Single ion effects at aqueous solid/liquid interfaces · 2014-03-26 · Single ion effects at...
Transcript of Single ion effects at aqueous solid/liquid interfaces · 2014-03-26 · Single ion effects at...
Christophe Labbez*, Bo Jönsson‡, Eric Lesniewska*, Michal Borkovec¥
Single ion effects at aqueous solid/liquid interfaces
* ICB, UMR 6303 CNRS - Université de Bourgogne, France‡ Theoretical Chemistry Department, Lund University, Sweden¥ Mineral, Analytical and Applied Chemistry Department, Geneva University, Switzerland
Experiments: S. G. Lemay et al, Phys. Rev. Lett. 93, 170802 (2004)Simulations: PRL 97, 068302 (2006)
ConclusionContext ResultsModel/Methods
Repulsion between oppositely charged particles
Amine-terminated silica surface
silica spheres
AFM experiments MC simulations
LaCl3
2x2m2,relative height 100nm
AFM tip with C-S-H
C-S-H surface
AFM experiments MC simulations
ConclusionContext ResultsModel/Methods
Ca(OH)2
Attraction between likely charged particles
P osmDL
=kBT∑ ci(mp)+Phc+PcorrPnet=P osm
DL−Posm
bulk
Gulbrand et al, P. J. Chem. Phys. 1984, 80, 2221.Kjellander et al, Chem. Phys. Lett. 1984, 112, 49.
Jönsson et al, Langmuir 2005, 21, 9211.Lesniewska et al, Langmuir, 2005, 21 7263
Outlines
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In the case of silica surfaces:
● How well the primitive model can describe the coupling between surface ionisation and ion adsorption ?
● Can the primitive model accurately describe the charge reversal in multivalent salt solutions ?
● What is the role played by the confinement ?
● Experiments on silica: - potentiometric titration, Dove et al - Charge reversal, Van Der Heyden et al
- Ion adsorption, Porus et al
- Dove, P. M.; Craven, C. M. Geochim. Cosmochim. Acta 2005, 69, 4963–4970.- van der Heyden, F. H. J.; Stein, D.; Besteman, K.; Lemay, S. G.; Dekker, C. Phys. Rev. Lett. 2006, 96, 224502.- Porus, M.; Labbez, C.; Borkovec, M. J. Chem. Phys, 2011, 135, 064701
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GCMC simulations
Bulk solution pH,
i,..., µ
N
≡Si-O- + H+ ≡Si-OH →←
• Electrolyte solution: primitive model
• Surface: discrete titratable sites
u r i , r j =z i z j e
2
40r r i−r j when (r
i – r
j) > (σ
i+σ
j)/2
u r i , r j =∞ otherwise
K0
Equilibriu
m
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Model for surface ionisation
≡Si-OH ≡Si-O- + H+
• Protonation and deprotonation of silica :
Non ideal term# site-site interactions site-ion interactions
→←
PB (1-pK Stern model)
MC
• Equilibrium constant is the activity product of the chemical species:
Labbez, C., Jönsson, B. Lect. Note in Comp. Sci. 2007, 4699, 66–72.
Ka
= = .aH a Si O aH cSiO SiO
aSiOH cSi OH Si OH
Ka
Simulations versus experiments
Figure: Experimental (points) and simulated (lines) surface charge density of silica particles dispersed in sodium salt for various concentrations.
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Silica in sodium salt
✔ GCMC: s=4.8 sites/nm2
pKa=7.7
dis
= 3.5 Å✔ Exp. data: Dove et al
Figure: Experimental (exp) and simulated (GCMC) surface charge density of silica particles dispersed in 200 mM sodium salt and 67mM and 1000mM calcium salt.
Simulations versus experiments
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Perfect agreement↳
✔ GCMC: same parameters as for Na+
✔ Exp. data: Dove et al
Calcium versus sodium
Ion-ion versus ion-site correlations
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•Two regimes: at low salt concentration and pH ion-site correlations play a more important role than ion-ion correlations. On contrary, at high salt concentration and pH the reverse is found.
The discrete nature of titrating sites and ion-site correlations are essential physical properties.
↳
-Lines with symbols: 1 M CaCl2
-Lines without symbols: 0.067 M CaCl2
Co
ntr
ibu
tio
n
pH
Ion-ion ― Ion-site ―
Charge regulation
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D D (Å)
Ion
isat
ion
fra
ctio
n (
)
-Low pH: monotonic decrease of upon decreasing surface separation-high pH: increase of with decreasing D !
0.2 mM CaCl2
[CoSep]3+
Charge reversal
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Charge reversal
Buffered at pH 7.5
• The titration of surface sites has a prominent role in the generation of CR for which both ion-ion and ion site correlations come into play.
Exp. data: van der Heyden et al
Ion adsorption (MC)
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pH 10
Ion adsorption (MC vs reflectometry)
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✔ Exp. data: Porus et al
Ion specificity (MC vs reflectometry)
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Small variation of the ion size is enough to reasonably describe ion specificity at silica/water interface
Ion adsorption (MC vs reflectometry)
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However, at low pH the adsorption cannot be correctly described unless a 2pK model is used
1 pK model 2 pK model
(MgCl2) (MgCl
2)
pKa=7.7 0.09 pK
a1 = 7.7, 0.91 pK
a2 = 8.2
Surface forces
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The 2 pK model largely overestimates the measured forces at low pH !
NaCl 1 mM
AFM experiments
Surface forces
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The 1 pK model allows for a reasonable description of the forces !
NaCl 1 mM
AFM experiments
Conclusions
• The dominating interaction which controls the charging process, ion adsorption and charge inversion of silica in multivalent electrolyte solutions is of purely electrostatic origin and strongly dependent on ion-ion and ion-site correlations;
• The weak ion specific effects at the silica/water interface can be satisfactorily described by a small variation of ion radii.
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C. Labbez, B. Jönsson, M. Skarba and M. Borkovec, Langmuir 2009, 25 7209–7213M. Porus, C. Labbez and M. Borkovec, J. Chem. Phys, 2011, 135, 064701
Acknowledgements
• Serge Lemay, Delft University of Technology, Netherlands
• Patricia M. Dove, Virginia Techs, USA
• Bernard Cabane, ESPCI, France
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