Influence of layer charge on hydration properties of synthetic octahedrally-charged Na-saturated trioctahedral swelling phyllosilicates

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Influence of layer charge on hydration properties of synthetic octahedrally-charged Na-saturated

trioctahedral swelling phyllosilicates

Doriana Vinci, Baptiste Dazas, Eric Ferrage, Martine Lanson, Valérie Magnin, Nathaniel Findling, Bruno Lanson

To cite this version:

Doriana Vinci, Baptiste Dazas, Eric Ferrage, Martine Lanson, Valérie Magnin, et al.. Influence of layer charge on hydration properties of synthetic octahedrally-charged Na-saturated trioctahedral swelling phyllosilicates. Applied Clay Science, Elsevier, 2020, 184, pp.105404. �10.1016/j.clay.2019.105404�.

�hal-02510330�

Influence of Layer Charge on Hydration Properties of Synthetic Octahedrally-Charged Na-

1

Saturated Trioctahedral Swelling Phyllosilicates

2

3

Doriana Vinci

^a,b

, Baptiste Dazas

^c

, Eric Ferrage

^c

,

4

Martine Lanson

^b

, Valérie Magnin

^b

, Nathaniel Findling

^b

, Bruno Lanson

^b,

*

5

6 7

a

Univ. Bari Aldo Moro, Dipartimento Scienze Terra & Geoambientali, Bari, Italy

8

b

Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, F-38000

9

Grenoble, France

10

c

Univ. Poitiers, CNRS, IC2MP, F-86000 Poitiers, France

11

12

* Corresponding author: bruno.lanson@univ-grenoble-alpes.fr

13

14

ABSTRACT

15

Smectite hydration impacts dynamical properties of interlayer cations and thus the transfer and fate

16

of H

2

O, contaminants, and nutriments in surficial environments where this ubiquitous clay mineral

17

is often one of the main mineral components. The influence of key crystal-chemical parameters,

18

such as the amount of charge or the presence of fluorine, rather than hydroxyl groups, in smectite

19

anionic framework, on hydration, organization of interlayer species, and related properties has been

20

described for tetrahedrally substituted trioctahedral smectites (saponites). Despite the ubiquitous

21

character of octahedrally substituted smectites, that make most of the world bentonite deposits,

22

the influence of charge location on smectite hydration properties has not received similar attention.

23

A set of octahedrally substituted trioctahedral smectites (hectorites) with a common structural

24

formula Na

x

Mg

6-x

Li

x

Si

8.0

O

20

(OH)

4

and a layer charge (x) varying from 0.8 to 1.6 was thus synthesized

25

hydrothermally. The distribution of charge-compensating Na

⁺

cations and of associated H

2

O

26

molecules was determined experimentally from the modeling of X-ray diffraction data obtained

27

along water vapor desorption isotherms. Consistent distributions of charge-compensating cations

28

and of associated H

2

O molecules were also computed from GCMC simulations as a function of layer

29

charge. Interlayer H

2

O contents [2.5-5.5 and 8.0-10.0 H

2

O molecules per O

20

(OH)

4

for 1W and 2W

30

hydrates, respectively] are similar in all Na-saturated smectite samples, independent of the location

31

and amount of their layer charge. In contrast to synthetic saponite, for which stability of most

32

hydrated layers was increased by increasing layer charge, the stability of synthetic hectorite

33

hydrates is only marginally affected by layer charge. Consistently, the layer-to-layer distance of Na-

34

saturated hectorite 2W (and 1W) layers is independent of layer charge (15.10-15.65 Å and 12.0-

35

12.6 Å, respectively). The contrasting hydration behavior of synthetic Na-saturated saponite and

36

hectorite is likely due to different electrostatic attraction between the 2:1 layer and interlayer

37

cation, the charge undersaturation of O atoms at the surface of hectorite 2:1 layer being more

38

diffuse compared to saponite. Combined with previous results on saponites, the present data and

39

sample set provides key constraints to assess the validity of force fields simulating clay-water

40

interactions for an unmatched variety of smectite with contrasting locations and amounts of layer

41

charge deficits.

42 43

1. Introduction

44

Smectites are clay minerals ubiquitous in both terrestrial and marine surface environments

45

and in sedimentary rocks where they are often one of the main mineral components. Smectite,

46

whose TOT or 2:1 layers include two tetrahedral sheets sandwiching an octahedral one, represents

47

the most hydrated pole of phyllosilicates and is a family name for all expandable varieties thereof.

48

Vermiculites have a similar layer structure and differ from smectites by their higher layer charge

49

deficit (1.2-1.8 and ≈0.4-1.2 e

^-

per O

20

(OH)

4

in vermiculites and smectites, respectively (Guggenheim

50

et al., 2006). This charge contrast does not appear to induce significant modification of hydration

51

behavior or of interlayer species distribution (Dazas et al., 2015). Both families of swelling

52

phyllosilicates are thus hereafter jointly referred to as “smectite”. Isomorphic substitutions by

53

lower-valence cations occur in either tetrahedral or octahedral sheets and induce a permanent

54

negative charge of the 2:1 layer, which is compensated for by the presence of hydrated cations in

55

the interlayer space. Hydration of interlayer cations controls in turn smectite colloidal behavior and

56

crystalline swelling (Mooney et al., 1952; Sato et al., 1992; Bérend et al., 1995; Boek et al., 1995;

57

Cases et al., 1997; Young and Smith, 2000; Marry and Turq, 2003), and plays a pivotal role in the

58

physical and chemical behaviors of surface environments and sedimentary rocks where smectite is

59

abundant (Burst, 1969; Bouma et al., 1977; Matsuda et al., 2004; Takahashi et al., 2005; Bittelli et

60

al., 2012). More specifically, smectite hydration strongly impacts dynamical properties of interlayer

61

cations and thus the transfer and fate of H

2

O, contaminants, and nutriments (Malikova et al., 2006,

62

2007, 2010; Marry et al., 2011; Tertre et al., 2011a, 2011b; Michot et al., 2012), but also smectite

63

ability to produce nanocomposites for a variety of applications (Fowden et al., 1983; Sinha Ray and

64

Okamoto, 2003; Pavlidou and Papaspyrides, 2008; Guo et al., 2018).

65

The stepwise expansion of the layer-to-layer distance with increasing water activity has long

66

been reported in the literature as the intercalation of 1, 2, and, less frequently, 3 planes of H

2

O

67

molecules in smectite interlayers, leading to the well-known 1W (d

001

= 11.6–12.9 Å), 2W (d

001

=

68

14.9–15.7 Å), and 3W (d

001

= 18–19 Å) hydration states, in addition to the dehydrated one (0W,

69

d001

= 9.6–10.2 Å – Nagelschmidt, 1936; Bradley et al., 1937; Mooney et al., 1952; Norrish, 1954;

70

Walker, 1956) As reported in the early work of Hendricks, the continuous change of apparent layer-

71

to-layer distance actually arises from the random occurrence of these different hydration states

72

interstratified in smectite crystallites (Hendricks, 1938). XRD profile modeling procedures were thus

73

developed to quantify smectite hydration heterogeneity as a function of relative humidity (Cases et

74

al., 1992, 1997; Bérend et al., 1995). By fitting positions and profiles of 00l reflections over a large

75

angular range, relative proportions of the different layer types can be determined together with the

76

basal distance and water content of the different layer types (Ferrage et al., 2005b, 2016). This

77

approach was used to determine the influence of the amount of charge, a key crystal-chemical

78

parameter, in tetrahedrally substituted trioctahedral smectites (saponites) on their hydration

79

behavior, the organization of their interlayer species, and related properties (Ferrage et al., 2010,

80

2011; Jimenez-Ruiz et al., 2012; Michot et al., 2012; Dazas et al., 2015). The presence of fluorine,

81

rather than hydroxyl groups, in smectite anionic framework and its influence on hydration was also

82

studied in both synthetic saponite and hectorite (Dazas et al., 2013). The much lower contents of

83

interlayer H

2

O molecules in the latter varieties reported in these studies challenged the analogy

84

between hydroxylated and fluorinated varieties. The contrast is enhanced for 2W hydrate [2.8-6.8

85

H

2

O molecules per O

20

(OH)

4

in fluorinated varieties compared to 8.0-10.0 in hydroxylated ones –

86

Dazas et al., 2013, 2015] thus limiting the presence of interlayer H

2

O molecules to the cation

87

hydration sphere.

88

Location of the layer charge deficit in the tetrahedral or octahedral sheets of the 2:1 layer is

89

another key structural parameter whose influence on smectite hydration has been insufficiently

90

described despite the ubiquitous character of octahedrally substituted smectites, that make most

91

of the world bentonite deposits (Kaufhold et al., 2002; Christidis and Huff, 2009). Natural

92

tetrahedrally and octahedrally charged smectites display indeed contrasting hydration behavior, the

93

former exhibiting higher hydration heterogeneity (Ferrage et al., 2005b, 2007). To avoid a possible

94

bias linked to the formation conditions of natural samples, the present study aimed at characterizing

95

the hydration of hectorite, an octahedrally substituted trioctahedral smectite, using synthetic

96

compounds obtained under the same conditions as previously studied saponites (Ferrage et al.,

97

2010; Dazas et al., 2015). Specifically, interlayer H

2

O contents were determined for a set of four

98

synthetic Na-saturated smectites with layer charges ranging from ∼0.8–1.6 e

^-

per O

20

(OH)

4

using

99

H

2

O vapor (de)sorption isotherms. The relative proportions of the different smectite hydrates and

100

the distribution of H

2

O molecules and charge-compensating Na

⁺

cations within smectite interlayers

101

were then determined from X-ray diffraction profile modeling for all smectites along the H

2

O vapor

102

desorption isotherm. Hectorite hydration was determined as a function of the octahedral layer

103

charge and compared to that of saponite samples having similar amounts of layer charge, and

104

similar interlayer cation. Finally, Monte Carlo simulations were performed in the Grand Canonical

105

ensemble to account for the respective distributions of interlayer species in both octahedrally and

106

tetrahedrally substituted Na-saturated smectites (hectorites and saponites, respectively) over a

107

wide compositional range.

108 109

2. Materials and Methods

110

2.1 Sample preparation 111

Hectorites with a common structural formula Na

x

Mg

6-x

Li

x

Si

8.0

O

20

(OH)

4

and a layer charge (x)

112

varying from 0.8 to 1.6 were synthesized hydrothermally from gel precursors prepared from solid

113

Na

2

NO

3

and Li

2

CO

3

carbonates, MgNO

3

solution; and TEOS as sources of Na, Lin Mg, and Si,

114

respectively (Hamilton and Henderson, 1968). Syntheses were performed for 4 weeks at a

115

temperature of 400 °C and a water pressure of 1 kbar in an externally heated Morey-type pressure

116

vessel with an internal silver tubing, starting from

≈2.5 g of gel and ≈33 mL H2O to reach the the required 117

pressure

. Synthesized hectorites are hereafter referred to as Hect-OH-x, x being the amount of layer

118

charge deficit per O

20

(OH)

4

. Layer charge of synthesized smectites was assessed from the

119

stoichiometry of gel precursors, after verifying the absence of impurity in the reaction products by

120

X-ray diffraction (XRD). After synthesis, all samples were Na-saturated by being shaken mechanically

121

in a 1 mol L

⁻¹

aqueous solution of NaCl (

≈3.0 g of synthesized smectite in ≈250 mL of solution)

for 24 h

122

before separation of the solid fraction by centrifugation (

≈15 000 g for 20 min)

. NaCl remaining in

123

excess was then removed by rinsing the solid three times for 24 h in deionized water (Siemens

124

UltraClear, 18.2 MΩ cm

⁻¹

). Finally, the solid fraction was recovered by centrifugation .

125

2.2 Experimental Sample Characterization 126

Water vapor sorption isotherms were collected volumetrically at 25 °C on hectorite powders

127

using a BELSORP-max instrument from BEL Japan. Lyophilized hectorite aliquots (∼100 mg) were

128

initially outgassed at 150 °C for 24 h under a residual pressure of 10

⁻⁵

−10

⁻⁴

Pa. Specific surface area

129

was determined for all Na-saturated samples using the same instrument and the BET method (Table

130

S1). For all samples, oriented slides were prepared by pipetting an aqueous clay suspension on glass

131

slides and drying it at room temperature. The amount of deposited material was systematically

132

weighed (

≈4.0-10.0 mg.cm^-2)

. XRD patterns were then recorded using a Bruker D8 diffractometer

133

operated at 40 kV and 40 mA and equipped with a MHG Messtechnik humidity controller coupled

134

to an Anton Paar CHC+ chamber. Intensities were measured with a SolXE Si(Li) solid-state detector

135

(Baltic Scientific Instruments) for 6 s per 0.04 °2θ step over the 2−50 °2θ Cu Kα angular range.

136

Divergence slit, the two Soller slits, the antiscatter, and resolution slits were 0.3°, 2.3°, 0.3°, and

137

0.1°, respectively. Samples were kept at 23 °C in the CHC+ chamber during data collection, whereas

138

the desired relative humidity (RH) value was maintained by using a constant flow of mixed

139

dry/saturated air. RH was continuously monitored with a hygrometer (uncertainty of

∼2% RH) 140

located close to the sample along the whole isotherm. Samples were first equilibrated at ∼95% RH

141

for 6-12 h before starting data collection. Along the desorption isotherm, samples were then

142

equilibrated for 2 h at each new RH value before collecting XRD data, hydration stability being

143

systematically checked by recording again the low-angle reflection after collection of a complete

144

XRD pattern.

145

2.3 XRD profile modeling 146

The algorithms developed initially by Drits, Sakharov, and co-workers (Drits and Sakharov,

147

1976; Drits and Tchoubar, 1990; Sakharov and Lanson, 2013) were used to fit experimental XRD

148

profiles using a trial-and-error approach. This approach relies on a user-driven optimization of a

149

structure model to fit XRD data. Although it precludes taking into account correlations between

150

parameters, owing to the lack of covariance matrix calculation, this approach has proven successful

151

for the structural characterization of defective and interstratified lamellar structures (Drits and

152

Tchoubar, 1990; Lanson, 2011). Instrumental and experimental factors such as horizontal and

153

vertical beam divergences, goniometer radius, and length and thickness of the oriented slides were

154

measured and introduced in the calculations without further adjustment. The mass absorption

155

coefficient (μ*) was set to 45 cm

²

g

⁻¹

(Moore and Reynolds, 1997). The lognormal distribution of

156

coherent scattering domain (CSD) sizes along the c* axis was characterized by a maximum value, set

157

to 80 layers and by a mean value (N – Drits et al., 1997). The z-coordinates of atoms constituting the

158

2:1 (or TOT) smectite layer were adjusted from those of phlogopite (ICSD #95346). The interlayer

159

configuration for trihydrated smectite layers (3W – d

001

= 18–19 Å) includes planes of interlayer H

2

O

160

molecules located at

≈

1.25 and ≈3.5 Å from the interlayer midplane which is hosting interlayer

161

cations (Dazas et al., 2014). Debye-Waller parameters proposed by these authors for these two

162

planes of H

2

O molecules (Bwat) were used (Dazas et al., 2014). For bihydrated layers (2W – d

001

=

163

14.9−15.7 Å), the interlayer configuration used includes one plane of H

2

O molecules on each side of

164

the interlayer midplane hosting cations (Ferrage et al., 2005a). This model is characterized by the

165

distance (Δd2W) between the interlayer midplane and each of the planes of H

2

O molecules. Debye-

166

Waller parameter was refined for these H

2

O molecules to account for their positional disorder

167

(Ferrage et al., 2005a). For monohydrated layers (1W – d

001

= 11.6−12.9 Å) both cations and H

2

O

168

molecules were located at the interlayer midplane. A similar configuration, devoid of interlayer H

2

O

169

molecules, was used for dehydrated layers (0W – d

001

= 9.6−10.2 Å). The overall interlayer water

170

content at a given RH was constrained by water vapor (de)sorption isotherms and not refined,

171

although the distribution of this global content between the different types of hydrated layers was

172

refined. Layer-to-layer distance, preferred orientation of the particles in the sample (σ*), N, and

173

Δd2W were considered as variable parameters. In addition, because of the weak bonds between

174

adjacent smectite layers, layer thickness was allowed to deviate from the average d001 value. This

175

cumulative deviation from the periodicity is described as a “disorder of second type” (Guinier, 1964;

176

Drits and Tchoubar, 1990), and accounted for by introducing a variance parameter σ

z

(Ferrage et al.,

177

2005a).

178

The fitting procedure is described in detail elsewhere (Ferrage et al., 2005a, 2005b, 2010).

179

Briefly, a main structure, periodic (that is with only one layer type) if possible, was used to reproduce

180

as much as possible of the data. If necessary, additional contributions to the diffracted intensity

181

were introduced to account for the misfit. Up to four interstratified structures, each with a different

182

composition (relative proportion of the different layer types), were necessary to reproduce some of

183

the XRD patterns because of the observed hydration heterogeneity. Interstratification of the

184

different types of hydrated layers was systematically random in all contributions to the diffracted

185

intensity. The use of several interstratified structures to fit the data does not reflect the actual

186

presence of different populations of particles in the sample, and layers with a given hydration state

187

were thus assumed to have identical parameters (layer thickness and interlayer configuration) in all

188

interstratified structures to reduce the number of variable parameters. Similarly, N and σ

z

189

parameters were considered identical for all interstratified structures used to fit a given pattern,

190

although these parameters were varied as a function of RH. The relative proportions of the different

191

interstratified structures and their compositions were also adjusted.

192

2.4 Grand Canonical Monte Carlo Simulations 193

Monte Carlo simulations were performed in the grand canonical ensemble to assess the

194

content of interlayer H

2

O and the equilibrium state of interlayer cations and H

2

O molecules (Adams,

195

1974, 1975; Allen and Tildesley, 2017). Simulations were performed at both 20 and 80% RH (1W and

196

2W hydration states, respectively) and 300 K using a homemade program (Delville, 1991; Rinnert et

197

al., 2005; Michot et al., 2007, 2012; Porion et al., 2008; Jimenez-Ruiz et al., 2012). The rigid

198

simulation box included three interlayers and related 2:1 layers that encompassed six and four unit

199

cells along the a and b directions, respectively. The chosen −a/3 layer displacement led to a face to

200

face configuration for ditrigonal cavities and allowed matching closely experimentally determined

201

H

2

O contents, whereas the actual value of this displacement did not influence significantly the

202

distribution of interlayer species along the c* axis. The layer-to-layer distance was set from

203

experimental d

001

values (Table 1) for Na

x

Mg

6-x

Li

x

Si

8.0

O

20

(OH)

4

compositions with x = 0.8, 1.0, 1.3,

204

and 1.6.

205

The SPC/E water model and a version of the ClayFF force field slightly modified by a cross-

206

validation of atomic density profiles derived from GCMC simulations and X-ray diffraction pattern

207

modeling were used to simulate clay−water interactions (Cygan et al., 2009; Ferrage et al., 2011).

208

GCMC simulations encompassed 5000 blocks, each with 10 000 elementary steps allowing

209

stabilization of interlayer H

2

O molecule content. At each step, one of the smectite interlayers was

210

selected randomly, and with an equal probability an attempt was made (i) to remove a randomly

211

chosen H

2

O molecule, (ii) to add an additional H

2

O molecule in a random configuration, or (iii) to

212

move a randomly chosen cation or H

2

O molecule. During calculations, an Ewald summation was

213

used in addition to the three-dimensional minimum image convention to ensure the convergence

214

of the electrostatic energy (Heyes, 1994). A final run including 5000 blocks of 5000 steps was

215

performed to generate the equilibrium properties of interlayer species and to derive three-

216

dimensional radial cation−H

2

O distribution functions.

217 218

3. Results and discussion

219

3.1 Water vapor desorption isotherms 220

Experimental isotherms are typical for sorption of polar molecules on swelling phyllosilicates

221

(Rouquerol et al., 2013), the two main plateaus at ≈6 and ≈12 mmol of H

2

O molecules per gram of

222

dry smectite corresponding to domains dominated by 1W and 2W smectite layers (Sing et al., 2008),

223

respectively, similar to saponite (Michot et al., 2005). Hectorite water vapor desorption isotherms

224

are essentially independent of the layer charge deficit. Capillary sorption of H

2

O molecules when

225

approaching water vapor saturation prevents determining the upper end of the 2W stability

226

domain. On the other end, this domain extends down to ∼60% RH for all samples. In all cases, H

2

O

227

content ranges from ∼15.0 to

∼11.0 mmol g⁻¹

over this domain (Fig. 1), and the 2W-to-1W transition

228

spreads over ∼20% RH. 1W stability domain thus extends from ∼40 % RH to

∼5% RH, with H2

O

229

contents ranging from ≈7.0 to ≈4.5 mmol g

⁻¹

over this domain for all hectorites (Fig. 1).

230

3.2 XRD profile modeling 231

XRD data and corresponding optimum fits are shown for all samples as a function of RH in Figs.

232

2-4. The relative proportions of the different contributions to the calculated intensity and their

233

compositions (proportions of the different layer types interstratified – See Fig. S1 for details) are

234

reported in Tables 2-4. These Tables also include the main structural parameters of crystals (N),

235

layers (layer-to-layer distance and its fluctuation – σ

z

), and interlayer H

2

O content and organization,

236

with values of Δd2W and B

wat

parameters (Ferrage et al., 2011; Dazas et al., 2015).

237

3.2.1 Hect-1.0-OH 238

Four contributions are used to fit XRD data recorded at 95% RH (Fig. 2), the main one being

239

an essentially periodic 2W smectite with d

001

= 15.55 Å. A second contribution (contribution #3 in

240

Table 2) contains 2W layers (‘layer’

is used hereafter instead of ‘unit structure’ to describe the assemblage 241

of the layer plus the interlayer that defines the unit-cell motif)

interstratified with minor 1W layers to fit

242

the high-angle asymmetry of smectite 001 reflection. The other two contributions contain 2W

243

interstratified with 3W layers to fit the low-angle asymmetry of this reflection. From 90 to 60% RH,

244

XRD patterns are fitted with three contributions (Fig. S1), the main one including mostly (≥ 97%) 2W

245

layers interstratified with 1W layers, whereas the second contribution keeps a minor content of 3W

246

layers down to ≈70% RH to account for the low-angle asymmetry of smectite 001 reflection. In the

247

third minor contribution, 2W and 1W layers are interstratified in similar proportions. The overall

248

proportion of 1W layers remains about stable (<5%) over the 95-70% RH range and increases

249

significantly (≈24%) at 55% RH as evidenced by the enhanced high-angle asymmetry of smectite 001

250

reflection, the low-angle asymmetry of 2W smectite 005 reflection, and the onset of 1W smectite

251

002 reflection at ≈14 °2θ (Fig. 2). At 55% RH, four contributions are thus necessary to fit XRD data

252

(Fig. S1). The proportion of the main contribution, overwhelmingly dominated by 2W layers, strongly

253

decreases from 67 to 41% (at 60 and 55% RH, respectively). The three additional contributions,

254

containing 2W and 1W layers in contrasting proportions (≈3:1, 1:1, and ≈1:4 ratios), account for 10%

255

or more of the scattered intensity. Dehydrated smectite layers are first detected at 55% RH,

256

although in minimal proportion (≈1% – Table 2). Down to 45% RH, the coexistence in the

257

experimental patterns of reflections typical for both 2W and 1W smectite (e.g. peaks both at ≈16

258

and ≈14 °2θ) requires four contributions to fit the data (Fig. S1). Over the 55-45% RH interval, the

259

main contribution changes to an essentially periodic 1W smectite (contribution #4 in Table 2),

260

whereas the proportion of the contribution dominated by 2W layers decreases (41 and 4% at 55

261

and 45% RH, respectively) together with its content of 2W layers (95 and 82% at 55 and 45% RH,

262

respectively). The proportion of 1W layers consistently increases with decreasing RH in the other

263

two contributions. From 40% RH, experimental XRD patterns are dominated by 1W smectite (d

001

=

264

12.56 Å), and the main contribution consistently is an essentially periodic 1W smectite (contribution

265

#4 in Table 2). At 40% RH, the other two contributions to the diffracted intensity contain both 2W

266

and 1W layers. With decreasing RH, the contribution dominated by 2W layers vanishes (contribution

267

#1 not detected at 35% RH), whereas the proportion of 2W layers steadily decreases to 0 at 25%

268

RH. At 25 and 20% RH, the two contributions to the diffracted intensity contain only 1W and 0W

269

layers (Fig. S1). The proportion of 0W layers exceeds 10% only at 5% RH, however (≈18% – Table 2),

270

the decreased proportion of the essentially periodic 1W smectite (49% at 5% RH) indicating the

271

onset of smectite dehydration (Fig. S1).

272

3.2.2. Hect-1.3-OH 273

At 95% RH, four contributions are needed to fit XRD data (Fig. 3), the main contribution

274

(contribution #1 in Table 3) being an essentially periodic 2W smectite. Two additional contributions

275

account for the low- and high-angle asymmetry of smectite 001 reflection (contributions #3 and #4,

276

respectively, in Table 3). These two contributions are also dominated by 2W layers but contain a

277

significant (>25%) proportion of 3W and 1W layers, respectively. A fourth minor contribution is

278

dominated by 3W layers at these high RH conditions. Decreasing RH value to 65%, the overall

279

contributions to the diffracted intensity remain stable. The most significant evolution is the

280

decreasing proportion of 3W layers in contributions #2 and #3, and the increase of 1W layer

281

proportion in contribution #4 (Table 3). Contribution #2, which is dominated by 3W layers, is not

282

detected at RH values lower than 75% RH. At 60% RH, the onset of 2W-to-1W transition is

283

characterized by the prevalence of 1W layers in contribution #4 as a result of the steady increase of

284

their proportion in this contribution. At this RH value, the essentially periodic 2W smectite remains

285

the main contribution, however, despite a significant decrease of its relative abundance (from 65-

286

70% to 55% at >60% RH and 60% RH, respectively – Table 3). Two other contributions, both

287

dominated by 2W layers, are necessary to fit the data. 3W layers persist in contribution #3, whereas

288

only 2W and 1W layers are present in the fourth contribution (contribution #2 in Table 3). The XRD

289

pattern collected at 55% RH was fitted with the same four contributions, although the relative

290

abundance of contribution #4, which is dominated by 1W layers is increased from ≈10 to ≈20% (at

291

60 and 55% RH, respectively – Table 3). Reflections typical for 1W smectite are visible in the XRD

292

pattern collected at 50% RH (Fig. 3) and are accounted for by a contribution overwhelmingly

293

dominated by 1W layers (90% of 1W layers in contribution #4 – Table 3). On the other hand,

294

contribution #1 remains dominated by 2W layers (93%), the main contribution to the diffracted

295

intensity at this RH value containing 2W and 1W layers randomly interstratified in almost equal

296

proportions, however. At 45% RH, the same three contributions are present. Their relative

297

abundances are significantly modified, however with the prevalence of the essentially periodic 1W

298

smectite (contribution #4 in Table 3), whereas the contribution of the essentially periodic 2W

299

smectite becomes marginal (6% - Table 3). This contribution decreases further (3%) at 40% RH. At

300

this RH value, 1W layers prevail in the other three contributions, the main one being an essentially

301

periodic 1W smectite (contribution #4 in Table 3), whereas the other two contributions account for

302

hydration heterogeneity. Contribution #2 contains only dehydrated layers interstratified with 1W

303

layers, whereas both 0W and 2W layers coexist with prevailing 1W layers in contribution #3 (Table

304

3). These three contributions allow fitting XRD data down to 5% RH. With decreasing RH, the

305

proportion of 2W layers in contribution #3 decreases steadily to become marginal (<10%) at 15%RH,

306

whereas the proportion of dehydrated layers increases in contribution #2, whose relative

307

abundance increases with decreasing RH (from ≈15% to ≈30% at 35% RH and 5-10% RH, respectively

308

– Table 3)

309

3.2.3. Hect-1.6-OH 310

As for the other two hectorite samples, XRD patterns collected at high RH values (Fig. 4) are

311

dominated by the contribution of an essentially periodic 2W smectite (>90% of 2W layers –

312

contribution #1 in Table 4). Additional contributions that contain either 3W and 2W (contributions

313

#2 and 3) or 2W and 1W layers (contribution #4) account for the low- and high-angle tails,

314

respectively, of smectite 001 reflection. With decreasing RH conditions, the relative abundance of

315

the most hydrated contributions decreases together with their content of 3W layers. At 75% RH,

316

the two contributions allowing fitting XRD data are a major contribution of essentially periodic 2W

317

smectite and a minor contribution with ≈10% of 3W layers interstratified with 2W layers.

318

Contributions having similar composition contribute also to XRD patterns collected at 70 and 65%

319

RH. A third contribution in which 2W layers are randomly interstratified with 1W layers accounts for

320

the high-angle asymmetry of smectite 001 reflection that starts developing at these RH values. This

321

asymmetry increases further at 60% RH, and requires the addition of a fourth contribution

322

incorporating a significant proportion (>30%) of 1W layers (contribution #4 in Table 4). At the same

323

time, the proportion of 3W layers in contribution #2 becomes minimal (3% - Table 4). At 55% RH,

324

1W layers become prevalent in contribution #3, although peaks typical for 1W smectite become

325

visible only at 50% RH (Fig. 4). Consistently, the relative abundance of contribution #4, in which 1W

326

layers prevail, increases from 12 to 23% (at 55 and 50% RH, respectively – Table 4) whereas that of

327

contribution #1, which is still dominated by 2W layers (90%) becomes marginal (7% – Table 4), the

328

main contribution including both 2W and 1W layers in similar proportions (58:40 ratio – contribution

329

#2 in Table 4). At 45% RH, the XRD pattern is dominated by 1W smectite, the main contribution to

330

the diffracted intensity being an essentially periodic 1W smectite (contribution #4 in Table 4),

331

whereas the other important contribution is also dominated by 1W layers. A third contribution with

332

equivalent proportions of 2W and 1W layers accounts for the limited low-angle asymmetry of

333

smectite 001 reflection. Contributions with similar compositions allow reproducing XRD data

334

collected at 40% RH, the relative contribution of the essentially periodic 1W smectite increasing

335

significantly, however (from 55 to 83% at 45 and 40% RH, respectively). This contribution

336

overwhelmingly dominates XRD patterns collected from 35 to 20% RH, coexisting only with a minor

337

contribution in which 1W layers represent ≈70% of the total layers at 35 and 30% RH (Table 4). When

338

decreasing further the RH conditions, this essentially periodic 1W smectite remains the main

339

contribution to the diffracted intensity, with additional contributions containing dehydrated

340

smectite layers interstratified with prevailing 1W layers, accounting for the high-angle tails of

341

smectite 001 reflection.

342

3.3. Evolution of synthetic hectorite hydration along the H2O vapor desorption isotherm 343

The XRD profile modeling approach used allows gaining quantitative insights into the overall

344

hydration of investigated hectorites despite intrinsic heterogeneity. In particular, by plotting the

345

relative proportions of the different layer types as a function of RH it is possible to evidence the

346

transition between different hydration domains (Fig. 5). Plots obtained for all hectorites are similar

347

(Fig. 5a), with two main plateaus corresponding to bi- and mono-hydrated domains extending from

348

90-60% RH and from 40-10% RH, respectively. Except for Hect-OH-1.3, the proportion of a given

349

layer type exceeds 90% on these plateaus thus indicating a low hydration heterogeneity, consistent

350

with previous reports on tetrahedrally substituted synthetic smectites (saponites – Ferrage et al.,

351

2010) and in contrast with natural montmorillonites and beidellites (Ferrage et al., 2005b, 2007). In

352

contrast to saponites, there is no significant influence of the amount of layer charge deficit on the

353

2W-to-1W dehydration behavior of hectorite which is similar to that of low-charge saponite [0.8

354

charge per O

20

(OH)

4

– Fig. 5b]. When increasing layer charge, dehydration of saponite occurs at

355

lower RH values, whereas hydration behavior of hectorite remains unchanged (Fig. 5c). The

356

similarity of hectorite hydration behavior with that of low-charge saponite possibly indicates that

357

the reduced charge undersaturation of surface O atoms in hectorite, compared to high-charge

358

saponite, is responsible for the lack of influence of layer charge on hectorite hydration behavior.

359

Relative proportion of 0W layers decreases with increasing layer charge under low RH conditions

360

(from 25.6% to 9.0% for Hect-0.8-OH and Hect-1.6-OH, respectively, at ≈5% RH), however,

361

consistent with the increased stability of higher hydration states towards low RH conditions

362

observed for saponites when increasing the layer charge. Finally, 3W layers are systematically

363

reported in hectorites under high RH conditions, in contrast with saponites (Ferrage et al., 2010;

364

Dazas et al., 2015). This difference most likely does not correspond to contrasting hydration

365

behaviors related to charge location but is rather due to the lack of a structure model for 3W layers

366

that prevented at the time an optimum fit of saponite XRD data recorded under these high RH

367

conditions. In the present study, 3W layers were considered to fit the low-angle tails of the first

368

smectite reflection despite the possible contribution of scattering effects over this angular range.

369

3.4. Layer-to-layer distance and content of interlayer water 370

3.4.1. 1W hectorite layers 371

The overall range of layer-to-layer distances (12.0-12.6 Å) and of interlayer H

2

O contents [2.5-

372

5.5 H

2

O molecules per O

20

(OH)

4

] is similar in both hectorite and saponite, independent of the layer

373

charge location (Fig. 6, Tables 2-4, and published data for Hect-OH-0.8 and Sap – Ferrage et al., 2010;

374

Dazas et al., 2013, 2015). This similarity of layer-to-layer distances and water contents evidences

375

the limited influence of the distribution of H

2

O molecules, present either as part of the cation

376

hydration sphere or filling interlayer voids, on layer-to-layer distances (Tables 2-4) although the

377

relative proportion of these different types of H

2

O molecules impacts their dynamical behavior

378

(Michot et al., 2012). In addition, the layer-to-layer distance of 1W layers varies in a similar way

379

along the H

2

O vapor desorption isotherm for all hectorite samples (Fig. 6a, Tables 2-4), consistent

380

with the limited influence of hectorite layer charge on their hydration behavior. Similarly, for a given

381

content of interlayer H

2

O, the layer-to-layer distance of 1W hectorites is increased compared to that

382

of saponite (Fig. 6b), most likely as a result of the reduced electrostatic attraction between interlayer

383

cations and the 2:1 layer. Minimal cation-layer electrostatic interactions in hectorite may be

384

responsible for the contrasting evolutions of 1W layer-to-layer distance as a function of interlayer

385

H

2

O content, compared to saponite. Hectorite layer-to-layer distance remains unchanged over a

386

wide RH range (≥20% RH) before decreasing sharply for lower H

2

O contents, whereas saponite layer-

387

to-layer distance decreases steadily with decreasing H

2

O content over the whole 1W stability

388

domain (Fig. 6). Despite the limited influence of layer charge on hectorite hydration behavior, the

389

evolution of 1W layer-to-layer distance as a function of interlayer H

2

O content is influenced by layer

390

charge. Fig. 6b shows indeed that an increased number of interlayer cations induces a faster

391

decrease of the layer-to-layer distance with decreasing H

2

O content (normalized to the number of

392

interlayer cations). This rate is specifically increased for both saponite and hectorite when layer

393

charge is equal or higher than 1.2 charge per O

20

(OH)

4

, and/or when the number of H

2

O molecules

394

is lower than ≈3.5 per interlayer Na

⁺

.

395

3.4.2. 2W hectorite layers 396

Layer-to-layer distances observed for 2W hectorite layers (15.10-15.65 Å) are larger compared

397

to those reported for saponite (14.80-15.40 Å – Fig. 7) despite similar overall H

2

O contents [8.0-10.0

398

H

2

O molecules per O

20

(OH)

4

– Tables 2-4]. In addition, layer-to-layer distances determined for

399

hectorites are all akin and similar to those reported for low-charge saponite (Sap-OH-0.8 – Ferrage

400

et al., 2010) indicative of a weak electrostatic attraction between the 2:1 layer and interlayer

401

cations. On the contrary, 2W layer-to-layer distances steadily decrease for saponite with increasing

402

layer charge (Fig. 7b). For all 2W hectorite and saponite samples, the 2W layer-to-layer distance

403

decreases steadily with the decreasing number of H

2

O molecules hydrating interlayer cations, the

404

range of distances for a given layer charge being more restricted for saponites compared to

405

hectorite, however (Fig. 7b).

406

3.4.3. Comparison with volumetric H2O vapor isotherms and GCMC calculations 407

Consistent with the constraints imposed on the modeling, overall H

2

O contents determined

408

from XRD modeling and from volumetric desorption isotherms are in general agreement, except

409

under high RH conditions (≥70% RH) for which capillary condensation becomes significant (Fig. 1b).

410

XRD modeling leads indeed to H

2

O contents ranging from 4.8-5.1 mmol per g of dry smectite and

411

from 11.7-12.5 mmol per g of dry smectite in 1W and 2W hectorite layers, respectively. Similar H

2

O

412

contents were determined from GCMC calculations for 2W layers (11.9-13.7 mmol H

2

O per g of dry

413

smectite – Figs. 1b, 8). Both GCMC and XRD profile modeling suggest a slight increase of H

2

O content

414

with increasing layer charge in these 2W layers, whereas an opposite trend is obtained from

415

desorption isotherms, possibly as the result of sample texture (Cases et al., 1997). For 1W layers,

416

H

2

O contents derived from GCMC calculations (5.5-6.4 mmol H

2

O per g of dry smectite) are

417

systematically slightly higher than those determined experimentally (Figs. 1b, 8).

418

3.5. Organization of interlayer species 419

3.5.1. 1W hectorite layers 420

Overestimation of H

2

O contents derived from GCMC calculation compared to experimentally

421

determined values (Figs. 1b, 8, 9) possibly results from the use of the SPC/E model for water, this

422

model leading to higher H

2

O contents compared to the SPC model (Ferrage et al., 2011). Although

423

leading to a slight overestimation of H

2

O contents, the SPC/E water model, allows a robust

424

determination of H

2

O organization in saponite over a range of layer charge similar to the one

425

investigated in the present study (Ferrage et al., 2011). Alternatively, experimentally determined

426

H

2

O contents may be slightly underestimated by an incomplete release of H

2

O molecules after

427

sample degassing (Michot et al., 2005; Rinnert et al., 2005; Ferrage et al., 2010). Additional

428

experimental measurements, from infrared spectroscopy and/or neutron diffraction on samples

429

equilibrated with D

2

O, would be needed to assess further the accuracy of present experimental

430

constraints.

431

The overall agreement between interlayer electron density distributions derived from GCMC

432

calculations and from modeling of XRD data (Figs. 8, 9) is satisfactory however, especially for high

433

layer charges. For low values of layer charge, the overestimation of H

2

O contents from GCMC

434

calculations leads to significantly higher electron density at the interlayer mid-plane (Fig. 8). In all

435

cases, modeling of XRD profiles leads to a significant broadening of interlayer electron density

436

distribution, consistent with the breadth of computed distributions. The latter distributions indicate

437

however that the broadening is mainly due to the splitting of the interlayer cation position, this

438

splitting increasing with increasing layer charge.

439

3.5.2. 2W hectorite layers 440

The interlayer cation site is split also for 2W hectorite layers, although the highest cation

441

density occurs at the interlayer mid-plane (Fig. 8). Despite their overall consistency, interlayer

442

distributions of H

2

O molecules derived from GCMC calculations and from XRD profile fitting exhibit

443

significant differences. A first difference arises from the sharpening of H

2

O molecule distributions

444

determined from XRD profile modeling with increasing layer charge, whereas the breadth of O

445

atomic distributions is essentially the same whatever the layer charge (Fig. 8). A second difference

446

comes from the position of these distributions with respect to the interlayer mid-plane. H

2

O

447

molecule distributions deduced from XRD profile modeling are systematically located at ≈1.40-

448

1.45 Å (Δd2W in Tables 2-4) from the interlayer mid-plane. The same position was obtained from

449

GCMC calculations performed for Hect-OH-0.8. For higher values of the layer charge, centers of O

450

atomic distribution derived from GCMC calculations are shifted however towards the 2:1 layer by

451

0.15-0.20 Å. In all cases, planes of H

2

O molecules are located at a realistic distance of 2.8-3.0 Å from

452

the 2:1 layer.

453

3.6. Comparison of synthetic hectorite and saponite hydration 454

No significant influence of the amount of layer charge on hectorite hydration behavior was

455

evidenced in the present study, the dehydration from 2W to 1W layers occurring systematically over

456

the same RH range (60-40% RH – Fig. 5). A slight shift of the 1W-to-0W transition towards lower RH

457

values was observed however with increasing layer charge (Figs. 1, 5). This latter shift is consistent

458

with the increased stability of the most hydrated layers towards lower RH conditions observed for

459

saponites (Ferrage et al., 2005b, 2007, 2010, 2011; Dazas et al., 2015). A similar shift was observed

460

however for the 2W-to-1W transition (Figs. 5b, c), in contrast to hectorites. Despite their contrasting

461

stability, the H

2

O content is systematically the same for a given hydration state and all smectites

462

investigated, whatever the location and the amount of layer charge deficit (Fig. 1) with ≈4-6.5 and

463

≈10-13 mmol H

2

O per gram of dry smectite in 1W and 2W layers, respectively. These H

2

O contents

464

are reduced by ≈30% by F-for-OH substitution in the anionic framework of the 2:1 layer, however

465

(Dazas et al., 2013).

466

Steric constraints imposed on these similar amounts of H

2

O molecules vary significantly as a

467

function of the location and the amount of layer charge deficit. For hectorites, layer-to-layer

468

distances are similar indeed for a given hydration state (H

2

O content) whatever the layer charge

469

(Figs. 6, 7). For a given H

2

O content, these distances are equal or slightly higher than those reported

470

for Sap-OH-0.8 (Ferrage et al., 2010). On the other hand, layer-to-layer distances decrease

471

significantly with increasing layer charge for saponites (Dazas et al., 2015). This contrasting evolution

472

of layer-to-layer distances is most likely due to the lower electrostatic attraction between the 2:1

473

layer and interlayer cations in hectorite compared to saponite owing to the weaker local charge

474

undersaturation of O atoms at the surface of hectorite layers. This lower local undersaturation, and

475

the associated reduced need for local charge compensation, leads to interlayer species being

476

located further away from the 2:1 layer surface (Figs. 8a, 8b) although the same LJ parameters were

477

used for O atoms at the surface of the 2:1 layer for GCMC simulations of both saponites and

478

hectorites. For hectorites, interlayer cations closest to the 2:1 layer are located indeed at ≈2.5 Å

479

from this layer, compared to 2.2-2.3 Å for saponites, whereas H

2

O molecules are located at ≈2.8-

480

3.0 Å, compared to 2.6-2.8 Å for saponites.

481

Both the lack of layer charge influence on the layer-to-layer distance observed for hectorites

482

and the more diffuse charge undersaturation of O atoms at the surface of hectorite 2:1 layer,

483

compared to saponite, result in similar distributions of interlayer species whatever the amount of

484

hectorite layer charge. As a consequence, H

2

O contents, position of interlayer cations and H

2

O

485

molecules with respect to O atoms at the 2:1 layer surface, and breadth of the distributions are

486

indeed essentially similar for all hectorites whatever the layer charge (Figs. 8a, S2). By contrast, the

487

decrease of the layer-to-layer distance observed for saponite with increasing layer charge induces

488

steric constraints on the distribution of interlayer species. Constraints are enhanced by the stronger

489

local charge undersaturation of O atoms at the surface of the 2:1 layer and the induced increased

490

hydrophobicity of the layer surface (Ferrage et al., 2010, 2011; Dazas et al., 2015). Increased steric

491

constraints in saponite compared to hectorite lead in turn to a stronger organization and to an

492

increased polarization of H

2

O molecules (Fig. 9).

493 494

4. Conclusion

495

The overall range of interlayer H

2

O contents [2.5-5.5 and 8.0-10.0 H

2

O molecules per O

20

(OH)

4

496

for 1W and 2W hydrates, respectively] is similar in both synthetic Na-saturated hectorite and

497

saponite, independent of their layer charge. The hydration behavior of both Na-saturated smectite

498

varieties differs however as the transition from 2W to 1W layers systematically occurs over the same

499

RH range for synthetic Na-saturated hectorite whereas the stability of 2W layers is increased

500

towards lower RH conditions for high-charge synthetic Na-saturated saponites compared to low-

501

charge ones. Consistent with this increased stability of most hydrated layers with increasing layer

502

charge, a slight increase of 1W layer stability towards lower RH conditions is observed for high-

503

charge Na-saturated hectorite compared to low-charge ones. 1W hydrates of synthetic Na-

504

saturated hectorite and saponite also display similar layer-to-layer distances (12.0-12.6 Å),

505

whatever their layer charge. Contrastingly, layer-to-layer distances of 2W Na-saturated saponite

506

layers (14.80-15.40 Å) are shorter than those of Na-saturated hectorite 2W layers (15.10-15.65 Å).

507

In addition, the amount of layer charge has essentially no effect on the layer-to-layer distance of

508

Na-saturated hectorite 2W layers whereas a significant decrease of the layer-to-layer distance is

509

observed with increasing charge for Na-saturated saponite 2W layers. The contrasting hydration

510

behavior of Na-saturated saponite and hectorite is likely due to different electrostatic attraction

511

between the 2:1 layer and interlayer cation. The charge undersaturation of O atoms at the surface

512

of hectorite 2:1 layer is indeed more diffuse compared to saponite, owing to the location of the

513

charge deficit in the octahedral sheet. As a result, Na

⁺

cations are located further away from the

514

surface of the 2:1 layer in hectorite compared to saponite. In addition, contrasting layer-to-layer

515

distances of synthetic Na-saturated saponite and hectorite 2W layers lead to distances between

516

interlayer H

2

O molecules and the surface of 2:1 layers shorter in the former compared to the latter

517

despite unfavorable undersaturation of O atoms. Finally, the present article complements the

518

dataset available for tetrahedrally charged synthetic Na-saturated smectites (saponites) to build

519

comprehensive sets of data and samples with contrasting locations and amounts of layer charge

520

deficits that provide key constraints to assess the validity of force fields simulating clay-water

521

interactions. The present work being limited to Na-saturated hydroxylated smectites, its extension

522

and applicability to other interlayer cations should be assessed to decipher the specific influence of

523

the charge-compensating cation. Previous combined experimental and computational studies

524

investigated the influence of interlayer cation nature on the structure and dynamics of interlayer

525

species for alkali and alkaline earth cations (Michot et al., 2012; Bowers et al., 2014; Reddy et al.,

526

2016). Michot et al. (2012) also demonstrated the validity for molecular dynamic studies of atomic

527

potentials derived from the collation of experimental XRD profile modeling and of GCMC

528

calculations for Na-saturated samples. Specific attention should be paid however to large alkali

529

cation having low hydration energy (e.g., K

⁺

, Cs

⁺

– Bowers et al., 2011; Loganathan et al., 2016),

530

owing to their specific properties.

531 532

Supporting information

533

Comparison between experimental and calculated XRD patterns for a selection of Hect-1.0-

534

OH X-ray diffraction patterns with the respective contributions of the various interstratified

535

structures to the calculated profiles. Comparison of the H

2

O contents determined from vapor

536

sorption isotherms and from GCMC simulations. Specific surface area determined with the BET

537

methods for all synthetic Na-saturated saponites and hectorites described.

538 539

Acknowledgments

540

DV thanks U. Bari from granting her a Ph.D. fellowship. The CNRS interdisciplinary défi Needs,

541

through its “MiPor” program, is thanked for the financial support to the present study. Constructive

542

comments of three anonymous reviewers on its initial version allowed improving the present article.

543 544

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545

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Christidis, G.E., Huff, W.D., 2009. Geological Aspects and Genesis of Bentonites. Elements 5, 93-98.

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578

Dazas, B., Ferrage, E., Delville, A., Lanson, B., 2014. Interlayer structure model of tri-hydrated low-charge 579

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580

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581

Dazas, B., Lanson, B., Breu, J., Robert, J.L., Pelletier, M., Ferrage, E., 2013. Smectite fluorination and its impact 582

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