5230
J. Phys. Chem. 1982,86,5230-5234
electron-donating substituents dissolved in dilute acid. This new charge transfer transition is attributed to loss of a proton in the excited-state species producing a fluorescent monocation (in the case of 5-amho derivatives) or a zwitterion (in the case of 5-hydroxy derivatives). These characterizations are supported by pK, studies,
NMR results, and low-temperature comparisons.
Acknowledgment. We thank Jerry A. Hirsch for helpful comments on the data, and Alan W. Douglas, Merck, Inc., Rahway, NJ, for providing the NMR data. P.C.T. also thanks Merck, Inc. for partial financial support.
Kinetic Studies on Protonation-Deprotonation of Phosphate Groups and Proton Intercalation-Deintercalation in CY- and y-Zirconium Phosphates Using the Pressure-Jump Technique Mlnoru Sasakl, Naokl Mlkaml, Tetsuya Ikeda, Kazuakl Hachlya, and Tatsuya Yasunaga Department of Chemistry, Faculy of Science, Hiroshima Universify, Hiroshima 730, Japan (Received: May 19, 1982; I n Final Form: July 20, 1982)
In aqueous suspensions of a- and y-zirconium phosphates, two relaxations of the order of milliseconds and seconds were observed by using the pressure-jump technique with conductivity detection. In both suspensions, the fast relaxation time decreases with particle concentration, and the slow one decreases and then approaches a constant value. The dependence of pH on the particle concentration in the zirconium phosphate suspensions revealed that the degree of dissociation of phosphate groups existing in layers and on the surface of zirconium phosphates decreases with increasing concentration of protons released. Taking into account the surface potential created by the negatively charged phosphate groups, the fast and slow relaxations were attributed to protonation-deprotonation of the phosphate groups on the surface and intercalation-deintercalation of protons in the interlayer of the a- and y-zirconium phosphates, respectively. The equilibrium constants of the protonation-deprotonation determined kinetically were in agreement with the acidity constant of phosphoric acid in homogeneous solution. The purpose of the present investigation is to elucidate Introduction kinetically the protonation of the phosphate groups and Layered intercalation compounds, in which adjacent intercalation of protons in a- and y-zirconium phosphates layers are held together primarily by van der Waals forces, by using the pressure-jump technique with conductivity are known to have host lattice sites for guest species, and detection. their physical properties are modified significantly by topotactic bulk reaction processes involving intercalation of Experimental Section the guest species.*-7 In particular, insoluble polybasic acid The pressure-jump apparatus used is the same as that salts of tetravalent metals exhibit the interesting property reported previously,12and the time constant of the pressure that free protons resulting from protonation of polybasic jump is 80 ps. acid groups in the layers are exchangeable selectively for a- and y-Zirconium phosphates Zr(HP04)2.H20and certain cations through the intercalation p r o c e s ~ . ~The ~ ~ ~ ~ *Zr(HP04)2.2H20, ~ respectively, were supplied by Dr. S. study of intercalation dynamics is necessary to attain a Yamanaka of the Faculty of Engineering of Hiroshima fundamental understanding of the nature of the selectivity University. The X-ray powder diffraction patterns of both in the cation intercalation. However, the usual intercasamples were the same as those reported1 and verified that lation process into the compounds is too fast to be observed both samples are not amorphous but layered structures. by ordinary methods. The size, shape, and uniformity of the particles of zircoVery recently, using the relaxation techniques such as nium phosphate were examined by using a scanning the pressure-jump method, we have performed kinetic electron microscope, and the micrographs are shown in studies on rapid reactions a t solid-liquid interfaces and Figures 1, a and b. As can be seen from this figure both in the micropores of solids, and many important kinetic crystal forms were plates, especially well-grown ribbonlike insights into these reactions have been obtained.'@15 ones in the y-zirconium phosphate. The particle sizes were (1) A. Clearfield and J. A. Stynes, J . Inorg. Nucl. Chem., 26, 117 (1964). ~_._.,. (2) R. Schollhorn, R. Kuhlmann, and J. 0. Besenhard, Mater. Res. Bull., 11, 83 (1976). (3) S. Yamanaka, Y. Horibe, and M. Tanaka, J. Znorg. Nucl. Chem., 38, 323 (1976). (4) S. Miyata and T. Hirose, Clays Clay Miner., 26, 441 (1978). (5) R. Schollhorn, Physica, 99B, 89 (1980). (6) P. Calombet and M. Danot, Physica, 99B, 117 (1980). (7) T. Hibma, Phrsica. 99B. 136 (1980). (8) G . Alberti, A&. Chem. Res., 11, 163 (1978). (9) S. Son, F. Kanamaru, and M. Koizumi, Inorg. Chem., 18, 400 (1979). (10) M. Ashida, M. Sasaki, H. Kan, T. Yasunaga, K. Hachiya, and T. Inoue, J . Colloid Interface Sci., 67, 219 (1978).
(11) R. D. Astumian, M. Sasaki, T. Yasunaga, and 2. A. Schelly, J . Phys. Chem., 85, 3832 (1981). (12) K. Hachiya, M. Ashida, M. Sasaki, H. Ken, T. Inoue, and T. Yasunaga, J. Phys. Chem., 83, 1866 (1979). (13) K. Hachiya, J. Sci. Hiroshima Uniu., Ser. A: Phys. Chem., 45, 157 (1981). (14) T. Ikeda, J. Nakahara, M. Sasaki, and T. Yasunaga, J . Phys.
Chem., submitted for publication. (15) M. Sasaki, N. Mikami, T. Ikeda, K. Hachiya, and T. Yasunaga, J. Phys. Chem., in press. (16) J. A. Davis, R. 0. James, and J. 0. Leckie, J . Colloid Interface Sci., 63, 480 (1978). (17) S. Yamanaka and M. Tanaka, J. Inorg. Nucl. Chem., 41, 45 (1979). (18) T. Ikeda, M. Sasaki, K. Hachiya, R. D. Astumian, T. Yasunaga, and 2. A. Schelly, J. Phys. Chem., 86, 3861 (1982).
0022-3654/82/2086-5230$01.25/00 1982 American Chemical Society
a-and yZkconium Phosphates
The Journal of Physical Chemktry, Vol. 86, No. 26, 1982 5231
3-1 1
2
0
4
Cp , 10 g dmm3 Flgure 3. Particle concentration dependences of the reciprocal fast relaxation times in the a-Zr*P and y-Zr*P suspensions at 25 OC: (0) cu-Zr.P, (0)y-ZrOP. r - -
Figure 1. Scanning electron micrographs of zirconium phosphates: (a) a-Zr*P, (b) y-ZrOP.
I
I
L
Flgure 2. Two typical relaxation curves in aqueous suspension of 7-Zr*P observed by the pressure-jump technique with conductivity detection at C , = 40 g dm-3 and 25 OC;sweep: (a) 50 and (b) 500 ms/division.
determined to be 1.2 f 0.4 and 7 f 2 pm for cy- and yzirconium phosphates, respectively, and were approximately uniform. Samples were washed several times with distilled water. Both aqueous suspensions of zirconium phosphates were very stable, with no sign of appreciable sedimentation over a period of 1h. All samples were prepared by dilution of the stock suspensions of zirconium phosphates in which the particle concentration, C,, was determined from the weight of the sample after drying. The potential of the suspensions was measured by the ultramicroelectrophoresis method.13
Results and Discussion Kinetic measurements were carried out in aqueous suspensions of cy- and y-zirconium phosphates, hereafter called a-Zr*P and y-ZroP, respectively, using the pressure-jump technique with conductivity detection. As is apparent from the relaxation curves shown in Figure 2, a and b, two relaxations were observed in both solutions, where the directions of both relaxation signals indicate a decrease in the conductivity of the suspensions during the relaxation. The amplitude of the two relaxations was drastically diminished by addition of about N solutions of hydrochloric acid and sodium hydroxide. No relaxation was observed in the supernatant solution of the suspensions. It is also well-known that the phosphate group in the zirconium phosphates dissociated. 1*3 These facts suggest that the relaxations observed are related to the protonation-deprotonation of the phosphate group. The particle concentration, C,, dependences of the fast and slow reciprocal relaxation times, 7f1and T;~, in the a-Zr-Pand y-Zr-P suspensions are shown in Figures 3 and
3P6 I '
2
0
4 Cp , 10 g dm-,
'4
Flgure 4. Particle concentration dependences of the reciprocalslow relaxation times in the a-Zr*P and 7-Zr.P suspensions at 25 OC: (0) a-Zr.P, (0)7-ZvP.
5
4
r
Q.
0
2 4 Cp , 10 g dm-3
Flgure 5. Dependences of pH on the particle concentration in the a-Zr-P and 7-Zr.P suspensions at 25 'C: (e) a-Zr*P, (0)y-ZrOP.
4, respectively. As can be seen from these figures,the value of 7~~in the a-Zr-Psuspension is nearly equal to that in the y-Zr*Psuspension, while the value of T Lin~ the former is 1order of magnitude smaller than that in the latter. In both suspensions, furthermore, 7 ~ ~increase ' s with C,, while the 7 ~ ~increase 's and then approach constant values. Since the phosphate group in the zirconium phosphate dissociates as described above,lv3 the pH values of the suspensions were measured and the results are shown in Figure 5. As can be seen from this figure, the acidic property of the phosphate group in the cy-Zr-P differs significantly from that in the y-ZrOP. The values of the concentration of protons released per unit particle con-
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The Journal of Physical Chemistry, Vol. 86, No. 26, 7982
Sasaki et al. 7
6 I
0
0.5
1
d
J-O
2
Flgure 8. Plots of pK, in eq 3 vs. a. The arrows show the values of pK$! (0)a-Zr.P, (0)y-2r.P.
4
C, , 10 g dm-? Flgure 6. Dependences of [H+],,/C, on the particle concentrations. The arrows show the values of [H+],JCp at a = 1: ( 0 )cy-Zr.P, (0) y-Zr.P.
0
2
C,
4 , log dm-3
Flgure 7. Dependences of a on the particle concentrations: (0) cy-Zr.P, (0)y-Zr-P.
centration, [H+],,/C,, which also means the amount of dissociated phosphate group in 1 g of the zirconium phosphate, were calculated and the results are shown in Figure 6. This figure shows that the fraction of the phosphate groups dissociated decreases with increasing C, and the concentration of protons released. The value of the degree of dissociation, cy, can be easily calculated by using the following relation: a = ([H+l,e/Cp)/([H+l,e/Cp)~,-~
where SH and S- denote the phosphate group and the deprotonated charged site, the subscripts 1 and s denote the layer and the surface, respectively, k is the rate constant, and K is the equilibrium constant. In mechanism I two relaxations should be observed only in the case in which the second step is the fastest. Taking into account the surface potential created by negatively charged phosphate groups,11J6the reciprocal relaxation time for the second step is given by
(2) where e is the elementary charge, \ko is the surface potential, kg is the Boltzmann constant, T is the absolute temperature, and the superscript int denotes intrinsic. In order to calculate the surface potential term in this equation, we estimated the values of the surface potential by the following theoretical treatment. The overall protonation-deprotonation equilibrium constant, KO,can be expressed as
KO=
( [ ( W i I + [(S-)J)[H+l -
(1)
-
where the value of cy becomes unity at infinite dilution: C, 0. From the plots shown in Figure 6, the values of mol g-' ([H+],e/C,)c -o were determined to be 2.54 X for the y-Zr.'P and 9.6 x lo* mol g-' for the a-Zr.P. The C, dependence of a estimated in the y-Zr.P suspension differs from that in the cy-Zr.P suspension as shown in Figure 7. Since two relaxations were observed in the present study, a simple one-step mechanism of protonation-deprotonation reported previouslylOJ1could not be applied to the analysis of the experimental results. Therefore, the following mechanism involving the proton intercalationdeintercalation in interlayers and protonation-deprotonation of the phosphate groups on the surface was proposed, considering the intercalation-deintercalation of OH- and C1- in hydrotalcite-like compounds reported previously: l5
In order to obtain the value of \ko, one can conveniently rewrite this equation as pKo = pKOint- e\k0/(2.303kBT) (4) The values of pKo were calculated by using the values of the ion exchange capacity of 3.30 X mol g-' for a-Zr.P and 3.20 X mol g-' for y-Zr-P,17and the plots of pKo vs. (Y are shown in Figure 8. The plot in the y-Zr-Psuspension shows good linearity for cy, while that in the a-Zr.P suspension a t high values of cy deviates from the straight line, which may be due to dissolved carbonate ion. From the straight lines of the plots, the values of pKokt in both suspensions were determined to be 5.78 for the cy-Zr.P and 3.60 for the 7-ZrOP. The cy dependences of pKo in the zirconium phosphates are significantly smaller than those in metal oxides such as Fe304." On the other hand, the value of the { potential measured was about 10 mV and
The Journal of Physical Chemistty, Vol. 86, No. 26, 1982 5233
a-and y-Zirconium Phosphates
3r-----16
[H*lexp(-&)(O) , 1 0-3 mol dm'3
a
6 k~T
L
10
0.5
'0L
1 4
K2
2
K p [$I
a
6
4
Figwe 10. Plots of (0) y-ZreP.
IH*lexp(-~)(e) 10%ol dm-) I
7;'
vs. the concentrationterm in eq 6: (0)a-ZrqP,
exp[-e\k,/(2kBT)] vs. the concentration term Figure 9, Plots of in eq 5: (0)a-Zr-P, (0) 7-Zr.P.
TABLE I: Static and Kinetic Parameters of Protonation-Deprotonation of the Phosphate Group in aand yZirconium Phosphates at 25 "C" 10-5 x 103 x
lo-,
x
k_,int,
~,int
k,int
mol-'
S-'
dm3 s-l
m ol dm-3
systems a-Zr.P y-2r.P
1.9
1.3
1.8 0.24
3 / 'm
1
. I l l
I
I
1.1 5.4
The value of the acidity constant of phosphoric acid in a homogeneous system is 7 . 5 2 . a
is relatively small compared with those in ferric oxides." These facts indicate that most of the dissociated phosphate groups exist in the interlayers rather than on the surface of the zirconium phosphate, i.e., [(S-),I
