determination of the specific surface area in swelling clays f. salles 1, j.m. douillard 1, o....
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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+
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