Langmuir 1991, 7, 394-397
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Investigation of Electric Dichroism of Cetylpyridinium Bromide by Spectroelectrochemistry with a Long Optical Path Length Thin-Layer Cell Shaojun Dong* and Yongchun Zhu Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Academia Sinica, Changchun, Jilin 130022, People's Republic of China Received August 16, 1988. I n Final Form: July 12, 1990 In this paper, the electric dichroism of cetylpyridinium bromide (CPB) has been found and studied by spectroelectrochemistry with a long optical path length thin-layer cell (LOPTLC) for the first time. The CPB molecule with a long carbon chain and a polar pyridinium ring is anisotropic in molecular configuration or in polarizability. In the electric field of a thin-layer cell, the CPB molecule reorientates along the direction of the electric field and exhibits electric dichroism, which results in the increase of absorbance of CPB in the UV-vis range. By use of in situ measurement of spectroelectrochemistry, the order parameters of long molecular axis (5' = 0.845) and short molecular axis (D= 0.155) and the angle between the long axis direction of the CPB molecule and the direction normal to the electrode surface (0 = 18'44') have been determined. These data were used to describe the state of arrangement of the molecules in the solution. The reorientation of CPB molecules is the result of the interaction between the anisotropic molecules and electric field. The effects of the concentration of CPB and of the applied electric field on the electric dichroism have been investigated. Electric dichroism was first reported by Kuhnl in 1939. But only a few papers examined it f ~ r t h e r . ~If-amolecule ~ with permanent electric dipole is anisotropic in molecular configuration or in polarizability and unbending in molecular long axis, such as sodium dodecylsulfonate or hexadecyltrimethylammonium, it shows also an anisotropy in absorbance; that is the molecule has a different molecular absorbance coefficient in different directions. Due to the influence of electric field on the CPB molecule, the molecule will reorientate in solution along the direction of the applied electric field, and the absorbance of the molecule will change before and after an electric field is applied. This phenomenon is called electric dichroism. The intensity of the electric dichroism relates not only to the anisotropy in molecular absorbance coefficient but also to the degree of the order arrangement and can be measured with order parameters. Recently, Yamaoka and Chirney5 described electric dichroism and electrochroism theoretically. Myrvld et a1.6 studied the electric dichroism of dye molecules added to a liquid crystal system with UV-vis spectrophotometry. In our laboratory, we found that the cation surfactant cetylpyridinium bromide (CPB) in 0.10 M KBr solution shows an electric dichroism under applied potentials and investigated by in situ spectroelectrochemistry in a long optical path length thin-layer cell (LOPTLC). Experimental Section Instruments and Reagents. Spectroelectrochemical experiments were carried out on a DMS-90 UV-vis spectrophotometer (Varian Instrument, Co.) combined with a Model 1286 electrochemicalinterface (SolartronInstrument). The LOPTLC' used in this experiment had an optical path length of 7.6 mm and
* To whom correspondence should be addressed.
(1) Kuhn, W.; Duhrkop, H.; Martin, H. 2. Phys. Chem., Abt. E 1939, 45, 121. (2) Houssier, C.; Fredericq, E. Biochim. Biophys. Acta 1964,88, 450. (3) Golub, Ye. I.; Dvorkin, G. A. Biofizika 1964, 9, 545. (4) Houssier, C.; Fredevicq, E. Biochim. Biophys. Acta 1966,120,113. (5) Yamaoka, K.; Charney, E. J. Am. Chem. SOC.1972, 94, 8963. (6) Myrvold, B. 0.;Kleboe, P. Acta. Chem. Scand., Ser. A 1985, 39, 137. (7) Gui, Y.-P.; Porter, M. D.; Kuwana, T. Langmuir, 1986, 2,4, 471.
Q743-7463/91/2407-0394$02.50/0
a large ratio of electrode area to solution volume, with glassy carbon (GC) as the working electrode. A platinum auxiliary electrode and saturated calomel reference electrode were used. All potentials were recorded and reported with respect to this reference electrode. The filling and drawing of the sample solution in and out of the LOPTLC were conducted by a vacuum system.8 CPB (reagent grade) was recrystallized 3 times with acetone and then dried. All other chemicals were of analytical grade. All experimentalsolutionswere prepared with double distilled water. Experimental Procedure. The procedure was as follows: (1)The GC electrode surface was polished and cleaned as shown in ref 8. (2) Place the pretreated electrode into LOPTLC, which was connected with a vacuum system. (3) Place LOPTLC in the optical path of the spectrophotometer. (4) Fill the cell with the sample solution with the help of negative pressure in the vacuum system and sweep the absorbance spectrum in the wavelength range from 500 to 200 nm with zero absorbance at 500 nm. ( 5 ) After the first sweep apply a given potential to the electrode and sweep continuously until a stable absorbance is obtained. Following procedures 1-5 described above, we obtained absorbance spectra with differentCPB concentrationsand different applied potentials; a the typical example is shown in Figure 1, where concentrations of CPB is 4.00 X M in aqueous solution containing 0.10 M KBr at an applied potential of -0.45 V. From Figure l , A l andAiiwereobtainedas0.140and1.286,respectively, at 260 nm, which represent the absorbances of CPB before and after an electric field was applied, respectively. Theoretical Considerations According to classical electromagnetic theory, light absorbance of a substance can be considered as a result of interactions between a molecular electric dipole and a light transition tensor. Assuming that the CPB molecule is of a bar-type electric dipole molecule in solution, its configuration is shown in Figure 2. The electric dipole of ground state PO and of excited state p1 and the absorbance transition tensor f i 0 of ground state are all located in the plane of the pyridinium ring and parallel to the long carbon chain of the CPB molecule. Figure 3 shows the experimental fixed coordinate system X,Y , 2, among which Z is the direction of the external field, and the molecule ~~
(8) Dong, Shaojun;Zhu, Yongchun; Cheng, Guangjin Langmuir 1991,
7, 389.
0 1991 American Chemical Society
Electric Dichroism of CPB
Langmuir, Vol. 7, No. 2, 1991 395
Figure 3. Coordinate systems of experimental and CPB molecule.
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Figure 1. Change of absorbance of CPB in electric dichroism: C c p ~= 4.00 X 10" M, E = -0.6 (V vs SCE),containing 0.10 M KBr aqueous solution.
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