A Theoretical Study of the Site Selectivity of the Zeolite Cation Ridge National Laboratory, Oak Ridge, Tenn., 1965. R. L. Firor and K. Seff, J . Am. Chem. SOC.,99, 4039 (1977). R. L. Firor and K . Seff, J . Am. Chem. Soc., 99, 1112 (1977). T. B. Vance and K. Seff, J . Phys. Chem., 7 9 , 2163 (1975). "International Tables for X-ray Crystallography", Vol. IV, Kynoch Press, Birmingham, England, 1974, pp 73-79. (16) "International Tables for X-ray Crystallography", Vol. IV, Kynoch Press, Birmingham, England, 1974, pp 149-150.
(12) (13) (14) (15)
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978
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(17) D. W. Breck, "Zeolite Molecular Sieves", Wiley, New Yok, N.Y., 1974, p 503. (18) B. L. Dickson and L. V. C. Rees in "Proceedinas of the Third International Conference on Molecular Sieves", B. Uytterhoeven, Ed., Leuven University Press, Belgium, 1973, p 324. (19) J. A. Rabo and P. H. Kasai, frog. SoMState Chem., 9, 1219 (1974). (20) C. F. Baes, Jr., and R. E. Mesmer, "The Hydrolysis of Cations", Wiley, New York, N.Y., 1976, pp 228, 241-242.
A Theoretical Study of the Site Selectivity of the Zeolite Cation. 1. Site Selectivities of Alkali and Alkaline Earth Metal Cations in Zeolite A Kiyoshi Ogawa, Hidaka High School, Hidaka-cho 079-23, Japan
Masahiro Nitta," and Kazuo Aomura Faculty of Engineering, Hokkaido University, Sapporo 060, Japan (Received September 9, 1977; Revised Manuscript Received May 2, 1978) Publication costs assisted by Hokkaido University
A theoretical interpretation is given for the site selectivities of the exchangeable alkali and alkaline-earth metal cations in zeolite A. Zeolite A was divided into two structures, ionic and covalent. The cation-lattice interaction energies consisting of the electrostatic, polarization, dispersion, repulsion, and charge transfer energies were calculated for the cations at the six- and eight-membered oxygen ring sites in a simplified model of zeolite A structure. As a result, the site selectivities determined for cations were as follows: Li+, Na+, Ca2+,and Sr2+ ions prefer the six-membered oxygen ring site, whereas K+, Cs+, and Ba2+ions prefer the eight-membered oxygen ring site. These results are in good agreement with those obtained experimentally. It was concluded that this theoretical method would have considerable use in the determination and understanding of the cation sites and adsorption characteristics of zeolite A and possibly zeolites X and Y.
I. Introduction Synthetic zeolite A is a crystalline aluminosilicate well known for its industrial applications as an adsorbent and molecular sieve, and is very important in fundamental research because of its sorptive and catalytic properties.' The characteristic properties of zeolite A are due to the crystal structure which contains interconnected channels and cavities of uniform sizes. Zeolite A consists of a framework formed of A104- and Si04tetrahedra together with sufficient additional cations to achieve electroneutrality. These cations are exchanged easily with various others, which occupy particular sites in the crystal. In the case of Nal.oo-A,2of which the ideal chemical composition is Na12(A1Si04)12, 12 Na+ ions are distributed among three kinds of sites, i.e., four-, six-, and eightmembered oxygen ring sites, and for brevity they are The eight sites named sites 111, I, and 11, re~pectively.~ I in a unit cell are occupied by eight Na+ ions and the three sites I1 are occupied by three Na+ ions. The twelfth Na+ ion occupies one of the eight sites 111. The unit cell of Nal,oo-Ais shown in Figure 1. The exchangeable cations have their own intrinsic site preference^,^^ that is to say, the three kinds of sites in Na-A zeolite are occupied in order, not at random, by the exchangeable cations. This site selectivity is of great importance in understanding the physicochemical properties of zeolite A, such as adsorption and catalysis. Reactant molecules come in contact with the surface of the internal cavity, passing through the eight-membered oxygen ring, i.e., site 11, and interact with the cations at sites I. The adsorption is prevented partially or completely if large cations are present at sites 11. Therefore, the catalytic and adsorptive properties of zeolite A are remarkably affected when the cations at sites I and 0022-3654/7a/20a2-1655$0 1.oo/o
I1 are replaced by others. The site selectivity of cations was first suggested on the basis of adsorption experiments. For example, the studies by Breck et al.Ib and Rees and Berry6 showed that abrupt changes in the amount of sorbed nonpolar gases such as oxygen and n-alkane occur a t ca. 30% exchange in (Keg, Na)-A and (Caex, Na)-A systems, respectively. They concluded that this behavior is due to the K+ ions preferentially occupying sites 11, so that the effective pore size decreases because its ionic radius (1.33 A) is larger than that of the Na+ ion (0.98 A). They also concluded that Ca2+ions preferentially occupy sites I, causing the effective pore size to increase because of vacancies created a t the eight-membered oxygen rings. However, there is no theoretical information available in the literature on the site selectivities of alkali, alkaline-earth, and transition metal cations. We present here a theoretical interpretation of the site selectivity of cations in zeolite A. This is done by evaluating the cation-zeolite lattice interaction energy at each cation site.
11. Simplified Structure Model of Zeolite A The bond lengths and angles chosen are those from a single crystal X-ray diffraction analysis of dehydrated Nal,oo-Azeolite.2 The structure adopted is a simplified model with high symmetry, which allows easy calculation of the potential energy: bond length (Si,Al)-0 = 1.66 A, bond angles 0-(Si,Al)-0 = 109.4', Si-0-A1 = 154.4'. We have assumed that these values remain unchanged in ion exchange since the zeolite framework is little perturbed by exchange of one or two cations. According to P a ~ l i n g , 'the ~ amount of ionic character of the (Si,Al)-0 bond can be calculated to be 50% from 0 1978 American
Chemical Society
1656
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978
K. Ogawa, M. Nitta, and K. Aomura
AXIS I
Figure 1. Unit cell structure of zeolite A without cations. Sites I, 11, and 111 are shown by three kinds of shading.
the electronegativity difference between (Si,Al) and 0 atoms. This indicates that both ionic and covalent bonding components of the framework of zeolite A must be considered in the calculation of the cation-lattice interaction energy. In the case of a purely ionic crystal, the framework is made up of Si4+,AP+, and 02-ions strongly joined together throughout the crystal by ionic forces. The ionic component of the bonding in zeolite A is optimized when tetrahedra of 02-ions form about the Si4+and AP+ ions; the tetrahedral configuration is determined by the radius ratios of Si4+and A13+ ions to 02-ion. It is assumed that the cation is located at a position which brings about the maximum potential energy at each site and that the cations cannot approach the lattice 02-ions more closely than the sum of the both ionic radii. In the case of a purely covalent crystal, the framework consists of Sio,Al-, and Oo atoms strongly joined together by covalent forces, and there is one negative charge on the Al- ion because it has four valence electrons (it isomorphously replaces a Sio atom). The negative charge is neutralized by the equivalent cation introduced into the zeolite structure. Each of the valence electrons of Sio and Al- are to be placed in the four sp3 hybrid orbitals with the coefficient of the 3s orbital equal to 0.50, according to the concept of hybridized orbital^.'^ The oxygen atom possesses six valence electrons: two of those are to be placed in the two equivalent s-p hybrid bond orbitals with the coefficient of the 3s orbital equal to 0.67, and the remaining four valence electrons are to be placed in the two equivalent s-p hybrid unshared-pair orbitals with the coefficient of the 3s orbital equal to 0.05, respectively. These four orbitals on the oxygen atom are mutually orthogonal. Each of the valence electrons of the oxygen atom which are placed in the bond orbitals takes part in the framework of a covalent bond, (Si,Al)-0, with a bonding electron of the Si atom or Al- ion placed in the sp3 hybrid orbital. Thus, the three-dimensional network of the zeolite lattice structure consists of Si04 and A104tetrahedra, sharing corners with other tetrahedra. It is assumed that the cation is located at a position where the potential energy is maximum at each site and that the cation cannot approach lattice oxygen atoms more closely than the sum of ionic radius of the cation and the van der Waals radius of the oxygen atom.
Ai Figure 2. Site I and symmetry axis I. Unshared-pair orbit+ are shown by two lobes on each oxygen. AI
f?
Si
- is
AI
Si
AI
AI
Flgure 3. Site I1 and symmetry axls 11. Unshared-pair orbitals are shown by two lobes on each oxygen.
those of zeolite lattice ions (i): +3.5 for Si and Al, -2.0 for 0 in the ionic structure, and -1.0 for A1 in the covalent structure, respectively, and rMiis the distance between the cation and lattice ion. The cation is located at a point on the axis through the center of each Axis I passes through the center of site I and is perpendicular to the plane determined by three oxygen atoms which are located more closely to the center of the six-membered oxygen ring. Axis I1 passes through two oxygens which are located furthest from each other in the eight-membered oxygen ring. The potential energy of the cation was calculated in order to find the optimum position of the cation on each axis, and the position which gives the largest potential is available for the following energy calculations. Axes I and I1 and the positions of the cation are shown in Figures 2 and 3, respectively. The potential energy calculation of @E was carried out for all ions of one unit cell and several unit cells surrounding it. The potential value beyond eighteen unit cells in each case of sites I and I1 gave only a negligible contribution to the sum of the potential at that position. (2) Polarization Energy aP.If a polarizable ion is placed in an external field, it will become polarized and a force of attraction will arise. The attractive force results from the interaction between the electrostatic field due to the zeolite lattice and the field-induced dipole of the cation. The polarization energy is derived from the attractive force described above and can be represented by the following expression
111. Energy Calculation (1)Coulombic Electrostatic Energy
@E. If the zeolite lattice ions are considered as point charges, the Coulombic electrostatic energy between the cation at a given site and zeolite lattice ions can be calculated for various points in the site from the formula
where qM and qi represent the charge of cation (M) and
where CYMis the polarizability,of cation, (d@,/dr) is the electrostatic field at the cation position along the axis and is obtained from a large scale plot of @E as a function of axial distance, r. The formulae given above and in the following involve a number of quantities relative to the zeolite ions and atoms. The numerical values adopted are listed in Table I.
The Journal of Physical Chemistry, Vol. 82,No. 14, 1978 1657
A Theoretical Study of the Site Selectivity of the Zeolite Cation
TABLE I: Values of Atomic and Ionic Parameters magnetic polarizability, susceptibility, electron radius, cm3 per parti- cm3 per parti- affinity, cation A cle x cle x IO3' eV 0.029 - 0.99 5.39 Lit 0.60 Na'
K+ Cs' CaZt SIz+ Ba2+ 02-
O0
0.98 1.33 1.54 0.99 1.13 1.35 1.40 1.40
0.190 0.840 2.44 0.471 0.863 1.560 3.89 0.84
- 6.95
- 27.54
-91.0 - 22.1 -46.17 - 76.4 - 20.92 -11.1
5.14 4.40 3.80 11.87 11.03 10.00
i
(3) Dispersion Energy @D. There is a momentary distortion of one ion or atom by the other, and this distortion is in phase with the motion of the changes in the ion producing it. Dispersion force arises from the induced dipole-induced dipole interaction. The dispersion energy is derived from this attractive force, and several formulae have been proposed to express it. We shall adopt the formulae of Kirkwood and Mullerg for the inverse sixth term and of Kiselev et for the inverse eighth and tenth terms in dispersion energy. The formula is as follows:
where
r
3
4
(f - + - :)+=I
ffi/ffM
ion where exchange of electrons between the two ions occurs. The repulsion energy is derived from this exchange force. We shall adopt the Lennard-Jones form D/r12, which leads to simpler calculations compared with other exponential forms. Applying the additivity principle as in the cases of other energies, we obtain an expression of the form
1
where m is the mass of an electron, c is the velocity of light, and a and x are the polarizability and the magnetic susceptibility of the ion or atom interacting with each other, respectively. For ionic and covalent structures, a very important component of the dispersion energy is that between the cation and the nearer oxygen ring oxygens a t each cation site. This is caused by the rapid decrease of @D with an increase of the distance, rMi. The contributions of Si and A1 to the dispersion energy are negligible because they are buried in tetrahedra of oxygens and because their polarizabilities and magnetic susceptibilities are small and large, respectively. (4) Repulsion Energy The repulsive force results from the close approach of the cation to the zeolite lattice
(4)
where DMiis a constant characteristic of the interacting pair, involving contributions of the electrostatic, polarization, and dispersion terms. This can be calculated on the basis of the balancing of attractive and repulsive forces at the cation position, A summation is made only over the oxygen ions or atoms a t the nearest oxygen ring because of the short range of the repulsion force. (5) Stabilization Energy due to Charge Transfer @.cT. Charge transfer should be considered only when the charge transfer structure of a given system is as energetically stable as its no-bond structure. The charge transfer in the ionic structure of zeolite A can be left out of the consideration because of instability of the charge transfer structure, This energy is considered only in the case of the covalent structure. The two unshared-pair orbitals on every oxygen atom forming the eight-membered oxygen ring project toward the center of the ring. These directed unshared-pair orbitals can overlap with similar orbitals, and the unshared-pair electrons become delocalized. In the case of the six-membered oxygen ring, six unshared-pair orbitals on the three oxygen atoms placed nearer the center of the ring project toward the center of the ring. The remaining oxygen orbitals project in the opposite direction from the center. Only in the orbitals projecting toward the center is overlap among unshared-pair orbitals on the oxygen atoms possible, and delocalization of unshared-pair electrons occurs. The energies E of the sets of delocalized orbitals are obtained by the roots of the secular determinant of the form lHij - SyEI = 0, where Hii is the Coulombic integral, which is approximately equal to the valence state ionization energy of the oxygen atom i. H,j is the resonance integral between oxygen atoms i and J, which is estimated by the Helmholz-Wolfsberg approximation,'l and Sij is the overlap integral between unshared-pair orbitals on oxygen atoms i and j. If a cation is present near the center of these rings, the unoccupied orbitals on the cation effectively overlap with delocalized unshared-pair orbitals on the ring oxygen atoms, and some degree of transfer of the unshared-pair electron to the unoccupied orbitals may occur. The stabilization energy is produced by this transfer of an electron. The calculation of the energy is carried out based on Mulliken's charge transfer theory.12 In a system which contains a cation with oxygen atoms surrounding it at a given site, the energy of the no-bond structure, ENB, consists of the electrostatic energies between charged ions (cation M+ and the framework ions) and the one-electron energy of a delocalized unshared-pair orbital on the ring oxygen atoms obtained above. The charge transfer structure is formed by the transfer of an electron from one of the delocalized unshared-pair orbitals on the ring oxygen atoms to one of the unoccupied orbitals on the cation. The energy of the system in the charge transfer structure, ECT, consists of the electrostatic energies among the ring oxygen Al- ions in the framework, ions donated an electron 06+, the cation which has accepted an electron M(n-l)+and other cations Mn+,and the one-electron energy of the unoccupied orbital on the cation. The latter is approximated by the
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K. Ogawa, M. Nitta, and K. Aomura
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978
TABLE 11: Energies in Ionic Structure
ENB-ERES HI 11 - GI II’ERES = HI 11 - GI II*ERES ECT- ERES
cation
site
Li+
I I1 I I1 I I1 I I1 I I1 I I1 I I1
Na’
K‘
cs+ sr2t
Ba2+
@E,
@P,
@D,
14.16 15.26 14.16 14.66 13.18 14.18 12.38 14.11 33.24 34.60 33.20 34.12 31.82 33.67
0.00 0.00 0.00 0.01 0.02 0.01 0.18 0.00 0.00 0.03 0.00 0.04 0.10 0.04
0.11 -0.06 0.05 -0.36 0.23 -0.12 0.12 -0.35 0.45 -0.53 0.32 -0.35 0.74 - 0 . 7 2 0.72 -0.33 0.66 -0.34 0.34 -0.77 0.79 -0.62 0.49 -0.75 0.94 -1.65 0.65 -0.76
eV
eV
eV
@R,
eV
Eion,
eV
14.21 14.95 14.27 14.44 13.12 14.16 12.58 14.50 33.56 34.20 33.37 33.90 31.21 33.60
TABLE 111: Energies in Covalent Structure cation site
Na’ K+ “‘
Sr2+
BaZt
I I1 I I1 I I1 I I1 I I1 I I1 I I1
@E,
@P,
@D,
4.48 3.33 4.48 3.12 3.89 2.99 3.16 2.94 13.95 11.62 13.95 11.44 13.16 11.20
0.00 0.00 0.00 0.00 0.05 0.00 0.14 0.00 0.00 0.01 0.00 0.01 0.01 0.01
0.01 0.02 0.08 0.05 0.18 0.13 0.31 0.31 0.22 0.14 0.36 0.21 0.40 0.28
eV
eV
eV
@R,
eV
@CT,
eV
-0.01 0.23 -0.12 -0.03 -0.13 -0.31 -0.13 -0.53 -0.15 -0.11 -0.70 -0.46 -0.41 - 0.41 -0.42
0.07 0.24 0.13 0.09 0.10 0.03 0.06 0.79 0.53 0.50 0.47 0.47 0.36
eV 4.71 3.30 4.77 3.17 3.90 3.09 3.11 3.16 14.85 11.60 14.35 11.72 13.39 11.43
dehydrated Nal,oo-Aby ion exchange. The energy at site I11 was not calculated since the Na+ ion at site I11 seems to be energetically the most unstable of the twelve Na+ ions. In fact, the potential energy cf the Na+ ion at site I11 (5.77 eV) was much smaller compared with those of other sites. Other cations also may not occupy this site in the initial ion exchange. In the case of the ionic structure of zeolite A, the potential energy of the cation, Ei,,, can be calculated with eq 6. The value of each energy term and the total energies at sites I and I1 are summarized for each cation in Table 11. In the case of the covalent structure, the energy E,, can be calculated with eq 7. Those values at sites I and I1 are summarized in Table 111. The value of Erd at sites I and I1 are obtained with eq 10 and are summarized in Table IV. As shown in Tables I1 and 111, most of Eionand E,, are due to @E, with other terms such as @p, @D, @R, and @cT making smaller contributions. When the interaction energy values are compared for sites I and 11, however, these small energy terms become important in determining which site has the higher energy because those values are almost of the same order of magnitude as the differences in Eion,E,,,, and Eredvalues between site I and site 11. In the case of the Cs+ ion, for example, in the covalent structure, site I possesses higher @E value than site I1 (Table 111). However, the E,,, value of site I is smaller than that of site 11. The Eionvalue of site I1 also is larger than that of site I (Table 11). As a result, Eredof the Cs’ ion shows that the value of site I1 is larger than that of site I (Table IV). Therefore, the Cs+ ion prefers site I1 and is located at this site selectively. It is reasonable to assume that the cation introduced by ion exchange preferentially occupies the site possessing higher potential energy and that it is more stabilized. As is evident from Table IV, there is an appreciable difference in Erealfor sites I and
A
Theoretical Study of the Site Selectivity of the Zeolite Cation
TABLE IV: Energies in "Real" Structure and Site Selectivity of Cation site selectivity Ereal,
cation Li Na
site
eV
I
9.46 9.13 9.52 8.81 si51 8.63 7.85 8.83 24121 22.90 23.86 22.81 22.30 22.52
11 +
K+
cs Caz+
sr*+ Ba2
I I1 I I1
I I1 I I1 I I1 I I1
this method
exptl
I
Ia
I
Ib
I1
IIC
I1
IId
I
Ib
I
Ie
I1
f
References 6, 8, and 16. References 14 and 15. References 6, 14, and 17. Reference 18. e Reference 19. f No data available.
11, and from a comparison between sites I and I1 in Ereal, the site selectivity of the exchange cation can be determined. The results for alkali and alkaline-earth metal cations are shown in the fourth column in Table IV. In general, a simplified interpretation has been that the site selectivities of cations may be related to the size of the cation, not the kind of cation. It would, therefore, be presumed that smaller cations such as Li', Na', Ca2+,and Sr2+prefer site I and larger cations such as K', Cs+, and Ba2+prefer site 11. However, the limit of the size is unclear in this ambiguous rule, and the site selectivity of the cation cannot be readily of intermediate size (e.g., Ag+ = 1.26 anticipated. The size of the exchangeable cation, which has an important effect on @E, must be one of the factors determining the site selectivity, though other physical properties of the cation such as polarizability, magnetic susceptibility, and electron affinity also should be the site selectivity determining factors. The ionic character (50%) of the zeolite crystal estimated here is an important assumption. If the estimation by the extended Huckel MO method is carried out, it gives a value of 53%. However, a change in ionic character by &5% does not give appreciable change in Eredand does not reverse the site selectivity. (2) Comparison with Experimental Results. As described a t the beginning of this paper, the site selectivities of the exchange cations in zeolite A were suggested by the experimental results of sorption characteristics and ionexchange behavior, and a part of them have been supported by the results of X-ray diffraction analysis. The problem of cation positions in zeolite A has been studied by many researchers among whom the study of Seff et al. by single crystal X-ray diffraction is especially excellent, and their results are the best available for the comparison. As can be seen in Table IV, there is good agreement for all cations between the present and experimental results, except for the Ba2+ion. It is interesting that the Ba2+ion prefers site 11, unlike other alkaline-earth metal cations. Unfortunately, there is no information delineating the cation sites and site selectivity of the Ba2+ion in dehydrated (Baex,Na)-A,because the thermal stability of zeolite A is reduced considerably when Ba2+ions are introduced into the crystal, and structure collapse occurs on calcination of hydrated (Ba",Na)-A. Dyer et al., however, have estimated from their thermal i n v e s t i g a t i o n ~ lthat ~ , ~ ~the first two Baz+ions occupy sites I1 in hydrated (Baex,Na)-A. We expect that the site selectivity of the Ba2+ion supposed
The Journal of Physical Chemistry, Vol. 82, No. 14, 1978 1659
here will be confirmed by the refined X-ray diffraction analysis and sorption measurement when a stable dehydrated sample is obtained. In Table IV, the site selectivity is that calculated for the first incoming cation into dehydrated zeolite Nal,m-A by ion exchange, as described before. The site selectivity of the incoming cation may vary with the degree of ion exchange, even if some of the sites preferred initially still remain unoccupied and available. When the exchange ion is a monovalent cation, as in the case of the (Liex,Na)-A at sites I and I1 should remain system, the values of Ere*] almost unchanged from the initial state of ion exchange to a fairly high degree of ion exchange, because the charge distribution in the crystal is similar. The incoming cations, therefore, do not enter less preferred sites while they occupy all of more preferred sites. On the other hand when zeolite A is exchanged with bivalent cations, as in the case of the (Caex,Na)-Asystem, the potential energies a t sites I and I1 should vary appreciably with the degree of ion exchange because of the nonuniformity in charge distribution and of large intercationic repulsion. In order to explain the sorption of gases by Ca-A as a function of its calcium content, Takaishi et al.23concluded, using percolation theory, that one Ca2+ion per unit cell of dehydrated Cal,m-Ais located at a site 11. This indicates that the site selectivity of the bivalent cation varies at a higher degree of ion exchange. Very recently, Seff et al.24have shown, using a single crystal X-ray diffraction, that the sixth Ca2+ion and the sixth Sr2+ion are located at site I1 in dehydrated Cal,m-Aand Srl.m-Asamples, respectively. The present theoretical method is also available for the interpretation of change in the site selectivity with the degree of ion exchange. This problem is under investigation and will be published in a separate paper. Furthermore, even for the same cation, the site selectivity may vary between hydrated and dehydrated states of zeolite A because of the structural difference between them and the energetic change upon coordination of the cation with water molecules as ligands. In general, the locations of the cations in hydrated zeolite A are surprisingly irregular and are quite different from those found in dehydrated form. In (Liex,Na)-A,for example, the Li+ ion prefers site 11in hydrated form14i22and prefers site I in dehydrated form.14J5 The site selectivity of the cation in hydrated zeolite A is also of interest in connection with the motion of cations in the cavity. This problem may possibly be treated by the present theory, but will be far more complicated due to the configuration and number of water molecules as ligands of the cation. It is concluded in this paper that the evaluation of the interaction energy of the cation-lattice is an effective method for the determination of the site selectivity of the cation in dehydrated zeolite A. This method is applicable for investigation of the cation sites of other zeolite types, such as X and Y, because there is no essential difference structurally from type A. The full validity of this method should be shown by more extensive studies on a variety of cations. The site selectivity of the transition metal cation in zeolite A will be given in a later part of this series. Acknowledgment. We are indebted to Dr. H. Itoh of the Research Institute of Catalyst of Hokkaido University for the programing of overlap integral calculations and to the Hokkaido University Computing Center (FACOM 230-75) for calculating the overlap integrals.
References and Notes (1) (a)D. W. Breck, "Zeollte Molecular Sieves",Wiley-Interscience, New York, N.Y., 1974; (b) D. W. Breck, W. G. Eversole, R. M. Milton, T. B. Reed, and T. L. Thomas, J. Am. Chem. Soc., 78, 5963 (1956).
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C. A. J. Hoeve and A. S.Tata
(2) R. Y. Yanagida, A. A. Amaro, and K. Seff, J. Phys. Cbem., 77, 805 (1973). (3) M. Nitta, K. Ogawa, and K. Aomura, Bull. Chem. Soc. Jpn., 48, 1939 (1975). (4) M. Nitta, S. Matsumoto, and K. Aomura, J. Catal., 35, 317 (1974). (5) M. Nitta, K Ogawa, and K. Aomura, J . Cbem. Soc., Faraday Trans. 1 , 72, 2893 (1976). (6) L. V. C. Rees and T. A. Beny, "Molecular Sieves", Society of Chemical Industry, London, 1968, p 149. (7) (a) L. Pauling, "The Nature of the Chemical Bond", 3rd ed, Cornell University Press, Ithaca, N.Y., 1960, Chapter 3; (b) bid., Chapter 4. (8) K. Seff and D. P. Shoemaker, Acta Crystallogr.,22, 162 (1967). (9) J. G. Kirkwood, Pbys. Z., 33, 57 (1932); H. R. Muller, Proc. R. SOC. London, Ser. A , 154, 624 (1936). (10) N. N. Avgul, A. A. Isirikyan, A. V. Kiselev, I. A. Lygina, and D. P. Poshkus, Bull. Acad. Sci. USSR, Div. Cbem. Sci. (Eng. Trans/.) 11, 1334 (1957).
(11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24)
M. Wolfsberg and L. Helmholz, J. Cbem. Phys., 20, 837 (1952). R. S. Mulliken, J. Am. Cbem. Soc., 74, 811 (1952). E. Clementi and D. L. Raimondi, J . Chem. Phys., 38, 2686 (1970). P. Collin and R. Wey, C. R . Acad. Sci. Paris, 270, 1069 (1970). T. B. Reed and D. W. Breck, J . Am. Cbem. SOC., 78, 5972 (1956). T. Takaishi, Y. Yatsurugi, A. Yusa, and T. Kuratomi, J . Cbem. SOC., Faraday Trans. 1 , 71, 97 (1975). N. V. Raghavan and K. Seff, J. Pbys. Cbem., 80, 2133 (1976). T. 8. Vance and K. Seff, J . Pbys. Cbem., 79, 2163 (1975). A. Dyer, W. Z. Celler, and M. Shute, Adv. Cbem. Ser., No. 101, 436 (1971). A. Dyer, R. B. Gettins, and A. Molyneux, J. Inorg. Nucl. Cbem., 30, 2823 (1968). H. S. Sherry, J . Pbys. Chem., 70, 1332 (1966). 2. Dizdar, J. Inorg. Nucl. Cbem., 34, 1069 (1972). A. Yusa, T. Ohgushi, and T. Takaishi, J . Pbys. Cbem. Solids, 38, 1233 (1977). R. L. Firor and K. Seff, J. Am. Cbem. Soc., in press.
The Structure of Water Absorbed in Collagen C. A. J. Hoeve* and A. S. Tata Department of Chemistry, Texas A&M Universify, College Station, Texas 77843 (Received December 2, 1977; Revised Manuscript Received April 26, 1978) Publication costs assisted by the National Science Foundation
At any fixed temperature between -90 and +30 "C the partial molar heat capacity of water absorbed in collagen fibers is independent of concentration from 0 to 0.5 g of water per gram of dry collagen. This result is in disagreement with all two-state models proposed. The high value of 22 cal deg-l mol-l for the partial molar heat capacity of water decreases gradually to a value of 9 cal deg-' mol-' at -90 "C, a value approaching that of ice. We conclude that water becomes vitreous in the vicinity of -100 "C. Our results are in agreement with the previously proposed one-state, liquidlike, model for water absorbed in collagen. According to this model water molecules are hydrogen bonded in long chains that diffuse through the interstices in liquidlike fashion.
Introduction The structure of water is important for interactions between proteins and for transport phenomena through membranes. These structures are difficult to study, however, since in general, the detailed protein shapes are unknown. A favorable exception is collagen, consisting of triple-stranded helices, which for our purpose can be regarded as rigid rods. According to X-ray diagrams the rods are packed in a complex 1attice.l The important features for this article are that the rods are aligned along the fiber axis and that the lattice expands2 commensurate with the water content. At least up to 0.5 g of water per gram of collagen is exclusively absorbed in the lattice interstices with diameters less than 15 A. A variety of studies have indicated that the structure of water absorbed in collagen is different from that of water in bulk. Broad-line NMR measurement^^-^ show two absorption lines instead of one. This is direct proof that the water molecules in collagen are not rotating isotropically. Moreover, their rotational relaxation times are considerably longer6 than in bulk. Our dielectric measurements7 indicate that the dielectric constant is high and that the relaxation spectrum is unusually broad, unlike that of bulk water. As further evidence for the difference, our measurements8 carried out at 30 "C show that the partial molar heat capacity of water in collagen is 22 cal deg-l mol-l. This value is to be compared with 18 cal deg-l mol-l for bulk water. For an extensive review of protein hydration the reader is referred to an excellent article by Kuntz and Kauzmann? 0022-3654/78/2082- 1660$0 1.OO/O
In order to explain the structure of water in collagen several models have been proposed. Mainly based on NMR results, Chapman et al.1° proposed that part of the water molecules are bound at sites, but that they exchange rapidly with freely rotating water molecules. Migchelsen and Berendsen6 proposed a similar two-state model. In their model the bound molecules form hydrogen-bonded bridges between collagen C=O and N-H groups in the manner suggested by Ramachandran and Chandrasekharanall In an interesting article Suzuki and Fraser12 presented polarized infrared absorption spectra of water absorbed in collagen. They interpreted the anisotropy to be consistent with bound water having an orientation as expected by Ramanchandran and Chandrasekharan. Recently,13 the amount of water incapable of freezing has been denoted "nonrotational, bound", water, whereas the amount that is freezable, has been termed "free". Hence considerable support exists for two-state models. On the other hand, based on dielectric7 and heat capacity measurements,8 we have proposed a one-phase model, consisting of chains of hydrogen-bonded water molecules. It is to be,emphasized, however, that the water molecules in this model are not fixed; they diffuse through the interstices preserving their mutual hydrogen bonding. This model is thus best described as liquidlike. Previously,8 we have carried out heat capacity measurements for water absorbed in collagen at 30 "C over a wide range of concentrations. We found that at all concentrations the partial molar heat capacity is 22 cal deg-' mol-l, considerably higher than the value of 9 cal deg-' mol-l for water 0 1978 American Chemical Society
