J. Phys. Chem. 1990, 94, 5351-5356 PC3S locates the chromophore in a more optimal position for photoionization than do the other molecules studied. Comparing the molecules CIPS and Cl2PS,we find that CI2PS has the higher photoionization yield. This result is consistent with the results found for alkylmethylviologen in micelles and vesic l e ~ . ~This ~ . is~explained ~ by a shift of the chromophore toward the organic phase which is caused by the long alkyl tail in CI2PS. However, note that this effect is smaller than the effect between PC3S and PC,S. It is also of interest to compare the photoionization yields of MP and C I P S in which the difference is the sulfonation of the phenothiazine moiety. In general it is expected that the C I P S yield will be higher since it is expected to be closer to the water phase. Table I shows that this is the case for the CTAB/C80H reverse micelle. But this is not the case for the CTAB/C60H, CTAB/C,OH, or AOT systems. In these cases it is suggested that the C I P S yield is anomalously low due to back electron transfer. In fact, the MP yield in CTAB/C80H seems anomalously low and suggests that MP is particularly well solubilized into the organic phase in CTAB/C80H. Figure 4 also shows that the photoionization yield wth I-octanol as cosurfactant is highest for P c 3 s but lowest for Pc6s, P c I 2 s , or M P compared to I-hexanol or 1-butanol cosurfactants. This might be explained by less alkyl chain bending with the less polar surfactant. Thus the photoionization yields seem sensitive to alkyl-chain-enforced location of the chromophore as well as modification of this location by the surfactant polarity. AOT is a surfactant with a branched alkyl chain. Figure 5 shows a schematic structure of AOT micelles. AOT micelles also (16) Colaneri, M. J.; Kevan, L.; Thompson, D. H. P.; Hurst, J. K. J . Phys. Chem. 1987, 91.4072.
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contain a branched organic solvent (isooctane) in the bulk phase. These branched chains seem to allow more alkyl chain bending for the straight-chain photoionizable molecules (PC3S, PC6S, and PCI2S)to locate the chromophore nearer to the interface. This is reflected by the similar photoionization yields of PC3S, PC6S, and PCI2Sin AOT reverse micelles. The high photoionization yield for MP in AOT reverse micelles suggests that MP is located more into the interface than the PC$ molecules which is consistent with the lower polarity of MP. Conclusions Photoionization of phenothiazinesulfonates can be observed in rapidly frozen reverse micelles. Using different alkylphenothiazinesulfonates, it is found that the photoionization yields depend on the distance between the chromophore and the micellar interface. The best position for photoionization in reverse micelles is location of the chromophore near the interface as shown by PCS molecules. However, if the chromophore extends too much toward the water phase, as in C,PS molecules, the yield is decreased, probably by back electron transfer. The alkyl chain length effects seem moderated by alkyl chain bending which is enhanced in branched-chain surfactant reverse micelles like AOT and in CTAB/C,OH reverse micelles with less polar C,OH cosurfactants. Variable alkyl chain lengths in photoionizable molecules in reverse micelle systems do serve to control the photoionization yield. The details of control are only partially understood but alkyl chain bending and back electron transfer, which depend on the interface polarity and the surfactant chain conformations, are likely important factors. Acknowledgment. This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U S . Department of Energy.
Oligomerization in Silica Sols B. P. Feuston+ and S. H. Garofalini* Interfacial Molecular Science Laboratory, Institute for Engineered Materials, and Department of Ceramics, Rutgers University. Piscataway, New Jersey 08855-0909 (Received: December 12, 1989; In Final Form: February 7, 1990)
An empirical potential containing two- and three-body terms for silica-water interactions is proposed, with the oxygen-oxygen
interaction treated identically in both the water and vitreous silica subsystems. The structures of water, H 2 0dimers, HzO trimers, and the H,SiO,-H,O complex obtained by molecular dynamics simulations are in reasonable agreement with X-ray diffraction of water and ab initio calculations of molecules. Simulations of monomeric silicic acid sols provide a microscopic description of water producing oligomerization. The intermediate activation complexes are found to involve ionized monomers and pentacoordinate silicon.
I. Introduction Interactions between water and silica play an important role in the physical properties of a wide variety of systems, e.g., hydroxylated silica, silica surfaces, and silicic acid. Understanding these interactions on the atomistic scale would enhance the design capabilities of microelectronic devices, catalytic supports, optical wave guides, and sol-gel processing. Ab initio computational methods have been used to study the interaction between a single water molecule and a silicic acid monomer (H,SiO4), a relatively "simple" system involving Si, 0, and H interaction^.^-^ Results of these calculations have led to useful predictions for reaction sites and reaction paths of watersilica surface interactions as well as oligomerization of silicic acid monomers. At present, these 'Postdoctoral Fellow of the Institute for Engineered Materials, Rutgers University, and the John von Neumann Supercomputer Center.
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computationally demanding quantum chemical approaches cannot be applied to large enough systems to study such phenomena as polymerization or the complete relaxation of the silica surface in the presence of water. A less computationally intensive approach, which necessarily involves larger approximations, is currently required to investigate these complex Si-0-H systems. To this end, an empirical three-body potential has been developed for simulating such physically diverse systems as H20-H4Si04, H4Si04-H4Si04,and H20-vitreous silica surface! In the present ( I ) Sauer, J.; Morgeneyer, C.; Schroder, K.J. Phys. Chem. 1984,88,6375. (2) Hobza, P.; Sauer, J.; Morgeneyer, C.; Hurych, J.; Zahradnik, R. J . Phys. Chem. 1981, 85, 4061. (3) Sauer, J. Chem. Phys. Lett. 1983,97, 275; J . Phys. Chem. 1987,91, 2315; Chem. Rev. 1989.89, 199. (4) Lentz, B. R.; Scheraga, H. A. Chem. Phys. 1973, 58, 5296. ( 5 ) Newton, M. D.; Jeffrey, G. A.; Takagi, S. J . Am. Chem. Soc. 1979, 101, 1997.
0 1990 American Chemical Society
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The Journal of Physical Chemistr,y, Vol. 94, No. 13, 1990
report, the ability of the potential to provide a reasonable description of intra- and intermolecular interactions is demonstrated. Results of molecular dynamics (MD) simulations of interacting H20-H4Si04 molecules have found ground-state structures and binding energies to be in good agreement with quantum chemical calculations. In addition, the structure of the H 2 0 dimer, H20 trimer, and liquid water are in fair agreement with ab initio calculations and available X-ray and neutron diffraction Application of the new potential in simulations of silicic acid sols yields a microscopic description of the oligomerization of two monomers. I n the simulation, water-producing condensation 2(Si(OH),)
-
(HO),SiOSi(OH),
+ H20
11. The Potential and Methodology The long-term research objective is to study the effects of water interacting with silicate systems through the MD simulation technique, thus requiring the development of suitable interatomic potentials. A realistic model potential should describe H20-H20 interactions reasonably well, allow for the dissociation and formation of H 2 0 molecules, and describe the incorporation of the water oxygen into the silicate structure and vice versa. Therefore, the oxygen-oxygen interaction must be treated identically in both the H 2 0 and Si02subsystems. The above constraints constitute severe acceptance criteria for the model potential and eliminate the possibility of using any of the several good water potentials presently available."-i4 To begin the search, the potential recently developed by the authors for vitreous silica (v-Si02) was adopted for the Si02s ~ b s y s t e m . ' ~This interatomic three-body potential for v-Si02has been implemented in simulations of bulk silica and sodium trisilicates as well as the annealed silica s ~ r f a c e . I ~ - ~ ' Results of these MD simulations have found (i) the bond lengths, bond angle distributions, and the static structure factor to be in good agreement with experiment; (ii) low concentration of bond defects (
