i. 0.079
107.38 106.39 106.05 105.39 104.90 104.65 104.55 104.01 102.74 102.29 101 .95 101.36 108.08 zk 0.24
1.776 2.812 3.443 3.972 4.439 4.857 5,314 5.870
88-08 87.11 86.97 86.78 86.36 86.35 86.16 85.83
3.781 3.941 5.270 5,276 5.848 6.433
88.77 rt 0.13
68.81 69.02 68.85 68.32 68.65 68.02
70.01 zk 0.22
* hois the limiting equivalent conductivity obtained by least squares from the experia Corrected for the conductivity of the solvent. mental A values. The hO has been divided by 1.022 to obtain the A'HMT given in Table 111. Conductances were measured a t two temperatures (24.93 and 33.46') with a Leeds and Northrup Jones bridge using two Leeds and Northrup cells with constants 10 and 1 cm-I. All runs were taken in a Townson and Mercer thermostatic bath (*0.01"). The limiting equivalent conductivity of KC1 in aqueous solution was measured using both black platinum electrodes and bright platinum electrodes. The A'H~O measured with black platinum electrodes was in agreement with the literature data within 2 ppt (see ref 7, Appendices 6.1 and 6.2, pp 463-465). The data obtained with bright platinum electrodes were h o b r pt
= 1.022A0~,0 (24.93')
A0br pt
= 1.007A0~,o (33.46")
where AOH~Ois the literature value. The KCl limiting conductivity values in HRIT-water mixtures have been obtained by a least-squares extrapolation of experimental data (corrected for the conductivity of the solvent, plotted as a function of (See Table I and 11.) The limiting conductivities have been divided by the correction factor 1.022 a t 24.93' and 1.007 at 33.46'; the corrected data are given in Table 111. Near-infrared spectra of 5% HzO in DzO against pure DzO have been taken as a function of temperature, by using the technique of Worley and Klot,.z.s For the measurements with HRIT, the same amounts of HhIT were put in the dilute H20 solution and in the DzO reference solution, and the spectra were recorded from 1350 to 1700 mp. A Beckman DK-2 spectrophotometer, equipped with a circulating-water thermostat, was used. Structural temperatures of water in the presence of HMT were evaluated according to Worley and Klotz, who proposed that equal structural temperatures for different solutions correspond to equal
dc.
Table I1 : Experimental Values of KC1 Equivalent Conductivity in Aqueous Hexamethylenetetramine Solutions (t = 33.46')"
2.326 2.978 3.021 3.712 3.707 4.205 4.351 4.641 4.908 5,464 5.676 6.002 6,688 AO*
150.96 150.34 150.26 149.93 149.78 149.38 149.08 148.93 148.73 147.97 147.76 147.43 147.15 153.16 i. 0.12
1.833 2.902 3.670 4.494 5,475 6.040 6.684
128.35 127.33 126.50 125.99 125.46 124.99 124.44
129.59 i 0.13
a Corrected for the conductivity of the solvent. b The A O values have been divided by 1.007 to obtain the AOHMT given in Table 111.
values of the ratio of the optical density of HDO solutions at 1556 mp to that at 1416 mp.
Results and Discussion Application of Worley and Klotdss analysis to our near-infrared data for H M T aqueous solutions leads to the water structural-temperature values which are reported in Figure 1 as a function of H M T concentrstion. These data strongly suggest that H M T acts as a structure-maker solute in water, in agreement with earlier indications. Our conductivity data also indicate that the limiting equivalent conductance of KC1 in HMT, A'HMT, is markedly reduced with respect to Volume 72,Number 7 July 1968
G. BARONE, V. CRESCENZI, AND V. VITAGLIANO
2590 Table I11
-
Molarity of HMT, M 1.0 Molality of HMT, m 1.13
0.5 0.53
A'HMT~
RKCI* IO~IHMT~ t*,d*Q O C
( AOH,O)o' t,' "C ti*,Q.h
OC
1.5
2.0
1.80
2.67
24.93
33.48
24.93
33.46
24.93
24.93
128.3 0.857 0.819 17.5 132.9 19.2 19.4
152.1 0,866 0.823 25.7 157.6 27.6 27.4
105.7 0,706 0.658 9.3 114.2 12.4 13.5
128.7 0.733 0.667 17.7 139.1 21.3 20.6
86.9 0.594 0.517 2.9 101.2 7.6 7.1
68.5 0.457 0.397 --5.3 83.1 0.6 0.1
-
Limiting equivalent conductance of KC1 in HMT aqueous solutions. Limiting equivalent conductivity ratio: R K C l = AOHMT/ c Relative fluidity of HMT aqueous solutions, taken from ref 2 and 3. d Temperature of water for which ( A ~ H , O ) ~=* AOHMT. (See text.) e Values derived from eq 2, with a molar volume of 130 cma/mol for HMT. Temperature of water a t which A0aao = ( ~ t o H , o ) for ~ KC1 (eq 2, see text). Q The values of t* and t were calculated from eq i and ii in the text. h Structural temperature of HMT aqueous solutions (see Figure 1) obtained from spectroscopic data. 0
AOH~O.
'
I
I
I
I
1
(or R K C= ~ 1). From the known dependence of A'H~O for KC1 on temperature, values of t* were derived for each HMT concentration considered. [Limiting equivalent conductivity of KC1 aqueous solutions and related temperatures (taken from ref 7, Appendix 6.2, p 454) have been fitted by a least-squares method to give the equations (i) A'H,o = 81.66 2.4801 0.01087t2 (3.153 X 10-6)t3 (standard deviation, k0.12) and 0.5078A0~,~ - (7.861 X (ii) t = -36.74 (A'H,o)~ (9.728 X lo-') (AOH,O)~(standard deviation, *00.05)for 0" 2 t 2 55'1. These values (reported in Table 111) are systematically lower than those evaluated from spectroscopic data (Figure 1). Neglect of the effect of obstruction that the relatively large, nearly spherical HMT molecules may cause t o the motion of ions is a possible cause of this discrepancy. Using the equation for the obstruction effect in the form6
+
+
1
2
3
that in pure water, A'H~O (see Table 111). However 0 ~all, cases o the value of the ratio R K C=~ A 0 ~ ~ ~ ~ / isA in higher than the relative fluidity of the solutions. I n our opinion a possible explanation of these data may be advanced as follows. Since HRfT increases the degree of local order in water, ions will move in HMT aqueous solutions a t a temperature t in a medium having a structure corresponding t o that of pure water a t a lower temperature t*. The value of t* should thus correspond to the structural temperature of HMT aqueous solutions. If this assumption were taken literally and if no other complicating effects were present, the temperature t* would be that for which 1*
= (A'HMT) t
The Joumal of Phgsical Chemistry
+
A'HMT _ _ _--1 - 1.54 (AOH,O)O 1- @
Figure 1. Structural temperature of H M T aqueous solutions, as measured from near-ir spectra* as a function of the H M T molality: 0, te = 25'; 0 , t e = 33.46'; 8 , t, = 50".
(A'H,o)
+
(1)
(2)
where @ is, in our case, the volume fraction of HMT, it was found that if a molar volume of 130 cm3/mol is taken for HMT, the (AOH,O)~values happen to be, in each case, just those corresponding t o the spectroscopic structural temperatures of the various HMT solutionsexamined (see t and ti, values in Table 111). It is interesting to point out that a volume of 130 cm3/mol is a very reasonable value for HMT, compared with the solid-state value of 104 cm3/mo19and the hydrodynamic value of 160-180 cma/mol obtained from diffusion and viscosity data.3 Admittedly, the final agreement, which relies mainly on the soundness of the structuraltemperature values as derived by a spectroscopic technique, may be t o some extent fortuitous. (9) G . W. Smith, J. Chem. Phys., 36, 3081 (1962).
AN ASYMMETRY-POTEXTIAL EFFECT The results reported here seem to favor the applicability of the concept of structural temperature as a means of explaining the behavior of aqueous solutions. This concept appears to be particularly applicable to
2591 the problem of the conductance of simple electrolytes in the presence of an excess of a nonionic third component. Nore extended studies along these lines might prove interesting.
An Asymmetry-Potential Effect across Gradient Permselective Membranes
by F. de Korosy The Negev Institute for Arid Zone Research, Beersheva, Israel
(Received January 28, 1968)
The preparation of gradient permselective membranes has been described. They are prepared by one-sided amination of PVC1 (anionic) or by one-sided sulfochlorination of polyethylene (cationic), with subsequent quaternization or hydrolysis, respectively. Gradient membranes of about 100 kilohm cm2 generate an emf and produce electrical energy between salt solutions of identical composition. This effect is due to the diffusion of solution to the edge of the membrane at the circumference of the measuring cell and to the concentration differences arising on this edge. The work of Liquori and Botr6 on nonuniform selective membranes2 explained how an electric potential difference can be temporarily established across such membranes when they are placed between two electrolyte solutions of the same composition. Their effect is due to different degrees of permselectivity of two sides of the membrane and to ions migrating into the internal, aqueous layers of the membrane from both of its sides. Some time ago we also worked with permselective membranes in which there was a built-in concentration gradient of their active sites from one face toward the other. We also observed in some cases that a potential difference of up to 80 mV arises across these membranes when they are fitted into a cell between two solutions of, say, KC1, of the same concentration on their two sides. It was even possible to extract currents of up to some lo-' A for several weeks from membranes about 5 cm2 in surface area. Eventually it became possible to explain this mysterious source of energy. Our explanation of the phenomenon was different from that of Liquori and Bot& and seems sufficiently interesting to warrant a short description.
Preparation of Gradient Permselective Membranes Two entirely different methods were used, one for anion-selective and the other for cation-selective membranes. I n the first instance, a PVC' sheet without plastifier, about 0.1 mm thick, was pressed against a rubber sheet adjoining a metal plate. There was a hole across both the metal and the rubber, so that a small container was
formed with the PVC sheet as its base. Tetraethylenepentamine was poured into this containera and the assembly was heated to 80" for various periods of time, The PVC sheet was aminated beginning from the side of its contact with the pentamine in a gradient toward the other side. This was evident from its color. It has originally been an opaque white, and upon amination it turned, successively, yellow, orange, and reddish brown. The side toward the amine became colored long before the other side and always remained darker than the other side. The amination gradient could also be established by nitrogen determination of razor scrapings from different layers, e.g.: surface layer of the light-colored side, 1.8% N; first layer below the surface, 2.5% N ; second layer below the surface, 3.3% N; third layer below the surface, 3.7% N; surface layer of the deep-colored side, 9.4% N. After amination the gradient membranes were left for 1 day in a 25% alcoholic solution of methyl bromide to alkylate their amino groups, whereby a certain number of quaternary amino groups were also formed, These impart strong anion-exchange properties to the membranes. In the second instance, polyethylene sheets 0.020.1 mm in thickness were photochemically sulfochlorinated from one of their sides in a streaming gas mixture of sulfur dioxide and chlorine ( 2 :1) in the light of one (1) (2) (b) (3)
Polyvinyl chloride. (a) A. hl. Liquori and C. BotrB, J . Phus. Chem., 71, 3765 (1967); A. M. Liquori and C. BotrB, Ric. Sci., 6, 71 (1964). F. de Korosy and Y. Shorr, Dechema Monograph., 47, 477 (1962).
Volume 72?Number 7 July 1968
