J. Phys. Chem. 1986, 90, 1126-1 129
1126
of ZnO photodecomposed is independent of the anion concentration up to M, except for the I- ion where the Z n O photodecomposition decreases as the concentration of I- ion increases. This behavior can suggest that, in this interval of anion concentration, OH- adsorption predominates over anion adsorption on the particle surface for C1-, Br-, and CH,COO- ions. However, a t higher anion concentrations the opposite effect takes place. As is well-known, the adsorption of negatively charged species on the semiconductor surface changes the flat-band potential to more negative values.23 In this way, from inspection of the different I-E curves obtained in the presence of each anion (Figure 4), it can be concluded that anion adsorption on the particle surface decreases in the order C1- > CH,COO- 2 Br- > I-. Furthermore, this order coincides with the tendency to form complexes between (22) T. Kobayashi, H. Yoneyama, and H. Tamura, J . Elecrroanal. Chem., 122, 133 (1981).
(23) A. J. Bard, F. R. F. Fan, A. S. Gioda, G. Nagasubramanian, and H. S. White, Faraday Discuss. Chem. Sor., 70, 19 (1980).
the anion and the Zn(I1) cation. According to Micka and Gerischer," this tendency is a measure of the adsorption extent of the anion on semiconductor surface. The difference between the Fermi level of the electrons in the ZnO particle under illumination, the quasi-Fermi level of the electrons, and the Fermi level in the dark (E;'] - E F d is ) negative in presence of the different anions. This implies that the Fermi level shifts toward the conduction band, the shift being greater as more electrons are excited to the conduction band, that ;s, when the potential of the photogenerated electrons is more negative. This explains the similar trends observed for E;" - EFdvalues and the corresponding potentials of the photogenerated electrons in the Z n O particle in presence of different anions.
Acknowledgment. This work was financially supported by a research grant from the CAICYT, for which we are very grateful. Registry No. ZnO, 1314-13-2; H20, 7732-18-5; Zn2+',23713-49-7: H,O,, 7722-84-1; I-, 20461-54-5; Br-, 24959-67-9; Cl-, 16887-00-6; CH,COO-, 71-50-1; Fe(CN),)-, 13408-62-3.
Electron Spin Echo Modulation of Doxylstearic Acid Probes of the Surface and Internal Structure of Lithium Dodecyl Sulfate Micelles: Comparison with Sodium Dodecyl Sulfate and Tetramethylammonium Dodecyl Sulfate Micelles Richard R. M. Jones,* Department of Chemistry, Wake Forest University, Winston-Salem, North Carolina 27109
Ren6 Maldonado,+E. Szajdzinska-Pietek,*and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77004 (Receiued: August 1, 1985; In Final Form: October 29, 1985)
Two-pulse electron spin echo modulation analyses of x-doxylstearic acid spin probes (x = 5 , 7, IO, 12, and 16) in frozen lithium dodecyl sulfate (LDS) micelles in D 2 0 and lithium dodecyl-12,Z2,12-d3 sulfate micelles in H 2 0 have been carried out. The results give information on the average conformation of the stearic acid chains and the distribution of D 2 0 and the -CD3 end groups in the micellar aggregates. These are compared to data from dodecyl sulfate micelles with sodium or tetramethylammonium counterions (SDS and TMADS, respectively) which were studied previously in a similar fashion. The LDS micelles show a less compact and more hydrated headgroup region than SDS micelles. This is interpreted to arise from the lithium counterions acting as spacers between the micellar headgroups. This affects not only the micellar surface structure as indicated above but also the internal micellar structure by inducing more disordered alkyl chain packing. The -CD3 surfactant end groups in LDS micelles are not concentrated at the micelle center but are broadly distributed throughout the micellar volume. The surface and internal structure of LDS micelles is quite comparable to that of TMADS micelles and is different from that of SDS micelles.
Introduction The structure and properties of ionic micellar aggregates are known to depend in part on the properties of the counter ion^.'-^ The effects of different alkali metal and tetraalkylammonium counterions in dodecyl sulfate micelles have been explored to some extent. From the determination of critical micelle concentrations by conductometry, Mukerjee et aL4 concluded for alkali metal ions that the hydrated ion interacts with the micelle and that smaller hydrated ions have a greater interaction with the micellar surface leading to a lower degree of counterion dissociation and a more compact double layer. The aqueous solubilities of potassium and cesium dodecyl sulfates are low a t room temperature so the best comparisons can be made between sodium dodecyl sulfate (SDS) and lithium dodecyl sulfate (LDS) micelles. 'Current address: Procter and Gamble Company, Miami Valley Laboratory, P.O. Box 391 75, Cincinnati, OH 45247. Current address: Institute of Applied Radiation Chemistry, Technical University of Lodz, Wroblewskiego 15, 93-590, Lodz Poland.
0022-3654/86/2090-1126$01.50/0
In a series of papeq6-I4 we have reported on the structural characteristics of various frozen micellar systems from the analysis (1) Doughty, D. A. J . Phys. Chem. 1983, 87, 5286. (2) Almgren, M.; Swarup, S. J . Phys. Chem. 1983, 87, 876. (3) Rodakiewicz-Nowak, J. J . Colloid Interface Sci. 1983, 91, 368. (4) Mukerjee, P.; Mysels, K. J.; Kapauan, P. J . Phys. Chem. 1967, 71, 4166. (5) Mysels, K. J.; Princen, L. H. J . Phys. Chem. 1959, 63, 1699. (6) Narayana, P.A,; Li, A. S. W.; Kevan, L. J . A m . Chem. Sot. 1981, 103, 3603. (7) Narayana, P. A,; Li, A. S. W.; Kevan, L. J. A m . Chem. Sot. 1982, 104, 6502. (8) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R . M. J . A m . Chem. SOC.1984, 106, 4675. (9) Maldonado, R.; Kevan, L.; Szajdzinska-Pietek, E.; Jones, R. R. M. J . Chem. Phys. 1984, 81, 3958. (10) Szajdzinska-Pietek, E.; Maldonado, R . ; Kevan, L.; Jones, R. R. M.; Coleman, M . J . J . A m . Chem. SOC.1985, 107, 784. ( 1 I ) Szjdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Berr, S. S.; Jones, R. R . M. J . Phys. Chem. 1985. 89. 1547.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 6, 1986 1127
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