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!