DETERMINATION of the SPECIFIC SURFACE AREA in SWELLING CLAYS

F. Salles1, J.M. Douillard1, O. Bildstein2, M. Jullien3 and H. Van Damme4

(1) ICGM, Université Montpellier – France (2) CEA, DEN, LMTE – CEA Cadarache – 13108 St Paul lez Durance – France (3) ECOGEOSAFE – Europole Arbois – Aix-en-Provence (4) ESPCI – 75231 Paris –France

Material and Method

Introduction and Principle

Aim of this study : determination of the reactive specific surface area and understanding of the hydration process upon water adsorption for samples saturated with alkaline cations

the amount of adsorbed water measured in swelling clay is different for each sample (with different interlayer cation ) = hydration capacity and competition

with the hydration capacity of the layer surface and the swelling the evolution of the specific surface area and therefore of the reactive surface area confirms this strong influence the value of the adsorption enthalpy gives us the controlling step as a function of the relative humidity and the nature of the interlayer cation

(see also F. Salles, J.M. Douillard, R. Denoyel, O. Bildstein, M. Jullien, I. Beurroies, H. Van Damme, J. Colloid Interf. Sci., 2009, 333, 510-522)

• Purified powder of

montmorillonites (Mont)

from the MX-80 bentonite

(octahedral substitutions)

saturated with a large

majority of Na+ and Ca2+ as

interlayer cations

During the water adsorption, the structure of the swelling clays is strongly modified due to interactions between water

molecules and the constitutive parts of the clay structure: the interlayer cation and the layer surface. The modification of the

interaction equilibrium between cations and layer surfaces is the cause of the interlayer swelling.

The understanding of the hydration process in swelling clays requires the determination of the reactive surface area and the

identification of the driving-force for water adsorption. For this purpose, the simultaneous measurement of water adsorption

isotherm and adsorption heat is performed for MX-80 (Wyoming) samples saturated with homoionic alkaline cations.

Material Experiments

• Samples are fine powder

heated at 150°C for 12h

under vacuum

• Continuous volumetric adsorption + Tian-Calvet Microcalorimeter

• Equilibrium time

imposed after adding

water: minimum 4 hours

• Duration: 7 days

Results and Interpretation

Conclusion

0,0 0,2 0,4 0,6 0,8 1,00

100

200

300

400

500

600

700 (Li)

Mas

s of

Ads

orbe

d w

ater

(m

g/g

of c

lay)

P/P0

0,0 0,2 0,4 0,6 0,8 1,00

50

100

150

200

250

300

350 (Na)

Mas

s of

Ads

orbe

d W

ater

(m

g/g

of c

lay)

P/P0

0,0 0,2 0,4 0,6 0,8 1,00

50

100

150

200

250

300(K)

Mas

s of

Ads

orbe

d W

ater

(m

g/g

of c

lay)

P/P0

0,0 0,2 0,4 0,6 0,8 1,00

200

400

600

800

1000

1200

Spec

ific

Sur

face

Are

a (m

²/g)

P/P0

0,0 0,2 0,4 0,6 0,8 1,00

50

100

150

200

250

300

350

Spe

cifi

c S

urfa

ce A

rea

(m²/

g)

P/P0

0,0 0,2 0,4 0,6 0,8 1,0

100

200

300

400

500

Spe

cifi

c S

urfa

ce A

rea

(m²/

g)

P/P0

0,0 0,2 0,4 0,6 0,8 1,00

50

100

150

200

250

300(Ca)

Mas

s of

Ads

orbe

d W

ater

(m

g/g

of c

lay)

P/P0

0,0 0,2 0,4 0,6 0,8 1,00

50

100

150(Na/Ca)

Mas

s of

Ads

orbe

d W

ater

(m

g/g

of c

lay)

P/P0

0,0 0,2 0,4 0,6 0,8 1,0

150

200

250

300

350

400

Spe

cifi

c S

urfa

ce A

rea

(m²/

g)

P/P0

0,0 0,2 0,4 0,6 0,8 1,00

100

200

300

400

500

600

700 (Cs)

Mas

s of

Ads

orbe

d W

ater

(m

g/g

of c

lay)

P/P0

0,0 0,2 0,4 0,6 0,8 1,0

180

200

220

240

260

280

300

320

Spe

cifi

c Su

rfac

e A

rea

(m²/

g)

P/P0

0,0 0,2 0,4 0,6 0,8 1,0

50

100

150

200

Spe

cifi

c S

urfa

ce A

rea

(m²/

g)

P/P0

0,0 0,2 0,4 0,6 0,8 1,0

20

30

40

50

60

70

80

90

100(Li)

He

ats

(kJ

/mo

l of

wa

ter)

P/P0

0,0 0,2 0,4 0,6 0,8 1,020

30

40

50

60

70

80

90

100(Na)

Hea

ts (

kJ/m

ol o

f wat

er)

P/P0

0,0 0,2 0,4 0,6 0,8 1,025

30

35

40

45

50

55 (K)

He

at

(kJ/

mo

l of

wa

ter)

P/P0

0,0 0,2 0,4 0,6 0,8 1,030

35

40

45

50 (Cs)

He

at

(kJ/

mo

l of

wa

ter)

P/P0

0,0 0,2 0,4 0,6 0,8 1,0

25

30

35

40

45

50

55

60 (Ca)

He

at

(kJ/

mo

l of

wa

ter)

P/P0

0,0 0,2 0,4 0,6 0,8 1,020

30

40

50

60

70

80

90 (Na/Ca)

Hea

t (kJ

/mol

of w

ater

)

P/P0

Interpretation of adsorption isotherms

• Theoretical isotherms according to Lecloux-Pirard or de Boer (nm = value for the mono-layer, C = BET constant, X=P/P0 and N = number of water layer)

nm and C are determined using BET model and N is calculated from thermogravimetric analysis

• Calorimetry: differential heat and integrated heat (= sum of differential heat divided by the adsorbed water amount for a given RH)

1N

1NN

m

XXC1C

C1X1

XNX1N1Xnn

Evolution of the specific surface area

Fitting of the adsorption isotherm using the theoretical isotherms

determine the evolution of the specific surface area during water adsorption

• Existence of plateaus on the isotherms

• All isotherms are of II or IV type

BET equation can be applied

•Maximal adsorbed water amount depends

on the nature of the interlayer cation: Li

> Cs > Na > K > Ca > Na/Ca

• Water affinity depends on the MX sample

• Existence of plateaus on the isotherms //

XRD data

• All isotherms II or IV type

BET equation can be applied

•The specific surface area are determined

using 12 Å as cross sectional area of the

water molecules

• Sequence of the adsorbed water amount

different of the one for cations in solution

the driving force for hydration is not

only the hydration energy of the interlayer

cation

•Strong evolution of the specific surface

area for small cations

• In the case of Cs-sample: no evolution

• For Na/Ca, less evolution than for the

other cations

Evolution of the adsorption enthalpy

Description of the differential (black) and integrated (red)

heat to discriminate the swelling and the hydration (for the

cation or for the layer surface) steps

• Evolution for the values at low RH values:

Li > Na > Na-Ca> Ca > K > Cs

• As a function of the RH:

- RH < 40%: exothermic peaks = HYDRATION

- RH > 50% : endothermic peaks = SWELLING

• For Na/Ca: we distinguish the hydration for both cations

Exchanged powders of MX-80 bentonite saturated

with alkaline or Ca2+ cations: Li+, Na+, K+, Cs+