ELECTROSTATIC INTERACTIONS

Applications in colloid scienceOverbeek Medal Lecture

Håkan WennerströmDivision of Physical Chemistry 1Lund University

Combine theory and experiment.Study model systems that can illustrate theory.Combine molecular, colloidal and thermodynamic perspective.Demonstrate relevance for some applications.

Strategy

MotivationElectrostatic intermolecular interactionsare the most important cause of molecularorganization and selectivity inbiological and colloidal systems

Counterion NMR

Söderman,Engström&HW J.Colloid.Interfacce Sci 1980

Approach:Measure quadrupolar splittingsfor lamellar liquid crystals

23Na

7Li

Na+ vs Li+

competition

Question:How are counterions interactingwith highly charged surfaces?

The Poisson-Boltzmann equation andthe cell model

HW,Jönsson,Linse J.Chem. Phys.1982

Ael = 12 ρΦdV − k ci∫ (lnci −1)dV − ni{ln(ni /V ) −1}dV

i∑∫

ε0εr∇2Φ = −e zi

i∑ ci0 exp(−zieΦ /kT)

Force /area = kT cii∑ (zw ) −σ w /(2ε0εr)

Contac value theorem:

Poisson-Boltzmann equation

Mean field free energy functional

Minimization leads to:Cell model

Ionic amphiphiles 1Question:What is the electrostaticcontribution to the formation ofa micelle?

Approach: Solvethe PB equation.

Theory and experiment forCMC ofalkyl sulfates

Gunnarsson, Jönsson,HWJ.Phys.Chem 1980

Ionic ampiphiles 2

Gunnarsson, Jönsson,HWJ.Phys.Chem 1980

Q:Micelle-micelle interactions?

A:PB equation.Cell model.Test for ion chemical potential.

Theory and experimentFor SDS

Calculated phasediagram!

Ionic amphiphiles 3

Jönsson,HW J.Phys Chem 1987

Q:Is phase behavior driven by electrostatics?A:Create model free energy.

Ion Correlations 1

Guldbrand,Jönsson,HW,Linse J.Chem.Phys. 1984

Q:Accuracy of mean fieldapproximation?

A:MC simulations!

Model:Planar charged walls.Only counterions

MC

PB

Complete breakdownof PB for high charge densities and high counterion valencies

Non-monotonicforce curvein transition region

Ion correlations 2

Khan,Jönsson,HW J.Phys.Chem 1985

Q:Experimental manifestationof correlation attraction?

A:Compare lamellar liquid crystals with monovalent anddivalent counterions!

NaAOT - CaAOT-water

Observations:Pure Na system swellPure Ca system no swellingCoexistence at intermediatemixing.Generic behavior.

Ion correlations 3

Guldbrand,Nilsson, Nordensköld, J.Chem.Phys 1986

Q:Is geometry importantfor attraction?

A:MC simulation onDNA-like system.

Result:Attraction also incylindrical systems.Less pronounced.

PB

MC

Ion correlations 4

Jönsson,HW,Nonat,Cabane Langmuir 2004

Q:Practical applicationof correlation attraction

A:Cohesion in cement

In wet cement “C3S”particles dissolve andcharged silica-sulfatenanoparticles precipitate.pH increases steadily.

Ion correlations 5

Measured interparticle force.AFM

MC simulation for varyingcharge density.Jönsson,HW,Nonat,Cabane Langmuir 2004

Domain correlations

Tsao,Evans,HW Science 1993

Perkin,Kampf,Klein Phys.Rev.Lett 2006

ForceArea

≈ − σ 2

2εrε0

{exp[(κ 2 + 3.4 /R2)1

2 h /2]+ exp[−(κ 2 + 3.4 /R2)1

2 h /2]}−2

Attractionobserved for patchysurfaces

The dielectric approximation 1Q:Accuracy of the dielectricmodel of water?

A:Quantitative test ofMC simulations relativeto experimental phasediagram.

Theory appears towork quantitatively even for separations of 1nm!?

Turesson,Forsman,Åkesson,Jönsson,Langmuir 2004

Khan,Jönsson,HW J.Phys.Chem 1985

The dielectric approximation 2

Carlsson,Fogden,HW Langmuir 1999

Q:Temperature dependenceof electrostatic interactions?

Observation:The dielectricpermittivitty of water has anomalous T-dependence!Electrostatic free energylarger at high T.

Test: Ionic microemulsions

Non-ionicsystem

Ionic system

Full lines:calculation basedon PB equation andcell model

PB theory describestemperature dependencequantitativly!

The dielectric approximation 3Q:Attractive electrostaticinteractions at short range?

Proton NMR,alkyl chain and CH3

a)

b)

c)A:Hydration of DNA- CTA+

Leal,Topgaard,Martin,HWJ.Phys.Chem2004

The dielectric approximation 4

Leal,Wadsö,Olofsson,Miguel,HW J.Phys.Chem B 2004

A:Calorimetric study of of DNA- CTA+

Free energy Enthalpy

Q:Thermodynamics of hydration?

The dielectric approximation 5

Leal,Moniri,Pegado,HW Submitted

Q:Effect of salt?

A:Add NaCl to dry DNA- CTA+.

Result:Salt increases swelling.Effect can be modeled using PBequation

Dissolution ofNaCl in complex

Lifshitz theory and classical electrostatics 1

HW,Daicic,Ninham Phys.Rev.A 1999

OH

H

O

HH

R

V(R)=− 3kTα2

(4πε0)2R6

DispersionDebye andKeesom interactionshave an identicalasymptotic form forthe free energy!!!(but for energy)

Lifshitz theory and classical electrostatics 2

HW Collids&Surfaces A 2003h

Dipolarfluid

Dipolarfluid

G /area = − kT32πh2 x 2

0

∞

∫ {(ε(0) +1ε(0) −1

)2ex −1}−1dx

Comment: An elementary useof Lifshitz theory would suggesta different expression!

Unresolved issues

The breakdown of the dielectric approximationInteractions at short range, adhesion, binding constantsAnions at polar-apolar interfaceThe distance dependence of the hydrophobic interactionDipolar correlations in waterClassical-quantum high T free energy equivalence

Acknowledgements

Thanks toBengt Jönsson: A master of electrostatic theory and much moreBo Jönsson: For all computer simulation contributionsSven EngströmOlle SödermanAndrew FogdenIngemar CarlssonJohn DaicicAnd all collegues at Physical Chemistry 1, Lund University