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Particle-laden interfaces are ubiquitous nowadays. The understanding of their properties and structure is essential for solving different problems of technological and industrial relevance;
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...stabilization of foams, emulsions and thin films. These rely on the response of the interface to mechanical perturbations. The complex mechanical response appearing in particle-laden interfaces requires deepening on the understanding of physico-chemical mechanisms underlying the assembly of particles at interface which plays a central role in the distribution of particles at the interface, and in the complex interfacial dynamics appearing in these systems. Therefore, the study of particle-laden interfaces deserves attention to provide a comprehensive explanation on the complex relaxation mechanisms involved in the stabilization of fluid interfaces.
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We studied the mechanical and structural properties of mixed surface layers composed by 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and silica nanoparticles (NPs). These layers are obtained by ...spreading a DPPC Langmuir monolayer on a colloidal silica dispersion. The transfer/incorporation of NPs into the DPPC monolayer, driven by electrostatic interactions, alters the molecular orientation, the mechanisms of domain formation, and consequently the phase behavior of the surface layer during compression. The investigation of these systems by means of complementary techniques (Langmuir trough, fluorescence microscopy, ellipsometry, and scanning electron microscopy (SEM)) shows that the incorporated NPs preferentially distribute along the liquid expanded phase of DPPC. The layer assumes the stable and homogeneous bidimensional structure of a two-dimensional (2D) analogue of a Pickering emulsion. In fact, the presence of particles provides a circular shape to the DPPC domains and stabilizes them against growth and coalescence during the monolayer compression.
Rhamnolipids are very promising sugar-based biosurfactants, generally produced by bacteria, with a wide range of properties that can be exploited at an industrial and technological level, e.g. in ...cosmetics, food science, or oil recovery, to provide benefits for human health and the environment. This has led to intensive research into optimizing their production to increase yields and minimize costs, which is challenging because biotechnological methods for rhamnolipid production result in complex product mixtures and require the introduction of complex separation strategies to ensure the purity of the rhamnolipid obtained. This is an important issue for the introduction of rhamnolipids to the market due to the differences that exist between the properties of the different congeners. This review attempts to provide an overview of the interfacial properties, potential applications, and recent advances in understanding the molecular mechanisms that govern the adsorption to interfaces and assembly in solution of rhamnolipids. In addition, the review also discusses some general aspects related to the production and purification methods of rhamnolipids, highlighting the need for further research to fully exploit their potential. It is hoped that this review will contribute to the growing body of knowledge about rhamnolipids and stimulate further research in this field.
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The adsorption of mixtures of poly(diallyldimethylammonium chloride) and sodium N-lauroyl-N-methyltaurate at the water/vapor interface has been studied using drop profile tensiometry and neutron ...reflectometry. This study sheds light on the mechanisms involved in the adsorption of polyelectrolyte–oppositely charged surfactants by the characterization of both equilibrium and dynamic features associated with the layer formation at the fluid interface. The results are discussed in terms of an adsorption−equilibration of the interfacial layers as a two-step process, with the initial stages involving the adsorption of polyelectrolyte−surfactant complexes formed in the bulk followed by the reorganization of the material at the interface. This work contributes to the understanding of the physicochemical features of systems that undergo complex bulk and interfacial interactions with importance in science and technology.
Fluid interfaces are promising candidates for the design of new functional materials by confining different types of materials, e.g., polymers, surfactants, colloids, or even small molecules, by ...direct spreading or self-assembly from solutions. The development of such materials requires a deep understanding of the physico-chemical bases underlying the formation of layers at fluid interfaces, as well as the characterization of the structures and properties of such layers. This is of particular importance, because the constraints associated with the assembly of materials at the interface lead to the emergence of equilibrium and dynamic features in the interfacial systems that are far from those found in traditional 3D materials. These new properties are of importance in many scientific and technological fields, such as food science, cosmetics, biology, oil recovery, electronics, drug delivery, detergency, and tissue engineering. Therefore, the understanding of the theoretical and practical aspects involved in the preparation of these interfacial systems is of paramount importance for improving their usage for designing innovative technological solutions.
The widespread of mixtures formed by polyelectrolytes and oppositely charged surfactants in different industrial sectors requires a thorough understanding of the physicochemical bases governing their ...interactions. These interactions lead to the formation of polyelectrolyte–surfactant aggregates, with the presence of such aggregates in the aqueous mixtures impacting decisively on their bulk and interfacial properties. Furthermore, the broad range of morphologies, sizes, and properties of the aggregates makes it necessary to obtain a comprehensive picture describing their behavior and properties to be able to exploit their possible applications. This review is aimed to shed light on some of the most relevant physicochemical aspects involved in the complexation process of polyelectrolyte–surfactant mixtures, paying special attention to the formation of nonequilibrium states (kinetically trapped aggregates), and how these impact on the interaction of the complexes with interfaces, both fluid/fluid and solid/fluid.
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•Oppositely charged polyelectrolyte-surfactant form complex in solution.•One-phase and phase-separated systems can be defined as the composition of the mixtures is changed.•Thermodynamic considerations do not account for the whole description of the physicochemical behavior of polyelectrolyte–surfactant mixtures.•Nonequilibrium effects impact decisively in the behavior of polyelectrolyte–surfactant mixtures.•Complexes formed in solution affect the properties of interfaces on their adsorption.
•Palmitic acid and silica nanoparticles at the water–hexane interface.•Emulsions between silica nanoparticle dispersions and palmitic acid hexane solutions.•Emulsion stabilization by the formation of ...nanoparticle–surfactant complexes at the droplet interface.•Dependence of emulsion stability on the liquid phases volume ratio and palmitic acid content.•Nanoparticle–surfactant complexes redispersion for surfactant adsorption on the nanoparticle exceeding the monolayer.
The interaction between palmitic acid (PA) and silica nanoparticles at the water/hexane has been studied in relation to the ability in the stabilization of water in oil emulsions. To this aim, emulsions formed by aqueous silica nanoparticle dispersions and PA solutions in hexane have been investigated. In contrast with other previous studies, where the interaction of nanoparticles and surfactant molecules occurs already in the bulk phases, here PA and silica nanoparticles are dissolved/dispersed in different phases and interact only at the liquid–liquid interface. As for other particle-stabilized emulsions, the investigated system can provide emulsions stable over very long time because of the formation of nanoparticle–surfactant complexes, which accumulate at the droplets interface, providing a steric hindrance of coalescence. However, the present study has pointed out that these enhanced stability conditions are achieved only when the amount of surfactant leads to the formation of single surfactant layers onto the particles surface. Whereas an excess surfactant amount interacts with the particles, the formation of multilayers would be possible, and the formed complexes can be redispersed again in the aqueous bulk phase. This recalls to a scenario where the degree of hydrophobization of the particles is critical for the emulsion stabilization. This can be easily tuned by the control of the surfactant concentration and the volume ratio between the liquid phases. Interfacial tension measurement of the same couple of liquids, has allowed us to explore in more detail such phenomena, revealing a non-monotonic evolution of the dynamic interfacial tension at large PA concentrations. Also these observations can be rationalized assuming the redispersion of the complexes mentioned above, which allows explaining the interfacial tension evolution on the basis of the succession of adsorption and desorption of the different species – nanoparticles, nanoparticle–PA complexes, PA molecules – involved.
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•Study of polyelectrolyte-glycolipid mixtures using experiments and theoretical calculations.•Bulk association follows similar behavior that the found in systems with synthetic ...surfactants.•Glycolipids enhance significantly the adsorption onto negatively charged surfaces.•Mixtures adsorb strongly onto hair fibers opening the road to its application in cosmetic.•Agreement between experimental results and theoretical calculations.
Pseudo-binary mixtures of different glycolipids, four different rhamnolipids (RL) and an alkyl polyglucoside (APG), with poly(diallyl-dimethylammonium chloride) (PDADMAC) have been studied in relation to their adsorption onto negatively charged surfaces to shed light on the impact of the molecular structure of surfactants from natural sources (instead of synthetic surfactant, such as sodium laureth sulfate) on the adsorption of hair-conditioning polymers. For this purpose, the self-assembly of such mixtures in aqueous solution and their adsorption onto negatively charged surfaces mimicking the negative charge of damaged hair fibres have been studied combining experiments and self-consistent field (SCF) calculations. The results show that the specific physico-chemical properties of the surfactants (charge, number of sugar rings present in surfactant structure and length of the hydrocarbon length) play a main role in the control of the adsorption process, with the adsorption efficiency and hydration being improved in relation to conventional sulfate-based systems for mixtures of PDADMAC and glycolipids with the shortest alkyl chains. SCF calculations and Energy Dispersive X-Ray Spectroscopy (EDS) analysis on real hair confirmed such observations. The results allow one to assume that the characteristic of the surfactants, especially rhamnolipids, conditions positively the adsorption potential of polyelectrolytes in these model systems. This study provides important insights on the mechanisms underlying the performance of more complex but eco-friendly washing formulations.
Synthetic micro/nanomotors (MNMs) are human-made machines characterized by their capacity for undergoing self-propelled motion as a result of the consumption of chemical energy obtained from specific ...chemical or biochemical reactions, or as a response to an external actuation driven by a physical stimulus. This has fostered the exploitation of MNMs for facing different biomedical challenges, including drug delivery. In fact, MNMs are superior systems for an efficient delivery of drugs, offering several advantages in relation to conventional carriers. For instance, the self-propulsion ability of micro/nanomotors makes possible an easier transport of drugs to specific targets in comparison to the conventional distribution by passive carriers circulating within the blood, which enhances the drug bioavailability in tissues. Despite the promising avenues opened by the use of synthetic micro/nanomotors in drug delivery applications, the development of systems for in vivo uses requires further studies to ensure a suitable biocompatibility and biodegradability of the fabricated engines. This is essential for guaranteeing the safety of synthetic MNMs and patient convenience. This review provides an updated perspective to the potential applications of synthetic micro/nanomotors in drug delivery. Moreover, the most fundamental aspects related to the performance of synthetic MNMs and their biosafety are also discussed.
For many years the determination of the shear viscosity of interfacial layers has been source of strong controversy. This is mainly because different techniques provided different values of such ...parameter, which leads in many cases to a puzzling interpretation of the experimental results. One possible explanation of this is the non-correct analysis of the hydrodynamic conditions of the measurement and, in particular, the assumption of some approximations that may not be necessarily valid in all cases. The introduction of hydrodynamic consideration in the study of interfacial shear rheology has helped to clarify some of the existing discrepancies between measurements performed using different devices, thus allowing one to establish clearly the viscosity range in which different techniques can operate with enough sensitivity for determining the interfacial shear viscosity. This review puts in perspective the most recent developments on the studies of the interfacial shear rheology of fluid/fluid interfaces, analyzing the strength and weakness of the different approaches.
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•Determination of interfacial shear viscosity presents implications in many fields.•Many techniques accessible to determine interfacial shear viscosity.•Physically reliable viscosities are only obtained using detailed hydrodynamic analysis.•Combining micro- and macro-rheological techniques is possible to cover a broad range of interfacial viscosities.
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