Microscopic ElectrostrictionThe effect of ions to the structure of water

Rosaria Mancinelli, Alberto Botti, Tommaso Corridoni,Silvia Imberti, Maria Antonietta Ricci, Alan Soper

DILUTE

D

SALINE

SOLUTION

What is the range of influence of the ion ?

What is the disturbance on the HB net?

?

Does it depend on the ionic species?Does it depend on solute concentration?

Jones-Dole’s Lawsolution viscosity as a function

of solute concentration

ions woth B>0, η↑

ioni con B<0, iη↓

Outline Outline & & ApproachApproach

• Neutron diffraction experiment (NDIS) coupled with EPSR simulations on aqueous solutions of hydroxides and salts;• Raman spectroscopy: OH- as a probe of its local environment;• Range and amplitude of the perturbation due to the presence of ions in solution;• microscopic electrostriction• “Magic number” of water molecules?• Conclusions & Speculations.

EPSREPSR

potenziale di riferimento additivo a coppieintramolecolare + intermolecolare

Scatola di simulazione ~ 500 molecole, lato ~ 25 Å

composizione, densità, T del campione

Monte Carlo con roto-traslazioni molecolari

Calcolo dei fattori di struttura

Differenza tra fattori di struttura calcolati e misurati

Trasformata di Fourier della funzione di fit

Fit di con

correzione cosiddetta empirica al potenziale di riferimento

piccoli, senza strutture?NO

Accumuloconfigurazioni

SI

Cl -

Na +

SOLUZIONI

SALINE

DILUITE

K +

NaCl KCl

Na+ K+

““NegligibleNegligible effecteffect of of ionsions on the HB on the HB structurestructure in in liquidliquid waterwater””Omta, Kropman, Bakker, Science, 301 (2003)

…different counterion: OH-

…here shown with its first hydration shell (Botti et al. JCP 2003)

2 3 4 5 6 70

2

4

6

g

XOw

r (A)

1:83 Li Na K

KCl

NaClNaCl 1:83

NaCl 1:17

pure water

I shell w-w II shell w-w

P ≈ 400 MPa

gOwOw (r) used as a yardstick tomeasure the extent of perturbation

due to charged solutes on theHB network of water

equivalentequivalent PressurePressure

sali

idrossidi

acidi

800

600

400

200

0

equi

vale

nt p

ress

ure

(MP

a)

12 3 4 5 6 7 8 9

102 3 4 5 6 7 8 9

100

no. water molecules / solute

NaOH

Only first shell water molecules disturbed by the ions?

First shell removal from the simulation box

Correct forexcluded volume

• Second peak still shiftedcompared to pure water!

• True for all water moleculesin the simulation box!

• First shell unchanged, second shell shrinked:

MicroscopicMicroscopic electrostrictionelectrostriction

OH- as a probe of its environment

First shell does not depend on

• ionic species• solute concentration

1500 2000 2500 3000 3500 4000

I is

o (a.

u.)

ν (cm-1)

KOH : n H2O

Solute concentration

6543210

I (a.

u.)

400035003000250020001500

ν (cm-1)

0.5

-0.5

1 10 100

3610

3620

3630

3640

1 10 100

30

40

50

Γ iso (

cm-1)

ν iso (

cm-1)

nw

LiOH NaOH KOH

nw

0 1 2 3 4 5 60

10

20

30

40

g

NaO

r (A)

water molecules per solute 83 50 38 28 12 9 6 3

1.5 2.0 2.5 3.0 3.5 4.00.0

0.5

1.0

1.5

2.0

n

NaO

r (A)

water molecules per solute 83 50 38 28 12 9 6 3

2.0

1.5

1.0

0.5

0.0

coor

dina

tion

num

ber N

a-O

12 3 4 5 6 7 8 9

102 3 4 5 6 7 8 9

100

no. water molecules / solute

Richards and Hall, 1929

Heat of dilution

Molar apparent volumeRoux et al, 1983

800

600

400

200

0

equi

vale

nt p

ress

ure

(MP

a)

12 3 4 5 6 7 8 9

102 3 4 5 6 7 8 9

100

no. water molecules / solute

NaOH

Meinhold and Smith, PRE 2005

• buckyball structures? (Peslherbe, Ladany, Hynes Chem. Phys. 2000)• structural transformation related to the high-pressureliquid-liquid transition of supercooled water?• MSD of hydrated protein….

ConclusionConclusion & & SpeculationsSpeculations

• structure of the ENTIRE solution refined against a set of diffraction data;• effect of ions on water not negligible and notlimited to first hydration shell;• similar to the application of pressure to pure water: microscopic electrostriction; • two “states” of water above and below the magic number;• hopes for Hofmeister?

Effetto di volume esclusoEffetto di volume escluso

OO totale

OO bulk

ρpv>ρmedia

SOLUZIONI

SALINE

DILUITE

Sistema uniformevincolato fuorisfere identiche non overlapping

1Miscele 1:83 no overlapdelle shell d’idratazione dei soluti

Raggio sfere identiche equivalenti

2Miscele più concentrate overlapdelle shell d’idratazione dei soluti

Correzione euristica

per KCl 0,6m

I(tv) S(q)

OK only for single elastic collisions

A neutron exchangesenergy and is involvedin multiple scattering

events

Pulsedsource

moderator

1 10 100 1000 meV

Pulsed beam of neutrons

TOF diffractometersample

I(tv|θ)

tv

S(q)

θNeutron production

Neutron detection

ContrastContrast MethodMethod -- NDISNDISbH = -3.74 fm ≠ bD = 6.67fm

H/D Isotopic Substitution: experiment with water are fully exhaustive!