My research has been devoted to the investigation of equilibrium and non-equilibrium properties of Soft Matter systems:
- Study of non-equilibrium processes, such as colloidal aggregation in 2 and 3 dimensions, simultaneous sedimentation/aggregation, colloidal deposition processes, including the hydrodynamic effects involved in these processes.
- Formation of nanostructures in charged colloidal systems
- Study of effective interactions (such as depletion forces), structure, phase behavior and interfacial properties of mixtures of nanoparticles.
- Study of structure, interactions involved in charged thermoresponsive microgel particles.
- Study of the permeation of ions and charged/neutral cosolutes inside microgel particles.
ONGOING PROJECTS
Unveiling the fundamental path for the design of stimuli-responsive smart nanomaterilas (GENESIS, Project No. PID2022-136540NB-00, 2023-2026)
PI’s: Ana B. Jódar Reyes /Alessandro Patti
Soft Matter is a broad research area embracing systems whose behaviour can be tuned by energy variations of the order of thermal fluctuations. Such sensible responsiveness can be controlled to produce specific target properties. If this awesome scenario was further enhanced by the system’s ability to reversibly adapt to external stimuli, switching between mutually exclusive configurations, one could design smart nanomaterials for an impressively wide spectrum of applications. Although stimuli-responsive smart nanomaterials are already being applied in e.g. medicine, materials templating, microfluidics3 and robotics, our understanding of their behaviour is still at an embryonic stage, where the fundamental path connecting the origin of interparticle interactions with the material’s structure and properties is still far from being fully paved. Such a knowledge gap is due to the fact that this path, originating at nanoscopic time and length scales and fully unfolding only at macroscopic scales, makes the study of the mechanisms controlling the behaviour of smart nanomaterials especially challenging.
Among smart responsive nanomaterials, which reversibly self-adapt their behaviour to environmental changes or external stimuli, one finds colloidal suspensions of nanoparticles (NPs). NPs can be distinguished according to their degree of softness, S_E≡(k_B T)⁄∆E, being the ratio between thermal and elastic energy5,6. Soft NPs, such as nanogels and polymer-grafted NPs, can expand or contract as a result of e.g., pH or temperature changes. Not only does this swelling/deswelling affect size and/or shape of NPs, but also their internal architecture and mutual affinity, eventually altering the properties of the whole system. Hard NPs, whose rigid geometry is practically insensible to pH or temperature changes, under some specific conditions, can respond to external magnetic or electric fields by assembling into nanostrings that modify the system rheology. Despite the widespread interest that soft-NP and hard-NP smart nanomaterials are generating, there are still many open questions, mostly related to the physics of their interaction-structure-property relationship, that directly determines their technological potential.
By building a comprehensive programme and an international team of experts in simulation, theory and experiments, GENESIS will investigate the fundamental mechanisms that rule the behaviour of responsive soft-NP and hard-NP complex fluids and their ability to reversibly adapt their structure and properties to external stimuli. GENESIS has three main goals: (1) understanding the microscopic origin of the interactions that govern the phase behaviour of smart stimuli-responsive fluids according to the degree of softness or shape anisotropy of their constituent NPs; (2) unveiling the physical parameters that control the kinetics of their responsiveness; and (3) measuring the impact of their stimuli-driven structural change on dynamical and rheological properties that determine the material’s response.
Computational modeling for the development of environmentally friendly polymer coatings (Project No. A-EXP-359-UGR23, 2024-2026)
PI: Irene Adroher Benítez / Supervisor: Arturo Moncho Jordá
Polymers are the main components in cosmetic formulations, particularly as coatings designed to alter the macroscopic properties of biological surfaces such as hair and skin. The physico-chemical composition of these biological substrates significantly influences the kinetics of polymer adsorption and the properties of the resulting coatings. These factors, in turn, dictate the cosmetic performance of polymers.
Research in environmentally friendly polymer applications is an expanding field, driven by increasing consumer demand for innovative products that strike a balance between efficiency and sustainability. The cosmetics industry should align this research with the principles of green chemistry, focusing on the design and production of polymers that are safe, biodegradable, and efficient, while minimizing the use of harmful compounds. In alignment with this goal, this proposal aims to develop a comprehensive computational framework for modeling and evaluating the performance of sustainable polymer coatings with interest in cosmetics.
To accomplish this, we intend to simulate the dynamics of polymer adsorption onto substrates that realistically represent biological surfaces. We will quantitatively characterize the resulting polymer films with respect to both their equilibrium and non-equilibrium properties. The simulations will include substrates with varying chemical compositions and surface topographies, with special emphasis on mimicking human skin and hair. Given the focus on sustainability, we will also develop polymer models of both synthetic and natural origin. A particular emphasis will be placed on polysaccharides, which are viewed as promising candidates for sustainable formulations due to their biodegradable nature.
PROYECTS COMPLETED
Physical Mechanisms Involved in the Stability and Controlled Release of Drugs through Exosomes and Adaptive Passive and Active Hydrogels (Project No. P20_00241, 2021-2023)
PI’s: Arturo Moncho Jordá / Ana Belén Jódar Reyes
In recent decades, multiple innovative strategies in Nanotechnology have emerged, related to the development of drug and biomolecule delivery systems (or vectors) with applications in Bionanomedicine. Research in this field is often conducted without considering the physical biomolecule-vector interactions that will determine their encapsulation/release, nor their colloidal properties. However, the success in developing these systems largely depends on having as much information as possible about molecular interactions, microscopic structure, and interaction with the biological environment in which they will operate. To this end, this research project combines experimental studies, theoretical approaches, and atomistic and coarse-grained simulations of colloidal systems of interest, such as Nanovectors. The goal is to analyze the physicochemical phenomena at the molecular, nanometric, and macroscopic scales of the Nanovector, with special attention to the kinetic modeling of the release of molecules of interest in the Nanovector, as well as the interactions that maintain them in conditions of colloidal stability. Exosomes and both active and passive hydrogels will be used as NanoV systems. Exosomes are natural Nanovectors that act as intercellular messengers, while hydrogels are adaptive nanoparticles capable of expanding or contracting in response to multiple external stimuli, thereby allowing control over the rate of encapsulation/release. Active hydrogels are out-of-equilibrium systems that can also autonomously change their size as long as a chemical energy source is provided, resulting in programmable oscillating responses. In both cases, the proposed studies for these materials are fundamental for developing future strategies to control and optimize their applicability in Bionanomedicine.
Interactions and collective properties of nanogel/microgel-based soft-matter systems of nanotechnological interest (Project No. FIS2016-80087-C2-1-P, 2017-2019)
PI’s: Arturo Moncho Jordá / Alberto Martín Molina
Understanding the equilibrium and the non-equilibrium structure, dynamics and local distributions of the different components of a colloidal suspension in terms of the inter-particle interactions is a challenge of great importance in Soft Matter or Colloidal Physics. In this project, we focus on colloidal systems formed by an aqueous suspension of neutral and charged microgels. The ability of microgels of swelling/shrinking in response to many external stimuli makes this kind of soft nanoparticles a very versatile system with multiple biotechnological and industrial applications. We aim to investigate the stability and structure of microgel dispersions (in bulk and adsorbed at fluid-fluid interfaces), the permeation of different kind of solutes (small proteins, reactants, ions, drugs and genetic material) into the microgel, and relate all these properties to the effective pair microgel-microgel and microgel-solute interactions. In particular, we will explore the role of the electrostatic, excluded-volume, water-mediated hydrophobic/hydrophilic, dispersive and elastic interactions. The experimental study will be complemented with theoretical modelling and coarse-grain simulations. Concerning theory, we will make use of methods based on integral equations, density functional theories for complex fluids and other thermodynamic and statistical-mechanical approaches. The combination of both procedures will allow estimating the degree of solute sorption onto the microgel and to determine the effective interactions between the microgel particle and the solute. On the other hand, microgels behave as model particles to explore collective effects (structure formation, dynamics and rheology) in dilute and dense dispensions of particles in the bulk or confined at fluid-fluid interfaces, and to investigate the effective microgel-microgel interactions involved in both geometries.
Regarding coarse-grained simulations, this project intends to compute the effective potentials and/or forces associated to the interaction between nanogel particles, and the interactions
between nanogels and other nano-objects (such as drugs or polyelectrolytes) through this computational technique. In all cases, we will consider systems with and without electric charge. In this way, the role played by excluded-volume and electrostatic interactions will be looked into in depth. We will pay special attention to: a) structural and geometrical parameters, such as the nano-object size, the length of the polymer chains of the nanogel, the degree of crosslinking and branching; b) electrostatic parameters, such as the electric charge or the spatial distribution of charged groups in the polymer network; c) parameters related with the solvent and the ions inside, such as the dielectric constant, the salt concentration and so on. In a subsequent stage, we will also study the effect of dispersion forces, which lead to phenomena of high ionic specificity. The structure factors characterizing the spatial ordering in concentrated suspensions of nanogels will be computed as well. This spatial ordering is originated by nanogel-nanogel forces.
Effective interactions in microgel suspensions
Understanding the equilibrium and the non-equilibrium structure, dynamics and local distributions of the different components of a colloidal suspension in terms of the inter-particle interactions is a challenge of great importance in Soft Matter or Colloidal Physics. In this topic, we focus on colloidal systems formed by an aqueous suspension of neutral and charged microgels.
A microgel (or nanogel) particle is formed by a cross-linked polymer network of colloidal size immersed in a solvent, which can be designed to swell or shrink in response to many external parameters, such as temperature, pH, and solvent quality among others. Due to their nanometric size, the timescale of the swelling response (which is roughly proportional to the square of the typical spatial dimension of the microgel) is of the order of seconds, which is very short compared to the ones observed in the so-called macroscopic gels. Furthermore, the soft and porous nature of the microgels allow them to be permeated by the solvent, ions and other neutral or charged macromolecules. The combination of these properties make microgel suspensions unique smart materials for industrial and biomedical applications, such as carrier particles for biomolecules or controlled drug release, filtration or stimuli controlled nanoreactors
We are currently investigating the stability and structure of microgel dispersions, the degree of swelling, and the permeation of different kind of solutes (small proteins, reactants, ions, drugs and genetic material) into the microgel, and relate all these properties to the effective pair microgel-microgel and microgel-solute interactions. In particular, we use theoretical methods and coarse-grained computer simulations to investigate the role of the electrostatic, excluded-volume, water-mediated hydrophobic/hydrophilic, dispersive and elastic interactions. Concerning theory, we make use of methods based on Ornstein-Zernike integral equations, density functional theories for complex fluids and other thermodynamic and statistical-mechanical approaches. The combination of these procedures allows us to predict the degree of solute sorption onto the microgel and to determine the effective interactions between the microgel particle and the solute. On the other hand, microgels behave as model particles to explore collective effects (structure formation, dynamics and rheology) in dilute and dense dispersions of particles in the bulk or confined at fluid-fluid interfaces, and to investigate the effective microgel-microgel interactions involved in both geometries.
These investigations and the future work is being financially supported by the following research projects founded by the Spanish Ministerio de Economía y Competitividad (MINECO), “Plan Nacional de Investigación, Desarrollo e Innovación Tecnológica (I+D+i)”
Project FIS2016-80087-C2-1-P: “Interactions and collective properties of nanogel/microgel-based soft-matter systems of nanotechnological interest”, 2017-2019
Project MAT2012-36270-C04-02: “Structure and interactions in systems of soft nanoparticles (nanogels and liposomes)”, 2013-2015
Hydrodynamics in sedimenting suspensions
Many industrial applications of colloidal suspensions depend critically on their behavior under non-equilibrium conditions. Such properties are, however, notoriously hard to calculate because of the long-ranged solvent induced hydrodynamic interaction. Partially for this reason, the vast majority of theoretical and computational treatments of the nonequilibrium regime have focused on hard-sphere particles. Thus our understanding of how attractive interparticle interactions affect the nonequilibrium behavior of colloidal suspensions is still in its infancy. This state of affairs stands in marked contrast to the equilibrium regime, where methods to calculate how interactions control phase behavior and interfacial properties are already well developed.
To address this fundamental question, we study the steady-state sedimentation of spherical particles through a viscous solvent at low Reynolds number using Stochastic Rotation Dynamics computer simulations. Besides its intrinsic interest for statistical mechanics, sedimentation is also important for understanding industrial applications such as paints, coatings, ceramics, food, and cosmetics. We also investigate the role that interparticle attractive interactions (reversible aggregation) have on the sedimentation rate and on the hydrodynamic fluctuations,, and how increasing the particle concentration enhances two competitive effects, namely cluster formation (which accelerates the sediemntation rate) and the hydrodynamic backfklow retardation.
Depletion interactions in binary colloidal systems
The interaction between two nanoparticles in solution can be qualitatively modified by the presence of smaller suspended particles. For example, the addition of nonadsorbing polymer in a good solvent can lead to attractive depletion interactions, as first explained over 50 years ago by Asakura and Oosawa. This depletion effect arise when two nanoparticles approach each other so that their exclusion volumes overlap, resulting in more free volume available for the polymer chains. The subsequent encrease of entropy translates into an effective depletion attraction between the two nanoparticles. The strength of the depletion attraction is roughly controlled by the polymer concentration, and the range of the attractive well is mainly determined by the size of the polimers. The depletion can cause important effects on the statics and dynamics of phase transitions, including fluid-fluid demixing, crystallization, gelation, or glass transitions, as well as affect the interfacial properties associated with phase coexistence.
In addition to depletion forces, another sort of effective interactions can emerge depending on whether the small component is attracted or repelled to the surface of the big nanoparticle. For the case of strong attraction, the small component tends to be attached to the surface. Therefore, when two large nanoparticles approach each other, the adsorbed layers of small ones overlap yielding effective repulsive forces between the large nanoparticles that stabilize the suspension. In the opposite case, i. e. long-range repulsion between big and small components, the small particles are excluded in the proximity of the big particle surfaces. Consequently, within the approach of a pair of big colloids, the overlap of the depletion regions induces a difference in osmotic pressure that is translated into an enhanced effective attraction. In this balance, the interaction between small particles plays also an important role, leading to other mechanisms such as bridging or repulsion though attraction effects.
Aggregation and heteroaggregation processes
Colloidal systems are composed of particles which are large compared to the solvent molecules, but still small to exhibit thermal motion. Under inestable conditions (particle interactions dominated by attractions) the particles stick on contact due to attractive forces forming larger clusters. Depending of the strength and range of such attracions, the aggregation may be clasified in different non-equilibrium regimes: diffusive-limited cluster aggregatio, reaction limited cluster aggregation, reversible aggregation and fragmentation,…
Aggregation of colloidal particles occurs in a wide variety of physical, chemical and biological processes. Hence, it is of great practical interest to predict the time evolution of the aggregating species from relatively simple theoretical expressions. Smoluchowski’s equation has been widely used for this purpose. This equation, however, needs a physically deduced aggregation kernel before meaningful conclusions may be drawn from its predictions. For this purpose, this equation is complemented with diffusion theory combined with concepts of fractal geometry and computer simulations, which have been the most important tools for describing kinetic and structural aspects of aggregation processes.