Water Orientation in Smectites Using NMR Nutation Experiments

Article pubs.acs.org/JPCC

Water Orientation in Smectites Using NMR Nutation Experiments Marc Fleury*,† and Daniel Canet‡ †

IFP Energies nouvelles, 1 avenue de Bois-Préau, 92852 Rueil-Malmaison, France Institut Jean Barriol (FR CNRS 2843), Université de Lorraine, FST, BP 70239, 54506 Vandeuvre-les-Nancy Cedex, France

‡

ABSTRACT: We observed the arrangement of water molecules when confined in the interlayer space of smectites using low field nutation experiments. Despite the presence of paramagnetic impurities that prevent such observation at high field, we can determine the amount of water in strong interaction with the clay surface, i.e., water molecules which are partially oriented within each platelet. The experiments do not require the orientation of the clay platelets relative to the magnetic field. We present the NMR theory, the signal processing, and the results on a series of 4 homoionic smectites (Na, Ca, Mg, and K) at different relative humidities in order to vary the water content and the basal spacing. In these systems, we found that between 0.5 and 2.5 water molecules per ion are oriented.

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molecules in a one or two layer situation.8,9 In fact, a Pake doublet still exists without macroscopic orientation of the sample and originates from water orientation within each platelet. The experimental Pake doublet is then accounted for by a so-called powder average on the random orientation of the normals to the platelet surface. For silica particles, the water structure at their surface has been studied by spectra obtained using the magic angle spinning technique,10 which evidently excludes any macroscopic orientation of the sample. In synthetic material, it is also possible to analyze the details of the Pake doublet shape compared to the theoretical predictions in order to deduce an exchange rate between oriented and “bulk” water.11 Such difficult analyses confirm a slow exchange between these two populations. In general for natural clay systems, the above-mentioned NMR techniques severely suffer from the presence of paramagnetic impurities that are always present, and in many situations, the measured spectra cannot be exploited. Recently, Trausch and Canet12 proposed using nutation experiments13 as an alternate way to detect and quantify oriented water. Essentially, they proposed to use the relaxation contrast to separate these two populations, bulk or mobile water being associated with similar values of T1 and T2, while oriented water has much shorter T2 values compared to T1, yielding a different signal. Here, we also use nutation experiments but we propose to decompose directly the signal in the time domain into two populations, oriented and mobile, without the need for a strong relaxation contrast. This methodology has been applied on a series of natural smectite samples with different counterions and relative humidities, and therefore, we implicitly vary the amount of oriented water to test the method. These samples have been analyzed in a

INTRODUCTION The dynamics and organization of water molecules are modified close to solid surfaces or in nanoporous system. This has been well-studied in clays1 and in particular swelling clays for which complex interactions exist as a result of high confinement, sheet-like pores with a spacing corresponding to 1 up to 3 water layers and the presence of counterions to compensate the net negative charges of the solid layers and allowing the access of water. Since the water structure is modified up to a distance of 10 Å, all the water molecules are potentially affected when considering the interlayer space of swelling clays such as smectites. Clays have been studied using a large variety of techniques, with differential scanning calorimetry (DSC), infrared spectroscopy (IR), X-ray scattering techniques (WAXS, SAXS), neutron scattering (QUENS), and NMR among the most important. Within the NMR technique, several choices are possible: high field spectroscopy such as MAS NMR spectroscopy2 to study the chemical environment of protons and other nuclei as well as the dynamics of water at short time scales,3,4 low field relaxometry to obtain the dynamic of water at long time scales5 (microseconds and more), and 1H and 2H low field relaxation6 to study accessibility and mobility. Dedicated NMR experiments are able to reveal the restricted mobility of water molecules: if the H−H vector has a fixed orientation relative to the imposed static magnetic field, the dipolar interaction will not be averaged by molecular motions and will produce a strong splitting of resonance peaks in standard spectra. The partial orientation of water molecules has been demonstrated without the need of Fourier transform by Woessner as early as 19687 using a detailed analysis of the free induction magnetization decay and using powder samples in which clay platelets are oriented relative to the magnetic field. With oriented samples, the analysis of the so-called Pake doublet spectra, together with basal spacing measurements, allows deducing the details of the configuration of water © 2014 American Chemical Society

Received: December 3, 2013 Revised: January 30, 2014 Published: January 30, 2014 4733

dx.doi.org/10.1021/jp4118503 | J. Phys. Chem. C 2014, 118, 4733−4740

The Journal of Physical Chemistry C

Article

previous paper14 in which we show that the amount of water in the interlayer space determined by low field NMR is linearly related to the basal spacing measured by SAXS. We first present the theory, then the data processing technique, and finally the results.

The exponential decay in eq 2 is reminiscent of 1/T2 in standard NMR experiments. Let us recall that 1/T2 is generally substituted by 1/T2* in which the inhomogeneity of the B0 field is accounted for. Hence, we shall substitute Rs* to Rs in order to account for the inhomogeneity of the rf field B1. Alternatively to an exponential decay, a Gaussian decay could be envisioned. This particular aspect will be discussed later. Finally, the expression of the observed signal can be written as

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THEORY In the presence of an rf field of amplitude B1 acting along the xaxis of the rotating frame, the nuclear magnetization, starting from its equilibrium value along Z (the direction of the static magnetic field B0) rotates, in the rotating frame, around the x axis. This type of rotation is called nutation. The evolution of the magnetization longitudinal component (MZ) can be derived from the Bloch equations and obeys the following differential equation:13 d2MZ dt 2

+ (R1 + R 2)

S(t ) = S0 exp( −RS*t )cos(2πν1eff t + Φ) + (dc)

where the effective nutation frequency expressed in Hz (ν1eff = ω1eff/2π) is defined in eq 3 and the phase angle Φ is such that tan Φ = −

dM Z + (ω12 + R1R 2)MZ − R1R 2M 0 dt

Here R1 and R2 are the relaxation rates at the field strength B1 = ω1/γ. The appropriate sequence is shown in Figure 1, in which

(dc) ≈ S0

+ exp( −RSt )[a

+b

(6)

Spd(t ) = ⎞ ⎛ t2 S0,pd exp⎜⎜− 2 ⎟⎟ ⎝ 2σpd ⎠

(2)

∫0

π

2 cos(ω1,pd 1 − RD2 ,pd /ω1,pd t )sin α dα

(8)

where Mzst = M 0 ω1eff

(7)

This form can be obtained after subtracting the dc component and rephasing of the data, as explained later. For simplicity, we skip the superscript *. The other type of water in the systems considered here is in strong interaction with the clay surface and/or counterions present in the interlayer space. This type of water is expected to give rise to a Pake doublet which will manifest itself by a distribution of resonance frequencies due to the H−H dipolar interaction which is no longer averaged to zero because water is oriented. Each resonance frequency (and its opposite in the elementary doublet) corresponds to a given orientation defined by the angle α between the H−H vector and the direction of the static magnetic field. In nutation experiments and in a similar way to eq 7, these different orientations have to be taken into account by the following integral.12

M z (t ) = sin(ω1eff t )]

R1R 2 4π 2ν12

Sfree(t ) = S0,freeexp( −RS ,freet )cos(2πν1eff t )

the variable t is the time during which the radio frequency field is applied. The π/2 hard pulse produces a signal which is a measure of MZ. Thus, without rf irradiation prior to this pulse, it is the equilibrium magnetization M0 which is measured. When ω1 is much larger than any relaxation rate (as this is the case in standard experiments), eq 1 reduces to d2Mz/dt2 + (R1 + R2)(dMz/dt) + ω12Mz = 0 leading to the following result: Mz(t) = M0 exp[−((R1 + R2)/2)t]cos(ω1t). Conversely, when ω1 is intentionally set at a low value, a simple biexponential decay is observed if 4ω12 > (R2 − R1)2 (this will not be considered here), or, more interestingly, if 4ω12 < (R2 − R1)2 damped oscillations are observed and accounted for by the complete solution of eq 1: cos(ω1eff t )

(5)

The decay in eq 2 describes the first type of water considered in this work, called free water. Here, for free water, relaxation times are dominated by paramagnetic impurities and the pore size, yielding very small T2 and a ratio T1/T2 close to 2. We will see on an example later that the phase and dc components are theoretically very small and, anyway, subjected to experimental artifacts. As a consequence, for cases where different types of water coexist, we shall approximate eq 4 as follows:

Figure 1. Nutation sequence. The radio frequency field (rf) B1 is applied for a variable time t incremented from 2 up to 2000 μs. The signal is acquired after a π/2 hard pulse. The rf phase is unchanged within the sequence so that the magnetization longitudinal component is actually measured (nutation takes place in the zy plane). The nutation curve is the signal amplitude (denoted S(t) in the text) vs t.

Mzst

R b = − Seff a 2πν1

(dc) stands for the dc component which arises from Mzst. Its expression, given below, has been simplified by assuming R1R2 ≪ ω12, a situation frequently encountered. Anyway, owing to the uncertainty affecting its experimental determination as will be seen later, this is not of great concern.

(1)

=0

(4)

R1R 2 ω12

+ R1R 2

where RD,pd is a difference of relaxation rates defined similarly to eq 3, and σpd is the typical relaxation time decay of this type of water, similarly to RS in eq 3. In 8, it is ω1,pd that depends on the angle α:

RS , D = (R 2 ± R1)/2

= ω1 1 − RD2 /ω12

b = RS(M 0 − Mzst)/ω1eff

a = M 0 − Mzst

ω1,pd =

(3) 4734

ω12 + u 2(3 cos2 α − 1)2

(9)



Article

Here u is half the dipolar splitting in rad s−1. In the case of totally rigid molecules, the splitting u is maximum and can be estimated using:

umax =

3 21 ℏγ 3 2 r

an uncertainty of about 0.05 kHz. Methods for obtaining relaxation times and T1−T2 maps are described elsewhere.14 The measured nutation signal was acquired in quadrature detection using the appropriate number of points (from 100 up to 200) and time interval such as to obtain the steady state magnetization at long times without oscillations. Then the steady state magnetization is subtracted from each real and imaginary part, and these two signals are rephased such as to zero the imaginary part at time t = 0, and normalized. An attempt to use the information contained in the steady state magnetization and phase is presented below. To calculate the parameters of the model, an objective function E which is the sum of the squared difference between the data and model at each experimental time is minimized with respect to S0, RS,free, ω1eff, σpd, RD,pd, u, and f using Matlab routines. Since the derivative of E was calculated analytically, the inversion is very robust and the occurrence of local minima severely reduced compared to finite difference calculations. This is particularly true for u. Indeed, when u increases, ω1,pd increases, whereas (1 − RD,pd2/ω1,pd2)1/2 decreases, and therefore, the product of these two terms is not very sensitive to the variations of u. Hence, even though we found u values in the appropriate range, there is a large uncertainty on this parameter.

(10)

Here r is the H−H internuclear distance and γ the proton gyromagnetic ratio. With typical values of r = 1.6 Å, the frequency splitting u/2π can reach more than 40 kHz. In reality, the orientation of water molecules is far from being rigid but should be viewed as limited in a certain range (as discussed before). Hence, the observed splitting is much smaller, of the order of 1 kHz8,9 and the order parameter u/umax usually does not exceed 0.1. With such typical value, the effect of u in eq 9 becomes significant when ω1 is of same order of magnitude, corresponding indeed to the chosen experimental values. However, as will be seen later, the nutation experiment is not sensitive enough with respect to u to yield precise values of this parameter. Note that we chose a Gaussian decay in eq 8 because it appears more appropriate for data fitting very likely in relation with the solid-like behavior of this type of water. Finally, if free and oriented water are both present, the total normalized signal can be expressed as S(t )/S0 = fSfree(t )/S0,free + (1 − f )Spd(t )/S0,pd

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(11)

TEST EXPERIMENTS Experiments on Doped Water. The above theory has first been checked with experiments involving an aqueous solution of MnCl2 leading to the following water relaxation times T2 = 0.25 ms, T1 = 1.36 ms at 23.7 MHz, that is, RD = 1.63 ms−1 and RS = 2.37 ms−1. Here, we try to analyze not only the effective frequency and relaxation information but also the steady state magnetization and phase information. Hence, the data processing described above was not applied. Four values of the rf field amplitude, B1, were considered. Experimental data have been analyzed according to eq 4. A typical nutation curve (corresponding to the smallest B1) is shown in Figure 2. The perfect agreement between experimental data and theory is remarkable and shows that the choice of an exponential decay in eq 4 is appropriate. Results in terms of the five parameters of eq 4 are gathered in Table 1.

where f is the fraction of free water and Sfree(t)/S0,free and Spd(t)/S0,pd are extracted from eqs 7 and 8, respectively (S0 is just a scaling factor). Altogether, seven parameters need to be deduced from the experimental data: S0, RS,free, ω1eff,σpd, RD,pd, u, and, more importantly, f.

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MATERIAL, NMR MEASUREMENTS, AND DATA PROCESSING The systems studied here are the same as in Fleury et al.,14 and we give here only the necessary information in the context of this work. We used natural montmorillonites clays (the most common smectite) for which ion substitution with Na, Ca, Mg, and K has been performed. The water content was varied by equilibrating the powder samples at 3 different relative humidities (RH = 11, 33, 75) for several weeks. The starting material, not exchanged and containing both Na and Ca counterions, was also equilibrated with ethylene glycol (EG, C2H6O2) at 100% EG relative humidity. The structural formula of these smectites is (Si3.99Al 0.01)(Al1.67Fe 2 +0.02Fe3 +0.01Mg 2 +0.30)O10 (OH)2 Na +0.26(H 2O)n

The NMR experiments have been carried out on a Maran Ultra proton spectrometer from Oxford Instruments with a proton Larmor frequency of 23.7 MHz and a 10 mm probe having a π/2 pulse duration of 2 μs. In these instruments, the (horizontal) static field is generated by permanent magnets and the rf probe is a simple (vertical) solenoid of 12 mm length approximately. Clay powders have been placed in 10 mm outer diameter glass tubes filled up to a height of 8−10 mm to maximize filling factor. The nutation experiments have been performed with γB1/2π values ranging usually from 2.2 kHz up to 6.6 kHz, generated by decreasing the rf power delivered to the probe. At a given rf power, γB1/2π frequencies were determined by measuring the length of a 90° pulse, resulting in

Figure 2. Experimental data points (dots) and fitted nutation curve (line) according to eq 4. Notice the dephasing and the dc component. 4735



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field is responsible for this discrepancy. Indeed, the dc component corresponds to the application of the B1 field for a long time with its amplitude occurring by its square (see eq 6) and thus by its variance. As a consequence, we are faced with an effective rf field amplitude which is smaller than its nominal value. Actually, the disagreement increases with the amplitude of the rf field confirming this feature. As seen from Table 2, a similar effect occurs for the phase angle Φ and its expected value given by eq 5. The immediate consequence of the discrepancies of Table 2 is that we cannot rely on the dc component value for determining any relaxation parameter. Another consequence, more detrimental, is the erroneous value of Φ when extracted from the nutation experiment. This can be easily understood by considering S(0) (the amplitude of the nutation curve extrapolated at t = 0) which provides the following relationship between Φ and (dc).

Table 1. Parameters Deduced from the Experimental Data Involving an Aqueous Solution of MnCl2a ν1 (kHz)

ν1eff (kHz)

S0

RS* (ms−1)

Φ (rad)

(dc)

2.95 3.71 5.65 7.68

2.46 3.33 5.41 7.50

1123.0 1181.8 1514.8 1236.7

2.38 2.43 2.55 2.73

−0.83 −0.67 −0.36 −0.35

67.2 53.9 50.3 36.4

a

The second column has been calculated from eq 3. The measured RS at 23.7 MHz is 2.37 ms−1.

The parameter which is determined without ambiguity is evidently ν1eff. In principle, it should easily lead to RD, and hence to both R1 and R2, but in practice ν1 cannot be determined with enough accuracy to provide these values. Of course the difference between ν1eff and ν1 increases when the B1 amplitude (or equivalently ν1) decreases. For instance, it can be seen that this difference is indisputably significant for ν1 = 2.95 kHz. Furthermore, RS* increases when ν1 increases. This feature is very likely due to B1 inhomogeneity which is known to increase with the B1 amplitude (the strength of a B1 gradient is roughly proportional to the B1 amplitude). It can be noticed that here the true value of RS is almost retrieved for ν1 = 2.95 kHz. In practice, it will be recommended to extrapolate RS* to ν1 = 0 in order to access RS reliably. As expected, the dc component (dc) and the phase angle Φ decrease when ν1 increases. It turns out however that the value of the dc component is not in agreement with eq 6, as seen in Table 2. It is very likely that, again, the inhomogeneity of the B1

cos Φ =

(dc)/S0 expt

(dc)/S0 theor

Φ(rad) expt

Φ(rad) theor

2.95 3.71 5.65 7.68

0.06 0.045 0.033 0.029

0.009 0.005 0.002 0.001

−0.83 −0.67 −0.36 −0.35

−0.15 −0.11 −0.07 −0.05

(12)

From eq 12, it can be seen that an erroneous value for (dc) will entail an erroneous value for Φ. From this test, we conclude that, among all the parameters which can be extracted from a nutation curve, only the effective frequency and the decay rate are reliable. Still, the latter parameter is obtained accurately only by inversion. It provides half the sum of the two relaxation rates R2 and R1 while the effective nutation frequency is linked to their difference. Unfortunately, because of the rf field inhomogeneity, the phase angle of the nutation curve and the dc component cannot provide any additional information. These conclusions are at least valid for our probe, and may vary from one apparatus to another. Effect of B1 Field. In the presence of free and oriented water, one may think at first glance that it will be difficult to extract seven parameters from a single curve, since the phase and dc information cannot be used. This difficulty can however be lifted if one chooses the appropriate B1 field in order to generate a contrast between the two populations. Taking parameters found later after fitting the data, this is clearly evidenced in Figure 3; for a frequency of 2.22 kHz, oriented water has a separate contribution at low frequency as well as a

Table 2. Experimental and Theoretical (Equation 6) Values of the Direct Current Component (Rescaled Relative to S0) and Experimental and Theoretical (Equation 5) Values of the Phase Angle Φ ν1 (kHz)

S(0) − (dc) S0

Figure 3. Frequency content of each contribution of the two population model in eq 11. The total simulated nutation curve is shown in the inset. Parameters: ν1 = 2.22 kHz (a); ν1 = 3.65 kHz (b); u/2π = 1 kHz, f = 0.5, 1/RS,free = 0.2 ms, σpd = 0.1 ms, 1/RD,pd = 0.08 ms. Oriented water has a strong contribution at low frequency compared to free water if the frequency ν1 is chosen appropriately. 4736



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As anticipated, the results of the nutation experiments indicate a significant fraction of oriented protons due to molecules present in the interlayer space. We see first a perfect match of the data with the model in the time domain (Figure 5). More importantly, we see a clear separation of the

large amplitude at zero frequency. When increasing only up to 3.65 kHz, both populations have a similar contribution in the same bandwidth, and therefore, the inversion will not give meaningful results. Hence, several B1 fields must be tested in order to obtain the typical signature of oriented water. Therefore, we show later both the data fitting in the time domain and the results in the frequency domain to highlight the frequency contrast. Experiment on a Glycolated Clay Sample. As a further test of the method, we studied the case of a glycolated smectite (i.e., a clay sample equilibrated with ethylene glycol). This is a well-known technique to discriminate between swelling and nonswelling clays. Indeed for smectites, the basal spacing observed by XRD lies in this case in a very narrow range (16.9− 17.1 Å), independently of the counterions, and the intercalated EG molecules are configured in a bilayer arrangement.15 Since we have a large molecule whose size is similar to the interlayer spacing, we expect to have a reduced rotational mobility and consequently a specific signature in terms of relaxation time. The T1−T2 map (Figure 4) indicates two proton populations

Figure 5. Nutation experiment (ν1 = 2.22 kHz) on a glycolated smectite sample (same sample as in Figure 4) in the time domain (dots) and after Fourier transform (dashed line). The theoretical curve in the time domain is obtained from eq 11 with the following fitted values: f = 0.53, ω1eff/ω1 = 0.98, 1/RS,free = 0.410 ms, σpd = 0.095 ms, 1/RD,pd = 0.095 ms, and u/2π = 0.69 kHz. In the frequency domain, the lowest curve indicates the contribution of oriented EG molecules.

contribution of the two molecule populations in the frequency domain. Indeed, oriented EG molecules contribute mostly at frequencies smaller than 2 kHz, yielding a significant amplitude at zero frequency in the spectra while free molecules contribute mostly around ν1 = 2.22 kHz. The fitted values are f = 0.53, ω1eff/ω1 = 0.98, 1/RS,free = 0.410 ms, σpd = 0.095 ms, 1/RD,pd = 0.095 ms, and u/2π = 0.69 kHz. First, the above fraction is comparable to the one obtained from relaxation time measurements at 23.7 MHz. Second, 1/RS,free is comparable to the relaxation time T2 of EG free molecules measured at 23.7 MHz (∼0.3 ms). The typical decay time of oriented molecules falls below 0.1 ms, as in the T1−T2 map (note that the multiexponential model which has been assumed to build this map may not be valid for the interlayer water and this may alter the value of f deduced from this experiment).

Figure 4. T1−T2 map for a smectite sample equilibrated with ethylene glycol (Na and Ca counterions). The different lines indicate T1/T2 = 1, 1.5, and 8.

with two different T1/T2 ratios (1.5 and 8). Note that this is not found when such smectite is saturated with water, in which case fast exchange is occurring.14 Hence, the population with the highest relaxation time is a signature of free EG molecules outside the interlayer space. The second population with the lowest relaxation time (and at the limit of resolution of T2 measurements) is interpreted as EG molecules inside the interlayer space, and since it has a very small T2 ( Mg > Ca > K) appears naturally, hence the capability of a given ion to capture water molecules in their nearest environment. We find mostly that between 0.5 and 2.5 water molecules per ion are oriented, and this appears an appropriate order of magnitude. This is however smaller than the solvation capability of ions (e.g., six water molecules for the Ca cation in a bilayer state). The results described above inevitably raise the issue of the mobility of molecules and exchange between the two populations (oriented or not). It should be emphasized first that we cannot reproduce the data with either only a population of free water or only a population of oriented water. Therefore, the observation of these two populations in the interlayer space (there is no external water for these values of relative humidity14) is by itself an indication of a slow exchange at the time scale of the measurement (typically the lifetime of the magnetization). From a detailed analysis of the shape of Pake doublets in a synthetic clay, an exchange rate of 1500 s−1 was estimated,11 and such order of magnitude is compatible with our observation. The exchange issue is also inherently present in other works; for example on vermiculite samples,2 both populations are present and also exchange at a rate compatible with the observation of two populations. On silica surfaces,10 a Pake doublet is surprisingly clearly detected despite the absence of strong confinement. The orientation of water molecules seems also in contradiction with their relatively high mobility, despite the high confinement; indeed from the NMR relaxation point of view, interlayer and external water exchange sufficiently fast such that no clear bimodal relaxation time distribution is observed at the millisecond time scale.6,14 From neutron-based techniques, measured translational diffusion coefficients (in a time window up to 1 ns) are still high,16 on the order of 4 × 10−10 m2/s. But here, we detect a strong perturbation of the reorientation of water molecules, a fast intramolecular process superposed to slower molecular motions.17

Figure 6. Typical example of a nutation curve (ν1 = 2.22 kHz) on a smectite sample (Mg counterions, relative humidity 33%). Data in the time domain are indicated by dots and after Fourier transform by a dashed line in the frequency domain. The line in the time domain is obtained from eq 11 with the following fitted values: f = 0.43, ω1eff/ω1 = 0.98, 1/RS,free =0.235 ms, σpd = 0.127 ms, 1/RD,pd = 0.092 ms, and u/ 2π = 0.50 kHz. In the frequency domain, the lowest curve indicates the contribution of oriented molecules.

Table 3. Results Obtained on the Different Smectitesa

a

RH (%)

f

water content (mg/g)

11 33 75

0.45 0.43 0.54

144 200 257

11 33 75

0.42 0.39 0.56

120 204 259

11 33 75

0.53 0.4 0.56

144 200 257

11 33 75

0.54 0.46 0.66

38 74 128

oriented water (mg/g)

Smectite Mg 79 114 118 Smectite Ca 70 124 114 Smectite Na 68 120 113 Smectite K 17 40 44

1/RS,free (ms)

σpd (ms)

0.144 0.235 0.356

0.083 0.127 0.152

0.132 0.326 0.394

0.081 0.142 0.153

0.164 0.195 0.394

0.065 0.094 0.154

0.137 0.183 0.256

0.059 0.088 0.105

The total water content is obtained from Fleury et al.14

values (ν1eff, RD,pd, and u) are not really meaningful because they depend on the accuracy on ν1, as detailed before. In general, we found ω1eff/ω1 > 0.93 (as expected due to a T1/T2 ratio not larger than 2), 1/RD,pd between 0.04 and 0.09 ms, and u/2π between 0.4 and 1kHz. Note that the T1−T2 map technique does not give interesting results in these situations due to its limited capability of resolving weak relaxation contrast. The frequency splitting u is consistent with expected values, corresponding to order parameters in the range 0.017− 0.045, indicating large orientation fluctuations. These values are slightly smaller than usually found at high field using synthetic 4738



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Figure 7. Measured amount of oriented water for different ions and relative humidity. The same data are plotted using different units: mg of oriented water per g of dry clay, number of oriented water molecules per structural unit (left), and number of oriented water molecules per ion (right).

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Combining 2H NMR Spectroscopy and Multiscale Numerical. J. Phys. Chem. C 2007, 5441−5453. (4) Porion, P.; Faugère, A. M.; Delville, A. Analysis of the Degree of Nematic Ordering Within Dense Aqueous Dispersions of Charged Anisotropic Colloids by 23Na NMR Spectroscopy. J. Phys. Chem. B 2005, 109, 20145−20154. (5) Delville, A.; Letellier, M. Structure and Dynamics of Simple in Heterogeneous Conditions: An NMR Study of the Clay-Water Interface. Langmuir 1995, 11, 1361−1367. (6) Montavon, G.; Guo, Z.; Tournassat, C.; Grambow, B.; Le Botlan, D. Porosities Accessible to HTO and Iodide on Water-Saturated Compacted Clay Materials and Relation with the Forms of Water: A Low Field Proton NMR Study. Geochim. Cosmochim. Acta 2009, 73, 7290−7302. (7) Woessner, D. E.; Snowden, B. A Study of the Orientation of Adsorbed Water Molecules on Montmorillonite Clays by Pulsed NMR. J. Colloid Interface Sci. 1969, 54−68. (8) Sanz, J.; Herrero, C. P.; Serratosa, J. M. Arrangement and Mobility of Water in Vermiculite Hydrates Followed by 1H NMR Spectroscopy. J. Phys. Chem. B 2006, 110, 7813−7819. (9) Tenório, R. P.; Engelsberg, M.; Fossum, J. O.; da Silva, G. J. Intercalated Water in Synthetic Fluorhectorite Clay. Langmuir 2010, 26, 9703−9709. (10) Totland, C.; Steinkopf, S.; Blokhus, A. M.; Nerdal, W. Water Structure and Dynamics at a Silica Surface: Pake Doublets in 1H NMR Spectra. Langmuir 2011, 27, 4690−4699. (11) Trausch, G.; Canet, D.; Turq, P. Decomposition of a NMR Signal Comprising a Single Resonance in Exchange with a Pake Doublet: Application to the Analysis of 1H NMR Spectra of Water Molecules Embedded in Synthetic Clays. Chem. Phys. Lett. 2008, 456, 262−267. (12) Trausch, G.; Canet, D. 1H Nutation Experiments Under LowAmplitude Radiofrequency Fields. Quantitative Analysis of a Complex NMR Signal Arising from Water in Clays and Including a Pake Doublet. Magn. Reson. Imaging 2007, 25, 525−528. (13) Trausch, G.; Canet, D.; Cadène, A.; Turq, P. Separation of Components of a 1H NMR Composite Signal by Nutation Experiments Under Low Amplitude Radio-Frequency Fields. Application to the Water Signal in Clays. Chem. Phys. Lett. 2006, 433, 228−233. (14) Fleury, M.; Kohler, E.; Norrant, F.; Gautier, S.; M’Hamdi, J.; Barré, L. Characterization and Quantification of Water in Smectites with Low-Field NMR. J. Phys. Chem. C 2013, 117, 4551−4560. (15) Lagaly, G.; Ogawa, M.; Dekany, I. Clay Mineral Organic Interactions. In Handbook of Clay Science; Bergaya, F., Theng, B. K. G., Lagaly, G., Eds.; Elsevier: New York, 2006; Vol. 1, pp 309−377. (16) Michot, L. J.; Delville, A.; Humbert, B.; Plazanet, M.; Levitz, P. Diffusion of Water in a Synthetic Clay with Tetrahedral Charges by

CONCLUSION We detected and quantified oriented water in smectites using nutation experiments. The limitations of nutation experiments to reveal relaxation times are first derived from experiments on doped water with a short relaxation time (T2 = 0.25 ms); we conclude that only the effective frequency and the decay rate can be determined accurately by inversion. In clays, a two component model allows determining the amount of oriented water in the interlayer space. When both oriented and free water are present, the B1 field must be chosen such as to produce a contrast between these two components in the frequency domain. The first component, free water, has an oscillatory behavior with a relatively narrow frequency bandwidth. The second component is a Pake doublet model taking into account the orientation of H−H vector relative to the B1 field and containing mostly very low frequency components. The five parameters of the model are determined by inversion in the time domain. We found that the amount of oriented water is ranked according to the ionic potential and that it does not vary above a relative humidity of 33%, corresponding to a bilayer arrangement of water molecules in the interlayer space.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: marc.fl[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Istvan Furo for useful discussion during the preparation of this paper, and Eric Kohler for suggesting the experiments involving ethylene glycol.

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REFERENCES

(1) Michot, L. J.; Villieras, F.; Francois, M.; Bihannic, I.; Pelletier, M.; Cases, J. M. Water Organisation at the Solid-Aqueous Solution Interface. C. R. Geosci. 2002, 334, 611−631. (2) Sanz, J. Nuclear Magnetic Resonance Spectroscopy. In Handbook of Clay Science; Bergaya, F., Theng, B. K. G., Lagaly, G., Eds.; Elsevier: New York, 2006; Vol. 1, pp 919−938. (3) Porion, P.; Michot, L. Structural and Dynamical Properties of the Water Molecules Confined in Dense Clay Sediments: a Study 4739



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Combined Neutron Time-of-Flight Measurements and Molecular Dynamics Simulations. J. Phys. Chem. C 2007, 111, 9818−9831. (17) Halle, B.; Wennerströ m, H. Interpretation of Magnetic Resonance Data from Water Nuclei in Heterogeneous Systems. J. Chem. Phys. 1981, 75.

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Water Orientation in Smectites Using NMR Nutation Experiments

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