Glasslike Behaviour in Aqueous Electrolyte Solutions (2008)

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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

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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

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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

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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

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

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THz Raman: OKE

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THz Raman: OKE 2

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Aqueous ionic solutions

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

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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

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

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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

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

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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

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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))

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

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NMA

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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)

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

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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)

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

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

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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))

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Oscillations Power law Exponential NMA on multiple timescales

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

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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.

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- 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

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Water

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Water can form glass Mishima group http://www.nims.go.jp/water/

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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

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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

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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))

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

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Electrolyte Solutions

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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

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

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

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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))

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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

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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))

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Jamming transition

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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

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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

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