Contact Angles of Microellipsoids at Fluid Interfaces

The wetting of anisotropic colloidal particles is of great importance in several applications, including Pickering emulsions, filled foams, and membrane transduction by particles. However, the combined effect of shape and surface chemistry on the three-phase contact angle of anisotropic micrometer and submicrometer colloids has been poorly investigated to date, due to the lack of a suitable experimental technique to resolve individual particles. In the present work, we investigate the variation of the contact angle of prolate ellipsoidal colloids at a liquid–liquid interface as a function of surface chemistry and aspect ratio using freeze-fracture shadow-casting cryo-SEM. The method, initially demonstrated for spherical colloids, is extended here to the more general case of ellipsoids. The prolate ellipsoidal particles are prepared from polystyrene and poly­(methyl methacrylate) spheres using a film stretching technique, in which cleaning steps are needed to remove all film material from the particle surface. The effects of the preparation protocol are reported, and wrinkling of the three-phase contact line is observed when the particle surface is insufficiently cleaned. For identically prepared ellipsoids, the cosine of the measured contact angle is, in a first approximation, a linearly decreasing function of the contact line length and thus a decreasing function of the aspect ratio. Such a trend violates Young–Laplace’s equation and can be rationalized by adding a correction term to the ideal Young–Laplace contact angle that expresses the relative importance of line effects relative to surface effects. From this term the contribution of an <i>effective line tension</i> can be extracted. This contribution includes the effects that both surface chemical and topographical heterogeneities have on the contact line and which become increasingly more important for ellipsoids with higher aspect ratios, where the contact line length to contact area ratio increases

Effect of Compatibilization on Interfacial Polarization and Intrinsic Length Scales in Biphasic Polymer Blends of PαMSAN and PMMA: A Combined Experimental and Modeling Dielectric Study

We describe an approach to tailor the dielectric interfacial properties of polymer blends by the interplay of compatibilizer effects on blend morphology and on blocking of charge carriers. A systematic study of the effect of the concentration of the compatibilizer, a random copolymer of poly­(styrene-<i>random</i>-methyl methacrylate) (PS-<i>r</i>-PMMA), on the interfacial properties of stacked polymer films and a phase separating blend of poly­[(α-methylstyrene)-<i>co</i>-acrylonitrile]/poly­(methyl methacrylate) (PαMSAN/PMMA) was performed. From dielectric spectroscopy, “conductivity free” dielectric loss spectra revealed interfacial blocking of charge carriers at low frequencies owing to the conductivity contrast of the blend components, resulting in a dielectric interfacial peak. From the dielectric response of stacked polymer films, the interfacial polarization was significantly suppressed when the thickness of the more conducting film approached the Debye length (<i>L</i>_D), on the basis of which <i>L</i>_D could be calculated using the Trukhan model. With increasing concentration of interfacially localized copolymer, an increase of the relaxation strength of the interfacial polarization occurred due to a pronounced decrease of <i>L</i>_D. The characteristics of the former were further investigated in biphasic polymer blends and were governed by the intrinsic length scale of the system, i.e., the ratio of structure dimension (<i>D</i>_<i>v</i>) to Debye length (<i>L</i>_D). Upon compatibilization, the interfacially segregated copolymer not only results in refinement of the PαMSAN phase but also provides an augmented conductivity difference leading to a slower dynamics of PαMSAN charges at the interface. This resulted in a substantial increase of the peak intensity of the interfacial polarization, which was attributed to a pronounced attenuation of <i>L</i>_D. The latter parameter was estimated using either the impedance formalism or the interfacial relaxation time. Similar to the stacked polymer films, upon compatibilization, an increase of the relaxation strength of the interfacial polarization occurred in biphasic polymer blends, which corroborated a pronounced decrease of <i>L</i>_D. Our results provide a methodology to characterize and tune the morphology and blocking of charge carriers with the aim of tailoring the dielectric interfacial properties of biphasic morphologies

Adsorption of Ellipsoidal Particles at Liquid–Liquid Interfaces

The adsorption of particles at liquid–liquid interfaces is of great scientific and technological importance. In particular, for nonspherical particles, the capillary forces that drive adsorption vary with position and orientation, and complex adsorption pathways have been predicted by simulations. On the basis of the latter, it has been suggested that the timescales of adsorption are determined by a balance between capillary and viscous forces. However, several recent experimental results point out the role of contact line pinning in the adsorption of particles to interfaces and even suggest that the adsorption dynamics and pathways are completely determined by the latter, with the timescales of adsorption being determined solely by particle characteristics. In the present work, the adsorption trajectories of model ellipsoidal particles are investigated experimentally using cryo-SEM and by monitoring the altitudinal orientation angle using high-speed confocal microscopy. By varying the viscosity and the viscosity jump across the interfaces, we specifically interrogate the role of viscous forces

Effect of Compression on the Molecular Arrangement of Itraconazole–Soluplus Solid Dispersions: Induction of Liquid Crystals or Exacerbation of Phase Separation?

Predensification and compression are unit operations imperative to the manufacture of tablets and capsules. Such stress-inducing steps can cause destabilization of solid dispersions which can alter their molecular arrangement and ultimately affect dissolution rate and bioavailability. In this study, itraconazole–Soluplus solid dispersions with 50% (w/w) drug loading prepared by hot-melt extrusion (HME) were investigated. Compression was performed at both pharmaceutically relevant and extreme compression pressures and dwell times. The starting materials, powder, and compressed solid dispersions were analyzed using modulated differential scanning calorimetry (MDSC), X-ray diffraction (XRD), small- and wide-angle X-ray scattering (SWAXS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and broadband dielectric spectroscopy (BDS). MDSC analysis revealed that compression promotes phase separation of solid dispersions as indicated by an increase in glass transition width, occurrence of a peak in the nonreversing heat flow signal, and an increase in the net heat of fusion indicating crystallinity in the systems. SWAXS analysis ruled out the presence of mesophases. BDS measurements elucidated an increase in the Soluplus-rich regions of the solid dispersion upon compression. FTIR indicated changes in the spatiotemporal architecture of the solid dispersions mediated via disruption in hydrogen bonding and ultimately altered dynamics. These changes can have significant consequences on the final stability and performance of the solid dispersions