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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - CONIN (Effects of confinement on inhomogeneous systems)

Teaser

Summary

The objective of the project is determination of universal features and specific properties of systems spontaneously ordering into spatially inhomogeneous structures, with special focus on effects of confinement. We pay particular attention to systems spontaneously forming ordered patterns, from thin films on solid surfaces through particles on interfaces to biological membranes and arid ecosystems. Our studies can help to understand origin of life, for which a confinement is believed to play an essential role, and can be exploited in innovative technology. In addition, we investigate ionic liquids/ionic-liquid mixtures near charged surfaces and in porous media, and mobile ions in intercalation compounds that are important in innovative electrochemistry. In particular, we search for systems and conditions that allow for efficient energy storage. The new results and theoretical approaches will help in future studies that may find applications in medicine, information technology, energy conversion and storage, and functional fabrics.
Spontaneous pattern formation occurs on different length scales, and typically is caused by competing tendencies in interactions between the considered objects â€“ from ions through nanoparticles, proteins, colloid particles to trees in semi-arid ecosystems. We focus on modeling the systems whose elements attract each other at short distances, but repel each other at larger distances (SALR interactions). In the case of particles, the repulsion is of electrostatic origin and the attraction can be induced by the solvent. Particles that attract each other at short distances can form clusters, but the repulsion at larger separation prevents the clusters from further growth. In the case of such particles different patterns were observed. Our purpose is on the one hand to develop new theoretical and simulation methods suitable for investigation of such systems. On the other hand, we try to find out how the fluctuations that destroy the order can be suppressed, and we investigate the effects of confinement and obstacles.
We are also interested in systems with weaker heterogeneities, in particular in room-temperature ionic liquids (RTIL) and mobile ions in solids. The properties of ionic liquids are determined by the specific interactions as well as by the Coulomb potential.There is competition between structure making and structure breaking effects which may have different manifestation next to interfaces. We intend to study structure of RTIL near a flat or a porous electrode, and effects of confinement on charging-discharging processes. Another open question is the vapour-liquid and/or liquid-liquid phase equilibria in ionic liquids and in the mixtures of RTILs with neutral components. Mutual effects of the phase transitions and the charge accumulated near an electrode are important for supercapacitors and energy storage devices. In addition, we plan experimental study of nanostructured surfaces by electrochemistry methods.

Work performed

The assumptions and approximations made in the well-established theories of fluids lead to very inaccurate predictions in the case of systems with spontaneous inhomogeneities. We have developed new theoretical methods by combining different theoretical approaches into a more general framework. The new theories have been verified by comparison with computer simulations. We have investigated pattern formation by the particles adsorbed on flat or curved surfaces, using different simulation methods for several model systems. We considered hexagonal boundaries for the particles on the interface, and cylindrical boundaries for particles in three-dimensional space. An interesting observation is formation of chiral structures made of stripes of particles adsorbed on a surface of a sphere, or inside a hexagonal boundary with a small obstacle attached to it, and by cylindrical clusters confined in a cylinder. Chiral nanostructures can be useful e. g. in analysis of chirality in proteins or amino acids. The patterns formed in confinement by the SALR particles are shown in Fig.1.
The well-established theories of electrolytes have been developed for very dilute systems. In the case of RTIL consisting solely of ions, these theories are not applicable. In our project, we have developed theoretical approaches with the size and shape of ions and the solvent molecules taken into account. In addition, we have developed simplified theories taking into account the competition between the short-range van der Waals and long-range Coulomb interactions. Using the new theories we have determined (i) the demixing phase transition in mixtures of ionic liquid and neutral solvent, (ii) vapour-liquid phase behaviour of ionic liquids confined in porous medium, (iii) distribution of ions and electrostatic potential in ionic liquids and ionic liquid mixtures near a charged surface, (iv) the effect of finite pore length on ion structure and charging, (v) electrical double layers close to ionic liquid-solvent demixing, (vi) the effect of the crystal field variation on the screening effects and electro-physical characteristics, (vii) non-monotonic concentration distribution of charge carriers in solid electrolyte between flat electrodes. Using Monte Carlo simulations, we investigated diffusion and electric conductivity in Coulomb systems. In Fig. 2 a model for a binary mixture of hard spheres and hard spherocylinders in a matrix of immobilized (yellow) hard spheres is illustrated.

Final results

We have developed new theoretical approaches suitable for inhomogeneous systems, and introduced several new, more realistic models. We have obtained numerous results concerning phase transitions and effects of confinement. We have shown how the size and shape of the confining walls can modify the patterns formed by the particles. We will continue our studies of the generic models of self-assembling systems and RTIL and RTIL mixtures by new theories and computer simulations. In particular, we will study effects of confinement on self-assembling systems in the case of adaptive boundaries and/or nontrivial shape of the confinement, and effects of confinement on the structure, phase transitions and diffusion properties of simple and complex ionic systems. Experimental studies of nanoparticles production and their assembly during formation of nanostructured surfaces will be continued.