Our research group is interested in physico-chemical aspects of condensed systems.
We aim to provide a better understanding of simple and complex liquids and solution through a combination
of simulations, theory, and experiment. We are mainly intersted in water and aqueous solutions of hydrophobic
and ionic solutes (simple electrolytes, synthetic polyelectrolytes, proteins). We are a part of the Chair of
Physical Chemistry at Faculty of Chemistry and Chemical Technology (University of Ljubljana, Ljubljana, Slovenia).
Protein aggregation plays a crucial role in various diseases, such as Alzheimer's, Parkinson's, Huntington's and others. At the same time, protein aggregation complicates development of biological drugs, as it is often challenging to formulate a stable protein solution. These processes are not well understood. Our aim is to provide new important insights using a combination of experiments and theory.
We model proteins as hard spheres with a number of attractive square-well sites on the surface, while water and salt are treated implicitly. Using Wertheim's thermodynamic perturbation theory, we studied salt-specific thermodynamics of protein aggregation.
Even such a simple model correctly predicts phase diagram of various proteins. It implies that the main characteristics of protein aggregation are captured by the simple patchy model.
Next we focused on salt-specificity of protein aggregation.Our model is able to correctly predict variations in phase diagrams with respect to the type and concentration of the added salt.
We are interested in having a simple model of water that can show competition between electrostatics and hydrogen-bonding in hydration of ions.
A 2D water model with dipole shows that small enough ions prefer to be electrostatically hydrated on the account of broken hydrogen bonds. As the ion size increases, hydrogen bonding becomes the dominant mechanism of hydration. Smaller cations order water molecules to the same level as larger anions. The difference ultimately originates in asymmetric distribution of charge on a water molecule.
How do different salts affect solubility of a nonpolar solute in water? We tackled the question by studying the thermodynamics of hydrophobic hydration around various ions. In agreement with the experiments, our model predicts largest salting-out of hydrophobes for smallest ions (large charge density). This is due to a compact structure of ionic hydration shell, which is an effectively excluded volume for a hydrophobe.
Systematic Study of Ion Pairing in Popular Water Models
We studied ion pairing for a full set of alkali halides in 5 popular water models using Molecular Dynamics simulations. Three different force fields for ion-ion and ion-water interactions were tested. Analyzing potentials of mean force, good correlation with Collins proposition ("Law of matching water affinities") was found: small-small and big-big ion pairs tend to associate, while small-big combinations tend to dissociate.
Fast Method for Ion-Ion PMF calculations
We develop an interpolation scheme, called i-PMF, that is capable of capturing the full set of PMFs for arbitrary combinations of ion sizes. The advantage of the interpolation process is computational cost (105 speed-up compared to simulations).
Microwaves induce fast rotational motion of water molecules. Subsequently, the excess rotational kinetic energy is transfered to translational motion, and the whole system heats up. However, under continuous irradiation, there is not enough time for relaxation, and the state is formally described with rotational temperature being greater than the translational one.
We developed a simulation protocol to simulate systems, where translational and rotational temperature differ. We employed home-developed molecular dynamics code to investigate the role of translational and rotational motion on aqueous solutions and at the same time gain insight into the microwave-specific effects on water, hydration of simple solutes, hydrophobic effect and ionic interactions.
Ionenes are toy molecules for studying the interplay between hydrophobic and charge-density effects
Despite many experimental and theoretical studies performed over the past five decades, understanding the solvation of complex molecules such as polyelectrolytes is — at best — semi-quantitative.
Aliphatic x,y-ionenes are polycations having quaternized nitrogen atoms in their backbone, where x and y denote numbers of methylene groups between the consecutive nitrogens:
They are ideal molecules for systematic studying of the balance between hydrophobic and charge effects in water. In addition, aqueous solutions of different ionenes (w/o added simple salts) display pronounced ion-specific effects.
We combine experiments (viscosity, osmotic coefficient, electric conductivity, transport numbers, dielectric relaxation spectroscopy, various calorimetric techniques, small angle neutron scattering), simulations (Molecular Dynamics), and theory (scaling theory, Manning theory, Poisson-Boltzmann cell model) to study polyelectrolyte solutions.
We're interested in statistical models of fluids that show anomalous properties typical of water. Next, we use these models to study the hydrophobic hydration.
Accounting for the orientational degrees of freedom
We developed an analytical version of a water model, first introduced by Ben-Naim and commonly called "Mercedes-Benz" (MB) water. The model explicitly accounts for the orientationally dependent hydrogen-bonding between water molecules. We have the model in 2D and 3D version. Test water can be in three different states with respect to its neighbouring water, depending on the separation distance and H-bond orientation:
Volumetric and thermal properties follow the same trends with temperature as in real water and are in good general agreement with simulations (density anomaly, minimum in the isothermal compressibility, decreased number of hydrogen bonds upon increasing the temperature etc.).
Next, we extended this water model to obtain expressions for thermodynamic quantities of hydrophobic hydration.
Our goal was to get a model that captures the physical balance between water's hydrogen bonding and van der Waals interactions and is simple enough to be nearly analytical and not heavily computational. As validation, we find that this model gives the same trends as reference simulations of the underlying model and gives qualitative agreement with experiments.
Core-softened models of fluids utilize pair potentials that are spherically symmetric with two characteristic lengths. There is no explicit treatment of orientational degrees of freedom.
The model qualitatively follows the same trends as water. It is able to reproduce water-like anomalous properties and that of the hydrophobic effect (minimum solubility for nonpolar solutes near ambient conditions, increased solubility of larger noble gases, etc.). Interestingly, this model yields similar results as more complex and computationally expensive models that take orientational dependence of H-bonding into account.
Disordered Porous Materials
Partly quenched systems containing charges
Partly-quenched systems are those in which some of the degrees of freedom are quenched and others are annealed. This systems can be used to describe disordered porous materials filled with liquid or solution. Continuum systems with quenched disorder (e.g. nonporous materials, gels, amorphous substances, clays, engineering composites etc.) are of great interest for medicine, separation science, and catalysis, as well as for various technologies.
Simulations and replica integral equation theory
We're interested in thermodynamical, dynamical, and structural properties of partly-quenched systems which contain charges (ions). We use computer simulations (Monte Carlo, Brownian Molecular Dynamics) and develop theory based on replica formalism (replica integral equation theory).
From the editorial:
The papers in this special issue of the Journal of Molecular Liquids have been submitted with the dedication to Prof. Vojko Vlachy on the occasion of his 70th birthday. Vojko Vlachy is today known as one of the leading scientists in the physical chemistry of water and liquids, his theoretical and experimental work has focused mainly on electrolyte and polyelectrolyte solutions, particularly including proteins and colloids. During his career he has, and still is collaborating with prominent scientist from around the world. As such, this special issue is a collection of papers coming from different research groups, reporting on recent experimental and theoretical developments in the area of simple liquids and macromolecular solutions.
On behalf of the Editorial Board of J. Mol. Liq., as well as personally, we wish Vojko a generous source of new ideas, plentiful scientific results and good health for many years to come! We also express our gratitude to the contributors of this special issue.
Ken A. Dill, Myroslav F. Holovko, Barbara Hribar-Lee, and Natalie Malikova
Paper in Chemical Reviews
How Water’s Properties Are Encoded in Its Molecular Structure and Energies
Emiliano Brini, Christopher J. Fennell, Marivi Fernandez-Serra, Barbara Hribar-Lee, Miha Lukšič, and Ken A. Dill
How are water’s material properties encoded within the structure of the water molecule? This is pertinent to understanding Earth’s living systems, its materials, its geochemistry and geophysics, and a broad spectrum of its industrial chemistry. Water has distinctive liquid and solid properties: It is highly cohesive. It has volumetric anomalies—water’s solid (ice) floats on its liquid; pressure can melt the solid rather than freezing the liquid; heating can shrink the liquid. It has more solid phases than other materials. Its supercooled liquid has divergent thermodynamic response functions. Its glassy state is neither fragile nor strong. Its component ions—hydroxide and protons—diffuse much faster than other ions. Aqueous solvation of ions or oils entails large entropies and heat capacities. We review how these properties are encoded within water’s molecular structure and energies, as understood from theories, simulations, and experiments. Like simpler liquids, water molecules are nearly spherical and interact with each other through van der Waals forces. Unlike simpler liquids, water’s orientation-dependent hydrogen bonding leads to open tetrahedral cage-like structuring that contributes to its remarkable volumetric and thermal properties.
From the editorial:
It is our privilege and a great pleasure to honor professor Ken A. Dill with this special issue of The Journal of Physical Chemistry B on the occasion of his 70th birthday that he celebrated in December 2017, as well as for his wide-ranging contributions to science. This is our way of thanking him for all that he has taught us throughout these years, and of wishing him many more years of scientific discoveries.
This special issue was organized by Guest Editors Barbara Hribar-Lee, Evangelos Coutsias, Miha Lukšič, and Chaok Seok.