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STUDY OF THE ARRANGEMENT AND OF THE
MOBILITY OF ADSORBED MOLECULES BY
NUCLEAR MAGNETIC RESONANCE
J. Fripiat
To cite this version:
J. Fripiat.
STUDY OF THE ARRANGEMENT AND OF THE MOBILITY OF ADSORBED
JOURNAL DE PHYSIQUE Colloque C4, supplkment a u no 10, Tome 38, octobre 1977, page C4-44
STUDY OF THE ARRANGEMENT AND OF
THE MOBILITY OF ADSORBED
MOLECULES BY NUCLEAR MAGNETIC RESONANCE
J . J. FRIPIATC.R.S.O.C.I. C.N.R.S., rue de la FCrollerie, 45045 OrlCans Cedex, France
Rksumh.
-
L'exposB a pour but de montrer I'utilitB d e la r6sonance magnktique (R.M.N.) dans 1'6tude du phenomkne d'absorption physique. Deux types de surfaces sont examin6s : celles de corps amorphes, orientkes au hasard (gels de silice) e t celles de silicates lamellaires turbostratiques (vermiculite e t hectorite) orient6es pr6f6rentiellement suivant un axe cristallographique (I'axe C perpendiculaire aux feuillets).Dans le premier cas, la R.M.N. pulsee permet de d6finir divers types de mouvement (rotation et diffusion) de la mol6cule sur la surface B condition de choisir adkquatement le noyau r6sonnant ; par exemple, pour I'alcool mkthylique en Btudiant les noyaux 'H e t 'H dans le groupe mCthyle. On montre que les informations recueillies suk les mouvements e t I'h6t6rogCnCitt tnerg6tique de la surface permettent de reconstituer les Bquations d'6tat du film de methanol adsorb&.
Dans le cas des surfaces orientCes, I'usage d e la R.M.N.
B
bande large et de la R.M.N. pulske, outre les informations sur la nature du mouvement permet de formuler un modkle d'organisation de la phase adsorbee. L'exemple trait6 concerne I'eau e t I'eau lourde. On montre aussi que la structuration du liquide bidimensionnel adsorb6 se reflkte dans I'allure des isothermes d'adsorption e t dans les valeurs des entropies d'adsorption.Mots clefs : Application de la R.M.N. C i 1'6tude de I'adsorption physique.
Abstract. - This review aims t o show the use of nuclear magnetic resonance (NMR) for the study of physical adsorption processes. Two types of surfaces are examined : those of amorphous solids oriented at random (silicagels) and those of layer lattice silicates (turbostratic smectites) preferentially oriented about one crystal axis, namely the C axis perpendicular t o the layers.
In the first case, pulse NMR permits t o define the various kinds of motions experienced by the adsorbed species. The molecule illustrating this case is methanol adsorbed on silicagels, the resonant nuclei being either 'H or 'H in the methyl group. From the information obtained on the motions and on the surface energetic heterogeneity, the equations of state of the adsorbate can be predicted.
In the case of preferentially oriented surfaces, wide-band and pulse NMR, beside the nature of the motions, give models for the structural arrangement of the adsorbate. Water and heavy water are taken as example. The nature of the molecular organisation is correlated with the shape of the adsorption isotherms and with the values of the adsorption entropy.
Key words : Application of NMR t o the study of physical adsorption.
1. Introduction.
-
This paper aims t o show that NMR spectroscopy is a usefull tool to study physical adsorption from various points of view. Both structural and dynamical aspects of physical adsorption may be accounted for by this technique under the condition that the magnitude of the specific surface area falls in the range of sensitivity of modern NMR instruments, namely if the number of resonant nuclei in the sample is let us say, higher than 10''. This implies finely divided solids with surface areas, internal and external, higher than about 100 m2/g. For surfaces which are randomly distributed with respect t o the direction of the magnetic field, the information that can be obtained from NMR is mainly concerned with surfaceheterogeneity and microscopic surface diffusion. For surfaces which have a preferential orientation, in addition, the structural organization within the adsorbate layer, if any, may be detected by NMR a s well as the life-time of those structural units.
Randomly distributed surfaces are characteristic of amorphous materials whereas surfaces with preferential orientation along one or three crystalline axes are found is turbostratic smectites or tridimensionally organized swelling lattices.
Proton and deuteron have been the most studied resonant nuclei and multinuclei modern NMR instruments have opened the field t o a larger variety of molecules. I3C, 7Li, Xe for instance, are now good candidates for further elaborate studies. This review
STUDY OF THE ARRANGEMENT AND OF THE MOBILITY C4-45
paper however is devoted to the adsorption of simple molecules such a s water o r methanol and of their deuterated homologues. Relationships between the thermodynamic data obtained for the adsorption processes and those derived from the NMR
experiments performed on the very same systems are of particular interest. The link between both sets of data may be introduced a s follows.
The motions experienced by physically adsorbed molecular species rule t o some extent the adsorption entropy a s well as the equation of state of the adsorbate. If TI is the bidimensional pressure,
namely, the free enthalpy per unit surface area,
T = - ( a F / a A ) , a the area occupied by the adsorbed molecule, and T the absolute temperature, the function cp,
depends on the mobility in the condensed phase, on the frequency of the various modes of vibration with respect t o the surface, on the interactions between neighbors, and so on. The time scale in the adsorption process is introduced through the correlation function G (7) defined a s
G ( T ) describes the evolution of the system adequately if function f contains the information about the molecular motions. Random reorientation or translational jumps are generally represented by G = ( f (0)
f
"(0) ) exp - (T /TJ where T, is thecorrelation time ; T, is either the time between jumps
or the time needed t o reorient the molecule by an angle of 2 IT rad [I].
The correlation time or times, if several kinds of motions occur, may b e measured in the range t o
10-'Os by pulse nuclear magnetic resonance because the longitudinal spin-lattice relaxation rate Ti-' is proportional t o the Fourier transform of G.
T , =
1;
G ( r ) cos or d~,
(3) a t the nuclear resonance w = 2 ITV.T, defines thetime scale of the microscopic event which causes relaxation.
The surface heterogeneity is marked by a
distribution of correlation times
where
p
is the spreading coefficient of the distribution function, and T, the average correlationtime
and where H i s the average activation enthalpy.
The systems t o be reviewed are methanol adsorbed by silica gels [2-41 and water adsorbed in the bidimensional interlamellar space of layer lattice silicates
[5-71.
Details on the NMR techniques are given in the above references. A particular emphasis will be placed on the physical significance of the correlation times and on the parameters derived therefrom and relevant t o a dynamic description of the physical adsorption of methanol and water.
Structural data can be obtained for turbostratic layer lattice silicates by using the variation of the doublet-shaped signal observed for water and heavy water with respect to the orientation of the C crystal axis with respect t o the magnetic field. Indeed oriented aggregates stacked upon each other along this axis can be easily obtained by slowly sedimenting an aqueous suspension of the microcrystals. The resulting structure is turbostratic which means that a and b axis are a t random.
2 . Randomly oriented surfaces.
-
The adsorption of methanol has been studied for two silica gels. The first, called Xerogel, is characterized by pores with an average diameter of 17.5A
and the other, called Aerogel, contains pores unavailable t o N, but available t o dipolar water of methanol molecules [8]. The diameter is probably smaller than 10A.
The adsorption isotherms have the shape predicted by the B.E.T. tyqe 11. We are dealing with surfaces with some degree of heterogeneity and the degree of coverage has a statistical meaning.In order to assign the correlation time to some motion, information must be obtained about the magnitude of the local magnetic field acting on the proton and arising from either other protons in the same or from other molecules or any other magnetic nuclei or paramagnetic centers. In the case where the number of paramagnetic centers is small (say, for instance, less than 50 ppm Fe3+ impurities) and that there are no magnetic nuclei other than the protons, the measurement of the proton second moment (the average quadratic local magnetic field) permits one to decide what the possible motions are to which the measured correlation time(s) may be assigned because the motions modulate the local field and provoke the relaxation.
In the Xerogel independently of the degree of coverage (O), the second moment a t a temperature of the order of - 140 "C corresponds t o a molecule in which the CH, group is already reorienting rapidly around the C , symmetry axis. By contrast, a t that temperature, there is no free rotation of the CH,
group in the Aerogel. When the'situations described by the Arrhenius plots, 1 and 2 in figure 1
C4-46 J. J. FRIPIAT
observed that the activation enthalpy for the CH,
rotation is 1.6 kcal mole-', whereas in the liquid state the activation enthalpy for diffusion is 3.2 kcal mole-'.
This remark and also what has been said about the low-temperature values of the second moment suggest that correlation times, 1 and 2 in figure 1 are those of translational jumps, whereas correlation time 3 is that of the methyl group rotation.
FIG. 2.
-
Variation of the correlation times observed a t three different degrees of coverage. 'H resonance in the CH,OD-XOD-
systems. In the enclosure the values of H observed a t sevendegrees of coverage.
FIG. 1 .
-
Correlation times observed a t the coverage 0 = 1.3 for various systems. (1) ZH resonance in the CD,OH-XOH system,p = 3 and
H=
5.4 kcal mole-'. (2) 'H resonance in the CH,OD-XOD system P = 3.25 andH=
5.5 kcal mole-'. (3) 'H resonance in the CH30H-AOH system = 0.8 andH=
2.32 kcal mole-'. X, Xerogel (average pore diameter : 17.5A)
; A, Aerogel (average pore diameter < 10A)
; ,; proton resonance frequencyin the 14-kgauss field used in the NMR instrument.
In the larger pores of Xerogel and in the temperature range - 140 t o
+
50 OC, the methanolwould thus diffuse while the methyl group is rotating freely. In the narrower pores of Aerogel and in the same temperature range, diffusion would not occur. Then the thermal activation results in a progressively freer rotation of the methyl group. In Aerogel at decreasing 6, the methyl group rotation becomes progressively hindered while in Xerogel, as shown by three examples in figure 2, the translational correlation time decreases with 6.
The activation enthalpy for diffusion obtained at different degrees of coverage is shown in the
enclosure. It increases from about 4 to about 6 kcal mole-' in passing from half to the complete monolayer content and then it decreases progressively toward the value obtained for the free liquid at 0
>
2. This indicates that the effect of the surface on the diffusional motions is still felt by molecules separated by more than two statisticallayers from the solid wall.
It is also interesting t o point out that in agreement with de Boer's [lo] deduction, the activation enthalpy is approximately half the isosteric heat of adsorption obtained from
Indeed [1], between 8 = 0.7 and 6 = 1, q,,
increases from 10 to 14 kcal mole-' and then i t decreases from 14 to 12 kcal mole-' in going from 0 = 1 to 0 = 1.3.
STUDY OF THE ARRANGEMENT AND OF THE MOBILITY (24-47
then the surface'diffusion coefficients shown by the solid line in figure 3 are obtained for the Xerogel at
+
25 "C. Between the half-monolayer and the monolayer content a rapid increase in that coefficient is observed. For Aerogel 121, the diffusion coefficient is probably smaller than lo-'' cm2 s-' since the translational motion is outside the range of observation, e.g., 7 ,+
S.FIG. 3. - Variation of the diffusion coefficient with respect to the degree of coverage, methanol in Xerogel. In the liquid phase
the diffusion coefficient is about 2.6 x lo5 cmZ s-'.
-
U may be approximated by
<,,
: for Xerogel,is,
is 12.5 kcal for 0<
1. As already shown by Thompson and Resing 1131, there must be a relationship between y and p, the spreading coefficient of the translational (average) activation enthalpy It may easily be shown thatP
= 1/(2 kTy 'I2),
(14) assumingU =
2 E. In figure 4A and 4X, it is shown that a good fitting between the equations of state and the observed experimental points may be obtained by takingP
= 3, y = 0.07 for Xerogel, and y = 0.344 and Cw / R T = - 0.77 for Aerogel. It seems indeed reaso- nable to observe an apparently less heterogeneous surface (higher y and smaller P ) for Aerogel than for Xerogel since, in the former, the translational contribution is negligible and therefore, the adsorbed molecules do not experience the various situations that they meet in diffusing on the surface of the latter.It is interesting to compare the equations of state 0.4 for mobile and immobile films [11] with the
information about the motions obtained so far. \ m A mobile film (Van der Waals equation) is
-
@ 0.2 expected for Xerogel ; therefore,m
E
0 0 2 a In-+-- I n p =-- bkT In K . (9) 1 - 0 1 - 0 u An immobile film (Fowler-Guggenheim equation) s 0 3 should be observed for Aerogel :co
' I n p =--1nK. In ---- -
1 - 0 kT
In [9], a and b are the usual Van der Waals
coefficients, whereas in [lo], Cw is the interaction 1 energy.
In order to fit the adsorption data the technique
proposed by Ross and Olivier [12] has been applied.
o
A Gaussian distribution of adsorption energy U is 5 10 15 t o r r assumed : FIG. 4. - Methanol adsorption isotherm for an immobile film
(A = Aerogel) and for a mobile film (X = Xerogel). Solid line,
0
=I
$(P, U) d ( u - (1 1) calculated ; open circles, experimental data at 293 K. In (A), y = 0.344 ; in (X), y = 0.07.where
d
( U -U)
cc exp[ - y ( U -U)ZI
.
(12) Let us come back to the correlation times and especially to line 1 in figure 1. It represents theY is the spreading coefficient of the Gaussian
correlation time obtained from the spin-lattice and is a measure the surface relaxation time at the deuterium resonance for the
heterogeneity
C4-48 J. J. FRIPIAT
CD30H-XOH systems ( I ) at three degrees of
coverage : 8 = 0.8, 1.3, and 1.7, respectively. There is in that case practically no influence by the degree of coverage. This is not surprising because the quadrupole-inner electrical field gradient interaction (the so-called quadrupole coupling constant, Q.C.C.) is the main contribution to the deuterium nuclear relaxation. In that case the correlation time has been assigned to molecules tumbling in surface potential wells. Indeed, this motion should imply an average activation enthalpy similar to that of diffusion, i.e., that of breaking hydrogen bonds, and it should be coverage independent since it does not include any cooperative effect in opposition t o diffusion.
3. Water in the interlamellar space of layer lattice
silicates.
-
Van Olphen [I41 has shown that the water adsorption isotherm for an Na (Llano) vermiculite is characterized by two well-defined steps corresponding to&,
X-ray spacings of 1 1.8and 14.8
A
for the one-layer and two-layer bidimensional hydrate, respectively.Using the variation of the splitting of the doublet-shaped NMR signal with respect to the orientation of the C* crystal axis it has been shown [6] that the two-layer hydrate has a particularly simple structure as indicated schematically in figure 5. Each Na' is the center of a regular octahedron composed of six water molecules. The water molecules reorient rapidly around their C2 symmetry axis at temperatures as low as - 70 "C. This is confirmed by the observation of one single doublet for the 2H resonance spectrum of D20 substituting H,O.
and C , occur at a frequency much higher than the doublet splitting, i-e., %- 2.3 x lo4 Hz.
Below 200 K, these rotational motions are progressively hindered : the doublet splitting becomes orientation independent and the band width increases. Molecules are frozen in various orientations and the spectrum becomes analogous with that of a polycristalline sample. Because the main interaction contributing to the spin-lattice relaxation time is that between the proton and the paramagnetic centers (Fe3+) randomly distributed in the lattice, the assignment of the correlation times is more indirect. This is generally the case for molecules adsorbed on solids with an iron content of the order of or higher than 1 000 ppm.
Two different motions bring about their contributions in two well-separated temperature domains. Their correlation times are shown in figure 6 by two linear functions 1 (7,) and 2 (7,).
2 3 4 7 8
(1000:T) K-$
FIG. 6.
-
Correlation times : (1) and (2), two-layer hydrate of a sodium vermiculite ; (3) and (4),-
one-layer hydrate of a lithium hectorite. For (I), p = 3 and H = 8.5 kcal mole-' ; for (2),p = 1.5 and
H =
5.5 kcal mole-', f o r (3) T A and (4) r a , = 0,H = 4.4 kcal mole-'.
STUDY OF THE ARRANGEMENT AND OF THE MOBILITY C4-49
almost exactly the value obtained a t room temperature for function 2 in figure 6. Therefore the corresponding motion may be assigned to diffusional jumps. The problem of assigning correlation time 1 to a specified motion is more complicated.
The rotation of the hydration shell around the C, axis, as we have seen, is requested to account for the doublet splitting. Such
a
rotation could be at the origin of the diffusion of the hydrated cations. Assuming for instance, 1 of the order of 50&,
the diffusion coefficient deduced from correlation time 1 in figure 6 at room temperature would be about lo-' cm2 s-I. This is the order of magni tude obtained by Calvet [16] in homoionic montrnorillonites at 20 OC by the radiotracer technique.Kadi-Hariifi et a1
11-71
have studied by NMR water and heavy water in a comparable environment but more depleted in iron than vermiculite namely the one-layer hydrate of a Li hectorite.'It contains about 80 ppm iron and therefore the spin-lattice relaxation vector contributing to the observed T , is of nuclei dipolar origin. Furthermore the degree of orientation along the C* axis of this turbostratic smectite is rather good as evidenced by the observation of eleven 001 Bragg reflections.These advantages are however partially balanced by the surface heterogeneity with respect to water adsorption : indeed no clear step is observed in the water adsorption isotherm. This heterogeneity arises from adsorption outside the intplamellar space and occuring either on the external surface (this contribution is important because of the small size of the hectorite microcrystals) or within the pores, on the broken edges of these crystals [17].
Statistically the one-layer hydrate of hectorite contains 6 water molecules per Li' but the 001 spacing (which is 12.6 A) does not allow the formation of either an octahedral or of a tetrahedral shell because the distance between oxygen atoms of neighboring sheets is 12.6-9.8 = 2.8
A.
An arrangement containing about 3 water molecules in contact with each Li' cation seems particularly favored since it remains stable under vacuum at room temperature. The model adopted for this one-layer Li-hectorite hydrate is easy to represent. In figure 5, replace Na' by Li' and remove one layer of water. Here again the ' H and 2H spectra consist essentially of an orientation dependent Pake's doublet between 300 K and 200 K but there are also contributions of molecules with various orientations.The doublet splitting and line width are compatible with a model where three water molecules coordinated to Li' are spinning rapidly about their C , axes. These axes are titled by 70' with respect to the
C* crystal axis and they reorient rapidly about the
latter. The water molecules which do not belong to the first coordination shell might be organized
differently. There is also a fraction of the water content made from molecules in the pores between particles. As the temperature decreases the number of water molecules in the latter situation increases at the expense of those in the interlamellar space.
There are thus three categories of water molecules. The measurement of the, spin-lattice relaxation time allows one to observe them separately. The interlamellar PC and interparticular
P , water molecules have well differenciated longitudinal relaxation times, T I C and T , , res- pectively, the exchange between them being slow (see Fig. 7). T I C averages the contributions of the
FIG. 7. -Variation of T I C at two orientations, 6 = 0 and 6 = 90" and T I , (8 = 0") with respect t o temperature. The A an B contributions to T I C are on the left and right hand side of the enveloppe. 8 is the angle between the C crystal axis and the
magnetic field.
water outside (A) and inside (B) the hydration shell, the exchange between the two populations being relatively fast (
-
s-' at 295 K). The two contributions T;; and T;: can be separated if is assumed that the B situation is thermally stable. It is proposed that above 200 K the Li' cation random walk on the surface averages the structural arrangement in the A and B populations.C4-50 J. J. FRIPIAT
the A and B populations separately. At decreasing temperature, molecules (A) leave the interlamellar space and P, increases. T , , and TI, are ruled by
anisotropic motions since they are orientation dependent. The complex motion made from a combination of a rotation about the C, axis and of a
rotation of the whole hydration shell (Li,H,Q) about its C , axis could account for T,,. Since the Li' cation diffuses with its hydration shell, this complex motion can be accounted for by a rotational diffusion and tis correlation time is that shown by 7,
in figure 6. The assignment of the correlation time
r A obtained from T , , to a well specified motion
cannot be proposed.
Observe that a room temperature the correlation times described by the Arrhenius plots 4 (7,) and 2
(r,, of vermiculite) have about the same values, the
activation energies being somewhat higher for r ,
than for 7,.
Van Olphen [14] has measured the isosteric heat of adsorption and the entropy of adsorption of water for the Na-vermiculite.
The isosteric heats of adsorption range between 10 and 15 kcal and between 21 and 32 kcal per cation gram for the one- and the two-layer hydrates, respectively. Integral entropies of hydration are lower than the entropy of liquid water in both cases.
Hence the water molecules in the adsorbed phase must have a higher degree of order than that existing in the bulk liquid in agreement with the NMR observations. No thermodynamic data are available for the Li hectorite.
4. Conclusions.
-
From the two sets of examples described in this review, if is permitted to conclude that NMR spectroscopy gives a deep insight into the arrangement of adsorbed molecules and their dynamic behavior in the adsorbed state. The main disadvantage of the technique is the rather poor sensitivity and therefore its restriction to finely divided solids with relatively large surface areas.In the cases of methanol adsorbed by the silicagels
as well as of water forming a bidimensional hydrate within the interlamellar space of layer lattice silicates it has been shown that NMR was able to detect the temperature range were translational and rotational motions become increasingly hindered and frozen.
As emphazised by Resing [IS] this does not indicate necessarily a phase change in the true sense of the word. Indeed if for some reason there is a broad distribution of correlation times, at a given temperature not all nuclei fulfill the condition that would narrow the NMR line. The critical correlation time 7 : is about the square root of the second
moment. These nuclei with a correlation time
T~
<
r z exhibit a motionally narrowed line. The1 thers with T~
>
rg
have a rigid lattice behavior. Asthe temperature is changed the distribution function of the correlation time (eq. 4) shifts with respect to
7 : . The result is that motional narrowing occurs at
different temperatures for different nuclei. This is what Resing has called the apparent phase transition. A semi empirical equation has been proposed to relate the temperature at which 50 % of the nuclei fulfill the condition where r ,
<
r z to theactivation enthalpy of the narrowing motion [19]. Another interesting feature is the general relationship between the shape of the water adsorption isotherm in smectite and the number of proton populations detected by NMR.
As said in the introduction the use of I3C and of other nuclei can be expected to bring about very significant contributions. For 13C bearing moiecules
chemical shifts are not only larger but internuclear dipolar broadnings are much less : therefore detailed information on the nature of the bonding to the surface can be obtained [20-221. Accordingly the distinction between different kinds of motion would become easier.
In another vein the NMR of nuclei of hydrated cations would also shed ligth on the interpretation of the protonic motions as shown recently by Conard [23] in a study of the 7Li resonance in hydrated hectorite.
References
[I] PFEIFER, H., Advances in Nuclear Magnetic Resonance Vol. 55 (Springer, New York), 1973.
[2] CRUZ, M. I., STONE, W. E. E. and FRIPIAT, J. J., J. Phys.
Chem. 76 (1972) 3078.
[3] CRUZ, M. I., VAN CANGH, L. and FRIPIAT, J. J., Acad. R.
Belg. Bull. Cl. Sci. 58 (1972) 439.
[4] SEYMOUR, S., CRUZ, M. I. and FRIPIAT, J. J., J. Phys.
Chem. 77 (1973) 2847.
[5] TOUILLAUX, R., SALVADOR, P., VANDERMEERSCHE, C. and FRIPIAT, J. J., Isr. J. Chem. 6 (1968) 337.
161 HOUGARDY, J., STONE, W. E. E. and FRIPIAT, J. J., 3.
Chem. Phys. 64 (1976) 3840.
171 KADI-HANIFI, M., FRIPIAT, 3. J . , CONARD, I . and STONE,
W. G. E., to be published.
[8] CRUZ, M. I., ANDRE, J., VERDINNE, L. and FRIPIAT, J. J.,
Quimica 69 (1973) 895.
[9] O'REILLY, D. E. and PETERSON, E. M., J. Chem. Phys. 55
(1971) 215.
[I01 DE BOER, J. M., The dynamical Chardcter o f Adsorption (Oxford Univ. Press. London), 1953.
[ I l l FRIPIAT, J. J., CHAUSSIDON, J. and JELLI, A., Chimie
Physique des Ph6nomdnes de Surface. (Masson, Paris)
1971.
[I21 Ross, S. and OLIVIER, J. P., On Physical Adsorption (Interscience, New York), 1964.
[13] THOMPSON, J. K. and RESING, H. A., J. Chem. Phys. 43
(1965) 3853.
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Proceedings of the International Clay Conference Tokyo [19] WAUGH, J. S. and FEDIN, E. I., Sov. Phys.-Solid State 4
Vol. I, p. 649 (Israel Univ. Press Jerusalem), 1969. (1962) 1633.
[15] OLEJNIK, S., STIRLING, G. S. and WHITE, J. H., Spec. [20] MICHEL, D., Z. Phys. Chem. Leipzig 252 (1973) 263.
Discuss. Faraday Soc. 1 (1970) 188. [21] KAPLAN, S., RESING, H . A. and WAUGH, J. S., J. Chem.
[16] CALVET, R., These, Faculte des Sciences de Paris (1972). Phys. 59 (1974) 568.
[17] PROST, R., These, Faculte des Sciences de Paris (1975), [22] GAY, I. D., / . Phys. Chem. 78 (1974) 38.
n° 11487, Serie AO. [23] CONARD, I., A.C.S. Symposium Series 34 (1976) 85. [18] RESING, H. A., Adv. Mol. Relaxation Processes 3 (1972)
199.
DISCUSSION
D. NICHOLSON. — In your correction for surface that TJT2 is much larger than that expected from
heterogeneity do you have to include any the B.P.P. theory. In your case, this could be assumption about the way in which the explained by a heterogeneous surface. However heterogeneity is spatially distributed ? For example, large values of TJT2 have been found even for quite
whether the sites are in homotattic patches or homogeneous surfaces, randomly distributed.
J. J. FRIPIAT. — a) I had only one point for this J. J. FRIPIAT. — The heterogeneity character is comparison and thus I cannot draw conclusions ; assumed to be statistically distributed. b) There are several situations where
T1 minimum/ T2 > 1.6. For instance, T2 can be
F. ROUQUEROL. _ Pour quelles raisons a f f e c t e d by fo t h e r relaxation mechanism than Tt,
. , , i - 4 . j „ A m a specified temperature range,
pensez-vous que votre aerogel contient des pores de r * &
diametre inferieur a 10 A ? Vos isothermes peuvent
tout aussi bien etre interpretees par une F. A. P U T N A M . — You showed very good
chimisorption. Par ailleurs, s'il existe reellement des agreement between the measured and theoretical micropores, l'isotherme d'adsorption sur 1'aerogel adsorption isotherms. Could you state how many devrait refleter une adsorption encore plus adjustable parameters were used to obtain this fit ? homogene que sur le xerogel, ce qui n'est pas le cas.
J. J. FRIPIAT. — Actually the number of J. J. FRIPIAT. — Le gel de silice que nous avons adjustable parameters is very small. There is only a appele Aerogel developpe une surface specifique translation along the ordinate and which means that (BET) a l'azote de l'ordre de 135 m2/g, tandis que la we fit with respect to the magnitude of the surface surface occupee par l'alcool etait de l'ordre de area.
300 m2/g- D'apres nos observations, cela provenait
de pores ayant un diametre legerement superieur a R K. THOMAS. — We have made more recent celui de la molecule. Que dans ce cas l'adsorption ait neutron scattering measurements on lithium davantage un caractere chimique que physique est montmorillonites. Though we would have expected probable, mais du point de vue de la N.M.R., c'est- t o s e e diffusion of two different types of water : a-dire du point de vue du mouvement observe, interlamellar and external, as your results suggest,
VAerogel parait plus homogene. Cela n'est guere w e c a n f i t t h e q u asi-elastic scattering assuming that surprenant s i c e mouvement est une rotation duCH3, a l l t h e p r ot o n s diffuse equally fast. The diffusion alors que dans le Xerogel, la N.M.R. voit coefficients appear to be consistent with your r , essentiellement une translation de la molecule. values.
