Liquid/Solid interfaces: Structure and dynamics from spectroscopy and simulations
CECAM-HQ-EPFL, Lausanne, Switzerland
Liquid-solid interfaces are ubiquitous and responsible for a number of phenomena encountered in biological, chemical and physical processes. Surface-induced changes of material properties are not only important for the solid support but also for the liquid itself. The complex chemistry at solid-liquid interfaces is fundamental to heterogeneous catalysis and electrochemistry and has become especially topical in connection with the search for new materials for energy production. The interactions at solid/liquid interface control crystallization and shape selective crystal growth. This is fundamental for example to the synthesis of nanoparticles with speciﬁc tailored shape/size.
In life science, the most important solid-liquid interfaces are the cell-membrane / water interfaces. Phenomena occurring 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. More recently the development of bio-compatible materials has lead to research on the interface between bio-compatible material and lipid/proteins in aqueous solution.
Gaining microscopic insight in the processes occurring at solid-liquid interfaces is therefore fundamental to a wide range of disciplines. Recently, a deeper insight has been gained thanks to major advances in both experimental techniques and computer simulations. Two years ago we organized a ﬁrst CECAM workshop aiming to bring together diﬀerent communities involved in microscopic understanding of interfaces. This included on the experimental side, experts from surface speciﬁc techniques, such as non-linear optical spectroscopy (Sum Frequency Generation Spectroscopy (SFG) and
Second Harmonic Generation (SHG)), surface sensitive X-ray scattering, in situ Scanning tunnelling microscopy (STM) and infrared reﬂection absorption spectroscopy, and on the other side experts from the theory/computer simulation community. Synergic interaction between theoretical modelling and experiments is fundamental in order to dissect the experimental results and to rationalize the diﬀerent factors that contribute to the interfacial properties. This ﬁrst workshop on solid/liquid interfaces provided the framework for people from both theoretical and experimental communities to describe the state -of-the-art developments in their respective ﬁelds. It gave the possibility to interact and trigger new collaborations, especially between theoreticians and experimentalists, bringing together several communities ranging from solid-state, surface science, liquid-state, biochemistry
and fostering the exchange ideas and methodologies, and contributing to create a network of collaborations. Selected peer reviewed contributions from the participants to the workshop are now available in a special issue of Journal of Physics: Condensed Matter (Marie-Pierre Gaigeot and Marialore Sulpizi 2012 J. Phys.: Condens. Matter 24 (12)).
The ﬁrst key topic includes the most recent progresses in vibrational sum frequencies generation, with particular focus on phase sensitive experiments and multidimensional spectroscopy. Phase sensitive SFG experiments have permitted to unveil the molecule orientation at the interface. For example in case of mineral surfaces they have permitted to address the details of surface including the types of functional groups present, their protonation state and bond orientations, and the nature of near-surface water organization.  YR
Shen who is a pioneer of this technique will be among our experimentalists invited speakers, together with M. Bonn, S.Roke, G. Richmond, H. Allen. Along with them theoreticians with large experience in the analysis and understanding of solid/liquid interfaces such as mineral surfaces [2–5]
(J.D. Kubicki, K. Leung) and will provide a picture of what is the current theoretical understanding. Another key aspect will be the recently developed
[6, 7] femtosecond SFG-2D-IR spectroscopy. This technique combines the interface selectivity with the time resolution and permits to address the energy transfer dynamics. A key question here is how does the bulk energy transfer compare to the surface one? Water is a striking example, due to its very rapid resonant (Foster) energy relaxation dynamics which it exhibits both in bulk and at the surface.  A quite interesting point is how interfaces with similar static SFG response can a have a diﬀerent relaxation dynamics. .
The second key topics on our program is how can simulation methods and new developments cope with the challenge of interpreting the new experimental results on interfaces. This points to the new method development aspect of our workshop.
On one side is the demand for high accuracy, including electronic structure eﬀects, such as electronic polarization. On the other hand is the need to accumulate the relevant statistics in order to compare the calculated signal to the experimentally recorded one. In this respect we plan to invite scientists from both the quantum mechanics ( A. Michaelides,  K. Leung, , [10–12] M. Sprik,  N. Marzari, A. Gross [14, 15]) and classical mechanics/multiscale modelling (P. Jungwirth, L.C. Ciacchi, [16–18] M. Salanne,  H. Heinz [20, 21]) communities and experts from non-linear quantum optics (A. Morita, Y. Nagata [22, 23]) to share their view and their solutions to the response functions calculations at interfaces.
A special chapter of our workshop will be dedicated to the role of ions at
solid/liquid interfaces. There has been a huge eﬀort in the discussion of ions location at the water/vapor interface in connection with the Hofmeister series and its impact on a series of phenomena involving interfaces, such as protein binding and folding (See for example Ref. 24). Among our participants we will have a leading expert in the ﬁeld, such as P. Jungwirth. [25, 26] More recently there has been also a lot of debate on the ions inﬂuence on the interface vibrational spectra of surface water. Pioneer work of H. Allen [27, 28] has shown how diﬀerent ions with diﬀerent surface propensity and diﬀerent solvation properties can have a quite distinct impact on the water/vapor interface spectroscopy.  However not less crucial is the role of ions at solid/liquid interfaces, which has been less characterized, but is fundamental to ﬁelds such as geochemistry, but also to the characterization of devices for energy production(photovoltaic, fuel cells, (bio-)sensors). The role of charge distribution at solid/liquid interface is a key step toward the design of new devices living in contact with electrolyte solutions. Among the young investigators, K. Campean  and J. Gibbs-Davis [31, 32] will present their ﬁrst results on the ions behaviour at solid/liquid interfaces.
Nano- and microparticles have optical, structural, and chemical properties that diﬀer from both their building blocks and the bulk materials themselves, due to the high surface-to-volume ratio. To understand the properties of nano- and microparticles, it is of fundamental importance to characterize the particle surfaces and their interactions with the surrounding medium. Recent developments of nonlinear light scattering techniques have resulted in a deeper insight of the underlying light-matter interactions. 
They have shed new light on the molecular mechanism of surface kinetics in solution, properties of interfacial water in contact with hydrophilic and hydrophobic [22, 23] particles and droplets, molecular orientation distribution of molecules at particle surfaces in solution,  interfacial structure of surfactants at droplet interfaces, acid-base chemistry on particles in solution, and vesicle structure and transport properties.
Finally a few selected talks will contribute to out last topic: Not only water.
Water has been the most widely studied liquid due to its universal presence and its very special phase diagram, however solid/liquid interface also involve diﬀerent liquids, where completely diﬀerent properties are involved. In the realm of new systems for energy production such as dye-sensitized solar cells and super capacitors, roomtemperature ionic liquids are considered the medium of choice. Thus, while the ionic liquid appears to be a promising medium,  there is almost no molecular-level information as to their interaction with the solid surfaces. Recent new investigations were
able to provide the orientation of room-temperature ionic liquid at the graphene-ionic liquid interface  and to describe the nature of an ionic liquid-(solid) salt interface. 
Marie-Pierre Gaigeot (LAMBE UMR8587 Université d'Evry val d'Essonne, Université Paris-Saclay) - Organiser & speaker
Marialore Sulpizi (Ruhr University Bochum) - Organiser