Interfacial forces between a silica particle and phosphatidylcholine monolayers at the air-water interface

Interactions between a responsive microgel monolayer and a rigid colloid: from soft to hard interfaces

Responsive poly-N-isopropylacrylamide-based microgels are commonly used as model colloids with soft repulsive interactions. It has been shown that the microgel-microgel interaction in solution can be easily adjusted by varying the environmental parameters, e.g., temperature, pH, or salt concentration. Furthermore, microgels readily adsorb to liquid-gas and liquid-liquid interfaces forming responsive foams and emulsions that can be broken on-demand. In this work, we explore the interactions between microgel monolayers at the air-water interface and a hard colloid in the water. Force-distance curves between the monolayer and a silica particle were measured with the Monolayer Particle Interaction Apparatus. The measurements were conducted at different temperatures and lateral compressions, i.e., different surface pressures. The force-distance approach curves display long-range repulsive forces below the volume phase transition temperature of the microgels. Temperature and lateral compression reduce the stiffness of the monolayer. The adhesion increases with temperature and decreases with a lateral compression of the monolayer. When compressed laterally, the interactions between the microgels are hardly affected by temperature, as the directly adsorbed microgel fractions are nearly insensitive to temperature. In contrast, our findings show that the temperature-dependent swelling of the microgel fractions in the aqueous phase strongly influences the interaction with the probe. This is explained by a change in the microgel monolayer from a soft to a hard repulsive interface.

Detaching Microparticles from a Liquid Surface

The work required to detach microparticles from fluid interfaces depends on the shape of the liquid meniscus. However, measuring the capillary force on a single microparticle and simultaneously imaging the shape of the liquid meniscus has not yet been accomplished. To correlate force and shape, we combined a laser scanning confocal microscope with a colloidal probe setup. While moving a hydrophobic microsphere (radius 5-10  μm) in and out of a 2-5  μm thick glycerol film, we simultaneously measured the force and imaged the shape of the liquid meniscus. In this way we verified the fundamental equations [D. F. James, J. Fluid Mech. 63, 657 (1974)JFLSA70022-112010.1017/S0022112074002126; A. D. Scheludko, A. D. Nikolov, Colloid Polymer Sci. 253, 396 (1975)] that describe the adhesion of particles in flotation, deinking of paper, the stability of Pickering emulsions and particle-stabilized foams. Comparing experimental results with theory showed, however, that the receding contact angle has to be applied, which can be much lower than the static contact angle obtained right after jump in of the particle.

The application of atomic force microscopy in mineral flotation

During the past years, atomic force microscopy (AFM) has matured to an indispensable tool to characterize nanomaterials in colloid and interface science. For imaging, a sharp probe mounted near to the end of a cantilever scans over the sample surface providing a high resolution three-dimensional topographic image. In addition, the AFM tip can be used as a force sensor to detect local properties like adhesion, stiffness, charge etc. After the invention of the colloidal probe technique it has also become a major method to measure surface forces. In this review, we highlight the advances in the application of AFM in the field of mineral flotation, such as mineral morphology imaging, water at mineral surface, reagent adsorption, inter-particle force, and bubble-particle interaction. In the coming years, the complementary characterization of chemical composition such as using infrared spectroscopy and Raman spectroscopy for AFM topography imaging and the synchronous measurement of the force and distance involving deformable bubble as a force sensor will further assist the fundamental understanding of flotation mechanism.

Rice Starch Particle Interactions at Air/Aqueous Interfaces-Effect of Particle Hydrophobicity and Solution Ionic Strength

Starch particles modified by esterification with dicarboxylic acids to give octenyl succinic anhydride (OSA) starch is an approved food additive that can be used to stabilize oil in water emulsions used in foods and drinks. However, the effects of the OSA modification of the starch particle on the interfacial interactions are not fully understood. Here, we directly measured the packing of films of rice starch granules, i.e., the natural particle found inside the plant, at air/aqueous interfaces, and the interaction forces in that system as a function of the particle hydrophobicity and ionic strength, in order to gain insight on how starch particles can stabilize emulsions. This was achieved by using a combined Langmuir trough and optical microscope system, and the Monolayer Interaction Particle Apparatus. Native rice starch particles were seen to form large aggregates at air/water interfaces, causing films with large voids to be formed at the interface. The OSA modification of the rice starches particles decreased this aggregation. Increasing the degree of modification improved the particle packing within the film of particles at the air/water interface, due to the introduction of inter-particle electrostatic interactions within the film. The introduction of salt to the water phase caused the particles to aggregate and form holes within the film, due to the screening of the charged groups on the starch particles by the salt. The presence of these holes in the film decreased the stiffness of the films. The effect of the OSA modification was concluded to decrease the aggregation of the particles at an air/water interface. The presence of salts, however, caused the particles to aggregate, thereby reducing the strength of the interfacial film.

Effect of the Addition of a Cross-Linker and the Water pH on the Physical Properties of Films of pH-Responsive Polymer Particles at Air/Water Interfaces

We investigated how the stiffness and physical properties of films of pH-responsive polystyrene nanoparticles with poly(2-(N,N-dimethylamino)ethylmethacrylate) hairs (PDMA-PS particles) at air/water interfaces could be varied by using the cross-linker of 1,2-bis-(2-iodoethoxy)ethane (BIEE) and by varying the water pH. The physical properties of the film were investigated by using a Langmuir trough, atomic force microscope, and monolayer particle interaction apparatus. Films without large (μm-sized) holes were prepared by reacting BIEE with the PDMA-PS particles in the ethanol spreading solution for times ≤6 h and then by spreading this solution on a pH 5.8 water surface. Longer reaction times or a pH 9.0 water surface gave films with large holes. The holes were the result of the presence of aggregates of various sizes, which could not efficiently pack in the film. The aggregates were formed by interparticle adhesions because of the BIEE presence and physical attractive forces. The pH 5.8 water gave less aggregation than the pH 9.0 water because of the stronger interparticle electrostatic repulsions that resulted from the higher particle charge in pH 5.8 water than pH 9.0 water. The holes in the films could be decreased by reacting BIEE with the PDMA-PS particles on the pH 5.8 or 9.0 water surface, while the film was compressed to give a close packing density. The stiffness of the film of PDMA-PS particles increased, when BIEE was reacted with the PDMA-PS particles in the spreading solution for 1 h and was then spread on a pH 5.8 water surface. The film stiffness, however, did not increase, when BIEE was reacted with PDMA-PS particles on the pH 5.8 or 9.0 water surface for 1 h. This stiffness difference was explained by the fact that the former method gave a thicker film than the latter method.

Physical properties of mixed Langmuir monolayers of polystyrene particles with poly(N,N-dimethylaminoethylmethacrylate) hairs and a poly(2-hydroxyethyl methacrylate) polymer at an air/water interface

The effect of adding a poly(2-hydroxyethyl methacrylate) (PHEMA) polymer to a Langmuir monolayer of polystyrene particles carrying poly(N,N-dimethylaminoethylmethacrylate) hair (PDMA-PS particles) at air/water interfaces on the physical properties of the monolayer was studied. The addition of PHEMA to a PDMA-PS particle monolayer at an air/water interface gave a polymer-like monolayer at low surface pressures and a particle-like monolayer at high surface pressures. The PDMA-PS particles formed small aggregates that were dispersed throughout the PHEMA monolayer at low surface pressures, a result suggesting that the particles were trapped in the PHEMA network. Monolayers of closely packed particles were observed at higher surface pressures, suggesting that PHEMA was squeezed-out at higher surface pressures. The stiffness of the mixed monolayer was independent of the surface pressure, but increased as the ratio of PHEMA in the mixed monolayer increased. This increased stiffness was explained by the immobilization of the PDMA-PS particles by PHEMA.

Hollow Mesoporous Silica Nanocarriers with Multifunctional Capping Agents for In Vivo Cancer Imaging and Therapy

Efficient drug loading and selectivity in drug delivery are two key features of a good drug-carrier design. Here we report on such a drug carrier formed by using hollow mesoporous silica nanoparticles (HMS NPs) as the core and specifically designed multifunctional amphiphilic agents as the encapsulating shell. These nanocarriers combine the advantages of the HMS NP core (favorable physical and structural properties) and the versatility of an organic-based shell (e.g., specificity in chemical properties and modifiability). Moreover, both the properties of the core and the shell can be independently varied. The varied core and shell could then be integrated into a single device (drug carrier) to provide efficient and specific drug delivery. In vitro and in vivo data suggests that these drug nanocarriers are biocompatible and are able to deliver hydrophobic drugs selectively to target tumor cells. After the break of the pH-labile linkages in the shell, the drug payload can be released and the tumor cells are killed.

Two-dimensional DPPC based emulsion-like structures stabilized by silica nanoparticles

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.

Determination and comparison of how the chain number and chain length of a lipid affects its interactions with a phospholipid at an air/water interface

We determined how the number of chains in a lipid and its chain length affects its interactions with a phospholipid model membrane, and whether the number of chains or the chain length of lipids affects their interactions with the phospholipids more. This was achieved by using a Langmuir trough and a fluorescence microscope to study the interactions of mono-, di-, and triglycerides with a phospholipid monolayer at an air/water interface. The effect of the number of chains in a lipid on its interactions with phospholipids at air/water interfaces was shown by surface pressure-area per molecule isotherms and their thermodynamic analysis to worsen as the number of alkyl chains was increased to be greater than one. An increase in the packing density decreased the mixing ability of the lipids with the phospholipids, resulting in the formation of aggregates in the mixed monolayer. The aggregation was explained by the intermolecular hydrophobic and van der Waals attractions between the lipid molecules. Fluorescence microscopy revealed partial mixing without aggregation for monoglycerides, but the presence of lipid aggregation for diglycerides and triglycerides. The effect of decreasing the chain length of triglycerides from a long chain to a medium chain caused the interactions of the lipids with the phospholipid molecules at the air/water interface to significantly improve. Decreasing the chain length of monoglycerides from a long chain to a medium chain worsened their interaction with the phospholipid molecules. The effect of decreasing the triglyceride chain length on their interactions with phospholipids was much greater than the effect of decreasing the number of alkyl chains in the lipid.

Mixed DPPC-cholesterol Langmuir monolayers in presence of hydrophilic silica nanoparticles

Langmuir monolayers of Cholesterol (Chol) and a mixture of Chol with 1,2-Dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC), at a ratio of 17:83 in weight, spread on pure water and on silica nanoparticle dispersions, have been investigated measuring the compression isotherms as well as the surface pressure response to harmonic area variation of the monolayer. Aim of this study was to evaluate the effects of the interaction of silica nanoparticles with Chol and the conditions for the incorporation in the monolayer. In previous works on different kind of lipid monolayers, it has been shown that hydrophilic silica nanoparticles dispersed in the sub-phase may transfer into the monolayer, driven by the interaction with the lipid molecules that make them partially hydrophobic. The results here obtained indicate that also for Chol and Chol-DPPC mixtures the presence of silica nanoparticles may have important effects on the phase behaviour and structural properties of the monolayer. As confirmed by complementary structural characterisations, BAM, AFM and ellipsometry, the principal effect of the nanoparticle incorporation is the disruption of the monolayer packing, owing to the alteration of the cohesive interactions of lipid components.

Electrochemical modeling of the silica nanoparticle-biomembrane interaction

The interaction of amorphous colloidal silica (SiO(2)) nanoparticles of well-defined sizes with a dioleoyl phosphatidylcholine (DOPC) monolayer on a mercury (Hg) film electrode has been investigated. It was shown using electrochemical methods and microcalorimetry that particles interact with the monolayer, and the electrochemical data shows that the extent of interaction is inversely proportional to the particle size. Scanning electron microscopy (SEM) images of the electrode-supported monolayers following exposure to the particles shows that the nanoparticles bind to the DOPC monolayer irrespective of their size, forming a particle monolayer on the DOPC surface. A one-parameter model was developed to describe the electrochemical results where the fitted parameter is an interfacial layer thickness (3.2 nm). The model is based on the adsorptive interactions operating within this interfacial layer that are independent of the solution pH and solution ionic strength. The evidence implies that the most significant forces determining the interactions are van der Waals in character.