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analysis. See S. Ono and S. Kondo, "Handbuch der Physik", S. Fiugge, Ed., 1960. (11) E. J. Slaats, J. C. Kraak, W. J. T. Brugman, and H. Poppe, J. Chromatogr., 149, 255 (1978). (12) E. A. DiMarzio, J. Chem. Phys., 35, 658 (1961). (13) A. Calli6 and G. Agren, Can. J . Phys., 53, 2369 (1975).
(14) (15) (16) (17) (18)
R. E. Boehm and D. E. Martire, Mol. Phys., 38, 1973 (1979). H. Colin, C. Eon, and G. Guiochon, J. Chromafogr., 122, 223 (1976). R. P. W. Scott and P. Kucera, J. Chromatogr., 149, 93 (1978). R. P. W. Scott and P. Kucera, J. Chromatogr., 171, 37 (1979). E. Grushka, Ed., "Bonded Stationary Phases In Chromatography", Ann Arbor Science, Ann Arbor, MI 1974.
ESR Spectra of Interstitial Hydrogen Atoms in K2SiF6 Akinorl Hasegawa,+" Kolchi Nishikida,$ and Ffrancon Wllliams4 Department of Chemistry, The University of Tennessee, KnoxvMe, Tennessee 379 16 (Received: May 16, 1980)
Well-resolved ESR spectra of hydrogen atoms trapped in KzSiF6have been observed at temperatures above -60 "C after the irradiation of KzSiF6powder at -196 "C. The spectrum recorded at -55 O C showed an excellent fit to the spectrum simulated under the assumption that the hydrogen atom interacts with six nearest-neighbor fluorine nuclei in octahedral symmetry. Being associated with the cubic structure of KzSiF6, this fact suggests that the hydrogen atoms are in the interstitial sites in KzSiF6surrounded with six nearest fluorine atoms in octahedral symmetry and eight next-nearest potassium atoms in cubic symmetry. Hydrogen atoms are also trapped in interstitial sites in (NH&SiF6 as well as those in KzSiFG.
Introduction Several inve~tigationsl-~ have been reported on the electron spin resonance of neutral hydrogen atoms generated in irradiated CaFz crystals. These hydrogen atoms are trapped in interstitial sites and in substitutional sites upon irradiation at room temperature and at -196 "C, respectively. In the interstitial site a hydrogen atom interacts with eight nearest-neighbor fluorine atoms,l while in the substitutional site the interaction takes place with six nearest-neighbor fluorine atomsa2 Although a number of other ESR studies have been reported on hydrogen atoms trapped in, e.g., alkali chlor i d e ~acidic , ~ glasse~,~ phosphates6 and rare-gas mat rice^,^ it may be mentioned that fluorine compounds yield the most detailed information regarding the trapping site because the surrounding fluorine nuclei bring about large superhyperfine couplings. We report here a well-defined ESR spectrum of interstitial hydrogen atoms generated in y-irradiated KzSiF6 powder showing superhyperfine interaction with six nearest-neighbor fluorine nuclei. Experimental Section ESR sample tubes containing the powder of potassium hexafluorosilicate or ammonium hexafluorosilicate (Research Organic/Inorganic Chemical Corp.) were sealed under vacuum and irradiated at -196 " C for a dose of 1 Mrd. ESR spectra were observed at various temperatures with an X-band spectrometer described elswhere.* Results and Discussion measured at The ESR spectrum of y-irradiated -196 "C was readily analyzed into three groups of resonances. These included intense signals from the ca. 508-G doublet attributable to isolated hydrogen atoms and another strong set of signals centered closely around the field position corresponding to the free-spin g factor. Much f Department of Chemistry, Faculty of Science, Hiroshima University, Higashi-sendamachi, Hiroshima 730, Japan. f Perkin-Elmer Japan Co., Ltd., SKF Building, Shiba Daimon 1-9-1, Minatoku, Tokyo 105, Japan.
0022-3654/80/2084-3630$01 .OO/O
TABLE I : ESR Parametersa for Hydrogen Atoms Trapped in K,SiF, a t -55 "C
g value
g = 2.0026
i. 0.0004
hf coupling, MHz aH = 1459.5
f
shf coupling, 0.5
M Hz AilF = 201 A I F = 138 a F = 159
After obtaining approximate values of the ESR parameters by computer simulation as described in the text, the final values listed here were determined by matrix diagonalization calculations which incorporated t h e higher-order effects from both t h e hydrogen hf and the fluorine shf couplings. a
more interesting, however, was the third resonance which consisted of a -500-G doublet with a complex substructure, the centers of the two patterns coinciding closely with the sharp lines from the hydrogen atom. On raising the temperature, the intensities of the hydrogen doublet and the lines near the g value of free spin decreased gradually, while the features of the third resonance changed discontinuously at -125 and -60 "C and the line width decreased remarkably. As shown in Figure la, the spectrum observed at -55 "C consists largely of this doublet with a well-resolved substructure. Since the doublet splitting is almost equal to that of the free hydrogen atom, the major hyperfine (hf) interaction is attributable to a hydrogen atom and the further splitting into some sharp lines may be due to superhyperfine (shf) interaction with fluorine nuclei surrounding the hydrogen atom. The g value and the hf coupling constant of the hydrogen atom were calculated by using a matrix diagonalization programg and are given in Table I. The line shape of these shf structures is of particular interest because the outermost lines are highly symmetric and the adjacent lines have a line shape characteristic of a powder pattern with an axially symmetric hyperfine anisotropy. Accordingly, by inspection, it was possible to obtain trial values of the parallel and perpendicular shf coupling constants from the separation of the singularities in the anisotropic features from the center and outermost lines, as shown in Figure la. Moreover, the shf structures 0 1980 American Chemical Society
The Journal of Physical Chemistty, Vol. 84, No. 26, 1980 3031
ESR Spectra of Interstitial Hydrogen Atoms u=9121.0MHz
I
H
60.45
LA”4
U 30.45
A E x t emum
0.45 500
-7440Cl
200 300 S hf S p l i t t i n g s(MH.7)
100
Figure 2, A dagrani showing the angular dependence of shf splittings of six fluorine nuclei in octahedral symmetry with the parameters listed in Table I.
Flgure 1. (a) The ESR spectrum of y-irradiated K2SiF6 recorded at -55 O C after irradiation at -196 O C . All and A I are the trial values of the parallel and perpendicular shf coupling constants. (b) A simulation spectrum calculated for the ESR parameters listed in Table I wlth a line width AHof 6 G.
consist largely of seven lines. This fact indicates that the trapped hydrogen atom is surrounded by six fluorine nuclei. I t was therefore iissumed that the hydrogen atom interacts with six nearest neighbor fluorine nuclei in octahedral symmetry. Cionsequently, these fluorine nuclei are geometrically equivalent to each other and were taken to have the axially symmetric shf coupling constants determined above. Then, the principal symmetry axes of the shf tensors of these fluorines are directed toward the hydrogen atom locateld in the center of the octahedron of these fluorine nuclei: their direction cosines being (+1,0,0], (-l,O,O}, (0,+1,0),(0,-1,0], (0,0,+1),and {0,0,-1]with reference to a Cartesian coordinate system with the hydrogen atom a t its origin. Therefore, there are three pairs of magnetically equivalent fluorine nuclei with octahedral symmetry. According to this assumption, simulation spectra were then calculated with a program written by Lefebvre and Maruani.lo The simulation spectrum, Figure lb, obtained for the ESR parameters listed in Table I is in complete agreement with the observed spectrum, as seen in Figure la. Such an excellmt fit could not be obtained for six fluorine nuclei in any other symmetry. It was therefore concluded that the hydrogen atoms formed in y-irradiated K2SiF6 are trapped in sites surrounded with six nearest fluorine nuclei in octahedral symmetry. If a first-order treatment is employed, for convenience sake, the shf splittings of these fluorine nuclei can be written by
((Ail2- AI2) sin2 6’ cos2 + A,2)1/2ml, + {(As2- AI2) sin 6’ sin2 C#J + A~z]1~2mI,, + ((All2A L 2 ) cos2 6’
+ A,2}1/2mIz
where 6’ and C#J are poIar angles for the direction of a magnetic field and ml,, mly, and mIz are the total magnetic quantum numbers of’two fluorine nuclear spins on the x, y, and z axes, respectively, being equal to 1,0, or -1. The angular dependence of these splittings is shown in Figure 2. The diagram exhibits the following two important points. (I) The positions of the outermost lines vary be-
Figure 3. The cubic structure of K,SiF6.
TABLE 11: Observed Isotropic shf CouplingaF, the Number of Interacting Fluorines, and the Hydrogen-Fluorine Distance R in Undistorted Fluorides hydrogen site no. of F R,A K,SiF, interstitial 6 2.318 CaF , 8 2.359 SrF , 8 2.54 BaF, 8 2.678 CaF, substitutional 6 2.725 SrF, 6 2.93
aF 159 104 75 41 91 49
ref this work 1 14 2 2 2
tween 477 MHz ((forx , y, or z axis, where the splitting is equal to the isotropic value for six equivalent fluorines) and 485 MHz (for C3 axis) from the center. These almost invariant positions of the outermost lines account for their highly symmetrical appearance in the observed and simulated powder spectra shown in Figure 1. (11) The positions of the singularities (All and A, from the outside and center lines) in the powder spectra correspond to the various “extrema” of the line positions in the diagram. K2SiF6has a cubic structure with an arrangement typified by that found for K2PtC16,11although hexagonal modification is also known.12 In cubic K2SiF6,a regular octahedron of fluorine atoms is placed around each silicon atom and these octahedral and potassium atoms are located exactly as for the calcium and fluorine atoms in CaF2,11J3as shown in Figure 3. The center of the unit cube is not occupied and remains as an interstice. The interstitial site is surrounded with six nearest fluorine atoms in octahedral symmetry and eight next-nearest potassium atoms in cubic symmetry, as shown in Figure 3. Therefore, if a hydrogen atom is trapped in the interstitial site, it would interact with the six geometrically equivalent nearest-neighbor fluorine atoms in octahedral symmetry. The center-center distance, R, between the hydrogen atom in the interstitial site and the nearest fluorine atoms can be calculated as R = ao(l- 2u)/2 = 2.318 A,since the edge length a. is 8.133 A and the parameter u is 0.215.1° The
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The Journal of Physical Chemistry, Vol. 84, No. 26, 1980
distance is realistic because subtracting the fluorine ionic radius of 1.36 A leaves 96% of the maximum radius, 1.00 A, of the hydrogen at0m.l In Table I1 we have compared the observed isotropic shf coupling constants for hydrogen atoms trapped in several f l u ~ r i d e s l ! with ~ J ~ the distance R of hydrogen-fluorine separation for the undistorted lattices in cases of interaction with either six or eight fluorine nuclei. As seen in Table 11, the large isotropic value obtained for K2SiF6correlates well with the short distance of 2.318 A. Thus, it was concluded that the hydrogen atoms observed at -55 "C are in octahedral interstitial sites in K2SiF6. The ESR spectrum of hydrogen atoms trapped in KzSiF6 changed remarkably at -125 and -60 "C with increase in temperature. These changes were irreversible. The spectra observed at -170 and -110 "C could not be reproduced by simulations assuming shf interaction with six nearestneighbor fluorine atoms in octahedral symmetry or trigonal symmetry. A similar irreversible change observed for hydrogen atoms in CaF, at -138 "C was shown to result from the irreversible movement of substitutional hydrogen atoms into interstitial sitessZ Therefore, these changes induced by warming suggest that the hydrogen atoms trapped in KzSiF6at low temperature migrate into more stable positions.15 On the other hand, the spectrum of hydrogen atoms annealed above -60 "C changed reversibly with temperature, especially in the line width and in the total spread of the shf structures. The latter varied linearly with temperature from 1066 MHz at -170 "C to 963 MHz at -15 "C, while the hydrogen hf coupling constant showed practically no change. The ESR spectra of y-irradiated (NH4),SiF6were also measured for the purpose of comparison with those of irradiated KzSiF6. The spectrum of trapped hydrogen atoms was also observed, while such irreversible changes in the spectrum as those described for KzSiF6were not observed. Through comparison of the spectrum observed at ca. -50 "C with that for K2SiF6shown in Figure la, it was interpreted that hydrogen atoms in (NH&SiF6 are also trapped in interstitial sites corresponding to those in K2SiF6. The total spread of the shf structure decreased linearly with increase in temperature from 844 MHz at -170 "C to 715 MHz at -20 "C. The observed hf coupling constant of the hydrogen, 1441 MHz, is smaller than that for K,SiF6 and the change of the value with temperature was extremely small. In contrast to the result for K2SiF6,there was no remarkable narrowing of the spectral lines on annealing at higher temperature for ("4)2SiF6, and the line width remains broad, AH = 18 G, even at -20 "C. This difference may be interpreted in terms of the relative facility of hindered rotation of SiF2- groups in the two cases: the groups in potassium salts begin the hindered rotation at -148 "C,16while they remain rigid in ammonium salts even at room temperature.17 The value of the hf coupling constant of the hydrogen atom trapped in &Sip6 is significantly larger than the value for atomic hydrogen, 1420 MHz. The wave function of the trapped hydrogen can be obtained approximately by the orthogonalization of the hydrogen 1s function to an internally orthogonalized set of the six fluorine ion core orbitals. Therefore, the hf coupling constant of the trapped hydrogen should be N2 times the atomic value, where N is the normalization factor and is larger than 1 owing to the admixture of the hydrogen 1s function into the fluorine ion core orbital^.^ If one neglects the overlap between the ion core orbitals, the admixture coefficients are just the
Hasegawa et al.
overlap integrals between the hydrogen 1s function and the ion core orbitals. Thus, the observed large positive shift in the hf coupling constant reflects a strong admixture of the hydrogen 1s function and the ion core orbitals of fluorines in octahedral symmetry resulting from the fact that the overlap integrals between them are lar e owing to the short center-center distance R of 2.318 . The shf coupling constants of fluorine nuclei for K2SiF6 are considerably larger than those for the other fluorides, as shown in Table 11. Since the shf coupling constants are due to the admixtures of the hydrogen 1s function with the fluorine ion core orbital^,^ these large values can also be interpreted with the large admixtures, or the large overlap integrals through the short distance R. A closer examination of the results, however, reveals more subtle effects which at first sight do not appear to be in accord with the straightforward predictions of the simple overlap model. Thus, in reverse order to the isotropic fluorine couplings given in Table 11,the anisotropic fluorine coupling for the KzSiF6center (2BF = A - uF = 42 MHz) is significantly less than that (70 MHz\ for the interstitial hydrogen atom in CaF21 although the H-F distances for these two centers are very similar (Table 11). Since the g factor of 2.0026 for the K2SiF6center is almost exactly the same as that for the CaFz center1 and in any case shows only a very slight positive shift from the freespin value (2.0023), the difference in the anisotropic couplings cannot be attributed to a greater charge transfer to the hydrogen atom7 in the case of K2SiFG.We are inclined, therefore, to the view that the above results reflect the slightly different nature of the fluorine ligands in the two cases. Accordingly, the higher s / p ratio of induced spin densities for the KzSiF6center is understandable if the fluorine orbitals derived from SiFa- possess more intrinsic s character, presumably as a result of their perturbation by the Si-F bonds, than those from F- in CaF2. The center discussed in this paper consisting of the hydrogen atom surrounded by six fluorine ligands can be crudely regarded as H(F),. It may be worth pointing out that H(F-), possesses 25 valence orbitals and 49 valence electrons, and is therefore isoelectronic with the SF6-and cll;"6radicals. Hence the semioccupied MO will be the very highest MO which is antibonding between the 1s orbital of the H atom and the 2p, orbitals of the six fluorine ligands, and brings about a very large s spin density on the central H atom. This is exactly analogous to the alg* semioccupied MO in the SF6- and ClF6 radicals.18
R
Acknowledgment. This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, United States Department of Energy (Document NO. ORO-2968-124). References and Notes (1) J. L. Hall and R. T. Schumacher. Phvs. Rev.. 127. 1892 (1962). (2j R. G. Bessent, W. Hayes, and J. W. l-bdby, Proc. R. SOC.London Ser A , 297, 376 (1967). (3) R. E. Shamu, W. M. Hartmann, and E. L. Yasaitis, Phys. Rev., 170, 822 119681. , (4) i.-M. Spaeth and H. Seidel, Phys. Status Su/idiB, 46, 323 (1971). (5) W. Koehnleln and J. H. Venable Jr., Nature (London), 215, 618 (1967). (6) Y. P. Virmani, J. D. Zimbrlck, and E. J. Zeller. J. Phys. Chem., 77, 2622 (1973). (7) J. R. Morton, K. F. Preston, S. J. Strach. F. J. Adrian, and A. N. Jette, J . Chem. Phys., 7 0 , 288. (1979). (8) K. Nlshlkida and F. Williams, Chem. Phys. Lett., 34, 302 (1975). (9) A Hasegawa and F. Wllllams, Chem. Phys. Leff. 46, 66 (1977). (IO) A Lefebvre and J. Maruani, J. Chem. Phys., 42, 1480 (1965). (11) R. W. G. Wyckoff, "Crystal Structures", 2nd ed, Vol. 3, Wiley, New Yark. - 1965. . OD 339-342. (12) K. H. Hellwege, Ed., "Landolt-Biirnsteln" I Band, 4 Teil, SprlngerVerlag, Berlln, 1955, p 67. (13) L. Kolditz and H. Preiss, Z . Anorg. Allg. Chem., 325, 245 (1963). I.
rP
J. Phys. Chem. 1980,84,3633-3638 (14) B. Welber, Phys. Rev., 136, 1408 (1964). (15) K. Matsukl, K. Olino and J . Sohma, Jpn. J . Appl. Phys., 17, 1707 (1978). (16) J. S. Waugh and E. I. Fedln, Fir. T'verd. Tela, 4, 2233 (1962).
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(17) R. Blinc and G. Lahajnar, J . Chem. Phys., 47, 4146 (1967). (18) (a) A. R. Bo&, J. R. Morton, and K. F. Preston, J . Phys. Chem., 80, 2954 (1976); (b) K. Nishlkida, F. Williams, G. Mamantov, and N. Smyrl, J. Am. Chem. SOC.,97, 3526 (1975).
NMR Investigations of Aggregation of Nonionic Surfactants in a Hydrocarbon MedCum Huigo Christenson, Stig E. Frlberg," Ch~smistryDepartment, University of Missouri- Rolla, Rolla, Missouri 6540 1
anld David W. Larsen Chemistry Department, lhiversity of Missouri-St.
Louls, St. Louis, Missouri 63 l;?1 (Received: June 23, 1980)
Proton NMR, carabon-13NMR, light-scattering,and density measurements were used to investigate association structures and molecular interactions during water solubilization in polyoxyethylene dodecyl ether/ benzene syekems at high surfactant concentrations. The results indicated an absence of association aggregates at low water concentrations. At water contents between 10 and 20%, association structures started to form, probably milcelles of the inverse type, in which the water-polyoxyethylene chain interaction is the predominant feature. There is evidence that the average chain conformation changes with water content, and that bound water molecules are distributed evenly along the polyethoxyethylenechain. At higher water concentration,the results indlicated a water-rich core to be formed, supported by the close resemblance of the water properties to these of native water.
Introduction There have been numerous studies of the phase behavior and water solubilization by nonionic surfactants of the polyoxyethylene type. Investigations have been concerned both with polydisperse commerci a1 surfactants'-* and monodisperse substances of high p~rity.~-l' Characteristic of' these nonionic surfactants is the pronounced dependence of phase behavior on the temperature and the nature of the hydrocarbon. S h i n ~ d a has ~ - ~systematized this complex behavior by introducing the concept of the HLB temperature. In order to clarify the dependence of water: solubilization upon temperature and hydrocarbon, it is necessary to determine the conditions for water-free surfactant molecules in hydrocarbon eolution. Such investigations have shown that polyoxyethylene surfactants do not form micelles in hydrocarbon solution,'JZ-'' a t least riot unless large amounts of water and surfactant are present. However, there is evidence that small amounts of water may, in some systems, promote micellization. These investigations have mainly been concerned with the determination of micellar size; information about specific solubilization sites for different hydrocarbon structures and about their molecular interactions with water and surfactants has so far been lacking. We found such information to be necessary for a complete understanding of the water solubilization in hydrocarbons by nonionic surfactants of the polyoxyethylene alkyl ether type. F'reliminary results demonstrated the interpretational difficulties of NMR spectra in concentrated systems due to the complex intermolecular interactions of the surfactant molecules. Against this background, our first investigationll was limited to sufficiently small concentrations for the surfactant molecules to exist as monomers. Under these conditions, the NMR resonances could be given an unambiguous interpretation and the changes broughk about by addition of water to the solution could be analyzed. 0022-3654/80/2084-3633$0 1.OO/O
Our results showed the aromatic hydrocarbon to be concentrated along the polar part of the surfactant. Addition of small amounts of water gave spectral changes that revealed a preferential water adsorption hydrogen bonded to the hydroxide group of the surfactant. Further addition of water gave a gradual retraction of the aromatic hydrocarbon from the polar chain caused by the advancing water. The understanding gained from the dilute solution studies gave a firm basis for an analysis of more concentrated solutionri with micellization. Hence it appeared logical to consider conditions at higher surfactant/ hydrocarbon ratios as a sequel to the preceding work. The object of the present article is to consider molecular interactions and associations in systems of benzene/polyoxyethylene dodecyl ethers/water at benzene/surfactant ratios giving maximum water solubilization. Experimental Section Materials. The polyoxyethylene dodecyl ethers, obtained from Niltkol Co., Japan, were well-defined compounds of purities 198%, as determined by gas chromatography. The polyoxyethylene dodecyl ethers will henceforth be referred to only by their prefixes, Le., penta for pentaoxyethylene dodecyl ether, etc. The benzene was Fischer certified and used without further purification, and the water was doubly distilled. Determinatioirz of Solubility Areas. The extensions of the water-poor isotropic liquid-phase regions were determined by direct titration of surfactant-benzene solutions with water. A short time (15 min-1 h) was allowed for equilibration at 30 OC. A t least 15 points were used for the determination of each solubility area. The following ineasurementa were all done on the system penta/benzene/water with a pentalbenzene ratio of approximately 3:l. This is the ratio that gives maximum water solubilization, 55% at 30 'C. The exact ratio varies between 7 6 2 4 and 74:26 depending on the lot number of 0 1980 American Chemical Society
