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A study of the orientation of adsorbed water molecules on montmorillonite clays by pulsed NMR
A Study of the Orientation of Adsorbed Water Molecules on Montmorillonite Clays by Pulsed NMR 1 D. E. W O E S S N E R A~D B. S. S N O W D E N , JR. Mobil Research and Development Corporation, Field Research Laboratory, Dallas, Texas
Received September 19, 1968 The study of clay water systems by pulsed nuclear magnetic resonance reveals that natural as well as synthetic clays have associated with them a certain amount of oriented water. Direct experimental evidence of oriented water between natural clay platelets is observed for montmorillonite at room temperature. Doublet splittings in the N1V[Rspectra indicate that the water molecules are preferentially oriented by the clay platelets. The occurrence of the doublet splitting was found to be affected by both particle size and temperature. Experimental evidence indicates that the transverse relaxation process above room temperature is not grossly affected by paramagnetic centers arising from the natural iron content of the clay lattice. Although it is more difficult to observe the doublet splitting in samples with a high iron content, these paramagnetie centers do not preclude its occurrence. INTRODUCTION
lite was observed (7) to collapse to a singlet under conditions of high t e m p e r a t u r e and small particle size. This coalescence of the doublet is interpreted (7) as an order-disorder transition in the adsorbed water. Steady state line-shape measurements h a v e also been reported (8) for water adsorbed on synthetic beidellite, synthetic saponite, and some natural montmorillonites. A doublet splitting of the proton N M R resonance of the adsorbed water was observed at low temperatures. This was interpreted as being caused b y relatively motionless water molecules. I n the case of natural montmorillonite an increase in t e m p e r a t u r e causes this low-temperature doublet to converge into a singlet. However, at higher temperatures, a doublet with a relatively small separation was observed in the synthetic d a y s . These authors (8) interpreted their observation of a single line for the interlamellar water in natural montmorillonite as resulting from rapid hydrogen exchange between different water molecules. T h e y also found that, in m a n y instances, the appearance of the doublet is influenced b y the size of the clay particles.
The interactions of water molecules with clay surfaces m a y result in a preferential orientation of the interfacial water in such a manner as to produce a relatively ordered system. The preferential ordering of the syst e m depends on the temperature, pressure, water content, and nature of the exchangeable cations. Transitions between a disordered system and a relatively ordered system can take place when one or more of the above conditions is changed. This t y p e of transition m a y be similar to the l a m b d a transition observed for a m m o n i u m halides (1-5), where a sudden change in volume occurs. Nuclear magnetic resonance ( N M R ) lineshape measurements b y steady state techniques have revealed ordered phases for w a t e r molecules adsorbed on some natural zeolites (6) and natural vermiculite (7). The preferred orientation manifests itself as a doublet in the proton N M R spectrum. The doublet in the N M R spectrum of vermieu1 Presented at the 9th Experimental NMI~ Conference in Pittsburgh, Pennsylvania, February 29-March 2, 1968. Journal of Colloid and Interface Science, Vol. 30, No. 1, May 1969
54
ORIENTATION OF ADSORBED WATER MOLECULES In this work we have applied various pulsed N M R techniques to the study of the occurrence of doublets as well as other properties of the interlamellar water in montmorillonite-water systems. PHYSICAL NATURE OF TIIE EXPERIMENT The clay systems studied are of the montmorillonite type composed of silicate and adsorbed water layers. The pulsed N M R measurement technique (9) provides a sensitive means for detecting the presence of a preferred orientation of the water molecules. This orientation results from the interaction of nonspherical water molecules with the surface of the clay platelets. A surface is known (10, 11) to affect an adsorbed molecule in such a way t h a t the magnetic dipole-dipole interactions within the molecule do not always average to zero. In a bulk liquid, where all molecular orientations are equally probable, these interactions always average to zero. Hence, the observation of a net intramoleeular magnetic dipoledipole interaction indicates that preferred orientations exist for adsorbed molecules (6). The nonzero intramolecular magnetic dipole-dipole interaction causes the observed N M R spectrum of the adsorbed water molecules to appear as a doublet; whereas a null intramoleeular magnetic dipole-dipole interaction, as in the case of bulk liquid water, produces a singlet spectrum. There is also a broadening of each of the individual doublet lines when this interaction is nonzero. In some cases, the broadening is greater than the doublet splitting, leading to an a p p a r e n t single broad line. In many clay-water systems it is possible to convert an apparent sing]et into a doublet by adjusting the temperature. It is desirable to obtain a resolved doublet since the doublet splitting can be used in studying the molecular orientations of the water on the clay surface. One effective means of detecting and measuring this doublet splitting is the utilization of N M R spin-echo techniques. The transverse nuclear magnetization expressions which are applicable to systems of two identical spin ~ nuclei experiencing mutual magnetic dipolar interaction can be calculated. If we neglect relaxation effects the expecta-
55
tion value of transverse magnetization following a 90°-180 ° pulse sequence is (M+(t))
= iMoe -iÈ°(t-2T)
cos
(co't).
[1]
The corresponding transverse magnetization following a 90090 ° pulse sequence is (M+(t)) =
(1/~)iMo{e-i~°°tT .cos
e -¢~°°(t-2:')} [~'(t
-
[2]
2~)].
The upper negative sign in the expression in curly brackets applies to the ease in which there is no phase shift between the two pulses; the lower positive sign applies when a 90 ° phase shift exists between the two pulses. In these equations, M0 is the equilibrium longitudinal nuclear magnetization, coo is the angular resonance frequency, and r is the time between the two pulses. For magnetic dipolar interaction in motionless molecules, J becomes co' = (N)z2~r-3(3 cos20 - 1),
[3]
where r is the length of the straight line connecting the nuclei, and 0 is the angle between this line and the direction of the stationary magnetic field, H0. When Eq. [3] is extended to include molecular motion, the angle 0 becomes time dependent. If the quantity (3 cos 20(t) - - 1) is transformed into the coordinate system fixed on the clay matrix, the following expression is obtained. (3 cos 20(t)
-
1)
= 3It sin 0' sin ~' + m sin 0' cos ¢' ÷ncos0'] 5-
[41
1.
In this equation, g, m, and n are the timedependent direction cosines of the nuclear interaction vector in the matrix coordinate system. The symbols 0' and ~' are two of the Euler angles (¢', 0t, Ct), which describe the orientation of the matrix coordinate system in the coordinate system attached to H0. Fluid motions cause g, m, and n to be time dependent, resulting in fluctuation in co'. When the correlation time (to) is sufficiently short, i.e., ( 3 / 2 ) ~ y ~ r - % ~ << 1, the quantity (3 cos "~0(t) - 1) may be replaced by its time-averaged value. In the ease of axial symmetry about the normal to a surface, Journal of Colloid and Inte,'face Science, Vol. 30, No. I, May 1969
56
WOESSNER AND SNOWDEN I
I
5
10
I
L
15
20
1.0
0
z~
Bt
Fro. 1. The time dependence of the transverse nuclear spin magnetization for a powder sample. TABLE I LOC.4.TIONS OF T H E Z E R O C R O S S I N G S AND T H E M A X I M A AND M I N I M A
FOR (COS (¢o't)>a~
Maximum, minimum, or zero value
Bt
Zero --0.22013 Zero -t-O.21642 Zero - O . 14406 Zero -t-0.15259 Zero -0.11595 Zero :{-0.12399 Zero - O.09992 Zero
2.16 3.61 5.63 7. O1 8.53 10.14 11.88 13.32 14.84 16.45 18.14 19.61 21.14 22.75 24.42
of <¢os(,~'t))~
! •
~0 lS
'
B(3 cos 2 0'
[51
1),
pulsed N M R determinations of T2 is the spin-echo amplitude at time t = 2r. An examination of Eqs. [1] and [2] reveals several characteristics of pulsed N M R signals obtained from m a n y real systems. The equation for 90°-180 ° pulse sequences shows t h a t
< COS (~o t)}av
where =
B = (3/8)~/~r-3<3 cos 2 0t' -
1>.
The quantity 0' is the angle between the direction of H0 and the normal to the surface; and 0't is the angle between the protonproton vector and the normal to the surface. The components of the magnetic dipoledipole interaction which are removed b y averaging would be expected to influence the relaxation times. T h e i m p o r t a n t quantity measured in Journal o Colloid and Interface Science, Vol• 30, No. 1, ?¢[ay 1969
f~/: J0
cos (Bt(3 cos ~ 0' -- 1))
[6]
• sin O'dOp. I t is possible to evaluate the above integral as a function of Bt. Figure 1 shows the functional dependence of ( cos (Jt))a~ on the quantity Bt. I t is possible to evaluate the doublet splitting constant B b y determining the times at which maxima, minima, or zero crossings occur (Table I). When a sample is a mixture of hydrogens,
ORIENTATION
OF ADSORBED
some of which have a doublet splitting and some of which do not, the zero of the transverse magnetization signal is apparently shifted down. This determination of the doublet splitting is complicated by a wide distribution of B values in the sample. A very wide distribution of B values causes the peaks and the valleys of this curve to "wash out" in such a manner that the signal appears to decay monotonically to zero. The detection of doublet splitting by the 90°-180 ° pulse sequence is therefore limited. The quantity (M+(2r)} following the 90 °90 ° pulse sequence decreases monotonically to zero, whereas the corresponding quantity following a 90°-180 ° pulse sequence usually decays more rapidly and may exhibit cyclic variations. This difference in decay rates can be used to detect doublet splitting. Some observed values of the splitting parameter (B) are considerably less than that expected for motionless adsorbed water molecules. This decrease in B results from a partial "averaging-out" of the nuclear magnetic dipole-dipole interaction due to molecular motion. Measurements of TI provide insight into this averaging process since the Tt values depend only on the "motionaveraged-out" component of this interaction. The relaxation rate (1/T~) is related (13) to the temperature-dependent correlation time (re) of the molecular motions as follows :
WATER
MOLECULES
57
T1 relaxation curves into meaningful components. It is therefore convenient to define a quantity (T1)ess as twice the value of the time required for the amplitude of the relaxation curve to decay to e-1/2 times its initial amplitude. In the case of a single-component curve, (T~)eN is the actual T~ value. The advantage of this definition of (T~)~ls is that it emphasizes the first part of the relaxation curve and is related to an effective average relaxation rate. The experimental T2 curves, obtained by use of the 90°-180 ° pulse sequences, and the experimental T3 curves, obtained from 90 °90 ° pulse sequences, are also complex. In order to compare the curves obtained from different samples, we define the following relaxation times for the clay-water samples. The time necessary for the amplitude of the (M+(2r)} curve following the 90°-180 ° pulse sequence to decay to I/e'th of its initial value will be defined as T:* (see Fig. 3). The corresponding decay time of the curve following the 90090 ° pulse sequence is defined as T~*. The quantity T3* is a measure of the width of the individual components of the NMR frequencies, whereas T2* is a function of T~* and the magnitude of the doublet splitting. Generally, T3* is greater than T2*. LO
I
1
0.9 0.8
1/T~ ~-~ r~/(1 + o~o%e2),
[7]
where ~o = the nuclear resonance frequency of the proton expressed in angular units. In the case of water molecules adsorbed on clays, the exact expression for I/T1 in terms of re is too complicated to evaluate. However, the proportionality given in Eq. [7] allows the investigation of the effect of temperature change on relaxation rates. The relaxation rate approaches its maximum value when c00re approaches unity. Since the correlation time increases as the temperature decreases, it is possible to find a value of correlation time from the maximum in relaxation rate observed in the T~-temperature dependence experiment. The T~ measurements reveal multicomponent spin-lattice relaxation (see Fig. 2). Generally, it is not possible to resolve these
e_l/2
c
&0,4
-- 0.3 = N
F 0.2
5
T, miIITseconds
i0
Fro. 2. The time dependence of the longitudinal nuclear magnetization for synthetic calcium-
saponite at +30°C. Journal of Colloid and Inter/ace Science,
Vol. 30, N o . 1, ~ I a y 1969
58
WOESSNER AND SNOWDEN
1.0
0.q 0.8
D.7
-~,,
?
J
~'
\~
I
I
I
II
THE IRON OXIDE CONTENTS OF THE NATURAL
\~ \\
1290°- 180° Pulse Sequence T3. . . . 90° - 90° Pulse Sequence
_
-~0.5 "~ 0.4 ~2~0.107
CLAYS
Clay
FeeO~ (%)
I:leetorite Tatatila montmorillonite Allt Ribheia saponite Otay montmorillonite Belle Fourehe montmorillonite Wyoming montmorillonite
~ l ~'s\\\k
0.6
Nile.-
TABLE
_
0.03 ~ 0.06 ~ 0.66 b 0.86 ~ 4.37 c 4.32 c
~R. E. Grim, "Clay Mineralogy," McGrawHill, New York, 1953, p. 371. hR. C. Mackenzie, Min. Mag. (London) 31,672 (1957). c B. B. Osthaus, "Clays and Clay Technology," Bull. 169, California Dept. of Natural Resources, Division of Mines, San Francisco, 1955, p. 96.
.... U~'~,,T3~:,f611sec
~0.3
0.1 --
0 -0./
I 0. I
I
(?.2
I
I
i
0.3
0.4
0.5
t, milliseconds
FIO. 3. The proton T~ and T~ curves obtained at -40°C from powdered ealeium-hectorite. In cases where the steady-state NMR spectrum is well resolved; the doublet splitting parameter predominates in determining T2* because T3* is much greater than T2*. In this work, we combined the Tx, T2*, and T~* measurements described above to study the rates of motion of water adsorbed on clays as well as the doublet splitting parameter. The clay systems examined are synthetic beidellite, synthetic saponite, natural saponite, natural hectorite, and several natural montmorillonites. Also, these clays are ion-exchanged to obtain clay samples with several different adsorbed cations. In addition to powdered clay samples, several oriented clays are examined. The iron oxide content of the natural clays is also considered (Table II). EXPERIMENTAL
PROCEDURE
A. Clay Samples and Their Preparation. Measurements were made on synthetic beidellite and saponite samples obtained 2 in the sodium adsorbed cation form. The Allt Rib2Obtained from Tem-Pres Research, Inc., State College, Pennsylvania. Journal
o f Colloid a n d I n t e r f a c e Sci ence,
Vol. 30, No. 1, May 1969
hein saponite 3 was measured without any chemical modification. This latter sample was dispersed in distilled water, and centrifuged, and then the clear supernatant liquid was decanted. The exchangeable cations in this sample are approximately 92 %
Ca ++ and 8 % Mg++. The NMR measurements were made on the hectorite which was commercially obtained 4 in the sodium form. The Tatatila montmorillonite5 occurs naturally (14) in the calcium form and was measured as such in the present experiments. The sodium form was also obtained by ion exchange in aqueous sodium chloride followed by dialysis in distilled water. The Otay montmorillonite6 was ion-exchanged in aqueous sodium ehloride and then dialyzed in distilled water. The solids which settled out were discarded. These solids probably consisted of sand and other nonclay materials. 3 This sample was obtained on loan from R. C. Mackenzie of the Macaulay Institute for Soil Research, Aberdeen, Scotland. 4 Obtained from the Baroid Division, National Lead Company, Houston, Texas. This sample was
designated B1-26. 6This clay is from Tatatila, Vera Cruz, Mexico. The sample was provided by the U.S. National Museum in Washington, D. C., catalog number 101836. 6 A.P.I. sample number fornia.
24 from Otay, Cali-
ORIENTATION OF ADSORBED WATER MOLECULES The Belle Fourche montmorillonite7 was prepared as follows. A I weight % suspension of the raw clay was maintained at 76.6°C for 24 hours. After another 24 hours at room temperature, those solids whieh had settled from the suspension were removed and sufficient HC1 was added to make the slurry 0.1 N. The resulting hydrogen elay was then washed with distilled water until the specific conductivity of the filtrate from a 1 weight % suspension had reached a value lower than 10.8 mhos. Four batches prepared in this manner were neutralized with lithium, sodium, potassium, and rubidium hydroxides. Equivalence points were determined by eonductometric titration. Samples of quartz-free montmorillonites with a settling radius less than one-tenth micron were obtained by centrifugation in a Sharples super-centrifuge. The Wyoming montmorillonites was received in the sodium ion-exchange form. The calcium forms of synthetic beidellite, synthetic saponite, hectorite, Otay montmorillonite, and Belle Fourehe montmorillonite were obtained from the sodium forms by ion exchange in aqueous calcium chloride. The products were repeatedly washed in distilled water and centrifuged in an International Centrifuge, ~{odel C 50, until the supernatant liquid remained cloudy aft~er centrifuging for an hour. The powder samples were prepared by freeze-drying the day-water pastes. After equilibrating at 25°C and 51% relative humidity for a month, the approximately 0.8 gm samples were sealed in 12 mm o.d. Pyrex vials. The oriented samples were prepared by allowing dilute aqueous suspensions of the clays to evaporate slowly in flat-bottomed glass containers. The resulting elay "plates" were cut into rectangles 7.5 )< 12 mm and equilibrated at 25°C and 51% relative humidity for a month. These rectangular plates were stacked in square Pyrex vials of 8 mm inside dimension and sealed. After being sealed, the samples were aged in boiling water for 8 hours. This procedure 7Purchased from the Hancock Mud Sales and Service Company, Houston, Texas. s Obtained from the Baroid Division, National Lead Company, Houston, Texas.
59
removed the temperature variation of relaxation times observed when a sample was measured at 25°C, raised to a higher temperature, and then remeasured at 25°C. B. Pulsed N M R Apparatus. The pulsed NMR measurements are made at 25 MHz with the conventional pulsed NMR apparatus (15). A gas-flow cryostat is employed to maintain the sample temperature within 4-0.1°K. The temperature of the sample is measured by means of a thermocouple junction placed in the gas stream near the sample container. The absolute temperature of the sample is known to 4-0.5°K. The T1 values are measured by the conventional 180°-90 ° pulse techniques (15). The transverse magnetization measurements at time t = 2r are made by use of 90°-180 ° and 90o-90 ° pulse sequences (9). In this latter pulse sequence the second 90 ° pulse is derived from a continuous rf signal the phase of which is 90 ° different from that of the initial 90 ° pulse. Experimentally, we obtain this pulse sequence by splitting the output of a crystal oscillator into two channels. One of these channels contains a variable delay line used as a phase shifter. The pulses result from gating the appropriate channel. These gated outputs are added and amplified to form the pulse sequence. An active damper (16) of pulse transients is employed because the T2* values are short. Careful measurements of the signal amplitude following a 90° pulse are made. Comparison of these amplitudes with the corresponding measurement on a water sample of known weight provides a means of obtaining the hydrogen content of the clay samples. This hydrogen content can be expressed as the ratio of the equivalent weight of water to the total weight of the sample. The hydrogen contents measurements are reproducible to within 4-2 %. RESULTS The (T1)e//-temperature dependences of the clays (see Figs. 4-6) show minima at approximately -40°C. For the clays studied the temperature at which these minima occur does not shift appreciably. Below -100°C, the (T1)ez values are relatively insensitive to changes in temperature. The (T1)ef/miniJournal of Colloid and Interface Science, VoL 30, ~o. I, May 1969
60
WOESSNEI~ A N D SNOWDEN 103
i
I
I
i
i
I
I
102
I
I
i
I
[
L
hectorite
102
1(]1
yca/cium - montrnorillonite 100
~
(Tatatila)
L
.~tmorillnnite
101
sap:nite (Nit Ribhe(n) I00 10-1
I 4
F
i0-I
3
~ 5
I I 6 7 103/T (I/°K)
I
to -2
proton (T1).sf obtained from several powdered clay samples. I
I
I
I
i
I
I
syntheticsodium- beWellite ~eidellite
102
"~hectorite "E ~01
' / , ' ° ~ s ousodi ~ ium- ~ o ntmorilionire
~(Tatatila) 10o
10-1
Fro.
I
I
3
5.
4
The
proton (T1)el] samples.
I
5
I
6
I(13[T(If°K)
temperature
I
7
dependence
~ff
Cal(CBie~;Ul-Fomu°n:hme~ ri II°n'te
I
8
l~m. ~. The temperature dependence of the
103
~
I
I
8
9
of
the
obtained from several powdered
Journal of Colloid and Interface Science, ¥ol. 30, No. 1, M a y 1969
I
3
I
4
I
5
I
6 i03 IT(I/°K}
I
7
I
8
F~G. 6. The temperature dependence of the proton (T1).is obtained from several powdered clay samples.
mum values of the sodium-exchanged synthetic clays are greater than those of the corresponding calcium clays, which have a higher water content. The (T1)ese minimum value of the natural saponite is considerably lower than that of the synthetic clays because of the presence of iron in the natural clay lattice (see Table II). In the natural clays, there appears to be a lowering of the (T1)ez minimmn with increasing iron content. There is a good correlation between the low-temperature (T1)eZ' values and the iron content of the clay. (See Fig. 7.) The T2-temperature dependence was also studied. Below -100°C, the T2* values are nearly constant. Figure 8 shows a transverse relaxation curve taken at a low temperature. Initially, the curve depicts a rapid decay of signal with time. After about 30 usec, the signal decay is arrested and becomes fairly constant. Then, at later times, the amplitude of the signal again decays rapidly. The initial decay is due to the protons of the adsorbed water molecules, whereas the remaining pla-
ORIENTATION i00
I
OF ADSORBED
sodium- heetorite
/\
I0
calcium montm~orillonite G
~
Z1.o
-
~
~
~i~,o~i~e~_~_~
.
e"
MOLECULES
61
leered by changes in temperature. On the other hand, the motions of the water molecules are m u c h less restricted and increase rapidly with temperature. Consequently, at high temperatures the adsorbed water molecules have a long relaxation time. The shape of the T2 curve changes radically with temperature above --100°C. The initial portion of the curve in Fig. 9 is due to the relaxation of the immobile lattice protons. The remain-
I
o~--calcium - bectori~e
~
WATER
calciurn- saponi~e (AI[i Ribhein)
\
(Otay)
~
I
~
1.0 0.9
O.L O.Ol
I
calcium - montnlorillonte~.~'~ (Belle Fourche)
O.tO
I. O
0.8 -iO 0.7
percent Fe20B
FIG. 7. The proton (T~)eH measured at --140°C in the powdered clays plotted versus the Fe203 content. teau and subsequent decay are produced b y the clay lattice protons. I f it is assumed t h a t plateau amplitude results from the signal generated by the clay lattice protons, then the amount of adsorbed water can be calculated from the plateau and total signal amplitudes (see Table I I I , in which the water content is expressed as X grams of water per M grams of clay). Since the lattice protons are relatively immobile, their transverse relaxation behavior is unaf-
~ 0.5--
.~ o.B
I
~ 0.4-0.3 g.2 B.I--
I
O.05
[
~, milliseconds
O. LB
O.1O
FIG. 8. The proton T2 curve obtained at -- 140°C from powdered synthetic calcium-saponite.
TABLE I I I T H E W A T E R CONTENTS X / M OF THE C L A Y SAMPLES EXPRESSED AS THE R A T I O OF THE WEIGHT OF W A T E R TO THE W E I G H T OF THE CLAY FOR THE P O W D E R SAMPLES D E R I V E D FROM THE T O T A L PROTON SIGNAL AMPLITUDE AND THE I~ELATIVE P L A T E A U AMPLITUDE
Clay
Sodium-beidellite (synthetic) Calcium-beidellite (synthetic) Sodium-saponite (synthetic) Calcium-saponite (synthetic) Calcium-saponite (Allt Ribhein) Sodium-hectorite Calcium-hectorite Sodium-montmorillonite (Tatatila) C alcium-montmorillonite (Tatatila) Caleium-montomorillonite (Otay) Calcium-montomorillonite (Belle Fourche)
Total proton signal amplitude
Relative plateau amplitude
X
0.125 0.166 0.140 0.197 0.197 0.103 0.175 0.157
0.270 0.220
0.100 0.149 0.113 0.187 0.206 0.085 0.174 0.133 0.253 0.320 0.202
0.227
0.258 0.178
0.275
0.200 0.135 0.240 0.155 0.250 0.110 0.060 0.055
Journal of Colloid and In~rfac~ Science, Vol. 80, No, 1, May 1969
62
WOESSNE~ AND SNOWDEN
1.0
I
i
I
_
0.9
0.8
0.7
~0.5
0.30.2 0. l
0.5
LO
1.5
t, milliseconds
FIG. 9. The proton T~ curve obtained at-t-90°C from powdered synthetic calcium saponite.
T2*-temperature dependence in many of the clay-water systems, there is an abrupt change in the T2*-temperature dependence at the temperature where the T2 curve first shows a doublet structure. As the temperature is increased, the doublet persists and T2* varies less with temperature until a temperature is reached at which the doublet coalesces into a singlet. As the temperature is further increased, the T2*-temperature dependence increases rapidly. The Ts* values of these clays were measured in the temperature range between - 1 0 0 ° and +80°C. The values of T3* are plotted in Figs. 10 to 13. Two types of temperature dependence of T3* are observed. The T~* values of the clays which do not exhibit a T2 doublet splitting increase steadily with temperature as shown in Fig. 14. i00
I
I
I
- - I T3*
I
I
I
I
* i
I
~+c~'9~ ~q,o, "~"~"'"*
\
1~.
I
q
7
l
o calcium- sap0nite (Ai[t Rlbhein) • synthetic calcium- sap0nite
~ * . ~ ...... -,
I
I
12"
- -
100
I
÷ synthetic s°~ium- sap°nite
o syntheticsodium- beideliite • synthetic calcium- beideliite
-.~""~--"~
1o-2
I
1
1
I
!
I
3
4
5
6
7
8
9
1DS[T (I /°K)
10-2/I
3
I
4
I
5
I
I
6 7 I031T{II°K)
[
FIG. 10. The temperature dependences
F~Q. 11. The temperature dependences of the prolGon T2* and T3* obtained f r o m several saponites.
I
8
9
of
the
1°°
proton T2* and T~* obtained from synthetic beidellites. ing portion of the curve is caused by the more labile adsorbed water molecules. In some intermediate region between the two extreme temperature regions described above, the amplitude of the T2 curve shows maxima and minima indicative of doublet structure. As the temperature is increased above --100°C, the relaxation time (T2*) of the adsorbed water in the clay increases rapidly until it is greater than that of the lattice protons. At this temperature, the T2 curve exhibits the doublet structure depicted in Fig. 3. In Figs. 10-13, which show the Journal o/Colloid and Interface Science, Vol. 30, No. 1, M a y 1969
i
I
i ___
\XQ
"~
o calcium - hectorile • calcium m0ntmordlomte
"~.~-q
=
I
T2*
- -
~'~.. ~ ~o--Ik--~-~¢~.
( T3~
(Tatatila)
\ \\
10-1
lo-2
I
5
I
4
I
5
I
5
I
7
los/T (t/°K)
Fro. 12. The temperature dependences of the p r o t o n T2* and T~* obtained f r o m several powdered calcium clays.
ORIENTATION
OF ADSORBED
i0 0
I "k
- T3~, - T2 o sodium - montmorillonite Tatatfla
( ~ o I \ \
",
C,
lO -B
I
I
3
4
o-.,
I 5
I
I
6
7
103/T (I/°K}
FIo. 13. The temperature dependences of the proton T~* and Ts* obtained from several sodium clays. ID 0
I
I
I
---
T3.
- -
T2*
I
o calcium- m0ntmorillonite
(Belle Fourche) • calcium - montrnorillonite (Otayl
"~ i0-1
Io-2
I
3
I
I
4
B
I
WATER
MOLECULES
63
synthetic calcium saponite shown in Fig. 15. Since the lattice protons are unaffected by temperature, the observed shift of the curve with temperature must be independent of them. This temperature dependence is greatest at the higher temperatures where the doublet splitting appears to "wash out." Several examples of this phenomenon are shown in Figs. 10-13. Direct measurements of the doublet splitting parameters obtained from the times at which the maxima and minima occur on the 90°-180 ° pulse sequence curves are given in Table IV. These measurements reveal only a minor decrease in the doublet splitting parameter with temperature. For this reason, the apparent "wash out" of the doublet splitting at high temperatures must be attributed to an increase in the number of water molecules having no doublet splitting. In the clays studied there is evidence t h a t in a small temperature range a transition occurs between oriented adsorbed water molecules which have a doublet splitting and adsorbed water molecules with no doublet splitting. The occurrence of this transi-
I 6
7
103/T (1/°K)
Fze. 14. The temperature dependences of the proton T2* and T~* obtained from several calcium montmorillonites. For these clays, the Ta*/T2* ratio is only slightly greater than unity. The temperature behavior of the T3* values of the clays which exhibit T2 doublet splitting is markedly different. Over much of the temperature range, Ta* is relatively constant and several times greater than T2*. The shape of the curve from which the Ts* values are obtained has been shown above to be indicative of the number of protons having a doublet splitting. Since the depth of the minima and the height of the maxima tend to increase with temperature for the clay-water systems, it is obvious that the number of protons with a doublet splitting decreases with temperature. This is illustrated by the measurements on
o. ~o.
0.
-& 0
O.t
0,2
0,3
0.4 0.5 t, milliseconds
0.6
0.7
0.8
E(~
FXG. 15. The proton 2"2 curves obtained at --30 °, +10 °, and -F90°C from powdered synthetic calcium saponite. Journal of Colloid and Interface Sclenve, Vol. 30, No. 1, M a y 1969
64
WOESSNER AND SNOWDEN TABLE IV THE DOUBLET SPLITTING CONSTANT B~ IN UNITS OF 10 4 I:~ADIANS/SEc~ DETERMINED AT THE INDICATED TEMPERATURES FROM MEASUREMENTS ON POWDERED CLAY SAMPLES Temperature (°C) Sample
Sodium-beidellite (synthetic) Calcium-beidellite (synthetic) Sodium-saponite (synthetic) Calcium-saponite (synthetic) Calcium-saponite (Allt Ribhein) Sodium-heetorite Calcium-hectorite Sodium-montmorillonite (Tatatila) C ~lcium-montmorillonite (Tatatila)
--90 --8O --7C --60 --50 --40 --30 --20 --1(
0
1.9 1.4 -- - - 1 . ~ - 3.9 3.] 3.5 2.( 1.c~ 1.9 1.~ 2.1 i2.~ 2.~ - - 2.71 - 2.3 14.1 1.~ 1.4 1.~ -3.~ ~.0 3 . ~ 4.11 3.6
12 1.111.o I 3? 3.5 3.2 1.7' 1.7 2.' 2.1 2.C - - 2.2 - - - 1.3
1912.0120
1.7 - -
1.3
1.7 - -
1.'
1.6 - -
~10 -I-20
+,~o
Lo
3.1 1.6 2.0 2.1
2.0
The dashed entries indicate ~ measurement was not made. The blank entries indicated no doublet observed. tion in both natural and synthetic clay-water systems suggests t h a t there is no inherent property in natural clays which precludes the existence of a doublet. I n natural vermiculite (7) the transition t e m p e r a t u r e was observed to increase with particle size. This increase in transition temperature is also observed when an oriented sample is compared to a powdered sample. The doublet splitting for oriented sodiumhectorite occurs in the t e m p e r a t u r e region - 7 0 ° C to ~-50°C, whereas t h a t of the powdered sample is between - 80°C and ~- 20°C. Also, the doublet occurs in oriented sodiummontmorillonite (Tatatila) at 25°C; in the powdered sample it is found only in the t e m p e r a t u r e range from - 9 0 ° C to - 4 0 ° C . These observations are understandable if orientation increases the effective particle size of the sample. The generality of the occurrence of the doublet splitting in oriented clay samples is confirmed b y the further observation of doublet splitting in sodium-montmorillonite (Otay) from - 2 0 ° C to + 4 0 ° C and sodiummontmorillonite (Wyoming) between - 3 5 ° C and ~-85°C as well as in oriented Belle Fourche montmorillonites in the lithium, sodium, potassium, and rubidium ion-exchanged forms. The corresponding powdered calcium montmorillonites exhibit no doublet Journal e/Co~lold and Interface Science, ¥ol. 30, No. I, May 1969
splitting. However, untreated 9 natural calcium montmorillonite (Tatatila) exhibits a doublet splitting at - 4 0 ° C but not at + 2 5 ° C . Thus it appears t h a t m a n y natural or synthetic montmorillonite clays exhibit a doublet splitting in s o m e t e m p e r a t u r e regions. The occurrence of the doublet splitting, however, can be obscured b y the condition and t r e a t m e n t of the sample. Examination of the powdered calcium samples listed in Table I I I indicates t h a t the probability of doublet splitting is decreased b y the presence of iron. The doublet splitting was observed in all but the O t a y and Belle Fourche powdered calcium-montmorillonite samples. F r o m Table I I it is evident t h a t these two samples h a v e the highest iron content of a n y of the montmorillonites studied. The doublet splitting in natural vermiculite (7) was found to have a (3 cos ~ 0t -- 1) dependence I° on the orientation of the clay plate in the magnetic field. I n this work measurements were made on several oriented clay samples placed at specific angles (0') in the magnetic field. Measurements of T2 were 9 This sample was merely cut to fit the sample vial from a larger sample obtained courtesy of the U. S. National Museum (see footnote 5). No other treatment of this sample was performed. 10The angle 0' is defined in Eq. [5].
ORIENTATION OF ADSORBED WATER MOLECULES
65
TABLE V
tially the same as the c o r r e s p o n d i n g values
T H E I-IYDROGEN CONTENT EXPRESSED AS TItE RATIO OF TttE TOTAL PROTON SIGNAL AMPLITUDE IN TERMS OE THE EQUIVALENT W E I G H T OF W A T E R (X) TO T/IN TOTAL W E I G H T OF TIlE SAMPLE (m) AND THE DOUBLET SPLITTING PARAMETER B DETERMINED FROM MEASUREMENTS ON THE ORIENTED CLAY SAMPLES
obtained from powdered samples. The T3* values depend on orientation.
x/m
Sample
Lithium-montmorillonite (Belle Fourche) Sodium-mmltmorillonite (Belle Fourehe) Potassium-montmorillonite (Belle Fourche) Rubidium-mongmorillonite (Belle Fourche) Sodium-montmorillonite (Wyoming) Sodium-montmorillonite (0tay) Sodium-montmorillonige (Tatatila) Sodium-hectorite
104B
(radials~ T~om~)" S65)
0.131
1.7
+25
0.137
1.5
+25
0.094
1.7
+25
0.102
1.8
+ 5
0.115
2.2
+25
DISCUSSION
The temperature dependence of the (T~)~ii values of the clays studied indicate that the interlamellar water is in rapid motion at temperatures near and above the (T1)~sI minimum temperature. This is evident from the fact that co0r~is approximately unity at the (Ti)~si minimum. Also the correlation 1.O
I
1
0.9 0.8
0.170
1.7
+10
0.152
2.2
+25
1.7
+25
0.113
9.7
.....
T 3 (90"- 90 ° Pulse Sequence)
- -
T 2 (90° - 180~ Pulse Sequence)
0'= 90" .SL0 o
0.6 .-x_
t\
0.5
\, \\ \
0.4
made on the m a j o r i t y of samples listed in Table V at 0' = 0 °, 55 °, and 90 °. Representative T~ and Ta curves for sodium montmorillonite (Wyoming) are given in Fig. 16 for 0t values of 0 ° and 90 ° . The 55 ° orientation is omitted from the figure because there is no evidence of splitting at this angle. Our T.. results on the oriented samples appear to verify the (3 cos 20' -- 1) orientation dependence. The Ta* values and doublet splitting parameters of the oriented clay samples are given in Tables V and VI. The values of the doublet splitting parameters are essen-
9.3 \
\\
\¶\
0.2
"q\
\\
\Q
0.1 0 -0.1
I 8
0.1
I
l
0.2 t, milliseconds
0.3
FIG. 16. The proton T2 and T3 curves obtained at 25°C from oriented sodium-montmorillonite (Wyoming bentonite).
TABLE VI T H E T3* VALUES OF SEVERAL ORIENTED CLAY SAMPLES MEASURED AT T H R E E D I F F E R E N T VALUES OF THE ORIENTATION 0 p OF TIlE CLAY IDLATES IN THE IV~AGNETIC FIELD Sample
Lithium-montmorillonite (Belle Fourche) Sodium-montmorillonite (Wyoming) Sodium-montmorillonite (Otay) Sodium-montmorillonite (Tatatilu) Sodium-hectorite
Tz*(O), (reset) Ta*(9O°), (reset) Tz*(55°), (reset) Tz*(O°)/Tz*(55°)
0.068 0.086 0.086 0.110 0.185
0.103 0.117 0.122 0.138 0.198
0.127 0.120 0.140 0.178 0.220
0.53 0.72 0.61 0.62 0.84
Journal of Colloid and Interface Science, VoI. 30, No. 1, May 1969
66
WOESSNER AND SNOWDEN
time at any given temperature is similar for the various clays studied because the (T1)~ss minima of the clays occur at approximately the same temperature. The (T1)ess values, however, differ widely with the various clays and seem to be related to the iron content. This relationship to iron content is not as direct at the (T~)eZ minimum temperature as it is at lower temperatures. The low-temperature (T1)ess values are inversely proportionM to the iron content (see Fig. 7). The observed low-temperature (T~)~ss values can be explained by the fact that at low temperatures the dominant relaxation mechanism is a proton magnetic dipole interaction with paramagnetic centers. The presence of iron would, of course, account for these centers in natural clays (17). If this latter relaxation mechanism is not dominant at low temperatures, the (T~)~ssvalues would be expected to be much greater at low temperatures. The complex relationship between iron content and (T1)~z minimum values is due to the competing processes determining the correlation time at the T1 minimum temperatures (17). The correlation time is determined by both the electron relaxation time of the paramagnetic center and the correlation time of the motions of the adsorbed water molecules. The observed T1 values indicate that the interlamellar water is in rapid motion. A simple interpretation (18) of this is that most of the interlamellar water is in a relatively free state, loosely bound with little intermolecular interaction. The interlamellar water is a dynamic adsorption phase with constant destruction and rebuilding of the structure; and thus any ordering of the water molecules has a purely statistieM character (19). Moreover, this description can apply both to the motion of the whole water molecules and also to the motion of their components (19). Ducros (20-22) questions the idea of rapid motion of only uni~ water molecules because this, together with the expected orientation effects, would lead to a doublet resonance contrary to the observations. (Such doublets do occur (6) for zeolitic water in minerals such as chabazite, stilbite, and edingtonite.) His explanation is that the interlamellar water comprises a highly ionJournal o/Collold and In~erfaee Science, Vol. 30, No. I, l~ay 1969
ized network in which the protons exchange at a rate 106 times per second between neighboring molecules. Tarasevich (19) agrees with the above explanation. He states that the existence of protons on the surface of clays has been experimentally established by (a) the spectroscopic determination of the presence of the hydronium ion in vermiculite, and (b) the formation of the ammonium ion by the interaction of ammonia with the adsorbed water on the surface of montmorillonite. He suggests that the presence of mobile protons oa the montmofillonite surface results from strong polarization of the OH bonds of the adsorbed water molecules by the oppositely charged poles of the oxygen layer of the clay and of the exchange cations. The exchange explanation for the absence of the expected doublet at certain temperatures in montmorillonite is certainly possible. An exchange rate comparable to or greater than the measured doublet splitting constant (B) would be necessary to "wash out" the doublet. The B values of the montmorillonite samples studied ranged from 1 X 10+~ to 4 X 10+4 rad/sec. This magnitude of B value indicates fairly rapid proton exchange is taking place in these samples. When the exchange rate decreases sufficiently, the doublet splitting is observed. This simple relationship between the exchange rate, the doublet splitting constant, and the observation of the doublet splitting is complicated by inherent widths of the doublet components. These widths are affected by the presence of paramagnetic centers in the sample; however, this effect is not large at temperatures about and above the T1 minimum temperature. Comparison of Figs. 11 and 12 indicates that calcium hectorite and calcium montmorillonite (Tatatila) have the same T3* values at the high-temperature "plateau" as the synthetic saponite samples, which have a much lower iron content. Above room temperature the T2* values of the high iron content calcium montmorillonite (Belle Fourehe and Otay) are approximately the same as that of the synthetic calcium saponite sample (see Figs. 11 and 14). If iron content were the controlling factor, these T2* and T3* values would be expected to be radically different. Other evidence that fac-
ORIENTATION OF ADSORBED WATER MOLECULES 1.0 0.9 0.8
•
CONCLUSIONS "\
\\ \\ \ ,, \\~k "\ \ \\ \ o~, \\\ \\ \\~
- - - - - - T3 (90°- 90° Pulse Sequence) T2 (90°- 180° Pulse Sequence) o calcium- monimorillenite (Otay) • calclum- montmorillonite ffatatila)
xe\\
0.7
0.6 ~-o.5
~, 0.4
\ \ ;,
"-..
_
o.3
0.2
-..q
--
0.I
-0.1
67
0.1
t,
i 0.2 mil]iseconds
l 0.3
FIe. 17. The proton T2 and T3 curves obtained at -20°C from several calcium-montmorillonites. tors besides paramagnetic centers control transverse relaxation at higher temperatures can be gained by comparison of the T3* values of the sodium and calcium ion-exchanged forms of the montmorillonite (Tatatila). These two forms of the same clay have the same iron content; however, their T3* values in the high-temperature plateau are v e r y different. This again shows iron content does not control the observation of doublet splitting through line broadening. This tentative conclusion is also supported by the following observation. Initially, the T2 curve in Fig. 17 of the high iron content sample decays more rapidly than that of the low iron content sample. At later times this decay sequence is reversed and the T2 curve of the lower iron content sample actually becomes negative and then increases to a positive value. This latter behavior is indicative of a doublet splitting. The results in Fig. 17 do not explain the effect of iron on the doublet splitting in the powdered samples, since the 90°-180 ° pulse sequence used to give the T2 curve effectively erases the magnetic field inhomogeneities introduced by the presence of iron in the clay lattice.
This study indicates that pulsed N M R techniques provide a sensitive means of detecting the doublet splitting of water on clay surfaces. This doublet splitting can be detected by examination of the T2* values obtained from the spin-echo amplitude following the 90°-180 ° pulse sequence or by comparing the T2* values to the Ts* values. These T~* values result from the spin-echo following a 900-90 ° pulse sequence in which there is a 90 ° phase shift between the two pulses. The occurrence of the doublet splitting was found to be affected by both particle size and temperature. Experimental evidence indicates that the transverse relaxation process above room temperature is not grossly affected by paramagnetic centers arising from the natural iron content of the clay lattice. Although it is more difficult to observe the doublet splitting in samples with a high iron content, these paramagnetic centers do not preclude its occurrence. REFERENCES
1. WOESSNER, D. E., AND SNOWDEN,B. S., JR., J. Phys. Chem. 71, 952 (1967). 2. WOESSNER~ D. E., AND SNOWDEN, B. S., JR., J. Chem. Phys. 47, 378 (1967). 3. WOESSNER, D. E., AND SNOWDEN, B. S., JR., J. Chem. Phys. 47, 2361 (1967). 4. WOESSNER~ D. E., AND SNOWDEN,B. S., JR., J. Phys. Chem. 72, 1139 (1968). 5. WOESSNER, D. E., AND SNOWnEN, B. S., JR., J. Colloid and Interfac. Sci. 26,297 (1968). 6. DvcRos, P., Bull. Soc. Franc. Mineral. Crist. 83, 85 (1960). 7. GRAHAM, J., WALKER, G. F., AND ~VEsT, G.
W., J. Chem. Phys. 40, 540 (1964). 8. HECHT, A. M., ~)UPONT, M., AND DUCROS, P.,
Bull. Soc. Franc. Mineral. Crist. 89, 6 (1966). 9. POWLES, J. G., AND MANSFIELD, P., Phys. Letters 2, 58 (1962). 10. WOESSNER, D. E., AND ZIMMERMAN, J. R.,
J. Phys. Chem. 67, 1590 (1963). 11. WOESSNER, D. E., J. Phys. Chem. 70, 1217 (1966). 12. LOOK, D. C., LOWE, I. J., AND NORTHBY, ,}'. A., J. Chem. Phys. 44, 3441 (1966). 13. ABRAGAM, A., "The Principles of Nuclear Magnetism," Chap. 8. Oxford University Press, London, 1961. 14. GRI~, R. E., "Clay Mineralogy." McGrawHill, New York, 1953. Journal of Colloid and Interface ~cience, Vol. 30, No. 1, May 1969
68
WOESSNER AND SNOWDEN
15. B~CaTA, J. C., Gv~rowsKY~ H. S., AND Wo~sSNEn, D. E., Rev. Sei. Instr. 29, 55 (1958). 16. McK.~v, R. A., AND WO~SSN~R, D. E., J. Sci. Instr. 43,838 (1966). 17. RESING, H. A., Advan. Mol. Relaxation Processes 1, 109 (1967). 18. SEA~s, R. E. J., New Zealand J. Sci. 3, 127 (1960). 19. TARASE¥ICIt, YU. I., OVCYIARENKO, F. D.,
Journal of Colloid and Interface Science, Vol. 30, No. 1, May 1969
MATYASI-I,I. V., ZVIANK,V. V., AND TORYANIK, ACL,, Dokl. Akad. N a u k S S S R 156, 926 (1964). 20. DucRos, P., AND DUPONT, iVY., Comp. Rend. 254, 1409 (1962). 21. Ducaos, P., AND DUPONT l-V~., Bull. Ampere (11th Colloq. Ampere) 11, 654 (1962). 22. Dcc~os, P., AND DVPONT M., Bull. Groupe Franc. Argiles 13, 59 (1962).
Received September 19, 1968 The study of clay water systems by pulsed nuclear magnetic resonance reveals that natural as well as synthetic clays have associated with them a certain amount of oriented water. Direct experimental evidence of oriented water between natural clay platelets is observed for montmorillonite at room temperature. Doublet splittings in the N1V[Rspectra indicate that the water molecules are preferentially oriented by the clay platelets. The occurrence of the doublet splitting was found to be affected by both particle size and temperature. Experimental evidence indicates that the transverse relaxation process above room temperature is not grossly affected by paramagnetic centers arising from the natural iron content of the clay lattice. Although it is more difficult to observe the doublet splitting in samples with a high iron content, these paramagnetie centers do not preclude its occurrence. INTRODUCTION
lite was observed (7) to collapse to a singlet under conditions of high t e m p e r a t u r e and small particle size. This coalescence of the doublet is interpreted (7) as an order-disorder transition in the adsorbed water. Steady state line-shape measurements h a v e also been reported (8) for water adsorbed on synthetic beidellite, synthetic saponite, and some natural montmorillonites. A doublet splitting of the proton N M R resonance of the adsorbed water was observed at low temperatures. This was interpreted as being caused b y relatively motionless water molecules. I n the case of natural montmorillonite an increase in t e m p e r a t u r e causes this low-temperature doublet to converge into a singlet. However, at higher temperatures, a doublet with a relatively small separation was observed in the synthetic d a y s . These authors (8) interpreted their observation of a single line for the interlamellar water in natural montmorillonite as resulting from rapid hydrogen exchange between different water molecules. T h e y also found that, in m a n y instances, the appearance of the doublet is influenced b y the size of the clay particles.
The interactions of water molecules with clay surfaces m a y result in a preferential orientation of the interfacial water in such a manner as to produce a relatively ordered system. The preferential ordering of the syst e m depends on the temperature, pressure, water content, and nature of the exchangeable cations. Transitions between a disordered system and a relatively ordered system can take place when one or more of the above conditions is changed. This t y p e of transition m a y be similar to the l a m b d a transition observed for a m m o n i u m halides (1-5), where a sudden change in volume occurs. Nuclear magnetic resonance ( N M R ) lineshape measurements b y steady state techniques have revealed ordered phases for w a t e r molecules adsorbed on some natural zeolites (6) and natural vermiculite (7). The preferred orientation manifests itself as a doublet in the proton N M R spectrum. The doublet in the N M R spectrum of vermieu1 Presented at the 9th Experimental NMI~ Conference in Pittsburgh, Pennsylvania, February 29-March 2, 1968. Journal of Colloid and Interface Science, Vol. 30, No. 1, May 1969
54
ORIENTATION OF ADSORBED WATER MOLECULES In this work we have applied various pulsed N M R techniques to the study of the occurrence of doublets as well as other properties of the interlamellar water in montmorillonite-water systems. PHYSICAL NATURE OF TIIE EXPERIMENT The clay systems studied are of the montmorillonite type composed of silicate and adsorbed water layers. The pulsed N M R measurement technique (9) provides a sensitive means for detecting the presence of a preferred orientation of the water molecules. This orientation results from the interaction of nonspherical water molecules with the surface of the clay platelets. A surface is known (10, 11) to affect an adsorbed molecule in such a way t h a t the magnetic dipole-dipole interactions within the molecule do not always average to zero. In a bulk liquid, where all molecular orientations are equally probable, these interactions always average to zero. Hence, the observation of a net intramoleeular magnetic dipoledipole interaction indicates that preferred orientations exist for adsorbed molecules (6). The nonzero intramolecular magnetic dipole-dipole interaction causes the observed N M R spectrum of the adsorbed water molecules to appear as a doublet; whereas a null intramoleeular magnetic dipole-dipole interaction, as in the case of bulk liquid water, produces a singlet spectrum. There is also a broadening of each of the individual doublet lines when this interaction is nonzero. In some cases, the broadening is greater than the doublet splitting, leading to an a p p a r e n t single broad line. In many clay-water systems it is possible to convert an apparent sing]et into a doublet by adjusting the temperature. It is desirable to obtain a resolved doublet since the doublet splitting can be used in studying the molecular orientations of the water on the clay surface. One effective means of detecting and measuring this doublet splitting is the utilization of N M R spin-echo techniques. The transverse nuclear magnetization expressions which are applicable to systems of two identical spin ~ nuclei experiencing mutual magnetic dipolar interaction can be calculated. If we neglect relaxation effects the expecta-
55
tion value of transverse magnetization following a 90°-180 ° pulse sequence is (M+(t))
= iMoe -iÈ°(t-2T)
cos
(co't).
[1]
The corresponding transverse magnetization following a 90090 ° pulse sequence is (M+(t)) =
(1/~)iMo{e-i~°°tT .cos
e -¢~°°(t-2:')} [~'(t
-
[2]
2~)].
The upper negative sign in the expression in curly brackets applies to the ease in which there is no phase shift between the two pulses; the lower positive sign applies when a 90 ° phase shift exists between the two pulses. In these equations, M0 is the equilibrium longitudinal nuclear magnetization, coo is the angular resonance frequency, and r is the time between the two pulses. For magnetic dipolar interaction in motionless molecules, J becomes co' = (N)z2~r-3(3 cos20 - 1),
[3]
where r is the length of the straight line connecting the nuclei, and 0 is the angle between this line and the direction of the stationary magnetic field, H0. When Eq. [3] is extended to include molecular motion, the angle 0 becomes time dependent. If the quantity (3 cos 20(t) - - 1) is transformed into the coordinate system fixed on the clay matrix, the following expression is obtained. (3 cos 20(t)
-
1)
= 3It sin 0' sin ~' + m sin 0' cos ¢' ÷ncos0'] 5-
[41
1.
In this equation, g, m, and n are the timedependent direction cosines of the nuclear interaction vector in the matrix coordinate system. The symbols 0' and ~' are two of the Euler angles (¢', 0t, Ct), which describe the orientation of the matrix coordinate system in the coordinate system attached to H0. Fluid motions cause g, m, and n to be time dependent, resulting in fluctuation in co'. When the correlation time (to) is sufficiently short, i.e., ( 3 / 2 ) ~ y ~ r - % ~ << 1, the quantity (3 cos "~0(t) - 1) may be replaced by its time-averaged value. In the ease of axial symmetry about the normal to a surface, Journal of Colloid and Inte,'face Science, Vol. 30, No. I, May 1969
56
WOESSNER AND SNOWDEN I
I
5
10
I
L
15
20
1.0
0
z~
Bt
Fro. 1. The time dependence of the transverse nuclear spin magnetization for a powder sample. TABLE I LOC.4.TIONS OF T H E Z E R O C R O S S I N G S AND T H E M A X I M A AND M I N I M A
FOR (COS (¢o't)>a~
Maximum, minimum, or zero value
Bt
Zero --0.22013 Zero -t-O.21642 Zero - O . 14406 Zero -t-0.15259 Zero -0.11595 Zero :{-0.12399 Zero - O.09992 Zero
2.16 3.61 5.63 7. O1 8.53 10.14 11.88 13.32 14.84 16.45 18.14 19.61 21.14 22.75 24.42
of <¢os(,~'t))~
! •
~0 lS
'
B(3 cos 2 0'
[51
1),
pulsed N M R determinations of T2 is the spin-echo amplitude at time t = 2r. An examination of Eqs. [1] and [2] reveals several characteristics of pulsed N M R signals obtained from m a n y real systems. The equation for 90°-180 ° pulse sequences shows t h a t
< COS (~o t)}av
where =
B = (3/8)~/~r-3<3 cos 2 0t' -
1>.
The quantity 0' is the angle between the direction of H0 and the normal to the surface; and 0't is the angle between the protonproton vector and the normal to the surface. The components of the magnetic dipoledipole interaction which are removed b y averaging would be expected to influence the relaxation times. T h e i m p o r t a n t quantity measured in Journal o Colloid and Interface Science, Vol• 30, No. 1, ?¢[ay 1969
f~/: J0
cos (Bt(3 cos ~ 0' -- 1))
[6]
• sin O'dOp. I t is possible to evaluate the above integral as a function of Bt. Figure 1 shows the functional dependence of ( cos (Jt))a~ on the quantity Bt. I t is possible to evaluate the doublet splitting constant B b y determining the times at which maxima, minima, or zero crossings occur (Table I). When a sample is a mixture of hydrogens,
ORIENTATION
OF ADSORBED
some of which have a doublet splitting and some of which do not, the zero of the transverse magnetization signal is apparently shifted down. This determination of the doublet splitting is complicated by a wide distribution of B values in the sample. A very wide distribution of B values causes the peaks and the valleys of this curve to "wash out" in such a manner that the signal appears to decay monotonically to zero. The detection of doublet splitting by the 90°-180 ° pulse sequence is therefore limited. The quantity (M+(2r)} following the 90 °90 ° pulse sequence decreases monotonically to zero, whereas the corresponding quantity following a 90°-180 ° pulse sequence usually decays more rapidly and may exhibit cyclic variations. This difference in decay rates can be used to detect doublet splitting. Some observed values of the splitting parameter (B) are considerably less than that expected for motionless adsorbed water molecules. This decrease in B results from a partial "averaging-out" of the nuclear magnetic dipole-dipole interaction due to molecular motion. Measurements of TI provide insight into this averaging process since the Tt values depend only on the "motionaveraged-out" component of this interaction. The relaxation rate (1/T~) is related (13) to the temperature-dependent correlation time (re) of the molecular motions as follows :
WATER
MOLECULES
57
T1 relaxation curves into meaningful components. It is therefore convenient to define a quantity (T1)ess as twice the value of the time required for the amplitude of the relaxation curve to decay to e-1/2 times its initial amplitude. In the case of a single-component curve, (T~)eN is the actual T~ value. The advantage of this definition of (T~)~ls is that it emphasizes the first part of the relaxation curve and is related to an effective average relaxation rate. The experimental T2 curves, obtained by use of the 90°-180 ° pulse sequences, and the experimental T3 curves, obtained from 90 °90 ° pulse sequences, are also complex. In order to compare the curves obtained from different samples, we define the following relaxation times for the clay-water samples. The time necessary for the amplitude of the (M+(2r)} curve following the 90°-180 ° pulse sequence to decay to I/e'th of its initial value will be defined as T:* (see Fig. 3). The corresponding decay time of the curve following the 90090 ° pulse sequence is defined as T~*. The quantity T3* is a measure of the width of the individual components of the NMR frequencies, whereas T2* is a function of T~* and the magnitude of the doublet splitting. Generally, T3* is greater than T2*. LO
I
1
0.9 0.8
1/T~ ~-~ r~/(1 + o~o%e2),
[7]
where ~o = the nuclear resonance frequency of the proton expressed in angular units. In the case of water molecules adsorbed on clays, the exact expression for I/T1 in terms of re is too complicated to evaluate. However, the proportionality given in Eq. [7] allows the investigation of the effect of temperature change on relaxation rates. The relaxation rate approaches its maximum value when c00re approaches unity. Since the correlation time increases as the temperature decreases, it is possible to find a value of correlation time from the maximum in relaxation rate observed in the T~-temperature dependence experiment. The T~ measurements reveal multicomponent spin-lattice relaxation (see Fig. 2). Generally, it is not possible to resolve these
e_l/2
c
&0,4
-- 0.3 = N
F 0.2
5
T, miIITseconds
i0
Fro. 2. The time dependence of the longitudinal nuclear magnetization for synthetic calcium-
saponite at +30°C. Journal of Colloid and Inter/ace Science,
Vol. 30, N o . 1, ~ I a y 1969
58
WOESSNER AND SNOWDEN
1.0
0.q 0.8
D.7
-~,,
?
J
~'
\~
I
I
I
II
THE IRON OXIDE CONTENTS OF THE NATURAL
\~ \\
1290°- 180° Pulse Sequence T3. . . . 90° - 90° Pulse Sequence
_
-~0.5 "~ 0.4 ~2~0.107
CLAYS
Clay
FeeO~ (%)
I:leetorite Tatatila montmorillonite Allt Ribheia saponite Otay montmorillonite Belle Fourehe montmorillonite Wyoming montmorillonite
~ l ~'s\\\k
0.6
Nile.-
TABLE
_
0.03 ~ 0.06 ~ 0.66 b 0.86 ~ 4.37 c 4.32 c
~R. E. Grim, "Clay Mineralogy," McGrawHill, New York, 1953, p. 371. hR. C. Mackenzie, Min. Mag. (London) 31,672 (1957). c B. B. Osthaus, "Clays and Clay Technology," Bull. 169, California Dept. of Natural Resources, Division of Mines, San Francisco, 1955, p. 96.
.... U~'~,,T3~:,f611sec
~0.3
0.1 --
0 -0./
I 0. I
I
(?.2
I
I
i
0.3
0.4
0.5
t, milliseconds
FIO. 3. The proton T~ and T~ curves obtained at -40°C from powdered ealeium-hectorite. In cases where the steady-state NMR spectrum is well resolved; the doublet splitting parameter predominates in determining T2* because T3* is much greater than T2*. In this work, we combined the Tx, T2*, and T~* measurements described above to study the rates of motion of water adsorbed on clays as well as the doublet splitting parameter. The clay systems examined are synthetic beidellite, synthetic saponite, natural saponite, natural hectorite, and several natural montmorillonites. Also, these clays are ion-exchanged to obtain clay samples with several different adsorbed cations. In addition to powdered clay samples, several oriented clays are examined. The iron oxide content of the natural clays is also considered (Table II). EXPERIMENTAL
PROCEDURE
A. Clay Samples and Their Preparation. Measurements were made on synthetic beidellite and saponite samples obtained 2 in the sodium adsorbed cation form. The Allt Rib2Obtained from Tem-Pres Research, Inc., State College, Pennsylvania. Journal
o f Colloid a n d I n t e r f a c e Sci ence,
Vol. 30, No. 1, May 1969
hein saponite 3 was measured without any chemical modification. This latter sample was dispersed in distilled water, and centrifuged, and then the clear supernatant liquid was decanted. The exchangeable cations in this sample are approximately 92 %
Ca ++ and 8 % Mg++. The NMR measurements were made on the hectorite which was commercially obtained 4 in the sodium form. The Tatatila montmorillonite5 occurs naturally (14) in the calcium form and was measured as such in the present experiments. The sodium form was also obtained by ion exchange in aqueous sodium chloride followed by dialysis in distilled water. The Otay montmorillonite6 was ion-exchanged in aqueous sodium ehloride and then dialyzed in distilled water. The solids which settled out were discarded. These solids probably consisted of sand and other nonclay materials. 3 This sample was obtained on loan from R. C. Mackenzie of the Macaulay Institute for Soil Research, Aberdeen, Scotland. 4 Obtained from the Baroid Division, National Lead Company, Houston, Texas. This sample was
designated B1-26. 6This clay is from Tatatila, Vera Cruz, Mexico. The sample was provided by the U.S. National Museum in Washington, D. C., catalog number 101836. 6 A.P.I. sample number fornia.
24 from Otay, Cali-
ORIENTATION OF ADSORBED WATER MOLECULES The Belle Fourche montmorillonite7 was prepared as follows. A I weight % suspension of the raw clay was maintained at 76.6°C for 24 hours. After another 24 hours at room temperature, those solids whieh had settled from the suspension were removed and sufficient HC1 was added to make the slurry 0.1 N. The resulting hydrogen elay was then washed with distilled water until the specific conductivity of the filtrate from a 1 weight % suspension had reached a value lower than 10.8 mhos. Four batches prepared in this manner were neutralized with lithium, sodium, potassium, and rubidium hydroxides. Equivalence points were determined by eonductometric titration. Samples of quartz-free montmorillonites with a settling radius less than one-tenth micron were obtained by centrifugation in a Sharples super-centrifuge. The Wyoming montmorillonites was received in the sodium ion-exchange form. The calcium forms of synthetic beidellite, synthetic saponite, hectorite, Otay montmorillonite, and Belle Fourehe montmorillonite were obtained from the sodium forms by ion exchange in aqueous calcium chloride. The products were repeatedly washed in distilled water and centrifuged in an International Centrifuge, ~{odel C 50, until the supernatant liquid remained cloudy aft~er centrifuging for an hour. The powder samples were prepared by freeze-drying the day-water pastes. After equilibrating at 25°C and 51% relative humidity for a month, the approximately 0.8 gm samples were sealed in 12 mm o.d. Pyrex vials. The oriented samples were prepared by allowing dilute aqueous suspensions of the clays to evaporate slowly in flat-bottomed glass containers. The resulting elay "plates" were cut into rectangles 7.5 )< 12 mm and equilibrated at 25°C and 51% relative humidity for a month. These rectangular plates were stacked in square Pyrex vials of 8 mm inside dimension and sealed. After being sealed, the samples were aged in boiling water for 8 hours. This procedure 7Purchased from the Hancock Mud Sales and Service Company, Houston, Texas. s Obtained from the Baroid Division, National Lead Company, Houston, Texas.
59
removed the temperature variation of relaxation times observed when a sample was measured at 25°C, raised to a higher temperature, and then remeasured at 25°C. B. Pulsed N M R Apparatus. The pulsed NMR measurements are made at 25 MHz with the conventional pulsed NMR apparatus (15). A gas-flow cryostat is employed to maintain the sample temperature within 4-0.1°K. The temperature of the sample is measured by means of a thermocouple junction placed in the gas stream near the sample container. The absolute temperature of the sample is known to 4-0.5°K. The T1 values are measured by the conventional 180°-90 ° pulse techniques (15). The transverse magnetization measurements at time t = 2r are made by use of 90°-180 ° and 90o-90 ° pulse sequences (9). In this latter pulse sequence the second 90 ° pulse is derived from a continuous rf signal the phase of which is 90 ° different from that of the initial 90 ° pulse. Experimentally, we obtain this pulse sequence by splitting the output of a crystal oscillator into two channels. One of these channels contains a variable delay line used as a phase shifter. The pulses result from gating the appropriate channel. These gated outputs are added and amplified to form the pulse sequence. An active damper (16) of pulse transients is employed because the T2* values are short. Careful measurements of the signal amplitude following a 90° pulse are made. Comparison of these amplitudes with the corresponding measurement on a water sample of known weight provides a means of obtaining the hydrogen content of the clay samples. This hydrogen content can be expressed as the ratio of the equivalent weight of water to the total weight of the sample. The hydrogen contents measurements are reproducible to within 4-2 %. RESULTS The (T1)e//-temperature dependences of the clays (see Figs. 4-6) show minima at approximately -40°C. For the clays studied the temperature at which these minima occur does not shift appreciably. Below -100°C, the (T1)ez values are relatively insensitive to changes in temperature. The (T1)ef/miniJournal of Colloid and Interface Science, VoL 30, ~o. I, May 1969
60
WOESSNEI~ A N D SNOWDEN 103
i
I
I
i
i
I
I
102
I
I
i
I
[
L
hectorite
102
1(]1
yca/cium - montrnorillonite 100
~
(Tatatila)
L
.~tmorillnnite
101
sap:nite (Nit Ribhe(n) I00 10-1
I 4
F
i0-I
3
~ 5
I I 6 7 103/T (I/°K)
I
to -2
proton (T1).sf obtained from several powdered clay samples. I
I
I
I
i
I
I
syntheticsodium- beWellite ~eidellite
102
"~hectorite "E ~01
' / , ' ° ~ s ousodi ~ ium- ~ o ntmorilionire
~(Tatatila) 10o
10-1
Fro.
I
I
3
5.
4
The
proton (T1)el] samples.
I
5
I
6
I(13[T(If°K)
temperature
I
7
dependence
~ff
Cal(CBie~;Ul-Fomu°n:hme~ ri II°n'te
I
8
l~m. ~. The temperature dependence of the
103
~
I
I
8
9
of
the
obtained from several powdered
Journal of Colloid and Interface Science, ¥ol. 30, No. 1, M a y 1969
I
3
I
4
I
5
I
6 i03 IT(I/°K}
I
7
I
8
F~G. 6. The temperature dependence of the proton (T1).is obtained from several powdered clay samples.
mum values of the sodium-exchanged synthetic clays are greater than those of the corresponding calcium clays, which have a higher water content. The (T1)ese minimum value of the natural saponite is considerably lower than that of the synthetic clays because of the presence of iron in the natural clay lattice (see Table II). In the natural clays, there appears to be a lowering of the (T1)ez minimmn with increasing iron content. There is a good correlation between the low-temperature (T1)eZ' values and the iron content of the clay. (See Fig. 7.) The T2-temperature dependence was also studied. Below -100°C, the T2* values are nearly constant. Figure 8 shows a transverse relaxation curve taken at a low temperature. Initially, the curve depicts a rapid decay of signal with time. After about 30 usec, the signal decay is arrested and becomes fairly constant. Then, at later times, the amplitude of the signal again decays rapidly. The initial decay is due to the protons of the adsorbed water molecules, whereas the remaining pla-
ORIENTATION i00
I
OF ADSORBED
sodium- heetorite
/\
I0
calcium montm~orillonite G
~
Z1.o
-
~
~
~i~,o~i~e~_~_~
.
e"
MOLECULES
61
leered by changes in temperature. On the other hand, the motions of the water molecules are m u c h less restricted and increase rapidly with temperature. Consequently, at high temperatures the adsorbed water molecules have a long relaxation time. The shape of the T2 curve changes radically with temperature above --100°C. The initial portion of the curve in Fig. 9 is due to the relaxation of the immobile lattice protons. The remain-
I
o~--calcium - bectori~e
~
WATER
calciurn- saponi~e (AI[i Ribhein)
\
(Otay)
~
I
~
1.0 0.9
O.L O.Ol
I
calcium - montnlorillonte~.~'~ (Belle Fourche)
O.tO
I. O
0.8 -iO 0.7
percent Fe20B
FIG. 7. The proton (T~)eH measured at --140°C in the powdered clays plotted versus the Fe203 content. teau and subsequent decay are produced b y the clay lattice protons. I f it is assumed t h a t plateau amplitude results from the signal generated by the clay lattice protons, then the amount of adsorbed water can be calculated from the plateau and total signal amplitudes (see Table I I I , in which the water content is expressed as X grams of water per M grams of clay). Since the lattice protons are relatively immobile, their transverse relaxation behavior is unaf-
~ 0.5--
.~ o.B
I
~ 0.4-0.3 g.2 B.I--
I
O.05
[
~, milliseconds
O. LB
O.1O
FIG. 8. The proton T2 curve obtained at -- 140°C from powdered synthetic calcium-saponite.
TABLE I I I T H E W A T E R CONTENTS X / M OF THE C L A Y SAMPLES EXPRESSED AS THE R A T I O OF THE WEIGHT OF W A T E R TO THE W E I G H T OF THE CLAY FOR THE P O W D E R SAMPLES D E R I V E D FROM THE T O T A L PROTON SIGNAL AMPLITUDE AND THE I~ELATIVE P L A T E A U AMPLITUDE
Clay
Sodium-beidellite (synthetic) Calcium-beidellite (synthetic) Sodium-saponite (synthetic) Calcium-saponite (synthetic) Calcium-saponite (Allt Ribhein) Sodium-hectorite Calcium-hectorite Sodium-montmorillonite (Tatatila) C alcium-montmorillonite (Tatatila) Caleium-montomorillonite (Otay) Calcium-montomorillonite (Belle Fourche)
Total proton signal amplitude
Relative plateau amplitude
X
0.125 0.166 0.140 0.197 0.197 0.103 0.175 0.157
0.270 0.220
0.100 0.149 0.113 0.187 0.206 0.085 0.174 0.133 0.253 0.320 0.202
0.227
0.258 0.178
0.275
0.200 0.135 0.240 0.155 0.250 0.110 0.060 0.055
Journal of Colloid and In~rfac~ Science, Vol. 80, No, 1, May 1969
62
WOESSNE~ AND SNOWDEN
1.0
I
i
I
_
0.9
0.8
0.7
~0.5
0.30.2 0. l
0.5
LO
1.5
t, milliseconds
FIG. 9. The proton T~ curve obtained at-t-90°C from powdered synthetic calcium saponite.
T2*-temperature dependence in many of the clay-water systems, there is an abrupt change in the T2*-temperature dependence at the temperature where the T2 curve first shows a doublet structure. As the temperature is increased, the doublet persists and T2* varies less with temperature until a temperature is reached at which the doublet coalesces into a singlet. As the temperature is further increased, the T2*-temperature dependence increases rapidly. The Ts* values of these clays were measured in the temperature range between - 1 0 0 ° and +80°C. The values of T3* are plotted in Figs. 10 to 13. Two types of temperature dependence of T3* are observed. The T~* values of the clays which do not exhibit a T2 doublet splitting increase steadily with temperature as shown in Fig. 14. i00
I
I
I
- - I T3*
I
I
I
I
* i
I
~+c~'9~ ~q,o, "~"~"'"*
\
1~.
I
q
7
l
o calcium- sap0nite (Ai[t Rlbhein) • synthetic calcium- sap0nite
~ * . ~ ...... -,
I
I
12"
- -
100
I
÷ synthetic s°~ium- sap°nite
o syntheticsodium- beideliite • synthetic calcium- beideliite
-.~""~--"~
1o-2
I
1
1
I
!
I
3
4
5
6
7
8
9
1DS[T (I /°K)
10-2/I
3
I
4
I
5
I
I
6 7 I031T{II°K)
[
FIG. 10. The temperature dependences
F~Q. 11. The temperature dependences of the prolGon T2* and T3* obtained f r o m several saponites.
I
8
9
of
the
1°°
proton T2* and T~* obtained from synthetic beidellites. ing portion of the curve is caused by the more labile adsorbed water molecules. In some intermediate region between the two extreme temperature regions described above, the amplitude of the T2 curve shows maxima and minima indicative of doublet structure. As the temperature is increased above --100°C, the relaxation time (T2*) of the adsorbed water in the clay increases rapidly until it is greater than that of the lattice protons. At this temperature, the T2 curve exhibits the doublet structure depicted in Fig. 3. In Figs. 10-13, which show the Journal o/Colloid and Interface Science, Vol. 30, No. 1, M a y 1969
i
I
i ___
\XQ
"~
o calcium - hectorile • calcium m0ntmordlomte
"~.~-q
=
I
T2*
- -
~'~.. ~ ~o--Ik--~-~¢~.
( T3~
(Tatatila)
\ \\
10-1
lo-2
I
5
I
4
I
5
I
5
I
7
los/T (t/°K)
Fro. 12. The temperature dependences of the p r o t o n T2* and T~* obtained f r o m several powdered calcium clays.
ORIENTATION
OF ADSORBED
i0 0
I "k
- T3~, - T2 o sodium - montmorillonite Tatatfla
( ~ o I \ \
",
C,
lO -B
I
I
3
4
o-.,
I 5
I
I
6
7
103/T (I/°K}
FIo. 13. The temperature dependences of the proton T~* and Ts* obtained from several sodium clays. ID 0
I
I
I
---
T3.
- -
T2*
I
o calcium- m0ntmorillonite
(Belle Fourche) • calcium - montrnorillonite (Otayl
"~ i0-1
Io-2
I
3
I
I
4
B
I
WATER
MOLECULES
63
synthetic calcium saponite shown in Fig. 15. Since the lattice protons are unaffected by temperature, the observed shift of the curve with temperature must be independent of them. This temperature dependence is greatest at the higher temperatures where the doublet splitting appears to "wash out." Several examples of this phenomenon are shown in Figs. 10-13. Direct measurements of the doublet splitting parameters obtained from the times at which the maxima and minima occur on the 90°-180 ° pulse sequence curves are given in Table IV. These measurements reveal only a minor decrease in the doublet splitting parameter with temperature. For this reason, the apparent "wash out" of the doublet splitting at high temperatures must be attributed to an increase in the number of water molecules having no doublet splitting. In the clays studied there is evidence t h a t in a small temperature range a transition occurs between oriented adsorbed water molecules which have a doublet splitting and adsorbed water molecules with no doublet splitting. The occurrence of this transi-
I 6
7
103/T (1/°K)
Fze. 14. The temperature dependences of the proton T2* and T~* obtained from several calcium montmorillonites. For these clays, the Ta*/T2* ratio is only slightly greater than unity. The temperature behavior of the T3* values of the clays which exhibit T2 doublet splitting is markedly different. Over much of the temperature range, Ta* is relatively constant and several times greater than T2*. The shape of the curve from which the Ts* values are obtained has been shown above to be indicative of the number of protons having a doublet splitting. Since the depth of the minima and the height of the maxima tend to increase with temperature for the clay-water systems, it is obvious that the number of protons with a doublet splitting decreases with temperature. This is illustrated by the measurements on
o. ~o.
0.
-& 0
O.t
0,2
0,3
0.4 0.5 t, milliseconds
0.6
0.7
0.8
E(~
FXG. 15. The proton 2"2 curves obtained at --30 °, +10 °, and -F90°C from powdered synthetic calcium saponite. Journal of Colloid and Interface Sclenve, Vol. 30, No. 1, M a y 1969
64
WOESSNER AND SNOWDEN TABLE IV THE DOUBLET SPLITTING CONSTANT B~ IN UNITS OF 10 4 I:~ADIANS/SEc~ DETERMINED AT THE INDICATED TEMPERATURES FROM MEASUREMENTS ON POWDERED CLAY SAMPLES Temperature (°C) Sample
Sodium-beidellite (synthetic) Calcium-beidellite (synthetic) Sodium-saponite (synthetic) Calcium-saponite (synthetic) Calcium-saponite (Allt Ribhein) Sodium-heetorite Calcium-hectorite Sodium-montmorillonite (Tatatila) C ~lcium-montmorillonite (Tatatila)
--90 --8O --7C --60 --50 --40 --30 --20 --1(
0
1.9 1.4 -- - - 1 . ~ - 3.9 3.] 3.5 2.( 1.c~ 1.9 1.~ 2.1 i2.~ 2.~ - - 2.71 - 2.3 14.1 1.~ 1.4 1.~ -3.~ ~.0 3 . ~ 4.11 3.6
12 1.111.o I 3? 3.5 3.2 1.7' 1.7 2.' 2.1 2.C - - 2.2 - - - 1.3
1912.0120
1.7 - -
1.3
1.7 - -
1.'
1.6 - -
~10 -I-20
+,~o
Lo
3.1 1.6 2.0 2.1
2.0
The dashed entries indicate ~ measurement was not made. The blank entries indicated no doublet observed. tion in both natural and synthetic clay-water systems suggests t h a t there is no inherent property in natural clays which precludes the existence of a doublet. I n natural vermiculite (7) the transition t e m p e r a t u r e was observed to increase with particle size. This increase in transition temperature is also observed when an oriented sample is compared to a powdered sample. The doublet splitting for oriented sodiumhectorite occurs in the t e m p e r a t u r e region - 7 0 ° C to ~-50°C, whereas t h a t of the powdered sample is between - 80°C and ~- 20°C. Also, the doublet occurs in oriented sodiummontmorillonite (Tatatila) at 25°C; in the powdered sample it is found only in the t e m p e r a t u r e range from - 9 0 ° C to - 4 0 ° C . These observations are understandable if orientation increases the effective particle size of the sample. The generality of the occurrence of the doublet splitting in oriented clay samples is confirmed b y the further observation of doublet splitting in sodium-montmorillonite (Otay) from - 2 0 ° C to + 4 0 ° C and sodiummontmorillonite (Wyoming) between - 3 5 ° C and ~-85°C as well as in oriented Belle Fourche montmorillonites in the lithium, sodium, potassium, and rubidium ion-exchanged forms. The corresponding powdered calcium montmorillonites exhibit no doublet Journal e/Co~lold and Interface Science, ¥ol. 30, No. I, May 1969
splitting. However, untreated 9 natural calcium montmorillonite (Tatatila) exhibits a doublet splitting at - 4 0 ° C but not at + 2 5 ° C . Thus it appears t h a t m a n y natural or synthetic montmorillonite clays exhibit a doublet splitting in s o m e t e m p e r a t u r e regions. The occurrence of the doublet splitting, however, can be obscured b y the condition and t r e a t m e n t of the sample. Examination of the powdered calcium samples listed in Table I I I indicates t h a t the probability of doublet splitting is decreased b y the presence of iron. The doublet splitting was observed in all but the O t a y and Belle Fourche powdered calcium-montmorillonite samples. F r o m Table I I it is evident t h a t these two samples h a v e the highest iron content of a n y of the montmorillonites studied. The doublet splitting in natural vermiculite (7) was found to have a (3 cos ~ 0t -- 1) dependence I° on the orientation of the clay plate in the magnetic field. I n this work measurements were made on several oriented clay samples placed at specific angles (0') in the magnetic field. Measurements of T2 were 9 This sample was merely cut to fit the sample vial from a larger sample obtained courtesy of the U. S. National Museum (see footnote 5). No other treatment of this sample was performed. 10The angle 0' is defined in Eq. [5].
ORIENTATION OF ADSORBED WATER MOLECULES
65
TABLE V
tially the same as the c o r r e s p o n d i n g values
T H E I-IYDROGEN CONTENT EXPRESSED AS TItE RATIO OF TttE TOTAL PROTON SIGNAL AMPLITUDE IN TERMS OE THE EQUIVALENT W E I G H T OF W A T E R (X) TO T/IN TOTAL W E I G H T OF TIlE SAMPLE (m) AND THE DOUBLET SPLITTING PARAMETER B DETERMINED FROM MEASUREMENTS ON THE ORIENTED CLAY SAMPLES
obtained from powdered samples. The T3* values depend on orientation.
x/m
Sample
Lithium-montmorillonite (Belle Fourche) Sodium-mmltmorillonite (Belle Fourehe) Potassium-montmorillonite (Belle Fourche) Rubidium-mongmorillonite (Belle Fourche) Sodium-montmorillonite (Wyoming) Sodium-montmorillonite (0tay) Sodium-montmorillonige (Tatatila) Sodium-hectorite
104B
(radials~ T~om~)" S65)
0.131
1.7
+25
0.137
1.5
+25
0.094
1.7
+25
0.102
1.8
+ 5
0.115
2.2
+25
DISCUSSION
The temperature dependence of the (T~)~ii values of the clays studied indicate that the interlamellar water is in rapid motion at temperatures near and above the (T1)~sI minimum temperature. This is evident from the fact that co0r~is approximately unity at the (Ti)~si minimum. Also the correlation 1.O
I
1
0.9 0.8
0.170
1.7
+10
0.152
2.2
+25
1.7
+25
0.113
9.7
.....
T 3 (90"- 90 ° Pulse Sequence)
- -
T 2 (90° - 180~ Pulse Sequence)
0'= 90" .SL0 o
0.6 .-x_
t\
0.5
\, \\ \
0.4
made on the m a j o r i t y of samples listed in Table V at 0' = 0 °, 55 °, and 90 °. Representative T~ and Ta curves for sodium montmorillonite (Wyoming) are given in Fig. 16 for 0t values of 0 ° and 90 ° . The 55 ° orientation is omitted from the figure because there is no evidence of splitting at this angle. Our T.. results on the oriented samples appear to verify the (3 cos 20' -- 1) orientation dependence. The Ta* values and doublet splitting parameters of the oriented clay samples are given in Tables V and VI. The values of the doublet splitting parameters are essen-
9.3 \
\\
\¶\
0.2
"q\
\\
\Q
0.1 0 -0.1
I 8
0.1
I
l
0.2 t, milliseconds
0.3
FIG. 16. The proton T2 and T3 curves obtained at 25°C from oriented sodium-montmorillonite (Wyoming bentonite).
TABLE VI T H E T3* VALUES OF SEVERAL ORIENTED CLAY SAMPLES MEASURED AT T H R E E D I F F E R E N T VALUES OF THE ORIENTATION 0 p OF TIlE CLAY IDLATES IN THE IV~AGNETIC FIELD Sample
Lithium-montmorillonite (Belle Fourche) Sodium-montmorillonite (Wyoming) Sodium-montmorillonite (Otay) Sodium-montmorillonite (Tatatilu) Sodium-hectorite
Tz*(O), (reset) Ta*(9O°), (reset) Tz*(55°), (reset) Tz*(O°)/Tz*(55°)
0.068 0.086 0.086 0.110 0.185
0.103 0.117 0.122 0.138 0.198
0.127 0.120 0.140 0.178 0.220
0.53 0.72 0.61 0.62 0.84
Journal of Colloid and Interface Science, VoI. 30, No. 1, May 1969
66
WOESSNER AND SNOWDEN
time at any given temperature is similar for the various clays studied because the (T1)~ss minima of the clays occur at approximately the same temperature. The (T1)ess values, however, differ widely with the various clays and seem to be related to the iron content. This relationship to iron content is not as direct at the (T~)eZ minimum temperature as it is at lower temperatures. The low-temperature (T1)ess values are inversely proportionM to the iron content (see Fig. 7). The observed low-temperature (T~)~ss values can be explained by the fact that at low temperatures the dominant relaxation mechanism is a proton magnetic dipole interaction with paramagnetic centers. The presence of iron would, of course, account for these centers in natural clays (17). If this latter relaxation mechanism is not dominant at low temperatures, the (T~)~ssvalues would be expected to be much greater at low temperatures. The complex relationship between iron content and (T1)~z minimum values is due to the competing processes determining the correlation time at the T1 minimum temperatures (17). The correlation time is determined by both the electron relaxation time of the paramagnetic center and the correlation time of the motions of the adsorbed water molecules. The observed T1 values indicate that the interlamellar water is in rapid motion. A simple interpretation (18) of this is that most of the interlamellar water is in a relatively free state, loosely bound with little intermolecular interaction. The interlamellar water is a dynamic adsorption phase with constant destruction and rebuilding of the structure; and thus any ordering of the water molecules has a purely statistieM character (19). Moreover, this description can apply both to the motion of the whole water molecules and also to the motion of their components (19). Ducros (20-22) questions the idea of rapid motion of only uni~ water molecules because this, together with the expected orientation effects, would lead to a doublet resonance contrary to the observations. (Such doublets do occur (6) for zeolitic water in minerals such as chabazite, stilbite, and edingtonite.) His explanation is that the interlamellar water comprises a highly ionJournal o/Collold and In~erfaee Science, Vol. 30, No. I, l~ay 1969
ized network in which the protons exchange at a rate 106 times per second between neighboring molecules. Tarasevich (19) agrees with the above explanation. He states that the existence of protons on the surface of clays has been experimentally established by (a) the spectroscopic determination of the presence of the hydronium ion in vermiculite, and (b) the formation of the ammonium ion by the interaction of ammonia with the adsorbed water on the surface of montmorillonite. He suggests that the presence of mobile protons oa the montmofillonite surface results from strong polarization of the OH bonds of the adsorbed water molecules by the oppositely charged poles of the oxygen layer of the clay and of the exchange cations. The exchange explanation for the absence of the expected doublet at certain temperatures in montmorillonite is certainly possible. An exchange rate comparable to or greater than the measured doublet splitting constant (B) would be necessary to "wash out" the doublet. The B values of the montmorillonite samples studied ranged from 1 X 10+~ to 4 X 10+4 rad/sec. This magnitude of B value indicates fairly rapid proton exchange is taking place in these samples. When the exchange rate decreases sufficiently, the doublet splitting is observed. This simple relationship between the exchange rate, the doublet splitting constant, and the observation of the doublet splitting is complicated by inherent widths of the doublet components. These widths are affected by the presence of paramagnetic centers in the sample; however, this effect is not large at temperatures about and above the T1 minimum temperature. Comparison of Figs. 11 and 12 indicates that calcium hectorite and calcium montmorillonite (Tatatila) have the same T3* values at the high-temperature "plateau" as the synthetic saponite samples, which have a much lower iron content. Above room temperature the T2* values of the high iron content calcium montmorillonite (Belle Fourehe and Otay) are approximately the same as that of the synthetic calcium saponite sample (see Figs. 11 and 14). If iron content were the controlling factor, these T2* and T3* values would be expected to be radically different. Other evidence that fac-
ORIENTATION OF ADSORBED WATER MOLECULES 1.0 0.9 0.8
•
CONCLUSIONS "\
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FIe. 17. The proton T2 and T3 curves obtained at -20°C from several calcium-montmorillonites. tors besides paramagnetic centers control transverse relaxation at higher temperatures can be gained by comparison of the T3* values of the sodium and calcium ion-exchanged forms of the montmorillonite (Tatatila). These two forms of the same clay have the same iron content; however, their T3* values in the high-temperature plateau are v e r y different. This again shows iron content does not control the observation of doublet splitting through line broadening. This tentative conclusion is also supported by the following observation. Initially, the T2 curve in Fig. 17 of the high iron content sample decays more rapidly than that of the low iron content sample. At later times this decay sequence is reversed and the T2 curve of the lower iron content sample actually becomes negative and then increases to a positive value. This latter behavior is indicative of a doublet splitting. The results in Fig. 17 do not explain the effect of iron on the doublet splitting in the powdered samples, since the 90°-180 ° pulse sequence used to give the T2 curve effectively erases the magnetic field inhomogeneities introduced by the presence of iron in the clay lattice.
This study indicates that pulsed N M R techniques provide a sensitive means of detecting the doublet splitting of water on clay surfaces. This doublet splitting can be detected by examination of the T2* values obtained from the spin-echo amplitude following the 90°-180 ° pulse sequence or by comparing the T2* values to the Ts* values. These T~* values result from the spin-echo following a 900-90 ° pulse sequence in which there is a 90 ° phase shift between the two pulses. The occurrence of the doublet splitting was found to be affected by both particle size and temperature. Experimental evidence indicates that the transverse relaxation process above room temperature is not grossly affected by paramagnetic centers arising from the natural iron content of the clay lattice. Although it is more difficult to observe the doublet splitting in samples with a high iron content, these paramagnetic centers do not preclude its occurrence. REFERENCES
1. WOESSNER, D. E., AND SNOWDEN,B. S., JR., J. Phys. Chem. 71, 952 (1967). 2. WOESSNER~ D. E., AND SNOWDEN, B. S., JR., J. Chem. Phys. 47, 378 (1967). 3. WOESSNER, D. E., AND SNOWDEN, B. S., JR., J. Chem. Phys. 47, 2361 (1967). 4. WOESSNER~ D. E., AND SNOWDEN,B. S., JR., J. Phys. Chem. 72, 1139 (1968). 5. WOESSNER, D. E., AND SNOWnEN, B. S., JR., J. Colloid and Interfac. Sci. 26,297 (1968). 6. DvcRos, P., Bull. Soc. Franc. Mineral. Crist. 83, 85 (1960). 7. GRAHAM, J., WALKER, G. F., AND ~VEsT, G.
W., J. Chem. Phys. 40, 540 (1964). 8. HECHT, A. M., ~)UPONT, M., AND DUCROS, P.,
Bull. Soc. Franc. Mineral. Crist. 89, 6 (1966). 9. POWLES, J. G., AND MANSFIELD, P., Phys. Letters 2, 58 (1962). 10. WOESSNER, D. E., AND ZIMMERMAN, J. R.,
J. Phys. Chem. 67, 1590 (1963). 11. WOESSNER, D. E., J. Phys. Chem. 70, 1217 (1966). 12. LOOK, D. C., LOWE, I. J., AND NORTHBY, ,}'. A., J. Chem. Phys. 44, 3441 (1966). 13. ABRAGAM, A., "The Principles of Nuclear Magnetism," Chap. 8. Oxford University Press, London, 1961. 14. GRI~, R. E., "Clay Mineralogy." McGrawHill, New York, 1953. Journal of Colloid and Interface ~cience, Vol. 30, No. 1, May 1969
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15. B~CaTA, J. C., Gv~rowsKY~ H. S., AND Wo~sSNEn, D. E., Rev. Sei. Instr. 29, 55 (1958). 16. McK.~v, R. A., AND WO~SSN~R, D. E., J. Sci. Instr. 43,838 (1966). 17. RESING, H. A., Advan. Mol. Relaxation Processes 1, 109 (1967). 18. SEA~s, R. E. J., New Zealand J. Sci. 3, 127 (1960). 19. TARASE¥ICIt, YU. I., OVCYIARENKO, F. D.,
Journal of Colloid and Interface Science, Vol. 30, No. 1, May 1969
MATYASI-I,I. V., ZVIANK,V. V., AND TORYANIK, ACL,, Dokl. Akad. N a u k S S S R 156, 926 (1964). 20. DucRos, P., AND DUPONT, iVY., Comp. Rend. 254, 1409 (1962). 21. Ducaos, P., AND DUPONT l-V~., Bull. Ampere (11th Colloq. Ampere) 11, 654 (1962). 22. Dcc~os, P., AND DVPONT M., Bull. Groupe Franc. Argiles 13, 59 (1962).