J. Phys. Chem. 1981, 85, 3543-3545
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Flgure 1. The pair correlation function, calculated by g ( r ) = 1 -t ( 2 n 2 r p J 1 ~ , b si ( s ) M ( s ) sin sr ds5.
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Flgure 3. (a) The experimental interference curve calculated by i ( s ) = I&) - C X , ( ~from ( S )the X-ray scattering data (solid line) compared to the interference curve, j ( s ) calculated from the Fe-CI and the nonbonded CI-CI atom pairs of tetrahedral FeC.,I (b) The experimental interference curve, i ( s ) compared to the interference curve calculated from FeCI,- and three CI-0 atom pairs.
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A.
ARDF contains three well-defined peaks; a sharp symmetrical peak centered at 2.26 A (Pl),and peaks centered at 3.08 A (P2) and at 3.72 A (P3). P1 is due to nearest-neighbor Fe-ligand interactions. The location of P1 is consistent with the Fe-C1 bond distances in tetrahedral complexeP but not the Fe-0 or Fe-Cl bond distances found in octahedral complexes.1° The area under P1 in D(r) (Figure 2) is 113 f 8 e2and may be correlated with either FeC14- (tetrahedral) or ca. FeC12(H20)4+(octahedral) as the average species in this solution or any combination to these species, but T(r) calculated for four Fe-C1 pairs per Fe3+,with the Fe-CI distance being 2.26 A, is in excellent agreement with D(r) in the region of the Fe-ligand bonding. T(r) calculated from the octahedral model, i.e. four Fe-0 and two Fe-Cl pairs per Fe3+,is not in good agreement with D(r) in the 2.0-2.5-A region. P2 is due primarily, if not exclusively, to hydrogenbonded 0-C1 atom pairs12-14 and cannot be utilized to establish (or eliminate) various coordination models for Fe3+ in this solution. (7) R. R. Richards and N. W. Gregory, J. Phys. Chem., 69,239 (1965). (8) M. J. Benne, F. A. Cotton, and D. L. Weaver, Acta Crystallogr., 23, 581 (1967). (9) N. C. Baenzler, Acta Crystallogr. 4, 216 (1951). (IO) M. D. Lind, J. Chem. Phys., 45, 2010 (1967); Acta Crystallogr. Sect. E , 26, 1058 (1970). (11) J. Waser and V. Schomaker, Rev. Mod. Phys., 25, 671 (1953). (12) J. N. Albright, J. Chem. Phys., 56, 3783 (1972). (13) S. C. Lee and R. Kaplow, Science, 169, 477 (1970). (14) L. S. Smith and D. L. Wertz, J. Am. Chem. Sac., 97,2365 (1975). 0022-3654/81/2085-3543$01.25/0
P3 is attributed to Cl-Cl atom pairs. That P3 occurs at (8/3) X 2.26 A indicates that FeC14- is “tetrahedral”. P3 can be accounted for in no other manner. In Figure 3a is a comparison of the experimentally obtained interference curve, i ( ~ )and , ~the ~ interference curve calculated from the structural details (model) of FeC14-, i.e., j ( s ) where j ( s ) = CxiSf;fj47rr2pij[sinsrlsr] dr. The correlation is, at best, only general. Significantly better agreement between i(s) and j ( s ) is found when the hydrogen-bonded C1-0 atom pairs (at ca. 3.08 A) are included in the model (Figure 3b). The best correlation, Le. minimizing Ci(s)- j ( s ) and minimizing C[i(s)- j ( s ) ] : is obtained when Ncl-0 CI 3. In conclusion our results show that tetrahedral FeC14is the average and probably the only important solute species in this solution prepared from anhydrous FeC13and concentrated hydrochloric acid and that the ARDF completely identifies the one-dimensional structural details of this complex as well as the hydrogen-bonded 0-C1 interactions. No evidence of the various octahedral species reported by Magini and Radnai is found. (15) i(s) = Icoh(s)
-
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Deparfment of Chemistry University of Southern Mississippi Hattiesburg, Mississippi 3940 1
Mlckey D. Luter David L. Wertz’
Recelved: July 1. 1981; In Flnal Form: September 9, 198 1
Penta- and Hexacoordlnated Sllicon Sites on Silica Surfaces
Sir: In a recent series of papers Morrow et al. described extensive and detailed infrared spectroscopic studies of silica which provided evidence that a new active site was formed when silica was degassed above 400 0C.1-6 The (1)Morrow, B. A.; Devi, A. J. Chem. Sac., Faraday Trans. I 1972,68, 403. ( 2 ) Morrow, B. A.; Cody, I. A. J. Phys. Chem. 1975, 79, 761.
0 1981 American Chemical Society
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The Journal of Physical Chemistty, Vol. 85, No. 23, 1981
new sites led to strong absorptions at 908 and 888 cm-l with an associated, weaker absorption at 940 cm-l, their results indicating that “...the 888-cm-l band (and the shoulder at 940 cm-’) is primarily associated with a Lewis center, possibly a trivalent electron-deficient surface silicon atom, or to a silicon atom which is in a favorable geometric configuration so as to accept an extra pair of electrons in order to achieve greater than fourfold coordination. The 908-cm-’ band would then be primarily associated with a reactive siloxane bridge or network which ruptures when dissociative chemisorption occurs.’’ In subsequent papers the site in its simplest form was thought of as an unsymmetrical siloxane bridge, one of the silicons being electron deficient4but “...the real site may be considerably more complex. Indeed, our postulated electron-deficient silicon atom may be separated from the siloxane bridge.”5 The model thus accounts for the apparent stoichiometry and spectroscopic results of the surface reactions which were observed, but the nature of the site is still in doubt. In general, it appears that, although silicon in greater than fourfold coordination was mentioned, the discussion of data and mechanisms was firmly anchored to the conventional concept of silicon in its normal quadrivalent, tetrahedral state. It is now proposed that an alternative approach with silicon in nontetrahedral configuration may be more fruitful. It is pertinent to note that the SiOz polymorph stishovite, which has a rutile-like structure in which silicon is octahedrally coordinated, shows strong absorptions in the 1000-800 cm-l region; there is a strong band centered at 885 cm-l overlapping other strong absorption which appears as a shoulder centering at 949 cm-’ on the 885-cm-’ band.e Such bands are not found with other silicas, fused or crystalline quartz, or coesite, in all of which silicon is tetrahedral; such polymorphs absorb near 1098-1077 cm-l. As pointed out by Lyon,G there is a parallel for Ge02,which shows a shift of the absorption peak from 870 cm-l for the GeOz having quartz structure to 715 cm-l for GeOzhaving rutile structure. It seems unlikely that the close correspondence of the absorptions of stishovite and the degassed silica bearing special sites is entirely fortuitous. A not unreasonable interpretation of the latter is that a stishovite-like layer, i.e., sites in which silicon was octahedrally coordinated, was formed when silica became activated and gave rise to the 940-, 908-, and 888-cm-l absorptions. Sites of such a nature might be thought to be unusual in the extreme, in view of the fact that the preponderance of the very voluminous literature dealing with silica and siliceous surfaces considers silicon to take part in surface reactions and to form surface species in its conventional quadrivalent, tetrahedral state. (There are a very few scattered remarks in the literature about sites using terminology such as “valence unsaturation” or “coordinationally unsaturated silicon” and this topic was very briefly taken up by Kiselev and L ~ g i nwho , ~ did not favor the concept.) This direct analogy of surface structures to the majority of silicon compounds is probably correct in most cases. However, analogies to less usual silicon compounds can reasonably be drawn. The literature contains many references to silicon in coordination other than four (ref 8, a not exhaustive list) and there is (3) Morrow, B. A.; Cody, I. A. J. Phys. Chem. 1976,80,1995. (4) Morrow, B. A.; Cody, I. A. J. Phys. Chem. 1976,80, 1998. (5) Morrow, B. A.; Cody, I. A.; Lee, L. S. M. J. Phys. Chem. 1976,80, 2761. (6)Lyon, R. J. P. Nature (London) 1962, 196, 266. (7) Kiselev, A. V.; Lygin, V. I. “Infrared Spectra of Surface Compounds”; Wiley: New York, 1975.
Comments
further support from this from the mechanisms of reactions of silicon compounds. It is important to note that many compounds form at ordinary temperatures and pressures. The earliest mention of silicon in other than 4-fold coordination in connection with surfaces seems to be that of W e ~ lit; ~was suggested that the presence of water on silica may cause the silicon to expand its coordination.1° (A quarter of century later, Pakll observed an intense absorption at 380-200 nm with silica gel but not with aerogel or quartz and ascribed the absorption to an increase in the coordination of surface silicon atoms to 6.) However, this and similar comments’ have lain dormant, possibly because little was known then about unusual silicon compounds. It is known now that there are numerous bulk compounds in which silicon is nontetrahedral, these providing ample precedents for the thesis that analogous nontetrahedral structures may form on surfaces, and there appear to be some data to support it. A deliberate search should be made for such structures because unusual valence states, stabilized at the surface, or induced by an adsorbate and stabilized at the surface, may account for the extraordinary chemical reactions found with some silicious s ~ r f a c e s l ~and - ~ ”may be important in the mech(8) Voronkov, M. G. Pure Appl. Chem. 1966,13,35. Pestunovich, V. A.; Voronkov, G. M.; Zelchan, G. I.; Lukevics, E.-J.; Libert, L. I.; Egorochkin, A. N.; Burov, A. N. Khim. Geterotsikl. Soedin., Sb 1970, No. 2, 339. Bleidelis, J. J.; Kemme, A. A.; Zelchan, G. I.; Voronkov, M. G. Khim. Geterotsikl. Soed. 1973, 617. Pestunovich, V. A.; Tandura, S. N.; Voronkov, M. S.; Baryshok, V. P.; Zelchan, G. I.; Glukhikh, V. I.; Engelhardt, G.; Witanowski, M. Spectrosc. Lett. 1978,11,339. Boal D.; Ozin, G. A. Can. J. Chem. 1973, 51, 609. Klanberg, K.; Muetterties, F. L. Inorg. Chem. 1968,71,155. Ralph, E. K.; Grunwald, E. J. Am. Chem. SOC. 1968, 90,515. Campbell-Ferguson, H. J.; Ebsworth, E. A. U. J. Chem. SOC.A 1966,1508. Wilkins, C. J.; Grant, D. K. J. Chem. SOC.1953,927. Jolibois, H.; Doucet A.; Janier Dubry, J. L. h o g . Nucl. Chem. Lett. 1976,12,759. Beattie, I. R.; Gilson, T.; Webster, M.; McQuillan, G. P. J. Chem. SOC. 1964, 238. Pullman, B. J.; West, B. 0. J. Inorg. Nucl. Chem. 1961,19, 262. Mille, D. B.; Sisler, H. H. J. Am. Chem. SOC. 1955, 77,4998. Burg, A. B. Ibid. 1954, 76, 2674. Durkin, T. R.; Schram, E. P. Inorg. Chem. 1972,11,1048. Benkeser, R. A. Acc. Chem. Res. 1971,4,94. Vandrish, G.; Onyszchuk, M. J. Chem. SOC.1970,3327. Wannagut, V.; Schwarz, R. Z. Anorg. Allg. Chem. 1954,227,73. Ring, M. A.; Jenkins, R. L.; Zanganeh, R.; Brown, H. C. J. Am. Chem. SOC. 1971,93,265. Bain, V. A.; Killean, R. G. G.; Webster, M. Acta Crystallogr., Sect. B 1969,25, 156. Boal, D. H.; Ozin, G. A. Can. J. Chem. 1972,50,2484. Hulme, R.; Leigh, G. J.; Beattie, I. R. J. Chem. SOC. 1960,366. Ebsworth, E. A. V. “Volatile Silicon Compounds”; MacMillan, New York, 1963. Voorhoeve, R. J. H. “Organohalosilanes”;Elsevier, New York, 1967. Bazant, V., Chvalovsky, V., Rathousky, J. “Organosilicon Compounds”; Czechoslovak Academy of Sciences: Praque, 1965. Stishov, C. M.; Popova, S. V. Geokhimiya 1961,10,837 (Engl. transl. 1961, 10, 923). Stoffler, D. J. Geophys. Res. 1971,76,5474. Akimoto, S.; Syono, Y . Ibid. 1969, 74,1653. Ahrens, T. J.; Takahashi, T.; Davies, G. F. 1970, 75, 310. Striefler, M. E.; Barsch, G. R. Ibid. 1976,81, 2453. Stober, W. Beitr. Silikose-Forsch. 1966,89, 1. Ringwood, A. E.; Reid, A. F.; Wadsley, A. D. Acta Crystallogr. 1967, 23, 1093. Bissert, G.; Liebau, F. Naturwissenschaften 1967, 56, 212. Liebau, F.; Bissert, G.; Koppen, N. 2.Anorg. Allgen. Chem. 1908, 359, 113. Liebau, F. Fortschr. Mineral. 1963,48, Suppl. 1,18. Liebau, F. Bull. SOC. Mineral. Cristallogr. 1971, 94, 239. Scholze, H.; Gliemeroth, G. Glastech. Ber. 1966,39, 279. Varma, S. P.; Bensted, J. Silic. Ind. 1973, 38, 29. Moenke, N. Naturwissenschaften 1964,51, 239. Edge, R. A.; Taylor, H. F. W. Nature (London) 1969, 224, 363. Klimovich, V. M.; Chuiko, A. A.; Tertykh, V. A,; Neimark, I. E. In “Adsorption and Adsorbenta”; Vol. 1, D. N. Strazhenko,Ed.; Wiley: New York, 1973; pp 149 ff. Dhar, S.K.; Doron, V.; Kirschner, S.J. Am. Chem. SOC.1958,80, 753. West, R. J. Am. Chem. SOC.1958, 80, 3246. Dilthey, V. Berichte 1903,36,923. Rosenheim, A.; Raibman, B.; Schendel,G. 2.Anorg. Chem. 1931,196,160. Schultz, G. V.; Haug, A. 2.Phys. Chem. (Frankfurt am Main) 1962,34,328. Wannagat, U. In “Advances in Inorganic Chemistry and Radiochemistry”; Vol. 6, H. J. Emeleus and A. G. Sharpe, Ed.; Academic Press: New York, 1964; p 227. Weiberg, E.; Amberger, E. “Hydrides of the Elements of Main Groups I-IV”;Elsevier: New York, 1971; pp 468 ff. Aylett, B. J. Adv. Inorg. Radiochem. 1963, 11, 249. (9) Weyl, W. A. “A New Approach to Surface Chemistry and to Heterogeneous Catalysis”; School of Mineral Industries, Pennsylvania State College: State College, PA, 1951; pp 43 ff. (10) Weyl, W. A.; Hauser, E. A. Kolloid 2. 1951,124, 72. (11) Pak, V. N. Zh.Fiz. Khim. 1975, 49, 2938. (12) Morterra, C.; Low, M. J. D., Ann. N.Y. Acad. Sci. 1973,220,133. (13) Low, M. J. D. J. Catal. 1974, 32, 103.
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J. Phys. Chem. 1981, 85, 3545-3546
anisms of some heterogeneously catalyzed reactions.
I
Editor’s Note: Frofew M o w has examined this comment and consklers lt to be an interesting interpretation of his work.
(14) Bianchi, D.; Gardes, G. E. E.; Pajonk, G. M.; Teichner, S. J. J. Catal. 1974,33, 145. (15) Hoang-Van, C.; Mazabrard, A. R.; Michel, C.; Pajonk, G. M.; Teichner, S. J. C.R. Acad. Sci., Ser. C 1975,281, 24. (16) Repellin, M.; Perrier, R.; Lamartine, R.; Bertholon, G.; Pajonk, G. M., C. R. Acad. Sci., Ser. C 1977, 285, 335. (17) Lacroix, M., Ph.D. Thesis, University Claude Bernard, Lyon, France, 1980.
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M. J. D. Low
Department of Chemistry New York University New York, New York 10003
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Received: June 1, 1981; In Final Form: June 18, 1981
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Effect of Impuritles on Partial Molal Volume and Critlcal Micelle Concentration of Sodium Dodecyl Sulfate. Correction of Micelle Aggregation Numbert
Sir: In a recent paper published in this journal, micelle aggregation numbers, N , were reported for the surfactant sodium dodecyl sulfate (SDDS) in various NaCl(aq) background solutions.’ These values were obtained by using equilibium ultracentrifugation in conjunction with isopiestic distillation experiments used to determine the prefential interactions which occur in multicomponent electrolyte systems. A commercial SDDS was used in the experiments, and, on the basis of good agreement between the measured values of the critical micelle concentrations (crnc) and partial molal volumes with those in the literature, this lot of SDDS was considered to be acceptably pure. The values obtained for the aggregation numbers using this SDDS have been shown to be in excellent agreement with the best available values.2 Current investigations of SDDS in higher NaCl(aq) concentrations, using a second sample of SDDS from the same supplier, indicated the molal volume values were anomalously high for this SDDS sample. A direct comparison of the two samples, by density measurements in HzO, showed that the second sample had higher values of the partial molal volume than the original sample. Because the purity of either sample of SDS was now in doubt, an investigation of the effects of possible impurities in SDDS on the molal volume and cmc properties was begun. If the original SDDS sample were found to be impure, the reported values for the aggregation numbers could possibly be corrected for the presence of the impurity. Four varieties of SDDS were available to be investigated: a specially prepared SDDS of high purity (RLB);3Lot C6A from Eastman Organic Chemicals (the same SDDS used in the previus investigation); Lot A7C from Eastman Organic Chemicals; and Lot 6550452 from BDH Biochemicals. In addition, two “impure SDDS” samples were prepared by using Lot A7C SDDS to which was added, respectively, 1.908% n-dodecyl alcohol (DDA) by weight (A7C*) or 1.930% NaCl by weight (A7C**). Densities of the aqueous solutions of these six SDDS samples were measured at 25.00 “C with a Picker densimetera4 Surface tension measurements were made at 25 “C (1) D. A. Doughty, J. Phys. Chem., 83,2621 (1979).
(2) J. P. Kratohvil, J. Colloid Interface Sci., 75, 271 (1980). (3) R. L. Berg, “Thermodynamics of Aqueous Sodium Dodecylsdfate”,
Energy Research and Development Administration, BETC/TPR-77/3, 1Q77 --. ..
(4) P. Picker, E. Tremblay, and C. Jolicoeur, J.Solution Chem., 3,377 (1974). 0022-3654/81/2085-3545$01.25/0
Flgure 1. Variation of vfor SDDS as a function of square root of molality: (A)A7C‘ *; (0)RLB; (0)C6A; (A)A7C; (B) BDH; (0) A7C’.
on three of the samples, RLB, A7C, and BDH, with a du Nouy tensiometer. The results of the density measurements for the six SDDS samples are summarized in Figure 1in which the partial molal volumes are plotted as a function of square root of the molality. The cmc’s were obtained by extrapolating the trends in density vs. molality plots from above and from below to the point of intersection. Five of the samples (excepting A7C*) had essentially identical cmc’s, averaging (8.28 f 0.17) X 10” m. Sample A7C* had a cmc m. AT, the change in partial molal of (7.45 f 0.43) X volume upon micellization, was obtained by extrapolating the trends in V from above and from below to the cmc and taking the difference a t the cmc. The six samples had essentially the same Avaveraging 11.51 f 0.11 cm3mol-l (sample A7C had a lower value, 11.03 f 0.78 cm3 mol-’, but the large standard deviation negates the significance of the difference). The results of the surface tension measurements were as follows: RLB, no detectable minimum; A7C, substantial minimum of 9 mN m-*, concentration at minimum 6.2 X m; BDH, shallow minimum of 1 mN m-l, concentration at minimum 5 X m. The most striking feature of Figure 1is the parallelism of the curves, including the curves of the two “impure SDDS” samples. The presence of impurities obviously affects the absolute value of the partial molal volumes but in a very regular manner. The invariance of A B indicates that AV values reported in the literature can apparently be accepted without much concern about the absolute purity of the surfactant used. Another important feature is the essentially insignificant effect the impurities have had on the cmc. This differs from the results of others who have reported very substantial effects caused by impurities on the cmc of SDDS as determined by conductometric or surface tension method^.^^^ My surface tension data on two of the commercial SDDS samples confirms this marked effect. The apparent discrepancy can be resolved by considering that density measurements, reported as partial molal volumes, represent a bulk property of the solution, so that the effects of the modest amounts of impurities are averaged over the entire solution volume. In contrast, if the impurity, such as DDA, is itself surface-active, it can have a substantial effect on those types of measurements that “act” where the impurity is concentrated, e.g., a t the surface of the solution or on the surface charge of the micelles. (5) B. R. Vijayendran, J. Colloid Interface Sci., 60, 418 (1977). (6) M. J. Rosen, J. Colloid Interface Sci., 79, 587 (1981).
0 1981 American Chemical Society
