Sept., 1956
ADSORBEDW A T E R
ON S I L I C A G E L BY
NUCLEARR E S O N A N C E
1157
'I'ECHNIQUES
A STUDY OF ADSORBED WATER ON SILICA GEL BY NUCLEAR RESONANCE TECHNIQUES BY J. R. ZIMMERMAN,B. G. HOLMES AND J. A. LASATER Magnolia Petroleum Company,Field Research Laboratories, Dallas, Texas Received February 24, 1966
Nuclear magnetic resonance measurements have been ohtained from the hydrogen nuclei of HzO adsorbed on silica gel I)y means of pulseti radio frequency techniques. The experimental data ai'e values of TP, the inverse line width parameter, as a function of adsorbed water vapor coverage on the silica gel. Insofar as nucleirr rehxation time ( " 2 ) is concei,ned, a twofold pliase system is observed a t large surfirce coverage which resuinahly distinguishes between protons in the water adsoi,b(:d i n and adsorbed on the monomolecular layer. At low s u r l c e coverages, 0 < 0.5, the measurements point. out the existence of two discrete adsorption energy sites. The relaxation data pertaining to the lower energy sites a t this lo\v surfiwe coverage strongly suggest that the relaxation effects due to interactions between adsorhed water molecules have heen directly observed. A qualitative comparison of these experimental results is made with dielectric measurements obtained by Iiut.osiLki. M'here two-phase systems exist, some sample quantitative measurements of the relative number of water molecules associated with each adsorption phase are given.
Introduction In recent years a number of new techniques have been applied in the study of adsorption phenomena, iiicludirig the studies of the kinetics of adsorption, of accommodation coefficients of materials with varying amounts of adsorbed material, of infrared, ultraviolet and visible light adsorption spectra of the adsorbed material, and of dielectric behavior of adsorbed polar molecules. The spectroscopic techniques are all limited severely by experimental difficulties peculiar to adsorption systems. In all of these techiiiques for studying adsorption phenotnena, the measurements are such as not to permit distinct idcn tification of individual components contributing to the tniilti-phase adsorbing system; but rather such measuremetits represent an unresolved composite of the individual components. The use of iiuclear magtietic resonance pulse techniques1 may, in some instances, be able t,o resolve these different adsorbing components for certain type!$ of adsorbate. Niiclear relaxation processesz~3arisc from the magnetic interactiotis between the tiucleus under study and its 10c:il field. The high frequency components of the local field determine the spin-lattice (TI)relaxation time; the litie widt,h parameter (1/T2) arises from the interactions with the low frequency spectrum of the local field and from a broadening associated with the life time of the spin state. If the nuclei of a system do not, all enjoy the same average local field interactions during the time of relaxation measurements, then wch a system might be defined as a multi-phase nuclear system. The i-th phase cat1 then be described in terms of a set of relaxation times 2 l l i and T2i. I n a two-phase system, if eit'her Tli and Tlj or Tzi and T,, can be accurately evaluated, then certain aspects of the two phases are subject to independent study by nuclear magnetic resonance techniques. The multiple phase nuclear system technique has been usedl extensively in the determination of moisture content in proteins and starches.4 In this instance the resonance line width of the hydrogens of the solid chemical structure is extremely broad, while the line width of the adsorbed water is, (1) E. Hahn, Ph:ys. Reu., 80, 580 (1950).
(2) N. Bloembergen, E. Purcell and R. Pound, ibid., 73, G79 (1948). (3) F. Blooh, ibid., 70, 460 (1946). (4) T . Shaw and R . Elsken, J . Cham. Phys., SI, 565 (1953)
in comparison, very narrow. Wilson and Pake5 have observed this multi-phase system behavior in the study of quasi-crystalline polymers. Norberg and co-workers6 have recently extended the earlier work by Pake by measuring several distinct Tz's for the protons in solid polyethylene. Tanaka7 has reported the observation of a multi-phase syst,em in adsorbed water on carbon. I n t8hisinstance a distinction was made between water adsorbed i l l and above the mono-molecular layer by ohserving a sharp resonance line superposed on a broad resonance line. The relaxation data which are described in the following pages have been obt8n.ined from nuclear magnetic resonance pulse (spinecho) techniques.' I n view of the excellent treatises now available, no general matheinaticnl introduction to the field of riuclear resonance and the related spin-echo t'echnique is present'ed. Argumerits relating adsorption phenomena to nuclear relaxation phenomena are intentionally presented on a qualitat'ive basis.
Experimental Adsorption Apparatus.-A sample of 10.5582 g. of Davison8 commercial silica gel was weighed into a specid sample holder equipped with a valve arrangement so that, it could be attached or unattached to a high vacuum adsnrption system without changing the pressure on the silica gel. The silica gel sample and holder were dried under v:tcuutn nim.) a t a t,emperature of approximately :300"for six la,^. It was found that the silica gel sample had decreased in weight by 0.4473 g. during this treatment. I t is believed that the silica gel was now dry and the dry wiglit of silica gel was tdien as 10.1109 g. The dried, evacuated silica gel and the vacuum pump were isolated; and water vapor was introduced into the mar& fold. The sample bulb W R ~opened to the manifold and water vapor was adsorbed on the silica gel. An oil (Octoil) manometer was used to determine the water vapor pressure; however, the amount of water adsorbed hy the silica gel was determined by direct weighing of the sample (bulb). This procedure was repeated for each nuclear magnetic resonance relaxation time measurement. Each measurement was performed at 2 5 " . Once the silica gel was saturated, a reverse procedure was used to desorb the adsorbed water. The amount ot water desorbed a t each st'ep was also determined by weighing the sample. h-o obvious hysteresis effects were ohserved
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(5) C. Wilson and G.Pake, J . Polymer Sci., 10, 503 (1953). (6) I. Lowe, L. Bowen and R . Norberg, Bull. A m . P h y s . Soc.. 30, 16 (1955). (7) K. Tanaka and K. Yamagata, Bull. Chem. Soc. J a p a n , 38, 90 (1955).
(8) A chemical analysis of this gel has been made. content is approximately 0.2% by weight.
The alumina
1158
J. R. ZIMMERMAN, B. G. HOLMES AND J. A. LASATER
Vol. 60
insofar as relaxation data are concerned over the range of coverages described in this paper. The surface area mensui'ements of the silica gel were obtained by a standard B.E.T. apparatus. Nitrogen was adsorbed at liquid air teinperatures. An oxygen vapor pressure thermometer was used to measure these temperatures. Helium \vas used to determine the dead space. The sample was outgassed 24 hr. at 110' before each adsorption run for area measurement. The average surface area was found to he 625 ni.2/g. Nuclear Relaxation Measurements.-In order to obtain relaxation data from a two-pulse spin-echo system (see Fig. 1) the magnitude of two types of signals are obtained:
CONDITION
7 >>Lw
,
F R E E DECAY SIGNAL
F R E E DECAY SIGNAL
I
SPIN-ECHO SIGNAL
Fig. I.-A N.M.R. schematic for two r.f. pulses: I, a plot of R.F. signal amplitude with time; 11, a plot of nuclear signal amplitude with time.
(1) the spin-echo signal which occurs at time 2 7 after the first rf pulse, where T is the time between pulses; and (2) the free decay signals which arise immediately after each of the rf pulses. These signal amplitudes are measured as a' function of 7 . The relaxation times ( 2 ' 2 ) are obtained from spin-echo signal amplitudes, whereas the spin-lattice relaxation times ( T I )are determined from free decay signal amplitudes. A t y ical plot of data for determining the relaxation times ( 8 2 ) for a water vapor adsorption of ( r / m ) = 0.571 is shown in Fig. 2 (curve 11). If this adsorbed system were of a single phase, such a plot of spin-echo amplitude, A(27) us. 7 would be a simple straight line. The resulting slope of curve I1 at large 7 corresponds to one phase of the system, while curve I, which is a plot of the difference between curve I1 and the extrapolated slope of curve 11, corresponds to a second phase of the adsorbing system. I n addition to the availability of relaxation time values, it is also possible, under suitable conditions as in Fig. 2 (see ratio A 0 , / A O 2 )to , obtain the relative amounts of water molecules in the two adsorbing phases. For clarifying purposes, measurements in this paper relate only to 2'2 relaxation data. Generally, the experimental data are not sufficient for an accurate graphical analysis of a two-phase system. I n order to extend the region of accurate evaluations, numerical analyses of the data by means of the Field Research Laboratories' Datatrong computer have been made. No further discussion of the evaluation of relaxation data will be given at this time. However, a theoretical analysis of two-phase nuclear systems and appropriate numerical procedures will be submitted for publication in the near future.
Results Relaxation ( T z )Data.-Relaxation (7'2) data were obtained over the water vapor adsorption range of 0.0431 5 ( z / m ) 5 0.571. Aplot of T 2 v s .( z / m )is shown in Fig. 3. e is the fraction of monomolecular coverage, where the cross-sectional are? of the water molecule was taken to be 10.6 A.2. For convenience of explanation, the curves of Fig. 3 niny be divided into regions of three different (9) h l a n i i f n c t r i r d b y R l w t r o n n t n Corporation of Pasadena. Cali-
fornia,
- .., . .
.
I
0
I
l
l
l
l
l
l
l
l
l
l
l
I
2
3
4
5
6
7
8
9
10
II
I2
T'
Fig. 2.-Transverse
x
IO3-
seconds,
relaxation times for H20 adsorbed on silica gel.
stages of adsorption corresponding t o coverages of (a) 6 > 2, (b) 2 > e > 0.5, (c) e < 0.5.
-,TzI
0
01
F'/m
Fig. 3.-Transverse
I 02
I 03
1
I
0 4
0 5
06
g m H 2 0 / g r n ADSORBENT.
relaxation time for protons of H20 adsorbed on silica gel.
(a) 0 > 2.-For vapor coverage e > 2, above approximately two monoinolecular layers, a distinct resolution of two groups of water molecules is obtained, the groups corresponding to relaxation times T22and Tzl (Fig. 3). This two-phase system distinguishes between protons in the water adsorbed in and adsorbed on the monomolecular layer; Le., the group corresponding t o the relaxation time T f lis to be considered as having closer association with the silica gel surface than the other group corresponding t o the relaxation time TB. I n a qualitative sense, the degrees of freedom of the more tightly hound group of water molecules would be expected
Sept., 1956
ADSORBEDWATERON SILICAGELBY NUCLEAR RESONANCE TECHNIQUES
to be restricted more than the degrees of freedom of the less tightly bound group and, therefore, TZl< Tz2. Of particular interest is the fact that the Tzl group a t high coverage has a relaxation time not much different from that of the water molecules in the monomolecular layer region, 8 = 1. The fact that Trl does increase slowly toward higher coverage is due to the supposition that, as the number of molecules in the outer layers is increased, the random motion of the molecules in the monomolecular layer is increased appreciably. The o')servr!d increase in Tzl as water vapor coverage is incrertsed is expected on the basis that such an increase in random molecular motions would tend t o average out some of the local field interactions which are responsible for relaxing the hydrogen nuclei ; namely, the intra- and inter-molecular magnetic interactions between the hydrogen nuclei of the water molecules. (b) 2 > 0 > 0.5.--In the neighborhood of 0 = 2, as the quantity ( z l m ) decreases, the T22 and Tzl data converge into a single phase system; ie., it is iinpossible to resolve different phases. For ( x / / n ) in the region of approximately 0.12 < (.z/m) < 0.20 (8 .75), the hydrogen nuclei of the mater molecules tend to enjoy the same average local magnetic field interactions during the time of the nuclear relaxation measurements. (c) e < 0 . 5 - A ~ water vapor coverage is reduced below a monomolecular layer, the adsorbed system continues to behave as a single phase system. However, in the region of 0 = 0.5, this single phase system shows a sharp increase in T 2 with further decrease in ( x / m ) . The most dominant local field interaction insofar as T z values of adsorbed water vapor on silica gel will arise from the nuclear spinspin interactions between hydrogen nuclei of the same molecule. However, another, though smaller, contribution to local field will arise from spinspin interactions between hydrogen nuclei of different molecules. This interaction can be effective if there is sufficient proximity between the molecules, e.g., during collision or when molecules are adsorbed on adjacent sites. With the logical assumption that the adsorbed phase c:tn be defined as a mobile film, the rather sudden increast in Tzl as coverage is decreased in the neighborhood of 8 = 0.5 can be qualitatively interpreted as due to a decrease in effective inter-molecular interactions of the adsorbed molecules. Below 8 == 0.5 no appreciable change in TZl is observed; hence, the average local field interactions responsible for nuclear relaxation remains approximately constant at low coverage. This increase in T,, is observed both in the process of desorption and in the process of adsorption. I n addition to the system described by T z l ,it is possible to resolve a second adsorbing phase a t low water vapor coverages. The mater molecules associated with this particular phase described by T t r in Fig. 3 are not a part of any of the other adsorption phases hereto discussed. Because of the short relaxation time, T,' 0.020 second, and the relatively small number of water molecules associated with this phase, it is only st low coverage that this particular phase is subject to relaxation
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1159
measurements. The molecules associated with this phase are bound considerably more tightly t o the surface than are any of the other phases described previously ( Tz'
