J. Phys. Chem. 1983, 8 7 , 5-6
5
Kinetic Study on Inclusion Compound Formation Reaction of P-Cyclodextrin Polymer with SCN- Using the Electric Field Pulse Technique Mlnoru Sasakl, Tetsuya Ikeda, Naokl Mlkaml, and Tatsuya Yasunaga' Department of Chemistry, Faculty of Science, Hiroshima University, Hiroshima 730, Japan (Received: August 20, 1982; In Final Form: October 21, 1982)
In aqueous suspensions of 0-cyclodextrin polymer containing SCN-, single relaxation was observed with the electric field pulse technique. The relaxation was attributed to the inclusion compound formation reaction of 0-cyclodextrin with SCN- on the polymer. The inclusion compound formation rate constant obtained was in good agreement with that reported in homogeneous systems, while the dissociation rate constant was two order of magnitude smaller than that in the latter systems. It was revealed that the stability of the inclusion compound on the polymer is fairly large compared with that in the homogeneous system.
Cyclodextrin has a hydrophobic void space capable of hosting anions, and an inclusion compound formation reaction of cyclodextrin with anions has been statically and kinetically investigated as a model of the primary steps of enzyme reacti0ns.l Catalytic activity and selectivity of the cyclodextrins increases by polymerization of the cyclodextrin.2 However, kinetic studies on solid-phase reactions with the highest selectivity have never been performed. Polymeric p-cyclodextrin (p-CD,) was prepared according to the method of Solms and EgL3 So that a stable aqueous suspension of the polymer (0-CD,) could be obtained, the sample was ground with a vibrating mixer mill, and the size of the resulting particles was smaller than 1 pm. Dissolution of (3-CD from the (3-CD, was not detectable by means of colorimetric analysis with p-nitrophen01.~ Kinetic measurements were carried out in aqueous suspensions of 0-CD, containing KSCN by use of the electric field pulse technique with conductivity detection where the time constant of the apparatus was 0.5 ps. A single relaxation was observed, where the direction of the relaxation signal indicates an increase in conductivity with application of the high electric field. The relaxation time was independent of the electric field intensity. These facts suggest that the relaxation observed may be due to a dissociation field effect on some chemical reaction. No relaxation was observed in a (3-CD, suspension, aqueous solution of KSCN, aqueous solution of /3-CD, or an aqueous solution of p-CD containing KSCN. The SCN- concentration dependence of the reciprocal relaxation time, 7-l, is shown in Figure 1. As can be seen from this figure, the value of 7-l increases with the concentration of SCN-. The adsorption isotherm of SCN- on the p-CD, is shown in Figure 2 and the adsorption is a Langmuir one, where the bulk concentration of SCN- was determined by means of colorimetric analysis with ferric ammonium sulfate at X = 460 nm. The total amount of the host sites on the P-CD, was determined to be 6.33 X mol g-l. This value indicates that the proportion of
effective host sites of (3-CD units is only 7.1%. The equilibrium constant of SCN- adsorption was determined to be 1.35 X lo2 mol-' dm3. On the other hand, the { potential measured is smaller than 10 mV, and thus the effect of potential on diffusion of SCN- from bulk phase to polymer surface can be neglected in the present system. It has been reported that a relaxation of the order of 10 MHz has been observed in aqueous solution of p-CD containing anions such as SCN- by use of an ultrasonic absorption spectrometer, and the relaxation has been attributed to an inclusion compound formation reaction of p-CD with the anions.' In the present system, however, this kind of relaxation was not observed in the 10-100MHz frequency range with the ultrasonic absorption spectrometer. The above facts suggest that the relaxation due to inclusion compound formation reaction may shift toward a slower time range resulting from a polymerization of (3-CD. According to the mechanism of the above reaction, the following simple mechanism can be considered: /3-CD,
+ SCN-
P-CD,-SCN-
(1)
where P-CD,-SCN- is the inclusion compound. The equilibrium constant obtained kinetically from the theoretical equation for 7-l, however, was one order of magnitude larger than that obtained statically. Therefore, this mechanism was excluded. Next, mechanism I was divided into two processes, the inclusion compound formation reaction and a conformational change of the inclusion compound: P-CD,
+ SCN- + P-CD,-SCN-
=(@-CD,-SCN-)*
(11)
In this mechanism, the theoretical equation for 7-l also could not explain the experimental results. Thus, mechanism I1 is eliminated as a possible mechanism. In aqueous solution of (3-CD, a conformational change of (3-CD has been reported.' Such a conformationalchange may exist in (3-CD,. Thus, the following mechanism is proposed D-CD;
W 4-CD,
P-CDp-SCN-
(111)
\ *
(1) R. P. Rohrbach, L. J. Rodriguez, E. M. Eyring, and J. F. Wojcik,
J. Phys. Chem., 81, 944 (1977).
(2) R. Breslow, H. Kohn, and B. Siegel, Tetrahedron Lett., 16, 1645 (lR']G\. ~ - -_,_ .
(3) J. Solms and R. H. Egli, Helu. Chim. Acta, 48, 1225 (1965). (4) F. Cramer, W. Saenger, and H. C. Spatz, J. Am. Chem. SOC.,89, 14 (1967).
SCN-
(step 1, K,)
(step 2, K,) where k and K are the rate and equilibrium constants, respectively. In the case that step 2 is very fast compared with step 1,however, the kinetic results could not be in-
0 1983 American Chemical Society
6
The Journal of Physical Chemistry, Vol. 87, No. 1, 7983
0
1
2 ISCN-I , lo? mol dm"
Letters
3
Flgure 1. SCN- concentration dependence of the relaxation time in an aqueous suspension of P-CD, containing KSCN at a particle concentration of C , = 30 g dm-3, € = 41 kV cm-', and 25 "C.
0
2
---[P-CD,I, 1
+
4
ISCN'] K, +
I
mol dm-3
1 +K;'
Figure 3. Plot of T-I vs. the expression between brackets in eq 1: (9) K , = 0.17; ( 0 )K , = 0.25. K , = 0.08; (0)
TABLE I: Static and Kinetic Parameters of the Inclusion Compound Formation Reaction of 8-CD Polymer
with SCN- at 25 C 10-7k,, mol-' dm's-' 2.8 (4.4)a
1O-*K
1 0 - ' k l , SKI 3.0 ( 4 . 4 X lo*)'
dm3 9.3 (9.9 X
101. K2
1.7
The values are those reported in a homogeneous system.'
0
2
1
ISCN-I
I
mol dm?
Figure 2. Adsorption isotherm of SCN- on P-CD, at C , = 30 g dm-3 and 25 "C.
terpreted. In the opposite case where step 1 is faster than step 2, the relaxation time for step 1 is 7-l
= kl([P-CD,]
+ [SCN-I) + k-1 =
with 1
KO = 1 + K2-l The experimental plot of 7-l vs. the expression between brackets is shown in Figure 3, where K 2 is an unknown parameter. As is seen from this figure, only for the case
of K 2 = 0.17 the experimental plots fall on a straight line through origin. This excellent linearity supports the validity of mechanism 111. The static and kinetic parameters obtained are listed in Table I, where the corresponding parameters reported in the homogeneous system are also listed. The inclusion compound formation rate constant is in good agreement with that reported in a homogeneous system,' while the dissociation rate constant is two order of magnitude smaller than that in the latter system. In conclusion, the inclusion compound of the 0-CD, with SCN- on the polymer is fairly stable compared with that of p-CD with SCN- in the homogeneous system, which may be due to an enlargement of the hydrophobic environment caused by polymerization of p-CD. The relaxation related to the very slow step 2 in mechanism I11 could not be observed by using both electric field pulse and pressure-jump techniques with conductivity detection. This fact may give further support of mechanism I11 in which ionic specie does not participate in the step 2. Preliminary relaxation experiments in aqueous suspensions of 8-CD, containing I- and Clod- by using the electric field pulse technique were performed and similar relaxations were found. Further studies of these systems will lead to quantitative clarification of the inclusion compound formation reaction of P-CD, with anions. Registry No. p-CD, 79647-56-6; KSCN, 333-20-0.
