ASYMMETRYPOTENTIAL EFFECTACROSS NONUNIFORM IONSELECTIVE MEMBRANES
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The Asymmetry Potential Effect across Nonuniform Ion Selective Membranes
by A. M. Liquori Centro Nazionale di Chimica della Macromalecole, Sezione I l l , Laboratorio d i Chimica Fisica, Istituto Chimico, Uniuersitb di Roma, Roma, Italy
and C. Botr6 Istituto di Chimica Farmaceutica, Universitb d i Roma, Roma, Italy Accepted and Transmitted by The Faraday Society
(March 13, 1967)
The unusual electrochemical behavior of previously described asymmetric membranes is explained using a model system as reference. It is concluded that these membranes act as microcells made of elements with different permselectivity coefficients separated by highly swollen gels of polystyrenesulfonate. Asymmetric membranes made of two films of collodion polystyrenesulfonic acid with different charge densities sandwiching a film of polystyrenesulfonic acid show, after sodium exchange, an electrochemical behavior very close to that of the previously described asymmetric membranes. The asymmetry potential between two identical electrolyte solutions separated by such a membrane is theoretically explained according to the thermodynamics of irreversible processes.
Asymmetric Membranes I n a previous paper' the electrochemical behavior of collodion polystyrenesulfonic acid membranes (CPS) characterized by different concentrations of polystyrenesulfonate in the two external layers, si and so, was described. A peculiar effect was observed when such membranes were put in contact with two solutions, i and o, of a uni-univalent electrolyte having identical composition. In fact, an electrical potential difference could be measured between the two identical solutions separated by the asymmetric membrane. Such an "asymmetry potential'' was found to increase with increasing the difference of fixed charge densities between the two external layers si and so. When the asymmetric membrane was put in contact with two solutions of a uni-univalent electrolyte at different mean activities, the observed membrane potential was found to depend on the orientation of the membrane with respect to the two solutions. Such a behavior is shown in Figure 1. Although in the previous paper' an attempt was made to interpret this peculiar electrochemical behavior on the basis of an extention of Meyer-Teorell theory, it will be shown here that a simple straightforward explanation can be given if the following model system is considered. Let si and so be
two ion selective membranes having different charge densities in contact with two uni-univalent electrolyte solutions (e.g., NaC1) i and o separated by a middle compartment containing a solution of the same uni-univalent electrolyte, solution "m."
When there is no current, flow across t,he system, an electrical potential difference is established between the solutions i and o given by
AI) = I)'
- I)" =
(I)' - I)") - (I)" - J.")
where
and
where aaiand
as" are
(1) A. M. Liquori and
the selectivity coefficients of the
C. BotrB, Ric. Sei., 6 , 71 (1964).
Volume 71,Number 18 November 1967
3766
A. M. LIQUORI AND C. B O T R ~
s
t 130-
Therefore, the asymmetry potential corresponds to the electrochemical potential difference of the jth ion between the two solutions, which a t the steady state is generated by the different ionic activities of the salt in the compartments i, o if the system is asymmetric (aBi# a"). When the two electrolyte solutions i and o have different mean activities, eq 2, 3, and 7 may be combined to give the membrane potentials
8OC c
6 0 ~ I I
A#' =
4 OL
RT
7''a
In
aii
AP* +F
a*
c 20L
I I
0'
:
corresponding to the two opposite orientations of the 3
-log a
0
Figure 1. Asymmetry potential A$* across an asymmetric membrane of the previously described type' as a function of the log a* of the two identical electrolyte (NaCl) solutions in contact with the membrane. The membrane potentials A$' and A$" observed for two opposite orientations of the asymmetric membrane with respect to the two bathing electrolyte solutions of NaCl. The mean activity a t in one electrolyte solution is fixed a t 1.7 x 10-2 whereas the mean activity of the other electrolyte solution is varied.
I
80-
-- -0 ---o--- I
two membranes si and so having different charge densities. If the two electrolyte solutions i and o have identical composition, a,' = a+' = a,, the electrical potential difference (eq 1) becomes
eoL
\
\ \
1
, P
, An electrical potential difference between the two identical electrolyte solutions i and o must, therefore, be observed which is due to the asymmetry of the system which imposes the condition that ani # do. It is in fact interesting to observe that if pio =
+
pio
pjl
+
ZjFGj
(5)
are the electrochemical potentials of an ion in the two solutions i and o when these have identical composition i
Pj =
The Journal of P h y h l Chemktry
I L
b \
\
\
\
.~jF$j
and pjl =
t
0
(6)
Figure 2. Asymmetry potential A$* between two NaCl solutions i and o in contact with two membranes si and so separated by a middle compartment containing NaCl solution having a mean activity of 3.37 x 10-1. Membrane potentials A$' and A$" between two solutions i and o separated by the same system, with opposite orientations. The mean activity of one solution is kept a t 1.7 X 10-2 whereas that in the other solution is varied.
i
ASYMMETRY POTESTIAL EFFECT ACROSS NONUNIFORM IONSELECTIVE MEMBRANES
system, namely, with the membrane si in contact with the solution i or 0, respectively. The behavior of A+*, A+’, and A+” is illustrated in Figure 2 for the mequiv of system aim = 3.37 X 10-l) d,i = s X S04/g, d,, = 5 X lo-’ mequiv of SOl/g, aSi = 0.04, cyso = 1.0. The very close analogy between the behavior of asymmetric membranes and the asymmetric model system considered above is clearly apparent from a comparison of Figure 2 and Figure 1. Such an analogy readily suggests that electrolyte is present in the regions of contact between the elements of an asymmetric membrane having different charge densities. This hypothesis is confirmed by a microscopic examination of the asymmetric membranes prepared as described in the previous paper. In fact, in the membrane elements with higher charge density the presence of very small granules of polystyrenesulfonic acid may be observed which is in excess with respect to that homogeneously dispersed in the collodion matrix. This is very likely the result of the very high concentration of the polyelectrolyte in the collodion solutions from which the high charge density layer was cast. It may, therefore, be concluded that when such granules of the sodium exchanged polyelectrolyte, highly swollen by water, happen to be in contact between two membranes with different charge densities, a system of asymmetric microcells is formed which is analogous to the model system described above. A confirmation of the above conclusion stems from the behavior of asymmetric membranes made by sandwiching a thin layer of polystyrenesulfonic acid between two layers, si and so, of CPS having the same charge densities as the external layers of the previously described asymmetric membranes. After exchange with a 0.5 N solution of NaCl, the asymmetric potential A +* and the membrane potentials A+’ and A+” were measured. The results are shown in Figure 3. The similarity between the behavior of this type of asymmetric membranes and the previously described ones is striking. Applying eq 8 and 9, the electrical potentials differences A+‘ and ,+If should be obtained from the asymmetry potential A+* and the activity ratios u+~/u+’ inserting the best values of aBiand as’. When this is done for the curves of Figures 1, 2, and 3, values of aal and a80 are obtained as given in Table I. The permselectivity coefficients of the previously described asymmetric membranes and the new type appear to compare rather satisfactorily. Experimental Section
Uniform Membranes. Uniform membranes of polystyrenesulfonic acid embedded in a collodion matrix
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1201
I 3 2 0
O
4
-log a
1
Figure 3. Asymmetry potential A$* and membrane potentials A$’ and A$’’ a N a + exchanged asymmetric membranes made of two layers si and so of CPS sandwiching a film of polystyrenesulfonic acid. The other conditions are the same as in Figure 1, but the ked mean activity is for the membrane potentials A$’ and A$”. 1X
Table I Model system Previous asymmetric membrane New asymmetric membrane
ago
di
1.00 0.80
0.04 0.25
0.81
0.17
were made according to Gregor and Sollner.* The polystyrenesulfonic acid obtained according to Seihof3 had an average molecular weight of about 9 X lo4. After purification, its acid value was 4.80 mequiv/ g, as compared with a theoretical value of 5.43 mequiv/g calculated assuming one sulfonic acid group per benzene ring. Membranes were cast from solutions of 4% collodion in a 1:2 mixture of alcohol and ether mg/ml of polystyrenecontaining 4 mg/ml or 4 X sulfonic acid. The charge densities of the membranes thus obtained were 5 X lo-’ mequiv of SO,/g and 5 X 10-4/mequiv of SOa/g, respectively. Total ex(2) H. P. Gregor and K. Sollner, J. Phgs. Chem., 58, 409 (1954). ( 3 ) R. Neihof, ibid., 58, 916 (1954).
Volume 71, Sumtter fB
AYovember1967
T. W. KEWTONAND N. A. DAUGHERTY
3768
change of the hydrogen ions was achieved by equilibrating the membranes with a 0.5 N NaCl solution. Asymmetric Membranes. The previously described asymmetric membranes were prepared according to the previous paper. The new asymmetric membranes were obtained by casting a thin membrane of CPS with a charge densitmyof 5 X 10-4 mequiv of SOd/g and then evaporating over it a water solution of polystyrenesulfonic acid having a concentration of 4 mg/ml. Finally a thin membrane of CPS with a charge density of 5 X lo-' mequiv of S04/g was cast over the film of polystyrenesulfonic acid. Total exchange of the hydrogen ions with sodium was achieved as for the uniform membranes.
Apparatus. (a) Electrical Potentials in the Model System. A three-compartment cell made of Pyrex glass was used for this experiment. The membranes si and so were clamped between two flanges. Saturated calomel electrodes were immersed through agar bridges in the solutions i and 0 , and the potential differences were recorded with a Pye precision potentiometer equipped wit'h a Multiflex Lange galvanomet,er with an internal resistance of 1200 ohms and a sensitivity of 4 X amp/mm. (b) Electrical Potential across Asymmetric Illembranes. The potential differences across asymmetric membranes were recorded with calomel electrodes in a standard cell similar to that previously described.'
The Kinetics of the Reaction between Neptunium(II1) and Iron(II1) in Aqueous Perchlorate Solutions'
by T. W. Newton and N.A. Daugherty Uniaersity of Calijornia, Lo8 Alamos Scientific Laboratory, Los Alamos, AVew Mexico
+
(Receiwd March 13, 1967)
+ +
Rates of the reaction Kp3+ Fe3+ = Np4+ Fez+ in aqueous perchlorate solutions are given by d[Np(IV)]/dt = [Np3+][FeS+](ko k-l[H+]-l) for acid concentrations ranging from 0.05 to 2.0 M a t constant ionic strength. The effect of temperature was determined from 0.5 to 36.7"; k-I is from about 4 to 30 times larger than ICo in this range. The temperature coefficient of k-1 was used to determine the activation parameters for the preFe3+ H 2 0 = [(Np)(OH)(Fe5+)]* H+, dominant net activation process Np3+ AF* = 13.66 kcal/mole (IC-1 = 600 sec-' at 25")) AH* = 14.55 f 0.1 kcal/mole, and AS* = 3.0 f 0.3 cal/mole deg. The results for this reaction are compared with those for the similar Pu(1V)-Fe(I1) and Np(1V)-Cr(I1) reactions.
+
Introduction A number of apparently simple oxidation-reduction reactions have been studied in which the net change is in the charge on the ions involved. Reactions of this type among aqua ions are usually inhibited by H+, showing that various hydrolyzed species are reactive intermediates. The plTp(III)LFe(III) reaction belongs in this class and its kinetics were studied for comparison The Journal of Physical Chemistry
+
+
with analogous reactions in order to obtain a better understanding of the factors which influence these rates. Recent success in the application of parts of Marcus' theory2to inner-sphere reactions3 suggests the examina(1) Work done under the auspices of the U. s. Atomic Energy Cornmission; presented in part at the 153rd National Meeting of the American Chemical Society, Miami, F I ~ .April , 1967.
