Qian Chen, Diao Guowang Received Mar. 2, 2004; Supported by the National Natural Science Foundation of China (Grant No. 20373060) Abstract The inclusion complex of b-cyclodextrin(b-CD) with 4-(2-pyridylazo)resorcinol(PAR) has been synthesized and characterized by UV-vis spectroscopy, FT-IR, and powder X-ray diffractometry. The results indicate that the ratio of host to guest in the inclusion complex is 2:1, which means that PAR molecule is enwrapped from both ends by the hydrophobic cavities of two b-CDs. At 25ºC, the dissociation constant, KD, of the inclusion complex is 3.5¡Á10-6mol·dm-3.Keywords Superamolecule, 4-(2-pyridylazo)resorcinol, Inclusion complex, b-Cyclodextrin 1. INTRODUCTION Cyclodextrins(CDs) are cyclic glucopyranose oligomers bearing a characteristic toroidal shape. Normally, CDs are cyclic oligomers of six, seven, or eight head to end linked a-D-glucopyranose units, denoted a-, b-, and g-CDs, respectively. The surface of the molecule is hydrophilic, while the cavity is relatively hydrophobic. Owing to their structural characteristics, CDs are capable of forming inclusion complexes with a variety of guest molecules by incorporating them within their hydrophobic cavities, and the properties of the inclusion complexes have been extensively studied[1-3]. CDs' capability of forming inclusion complexes could be applied to reduce pollution, due to the stable inclusion complexes of CDs and pollutants. Since 1990s', the study of CDs for controlling pollution has been applied in practice step by step. Murai et al[4] reported the reclaiming of the nonionic surfactants in water with b-CD polymers. Sheremata et al[5] studied cyclodextrins for desorption and solubilization of 2,4,6-trinitrotoluene and its metabolites from soil. Brusseau et al[6] used cyclodextrins as a solubility-enhancement agent for remediation of a tetrachloroethene-contaminated aquifer. However, some approaches must be applied to decompose the pollutants, and the effect of cyclodextrins on the decomposing process of the pollutants has aroused great interests of the environmental scientists. 4-(2-pyridylazo)resorcinol(PAR) is widely used as indicator, and azo dyestuff. PAR is insoluable in water, but easily dissolved in organic solvents like alcohol, ether, etc[7]. The PAR molecule, consists a pyridyl group which strongly constrains its biotic decomposition, and a resorcinol group which causes pollution. As to other azo dyestuffs, it is difficult to dispose pollutants in their application. Actually, pollution caused by dyes is one of the major hazards to environment. Nowadays, the pollutants are normally treated by chemical, biochemical and physical methods and the processes are usually complicated. Sometimes, it requires expensive equipments. In this article, the thermodynamics of the inclusion complex of b-CD with PAR with spectroscopy have been investigated. 2. EXPERIMENTAL 2.1 Reagents b-CD(Shanghai Chemical Reagents Company) was twice recrystallized before using. PAR (A.R., Shanghai Chemical Factory) and other chemical reagents were of analytical pure and used without further purification. Double distilled water was used to prepare all solutions. 2.2 Apparatus To determine the dissociation constant of the inclusion complex, UV-vis spectra were taken between 200-600nm using a UV-2501(Shimazu) double-beam spectrophotometer with a stoppered quartz cell of 1.00 cm path length. The IR spectra were taken by a FTIR spectrometer (Nicolet 740) with KBr films. Powder X-ray diffractospectra were taken by a M03XFH (MAC Science) diffractometer. 3 RESULTS AND DISCUSSION As described before, PAR is difficult to be dissolved in water. In our experiments, PAR aqueous solution was prepared with excess addition of solid PAR and vigorously stirring of the mixture. The clear solution was obtained by filtering. The concentration of PAR was determined by UV-vis spectroscopy. The molar absorption coefficient of PAR at 25ºC was determined as follows: 0.01mmol PAR was dissolved in 10mL ethyl alcohol. Then a given amount of the solution was taken with a micro-injector and injected into a given amount of water to form PAR aqueous solution. The UV-vis spectra of the PAR aqueous solution were recorded and shown in Fig.1. At 25ºC, the wavelength of maximum absorption is 408 nm. From the peak intensity, the molar absorption coefficient can be calculated. The average value of the molar absorption coefficient, e , of PAR in aqueous solution is 1.82¡Á104 mol·dm-3·cm-1. Fig.1 At 25ºC, UV spectra of PAR aqueous solution with different concentrations of PAR. cPAR/10-6 mol·L-1: a.5.0; b.8.0; c.10; d.15; e.20; f.25; g.30 3.1 Formation of PAR-(b-CD)n inclusion complex and
its dissociation constant In fact, the PAR monomers
and the inclusion complexes coexisted in the solution, therefore, UV-vis absorption shown
in Fig.2 with different b-CD concentration is the superposition of those occurred by PAR
monomers and PAR-(b-CD)n
inclusion complexes, where n is the number of b-CDs in the inclusion complex. The intensity of UV-vis absorption
at a given wavelength will only depend on the amount of PAR monomers and PAR-(b-CD)n inclusion complexes
while b-CDs at the
mentioned wavelength were considered to have no absorption. Therefore, if n=1 in the
inclusion complex, the reaction in the solution is The results coincide with those reported by
reference [10]. All above absorption peaks can be found in Fig.5c, which
indicates that the absorption peaks of PAR are kept in the mechanical mixture. However,
from the IR spectrum of inclusion complex listed in Fig.5d, the original absorptions of PAR disappear comparing to Fig.5b and c but some new absorption
peaks occur which confirms that the formation of the inclusion complex because of the
interaction between b-CD
and PAR while PAR enters into the cavity of b-CD. PAR is incorporated within the hydrophobic cavity of b-CD, so the peaks in the donor's
infrared spectrum get broader and weaker. The combination of donor and acceptor is based
on the main binding forces including hydrogen bonding, Van der Waals forces, hydrophobic
interactions[11]. Because there is no chemical bond formed for this type of
inclusion complex, the shift of donor's absorption peak is not very striking[12]. 3.4 Powder X-ray diffractometry The authors also acknowledge the financial support from the Natural Science Foundation of Educational Committee of Jiangsu Province of China (Grant No. 03KJB150151), the Fund of Ministry of Education of China, and the Fund for an Academic Leader of QingLan Project of Institutions of Higher Learning in Jiangsu of China, which is a source for bringing up the future famous scientists. REFERENCES |