Final Report Summary - STCSCMBS (Statistical Thermodynamics and Computer Simulations of Complex Molecules in Bulk and at Surfaces)
The project „Statistical Thermodynamics and Computer Simulations of Complex Molecules in Bulk and at Surfaces” was devoted to the theoretical and computer simulation studies of nonuniform fluids involving complex molecules. It comprised of three work packages connected with investigations of (i) fluids in contact with tethered layers formed on surfaces and in pores, (ii) substrate driven self-assembly of supramolecular structures formed by complex organic molecules, and (iii) substrate induced self-assembly of nanoparticles with chemical dichotomy. The investigations have been carried out by researchers from four scientific institutions: Maria Curie-Skłodowska University, Lublin, Poland; Technical University Berlin, Germany, Institute for Condensed Matter Physics, Lviv, Ukraine and Universidad National Autonoma de Mexico, Mexico City, Mexico. During the project, we have published 20 scientific papers and the next 7 papers have been submitted for publication. A great majority of the papers have been published in top-ranked scientific journals. Our results have also been presented during 31 scientific conferences and workshops. The web address of the project is: http://stcscmbs.umcs.lublin.pl/(öffnet in neuem Fenster) .
One group of problems involved the description of effects of tethered brushes on the structure and thermodynamic properties of confined single and multicomponent fluids, in particular, on the selective adsorption, surface phase transitions, surface-induced phase separation, the formation of long-ranged fluid structures and many others. The systems involving grafted chains are not only interesting from the academic point of view, but they are also widely used in surface modification of various materials because of their special properties as biocompatible layers, environmental response materials, and on-fouling coatings. We also studied the systems of colloidal particles dispersed in a liquid crystalline environment. This is a fascinating class of soft matter composite materials with rich physics and inexhaustible potential for technological innovation. In fact, nowadays the so-called liquid crystal colloids form the basis for advanced functional and structural materials, engineering and household devices and products, and play a crucial role in various industrial applications. Therefore, understanding the properties of such systems is one of fundamental problems for science and technology. Finally, we studied the structure and the thermodynamic properties of Janus particles at interfaces. The term “Janus particles” designates spherical particles with one hydrophilic and the second hydrophobic hemisphere face properties. The presence of these two ingredients yields a large variety of self-assembled structures on different length scales. These range from molecular size micelles to mesoscopic membranes, foams, and lamellar phases. In practical applications, amphiphiles are used in a variety of contexts, e.g. to reduce the surface tension in complex mixtures, tertiary crude-oil recovery or as detergents; they can be also employed in synthesis of nanoporous materials, thin films for lithographic processes, for enhancement of chemical reactions and to stabilize bundle and network formation in solutions of carbon nanotubes. Moreover, the Janus particles can also be viewed as a coarse-grained model of amphiphilic molecules. Thus, it is not surprising that the investigations of Janus particles has attracted a great attention recently.
The basic achievements of our studies can be divided into two categories. The first one involved the development of new theoretical approaches and computer simulation techniques. The second was the application of these approaches (and also other well-established methods) to investigate the behavior of selected systems and to describe the phenomena occurring in them. We have proposed new versions of the density functional theory and new elements of the dissipative particles dynamics simulation method for the studies of fluid/tethered and colloid/liquid crystal systems. Novelties of our density functional theory have relied on: (1) accounting for a possibility of Coulomb interactions in the system; (2) taking into account a possibility of anisotropic interactions between fluid molecules and between fluid molecules and chains; (3) considering the cases when the walls are permeable to one of the component of a fluid. In order to handle such problems, we have proposed adequate forms of the free energy functionals involving the terms due to electrostatic forces and/or the terms allowing for the description of fluids in contact with semi-permeable membranes. In the case of simulation studies, we have also developed new methods for the dissipative particle dynamics (DPD) simulations: the method for accounting for the presence of walls in the simulations of confined fluids, the method for effective simulations of flows through narrow pores and the method for simulating colloid-liquid crystal mixtures.
In several papers, we have discussed phase transitions. In particular, using the density functional approaches, we have found new types of transitions in the systems involving fluids and chains tethered by their both ends to the opposite walls of slit like pores. In such systems, in addition to “usual” surface transitions such as capillary condensation, layering and wetting transitions, one can observe the transitions connected with the symmetry breaking and the return to symmetry in the distribution of the segments of chains. The new transitions can be of the first or of the second order, they can join the envelopes of surface transitions at critical end or at tricritical points. In the case of binary mixtures, which exhibit miscibility transition in the bulk, the scenario of phase transformations can be much more complex and fascinating, due to the competition between surface transitions, breaking and re-entrant recovery of symmetry transitions and the demixing transition. We have found a possibility of wetting and layering transitions at the external surfaces of semipermeable membranes that were built of layers of tethered chains. We have also demonstrated that the presence of a layer of tethered chains may enhance or inhibit the formation of orientationally ordered structures formed by amphiphilic (Janus) molecules at surfaces, depending on the character of interactions between the amphiphiles and the chain segments. The phase transitions in monolayers of rigid and flexible chains, as well as in monolayers of Janus-like particles have been studied by Monte Carlo simulations. In both cases, we have evaluated the phase diagrams, and identified differently ordered phases (e.g. lamellar) by employing appropriate methods of analysis. We have also carried out Monte Carlo simulations of phase transitions in confined lattice Janus fluids and a rich variety of different phases has been found. Finally, we have developed a density functional approach suitable to describe patchy colloids with two different off-center sites A and B and the allowed formation of double AB bonds. The proposed approach has been based of the fundamental measure theory and on the second order perturbation theory of Wertheim. For that model, a re-entrant phase behavior in bulk system has been found. Our calculations revealed that the re-entrant phase diagrams also appear in confined systems and the both lower and upper critical temperatures are independent of the pore width.
The anisotropic interactions between amphiphilic molecules and between amphiphiles and ions can lead to an interesting phase behavior at surfaces. We have demonstrated that addition of an appropriate amount of ions or the change of the wall charge can stimulate the formation of long-ranged orientationally (and in some cases also translationally) ordered structures. This is an example of a stimuli-responsive system, in which an external stimuli (e.g. the electrostatic potential) can lead to structural changes.
In several works, we have studied an effective force acting between surfaces (or colloidal particles). We have proposed new approaches that allow for evaluation of the force between two surfaces modified with polyampholytes, as well as between two surfaces immersed in the fluid of Janus particles. We have also proposed a theory aimed at the interpretation of the Atomic Force Microscope results obtained for modified surfaces. The theory allows for calculation of the force acting on a particular segment and the average force extorted by an apparatus tip during its movement over a modified surface. We have also explained the mechanism of adsorption of fluids on modified surfaces, changes in the selectivity of adsorption and changes in the characteristics of the tethered layer during adsorption of single and two-component fluids. Under certain conditions, we have found “unusual” variation of the height of tethered layer with temperature and with the amount of adsorbed species. Also, we have derived a theory that describes the behavior of tethered layers built of polyampholytes and adsorption of ions on such surfaces. Our approach allows for the evaluation of the dependence of the electrical double layer capacitance on the pore width in pores with the walls modified by polyampholytes. The investigations of the electric double layer capacitance of systems with nanopores are of particular importance for the development of new energy storages called supercapacitors.
The mesoscale DPD simulations have been used to study the microphase separation in binary partially miscible fluids confined in slit-like pores with modified walls. Our main interest has been to determine how the possible morphologies (lamellar, pillar-like, cylindrical, etc.) formed inside the pore depend on the geometrical parameters of the system. To describe the observed morphologies, we have calculated several characteristics (the density and temperature profiles, the radii of gyration for the attached polymers, and the minimum polymer-polymer distances in the direction parallel and perpendicular to the walls). The results obtained for the systems under equilibrium condition have been next used as starting points for the simulation of flows. We have found that some of equilibrium morphologies are sensitive to
the flow rate. For moderate flow rates, the steady-state has been found to consist of lamellar morphology with flow occurring mainly in the center of the channel. At sufficiently high flow rates, we have observed the flow-induced mixing in the center of the channel. The mesoscopic simulation technique has been also used in the studies of colloid-liquid crystal mixtures using different models for colloidal particles. In addition, a grafted colloid that mimics a gold nanoparticle with mesogen-modified surface has been used. The nematic phase has been modelled explicitly via soft spherocylinders interacting through an appropriately selected potential. In addition, estimates for the nematic-isotropic transition and the coherence length have allowed to establish a relation between the energy and the length scales with respect to the experimental systems. The models used exhibit defect topologies, namely that of a Saturn ring and a boojum defect for homeotropic and planar surface anchoring, respectively. In the grafted colloid model we have tuned the anchoring strength through the density of the mesogenic shell on the surface. We also found the biaxial boojum effect for the special case of longitudinal planar anchoring.
The scientific discoveries and findings of the project are mainly addressed to scientists and post-graduate students in the field of statistical thermodynamics of soft matter, and should help in a better understanding of a variety of systems involving complex molecules and surfaces. Our results can be used to interpret experimental observations and may also of importance for the development of new materials. It is important to stress that the collaboration between scientists from different centers increased scientific potential of all partners involved. During the project, we have collaborated with people not formally involved in the project. This mainly concerns two groups: from UMCS and from ICMP. The collaboration is being continued. At the moment, two consecutive scientific works are under preparation and we plan to continue the studies that have been the involved in the project.
One group of problems involved the description of effects of tethered brushes on the structure and thermodynamic properties of confined single and multicomponent fluids, in particular, on the selective adsorption, surface phase transitions, surface-induced phase separation, the formation of long-ranged fluid structures and many others. The systems involving grafted chains are not only interesting from the academic point of view, but they are also widely used in surface modification of various materials because of their special properties as biocompatible layers, environmental response materials, and on-fouling coatings. We also studied the systems of colloidal particles dispersed in a liquid crystalline environment. This is a fascinating class of soft matter composite materials with rich physics and inexhaustible potential for technological innovation. In fact, nowadays the so-called liquid crystal colloids form the basis for advanced functional and structural materials, engineering and household devices and products, and play a crucial role in various industrial applications. Therefore, understanding the properties of such systems is one of fundamental problems for science and technology. Finally, we studied the structure and the thermodynamic properties of Janus particles at interfaces. The term “Janus particles” designates spherical particles with one hydrophilic and the second hydrophobic hemisphere face properties. The presence of these two ingredients yields a large variety of self-assembled structures on different length scales. These range from molecular size micelles to mesoscopic membranes, foams, and lamellar phases. In practical applications, amphiphiles are used in a variety of contexts, e.g. to reduce the surface tension in complex mixtures, tertiary crude-oil recovery or as detergents; they can be also employed in synthesis of nanoporous materials, thin films for lithographic processes, for enhancement of chemical reactions and to stabilize bundle and network formation in solutions of carbon nanotubes. Moreover, the Janus particles can also be viewed as a coarse-grained model of amphiphilic molecules. Thus, it is not surprising that the investigations of Janus particles has attracted a great attention recently.
The basic achievements of our studies can be divided into two categories. The first one involved the development of new theoretical approaches and computer simulation techniques. The second was the application of these approaches (and also other well-established methods) to investigate the behavior of selected systems and to describe the phenomena occurring in them. We have proposed new versions of the density functional theory and new elements of the dissipative particles dynamics simulation method for the studies of fluid/tethered and colloid/liquid crystal systems. Novelties of our density functional theory have relied on: (1) accounting for a possibility of Coulomb interactions in the system; (2) taking into account a possibility of anisotropic interactions between fluid molecules and between fluid molecules and chains; (3) considering the cases when the walls are permeable to one of the component of a fluid. In order to handle such problems, we have proposed adequate forms of the free energy functionals involving the terms due to electrostatic forces and/or the terms allowing for the description of fluids in contact with semi-permeable membranes. In the case of simulation studies, we have also developed new methods for the dissipative particle dynamics (DPD) simulations: the method for accounting for the presence of walls in the simulations of confined fluids, the method for effective simulations of flows through narrow pores and the method for simulating colloid-liquid crystal mixtures.
In several papers, we have discussed phase transitions. In particular, using the density functional approaches, we have found new types of transitions in the systems involving fluids and chains tethered by their both ends to the opposite walls of slit like pores. In such systems, in addition to “usual” surface transitions such as capillary condensation, layering and wetting transitions, one can observe the transitions connected with the symmetry breaking and the return to symmetry in the distribution of the segments of chains. The new transitions can be of the first or of the second order, they can join the envelopes of surface transitions at critical end or at tricritical points. In the case of binary mixtures, which exhibit miscibility transition in the bulk, the scenario of phase transformations can be much more complex and fascinating, due to the competition between surface transitions, breaking and re-entrant recovery of symmetry transitions and the demixing transition. We have found a possibility of wetting and layering transitions at the external surfaces of semipermeable membranes that were built of layers of tethered chains. We have also demonstrated that the presence of a layer of tethered chains may enhance or inhibit the formation of orientationally ordered structures formed by amphiphilic (Janus) molecules at surfaces, depending on the character of interactions between the amphiphiles and the chain segments. The phase transitions in monolayers of rigid and flexible chains, as well as in monolayers of Janus-like particles have been studied by Monte Carlo simulations. In both cases, we have evaluated the phase diagrams, and identified differently ordered phases (e.g. lamellar) by employing appropriate methods of analysis. We have also carried out Monte Carlo simulations of phase transitions in confined lattice Janus fluids and a rich variety of different phases has been found. Finally, we have developed a density functional approach suitable to describe patchy colloids with two different off-center sites A and B and the allowed formation of double AB bonds. The proposed approach has been based of the fundamental measure theory and on the second order perturbation theory of Wertheim. For that model, a re-entrant phase behavior in bulk system has been found. Our calculations revealed that the re-entrant phase diagrams also appear in confined systems and the both lower and upper critical temperatures are independent of the pore width.
The anisotropic interactions between amphiphilic molecules and between amphiphiles and ions can lead to an interesting phase behavior at surfaces. We have demonstrated that addition of an appropriate amount of ions or the change of the wall charge can stimulate the formation of long-ranged orientationally (and in some cases also translationally) ordered structures. This is an example of a stimuli-responsive system, in which an external stimuli (e.g. the electrostatic potential) can lead to structural changes.
In several works, we have studied an effective force acting between surfaces (or colloidal particles). We have proposed new approaches that allow for evaluation of the force between two surfaces modified with polyampholytes, as well as between two surfaces immersed in the fluid of Janus particles. We have also proposed a theory aimed at the interpretation of the Atomic Force Microscope results obtained for modified surfaces. The theory allows for calculation of the force acting on a particular segment and the average force extorted by an apparatus tip during its movement over a modified surface. We have also explained the mechanism of adsorption of fluids on modified surfaces, changes in the selectivity of adsorption and changes in the characteristics of the tethered layer during adsorption of single and two-component fluids. Under certain conditions, we have found “unusual” variation of the height of tethered layer with temperature and with the amount of adsorbed species. Also, we have derived a theory that describes the behavior of tethered layers built of polyampholytes and adsorption of ions on such surfaces. Our approach allows for the evaluation of the dependence of the electrical double layer capacitance on the pore width in pores with the walls modified by polyampholytes. The investigations of the electric double layer capacitance of systems with nanopores are of particular importance for the development of new energy storages called supercapacitors.
The mesoscale DPD simulations have been used to study the microphase separation in binary partially miscible fluids confined in slit-like pores with modified walls. Our main interest has been to determine how the possible morphologies (lamellar, pillar-like, cylindrical, etc.) formed inside the pore depend on the geometrical parameters of the system. To describe the observed morphologies, we have calculated several characteristics (the density and temperature profiles, the radii of gyration for the attached polymers, and the minimum polymer-polymer distances in the direction parallel and perpendicular to the walls). The results obtained for the systems under equilibrium condition have been next used as starting points for the simulation of flows. We have found that some of equilibrium morphologies are sensitive to
the flow rate. For moderate flow rates, the steady-state has been found to consist of lamellar morphology with flow occurring mainly in the center of the channel. At sufficiently high flow rates, we have observed the flow-induced mixing in the center of the channel. The mesoscopic simulation technique has been also used in the studies of colloid-liquid crystal mixtures using different models for colloidal particles. In addition, a grafted colloid that mimics a gold nanoparticle with mesogen-modified surface has been used. The nematic phase has been modelled explicitly via soft spherocylinders interacting through an appropriately selected potential. In addition, estimates for the nematic-isotropic transition and the coherence length have allowed to establish a relation between the energy and the length scales with respect to the experimental systems. The models used exhibit defect topologies, namely that of a Saturn ring and a boojum defect for homeotropic and planar surface anchoring, respectively. In the grafted colloid model we have tuned the anchoring strength through the density of the mesogenic shell on the surface. We also found the biaxial boojum effect for the special case of longitudinal planar anchoring.
The scientific discoveries and findings of the project are mainly addressed to scientists and post-graduate students in the field of statistical thermodynamics of soft matter, and should help in a better understanding of a variety of systems involving complex molecules and surfaces. Our results can be used to interpret experimental observations and may also of importance for the development of new materials. It is important to stress that the collaboration between scientists from different centers increased scientific potential of all partners involved. During the project, we have collaborated with people not formally involved in the project. This mainly concerns two groups: from UMCS and from ICMP. The collaboration is being continued. At the moment, two consecutive scientific works are under preparation and we plan to continue the studies that have been the involved in the project.