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


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


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


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


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.


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


Perfect agreement↳

✔ GCMC: same parameters as for Na+

✔ Exp. data: Dove et al

Calcium versus sodium

Ion-ion versus ion-site correlations


•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


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


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)


pH 10

Ion adsorption (MC vs reflectometry)


✔ Exp. data: Porus et al

Ion specificity (MC vs reflectometry)


Small variation of the ion size is enough to reasonably describe ion specificity at silica/water interface

Ion adsorption (MC vs reflectometry)


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


The 2 pK model largely overestimates the measured forces at low pH !

NaCl 1 mM

AFM experiments

Surface forces


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.

ConclusionsContext ResultsModel/Methods

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


single ion effects at aqueous solid/liquid interfaces · 2014-03-26 · single ion effects at...

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