Liquid–solid interfaces: structure and dynamics from spectroscopy and simulations

Abstract

Liquid–solid interfaces play an important role in a number of phenomenaencountered in biological, chemical and physical processes. Surface-inducedchanges of the material properties are not only important for the solid support butalso for the liquid itself. In particular, it is now well established that water at theinterface is substantially different from bulk water, even in the proximity ofapparently inert surfaces such as a simple metal.The complex chemistry at liquid–solid interfaces is typically fundamental toheterogeneous catalysis and electrochemistry, and has become especially topicalin connection with the search for new materials for energy production. A quiteremarkable example is the development of cheap yet efficient solar cells, whosebasic components are dye molecules grafted to the surface of an oxide materialand in contact with an electrolytic solution. In life science, the most importantliquid–solid interfaces are the water–cell-membrane interfaces. Phenomenaoccurring at the surface of phospholipid bilayers control the docking of proteins,the transmission of signals as well as transport of molecules in and out of the cell.Recently the development of bio-compatible materials has lead to research on theinterface between bio-compatible material and lipid/proteins in aqueous solution.Gaining a microscopic insight into the processes occurring at liquid–solidinterfaces is therefore fundamental to a wide range of disciplines. This specialsection collects some contributions to the CECAM Workshop 'Liquid/Solidinterfaces: Structure and Dynamics from Spectroscopy and Simulations' whichtook place in Lausanne, Switzerland in June 2011. Our main aim was to bringtogether knowledge and expertise from different communities in order to advanceour microscopic understanding of the structure and dynamics of liquids atinterfaces. In particular, one of our ambitions was to foster discussion betweenthe experimental and theoretical/computational communities.On the experimental side, surface specific techniques, such as non-linearoptical spectroscopy (sum frequency generation spectroscopy (SFG) and secondharmonic generation (SHG)), surface sensitive x-ray scattering, in situ scanningtunneling microscopy (STM) and infrared reflection absorption spectroscopyprovide information on layers of nanometric thickness at the interface. On theother hand, it is quite clear that the experiments require theoretical modelling inorder to dissect the experimental results and to rationalize the different factors thatcontribute to the interfacial properties. In this respect molecular dynamicssimulations are a major tool. While many successes have already been achievedwith molecular dynamics simulations based on empirical force fields, firstprinciples molecular dynamics simulations are now emerging as the other majorapproach where structure and reactivity are treated in a consistent way. Recentprogress within the past 3–5 years on efficient treatments of basis sets and longrange interactions in density functional theory (DFT) indeed extend suchsimulation capabilities to hundreds and thousands of atoms, thus allowing realisticmodels for interfaces to be tackled, maintaining first principles quality. Most ofthese simulations bring information on the structural organization of the solvent inthe interfacial region between the solid and the liquid, but very few investigate thesupplementary challenge of extracting vibrational spectroscopic fingerprints ofthe interface and, in particular, the direct modeling of the vibrational sumfrequency generation (VSFG) non-linear spectra.The present special section reports an interesting contribution from the groupof R Y Shen who pioneered VSFG optical experiments. They show how VSFGmeasurements can be used to unravel the behavior of interfacial water on aluminaAl2O3 as a function of pH. The groups of A Hodgson and C Busse respectivelyprovide complementary experiments based on low energy electron diffraction(LEED), He atom scattering (HAS) and STM, to investigate the organization ofwater on metal, namely Pd(111) and Pt(111). Direct measurements ofhyperpolarizabilities for non-linear spectroscopy can be made throughhyper-Rayleigh scattering experiments, which are presented here by the group ofP F Brevet on gold and silver nanoparticles.From the point of view of molecular dynamics simulations of interfaces,complementary levels of calculations are presented in this special section. Thegroups of K Leung, M-P Gaigeot, M Sulpizi and M Sprik provide theoreticalinvestigations with DFT-based molecular dynamics simulations. Leung et al andGaigeot et al address the hot topic issue of the reactivity of oxides surface sitesand especially reliable methods to calculate pKas of these sites, with simulationstaking into account both the solid and the liquid explicitly, and at the same firstprinciples level of theory. Gaigeot, Sprik and Sulpizi furthermore combine theinformation on the structural organization of liquid water at the interface withquartz and alumina via pKa calculations and vibrational features (and theirmicroscopic assignments). Mixed quantum/classical molecular dynamics(QM/MM) simulations are presented by Ishiyama and Morita for the investigationof another topical interface, i.e. the liquid–air interface. They provide thetheoretical VSFG spectrum of the water–vapor interface and some understandingat the microscopic level of the experimental vibrational features.Molecular dynamics simulations based on empirical force fields have beenapplied to investigate hydrophobic interfaces by the groups of B Space andP Carloni. Carloni et al address salt effects at water–hydrophobic interfaces,investigating how the salts affect the structural organization of water at theseinterfaces. Space et al provide theoretical approximations to VSFG calculationsin the special case of the carbon tetrachloride–water interface and the assignmentsof the experimental recorded signatures.'More traditional' DFT static calculations can be applied to complex objectsat interfaces, providing their vibrational spectra, and two papers in this specialsection illustrate such approaches. Ceccet et al extract first hyperpolarizabilitytensors from DFT calculations on aliphatic chains and simulate the related VSFGspectra. They also investigate the effect of different functionals on the finalsignatures. Liegeois et al investigate functionalized surfaces, mainly focusing onIR and Raman spectral features, and provide very precise vibrational assignmentsdepending on chemisorption or physisorption of the adsorbed molecules.We are grateful to all the authors for their contributions to this special sectionand we hope that readers will enjoy this collection of papers and that they willfind further motivation to investigate and understand the complex phenomenaoccurring at interfaces.Liquid–solid interfaces contentsThe interfacial structure of water/protonated α-Al2O3 (112¯0) as a function of pH J Sung, Y R Shen and G A WaychunasStrain relief and disorder in commensurate water layers formed on Pd(111) F McBride, A Omer, C M Clay, L Cummings, G R Darling and A HodgsonH2O on Pt(111): structure and stability of the first wetting layer Sebastian Standop, Markus Morgenstern, Thomas Michely and Carsten BusseEffect of a thioalkane capping layer on the first hyperpolarizabilities of gold and silver nanoparticles Yara El Harfouch, Emmanuel Benichou, Franck Bertorelle, Isabelle Russier-Antoine, Christian Jonin, Noelle Lascoux and Pierre F BrevetPredicting the acidity constant of a goethite hydroxyl group from first principles Kevin Leung and Louise J CriscentiOxide/water interfaces: how the surface chemistry modifies interfacial water properties Marie-Pierre Gaigeot, Michiel Sprik and Marialore SulpiziVibrational spectrum at a water surface: a hybrid quantum mechanics/molecular mechanics molecular dynamics approach Tatsuya Ishiyama, Hideaki Takahashi and Akihiro MoritaA theoretical study of the sum frequency vibrational spectroscopy of the carbon tetrachloride/water interface Anthony J Green, Angela Perry, Preston B Moore and Brian SpaceSalt effects on water/hydrophobic liquid interfaces: a molecular dynamics study Chao Zhang and Paolo CarloniDensity functional theory-based simulations of sum frequency generation spectra involving methyl stretching vibrations: effect of the molecular model on the deduced molecular orientation and comparison with an analytical approach F Cecchet, D Lis, Y Caudano, A A Mani, A Peremans, B Champagne and J GuthmullerTowards modelling the vibrational signatures of functionalized surfaces: carboxylic acids on H–Si(111) surfaces Conrard Giresse Tetsassi Feugmo, Benoît Champagne, Yves Caudano, Francesca Cecchet, Yves J Chabal and Vincent Liégeois