Interaction between a silica particle and the underside of a polymer monolayer at the air/water interface in the presence of an anionic surfactant

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.

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.

Interaction between graphene oxide and Pluronic F127 at the air-water interface

Triblock copolymer Pluronic F127 (PF127) has previously been demonstrated to disperse graphene oxide (GO) in electrolyte solution and block the hydrophobic interaction between GO and l-tryptophan and l-tyrosine. However, the nature of this interaction between PF127 and GO remains to be characterized and elucidated. In the present study, we aimed to characterize and understand the interaction between GO and PF127 using a 2-dimensional Langmuir monolayer methodology at the air-water interface by surface pressure-area isotherm measurement, stability, adsorption, and atomic force microscopy (AFM) imaging. Based on the observation of surface pressure-area isotherms, adsorption, and stability of PF127 and PF127/GO mixture at the air-water interface, GO is suggested to change the conformation of PF127 at the air-water interface and also drag PF127 from the interface to the bulk subphase. Atomic force microscopy (AFM) image supports this assumption, as GO and PF127 can be observed by spreading the subphase solution outside the compressing barriers, as shown in the TOC graphic.