Physics and Chemistry at Fluid/Fluid Interfaces
- Marcello Sega (Helmholtz-Institut Erlangen-Nürnberg, Germany)
- Pal Jedlovszky (Eszterházy Károly University, Eger, Hungary, Hungary)
At the macroscopic, continuum level, liquid/liquid and liquid/gas interfaces are crucial to define ourselves and the world around us, and yet, when it is our turn to define them, especially at the microscopic, atomistic level, we often run into trouble.[Row86]
The study of liquid/vapour interfaces by means of computer experiments is as old as the field itself[Cro71] and, since the very beginning, fundamental questions such as the shape of the interfaces or the presence of molecular layers, have been lively debated[Cha75]. Developments in experimental techniques[Reg95,Mag95] and in computational approaches[Cha03] have later shown that the molecular layers are indeed observable in liquid metals, and that the intrinsic structure of the interface of liquids resembles that of their radial distribution function. Still, a clear, parameter-free definition of what interfacial atoms are has yet to be proposed[Jor10,Seg13,Sch15], but this did not prevent investigations on interfacial properties to be extended from simple and molecular liquids to complex ones, including polymers, bilayers, micelles, proteins and colloids[Bel16].
The field is in continuous development: recent results on simple liquids, based on larger and more accurate simulations and new theoretical analysis, have shown the limitations of the capillary wave theory and the importance of lateral correlations[Fer13,Cha16]; New studies on the properties of ions at the water/vapour interface are changing our view on their organization[Jun06,Net12] and on the once-believed universal Hofmeister series[Jun14,Sch16]; The surface roughness has been found to play an important role in determining activation energies[Nel10], and the need has been underlined, to revise our theoretical approaches to describe chemical reactions occurring at or in the immediate vicinity of liquid/liquid interfaces[Ben15]; Several simulation studies underline the importance of hydration layers being carried along together with charged solutes when crossing liquid interfaces[Dar13,Kik15]; The study of the distribution of stresses across liquid interfaces, with all the problems connected to its non-locality, as well as the proper evaluation of the surface tension have also seen a rise in interest in recent years[Oll09,Cho16]; Finally, non-equilibrium phenomena such as the evaporation of solutes with Super- and Sub-Maxwellian kinetic distributions[Kan16], and the coexistence of liquid- and vapour-like phases in active matter[Pry16], are testing the limits of validity and applicability or thermodynamics in our understanding of the physics and chemistry at fluid/fluid interfaces.
The workshop will focus on recent development and possible future directions regarding (a) intrinsic properties of fluid/fluid interfaces (b) partially miscible systems (c) issues of non-locality and approaches beyond the capillary-waves theory (d) interfacial properties of membranes (e) reactions at interfaces (f) calculation of local stresses (g) non-equilibrium systems: active matter and non-Maxwellian distributions.
In particular, the following issues will be tackled, besides, of course, those problems that will spontaneously arise during the discussions.
* Molecular layers: while the exisence of molecular layers at interface is now generally accepted, it is not yet clear which criterion should be used to associate molecules to the successive layers. Current solutions include applying repeatedly the algorithm used to identify interfacial molecules but the presence of a free parameter poses a consistency problem that needs to be discussed.
* Intrinsic properties: the intrinsic Helmholtz free energy profile, defined by a Boltzmann inversion of the intrinsic density profile, can be easily calculated, but it is not clear what its physical relevance is. In particular, it will be discussed about possible ways to relate it to kinetics of processes such as adsorption/desorption or evaporation/condensation, as well as to the problem of non-Maxwellian distributions in out-of-equilibrium evaporation of solutes.
* Active matter: to be discussed are the possible violation of the law of corresponding states, finite size effects and the existence of critical exponents, as well as the possibility of calculating osmotic pressure profile for this kind of systems.
* Partially miscible liquids: for these systems the definition of a dividing surface is more challenging than for fully demixing systems. Some strategies have already been proposed, but fail in determining interfaces if one one of the two phases is percolating, and new approaches are needed.
* Tolman length: strategies for the determination of the Tolman length for planar and curved interfaces should be discussed. For the planar systems, the value of the surface of tension as obtained through the pressure profile is not unique, and depends on the chosen path.
* van der Waals long-range corrections: the importance of long-range corrections to the dispersion term is increasingly being recognized, however a large body of results and methods is still based on simple cut-offs. The impact on interfacial properties should be discussed, along with best practices and strategies for an effective transition to a new standard in treating dispersion forces.
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