Glasslike Behaviour in Aqueous Electrolyte Solutions (2008)


No comments posted yet


Slide 1

Glasslike Behaviour in Aqueous Electrolyte Solutions: understanding changes in ultrafast relaxation dynamics near ions David Turton, Neil Hunt, & Klaas Wynne Department of Physics, SUPA, University of Strathclyde, Glasgow G4 0NG, Scotland, UK

Slide 2

Summary Terahertz stuff Aqueous ionic solutions A conundrum Glasslike behaviour in N-methylacetamide (NMA) Ultrafast Kerr effect studies Going slower & slower Glasses Water as a probe Aqueous electrolyte solutions Kerr studies Dielectric studies Jamming

Slide 3

Why THz spectroscopy? H-bond bend & stretch Water librations Rotational diffusion THz spectra are diffuse but: Are on timescale of chemical/biological reactions Are on timescale of MD simulations Spectra may be inhomogeneous

Slide 4

THz IR THz setup: 0.1-2.5 THz (3-80 cm-1) FTIR: 0.5-20 THz (15-600 cm-1) DRS: 100 MHz-100 GHz

Slide 5

THz plasmonics Pitch: 500nm High section: 340 nm Etch depth: 40 nm Welsh, Hunt, & Wynne, PRL 98, 026803 (2007)

Slide 6

THz Raman: OKE

Slide 7

THz Raman: OKE 2

Slide 8

Aqueous ionic solutions

Slide 9

What about water? Water’s 4 hydrogen bonds lead to tetrahedral structure But water is not ice!

Slide 10

Why study salt solutions…?! Surfaces of proteins and active sites of proteins are charged  changes water structure  affects reaction rates Thermodynamic effects  Hofmeister series, protein stability Sugars act similarly and are responsible for formation of ice cream

Slide 11

Electrolyte solutions Do ions alter the water’s hydrogen-bond network? When salts are added to water, the viscosity and density increase

Slide 12

Viscosity described by Jones-Dole: Viscosity related to (diffusive) rotational relaxation by Stokes-Einstein-Debye (SED): Jones-Dole B coefficient is often used to classify ions as either structure makers (kosmotropes) or breakers (chaotropes). They strengthen or weaken hydrogen-bond network Standard picture

Slide 13

Water clusters… “This is shown as an indicative example of the type of structure expected as water is (super)cooled”…

Slide 14

According to Laage & Hynes (PNAS 104, 11167 (2007)), the slow fraction represent exceptionally “slow waters” In fact, rotational diffusion next to Cl- is faster: 0.8 (2003 Science) Ultrafast pump-probe says: no

Slide 15

NMR relaxation from fluctuating electric-field gradients  Fluctuations slow down with concentration NMR & viscosity say: yes Viscosity increase scales with surface charge density ion (Struis, JPC 93, 7943 (1989))

Slide 16

Experiments give opposite results. This strange paradox needs to be resolved

Slide 17


Slide 18

NMA Contains the peptide linkage and is thought to form hydrogen-bonded chains in the liquid phase We perform OHD-OKE experiments with an oscillator (800 nm, 18 fs, 76/84 MHz)

Slide 19

Co-operativity Integrated far-IR spectrum is dependent on temperature Cole, JCP 68, 509 (1964) (Hunt & Wynne, CPL 431, 155 (2006))

Slide 20

Fractional SED with Ea/kB = 1200 K and  = 1.04 (T > 383 K) vs.  = 0.3 (T < 383 K) “Slipping” below 383 K Can be fit with: This is the fractional Stokes-Einstein-Debye effect (fSED)

Slide 21

DMA: no hydrogen bond Perfectly boring as expected as no hydrogen bonds (Viscosity and t2 increase as water is added)

Slide 22

Acetamide: as NMA Shows fSED again as expected (Hunt, Turner, Tanaka, & Wynne, JPCB 111, 9634-9643(2007))

Slide 23

Glasses and super-cooled liquids When a glass forms from a liquid, molecules get ‘stuck’ in non-equilibrium α relaxation diverges at Tg β relaxation remains Arrhenius (Debenedetti, Nature 410, 259 (2001))

Slide 25

Oscillations Power law Exponential NMA on multiple timescales

Slide 26

Spectral representation dynamics Debye ( relaxation) Cole-Cole ( relaxation) α β

Slide 27

Nonlinear curve fits are performed in the time domain Fit to the OKE signal for NMA at 300 K (black) with decomposition into α and β components of relaxation.

Slide 28

- and -relaxation follow macroscopic viscosity Both D and CC can be fitted by an Arrhenius dependency with a very similar activation energy of ~2400 K macroscopic viscosity fits Ea/k = 1500 K α β

Slide 29


Slide 30

Water can form glass Mishima group

Slide 31

OKE of Water Super-cooled water fits KWW (Righini, Nature 428, 296 (2004)) Water fits to Cole-Cole  Is this β-relaxation? 2 = 0.61ps,  = 0.86 Timescale for H-bond breaking

Slide 32

Dielectric relaxation α β Dielectric data water at 25º C At room temperature fits Debye curve with: 1 = 8.38 ps Thus: 1/3 = 2.8 ps Compare with: 2 = 0.61 ps

Slide 33

Water: the ideal probe Dipole moment large   DRS sensitive to molecular rotations Polarisability very isotropic  OKE sensitive to “translations” (Ratajska-Gadomska, JCP 116, 4563 (2002))

Slide 34

Water really is an ideal probe Water Methanol (Buchner et al., PRL 95, 197802 (2005)) α α β β β β α2 α2

Slide 35

Electrolyte Solutions

Slide 36

OKE on electrolyte solutions OKE fit to Cole-Cole DRS fit to Cole-Cole OKE was performed on aqueous NaCl and MgCl2 solutions DRS on MgCl2 solutions

Slide 37

OKE on solutions t2 increases with concentration β decreases with concentration MgCl2 NaCl NaCl Electrolyte solutions are more inhomogeneous

Slide 38

DRS on solutions Thus, rotations remain constant while translation slow down with concentration

Slide 39

Under isothermal conditions we use: where x0 is the glass-transition (jamming) concentration Vogel-Fulcher-Tammann behaviour Angell has shown that salts increase the glass transition temperature of water. This follows VFT equation as a function of temperature (Angell & Sare, JCP 52, 1058 (1970))

Slide 40

VFT fits VFT fit to viscosity: x0 = 12.0 M, B = 3.4 VFT fit to NMR: B = 4.5/4.1 VFT fit to OKE: B = 0.5

Slide 41

Residence times & structure Residence times typically come from MD Na+: 14-34 ps Mg2+: 228-422 ps Fe3+: 10-5-10-3 s Cl-: 3.3 ps* H2O: 4.2 ps* 4M MgCl2 - 50 M H2O (* PNAS 104, 11167 (2007))

Slide 42

Jamming transition

Slide 43

Conclusions “Glass-like behaviour” in NMA at room temperature: separate  and -relaxation Water accidentally an ideal probe molecule: DRS measures rotation (), OKE translation () Jamming transition of the ions in electrolyte solution ions form a glass water translations inhibited water rotations unhindered

Slide 44

Acknowledgements Collaborators: Richard Buchner (Institut für Physikalische und Theoretische Chemie, Universität Regensburg) Glenn Hefter (Chemistry Department, Murdoch University) C. Austen Angell (Dept. of Chem. & Biochem., Arizona State) Hajime Tanaka (Institute of Industrial Science, University of Tokyo) Thanks to: Jason Crain Leverhulme Trust

Summary: Talk about ultrafast relaxation in aqueous electrolyte solutions

Tags: ultrafast thz terahertz water liquid glass

More by this User
Most Viewed
Previous Page Next Page
Previous Page Next Page