Effect of polymer-surfactant association on colloidal force

Magnetic nanofluids (Ferrofluids): Recent advances, applications, challenges, and future directions

Impelled by the need to find solutions to new challenges of modern technologies new materials with unique properties are being explored. Among various new materials that emerged over the decades, magnetic fluids exhibiting interesting physiochemical properties (optical, thermal, magnetic, rheological, apparent density, etc.) under a magnetic stimulus have been at the forefront of research. In the initial phase, there has been a fervent scientific curiosity to understand the field-induced intriguing properties of such fluids but later a plethora of technological applications emerged. Magnetic nanofluid, popularly known as ferrofluid, is a colloidal suspension of fine magnetic nanoparticles, has been at the forefront of research because of its magnetically tunable physicochemical properties and applications. Due to their stimuli-responsive behaviour, they have been finding more applications in biology and other engineering disciplines in recent years. Therefore, a critical review of this topic highlighting the necessary background, the potential of this material for emerging technologies, and the latest developments is warranted. This review also provides a summary of various applications, along with the key challenges and future research directions. The first part of the review addresses the different types of magnetic fluids, the genesis of magnetic fluids, their synthesis methodologies, properties, and stabilization techniques are discussed in detail. The second part of the review highlights the applications of magnetic nanofluids and nanoemulsions (as model systems) in probing order-disorder transitions, scattering, diffraction, magnetically reconfigurable internal structures, molecular interaction, and weak forces between colloidal particles, conformational changes of macromolecules at interfaces and polymer-surfactant complexation at the oil-water interface. The last part of the review summarizes the interesting applications of magnetic fluids such as heat transfer, sensors (temperature, pH, urea detection, cations, defect detection sensors), tunable optical filters, removal of dyes, dynamic seals, magnetic hyperthermia-based cancer therapy and other biomedical applications. The applications of magnetic nanofluids in diverse disciplines are growing day by day, yet there are challenges in their practical adaptation as field-worthy or packaged products. This review provides a pedagogical description of magnetic fluids, with the necessary background, key concepts, physics, experimental protocols, design of experiments, challenges and future directions.

From Individual Liquid Films to Macroscopic Foam Dynamics: A Comparison between Polymers and a Nonionic Surfactant

Foams can resist destabilizaton in ways that appear similar on a macroscopic scale, but the microscopic origins of the stability and the loss thereof can be quite diverse. Here, we compare both the macroscopic drainage and ultimate collapse of aqueous foams stabilized by either a partially hydrolyzed poly(vinyl alcohol) (PVA) or a nonionic low-molecular-weight surfactant (BrijO10) with the dynamics of individual thin films at the microscale. From this comparison, we gain significant insight regarding the effect of both surface stresses and intermolecular forces on macroscopic foam stability. Distinct regimes in the lifetime of the foams were observed. Drainage at early stages is controlled by the different stress-boundary conditions at the surfaces of the bubbles between the polymer and the surfactant. The stress-carrying capacity of PVA-stabilized interfaces is a result of the mutual contribution of Marangoni stresses and surface shear viscosity. In contrast, surface shear inviscidity and much weaker Marangoni stresses were observed for the nonionic surfactant surfaces, resulting in faster drainage times, both at the level of the single film and the macroscopic foam. At longer times, the PVA foams present a regime of homogeneous coalescence where isolated coalescence events are observed. This regime, which is observed only for PVA foams, occurs when the capillary pressure reaches the maximum disjoining pressure. A final regime is then observed for both systems where a fast coalescence front propagates from the top to the bottom of the foams. The critical liquid fractions and capillary pressures at which this regime is obtained are similar for both PVA and BrijO10 foams, which most likely indicates that collapse is related to a universal mechanism that seems unrelated to the stabilizer interfacial dynamics.

Inter-droplet force between magnetically polarizable Pickering oil-in-water nanoemulsions stabilized with γ-Al2O3 nanoparticles: Role of electrostatic and electric dipolar interactions

HYPOTHESIS

The presence of nanoparticles at oil-water interface influences the interaction forces between Pickering emulsions. When charged nanoparticles are at the oil-water interface of an electrostatically stabilized emulsion, in addition to the screened Coulombic interaction, electric dipolar force also influences the total inter-droplet force profiles. An in-depth understanding of the effects of such electric dipolar forces is essential for designing colloidally stable Pickering nanoemulsions for various applications.

EXPERIMENTS

Inter-droplet forces between γ-Al₂O₃ nanoparticle stabilized oil-in-water nanoemulsion, containing superparamagnetic nanoparticles (magnetically polarizable) in the oil phase, are measured using the magnetic-chaining technique at different pH and salt concentrations. The role of mono-, di- and tri-valent salts on the inter-droplet force profiles are assessed.

FINDINGS

Force measurement studies reveal a lowering of inter-droplet spacing, within the linear chains, for higher salt concentrations due to an increased screening. Strong interfacial attachment of the charged nanoparticles results in the formation of an asymmetric charge cloud leading to an electric dipolar interaction. Incorporating the contributions of electric dipolar and screened Coulombic interactions, the theoretically estimated total repulsive force magnitudes are in good agreement with the experimental data. The obtained results offer better insights into the nature of colloidal force between charged particle stabilized nanoemulsions.

Influence of Ag+ interaction on 1D droplet array spacing and the repulsive forces between stimuli-responsive nanoemulsion droplets

This paper reports results on the effect of interaction of Ag(+) on 1D droplet array spacing and the repulsive forces between stimuli-responsive nanoemulsion droplets, stabilized with an anionic surfactant--sodium dodecyl sulfate--and a diblock polymer--poly(vinyl alcohol)-vinyl acetate. The repulsive interaction is probed by measuring the in-situ equilibrium force-distance in the presence of Ag(+) using the magnetic chaining technique. At a constant static magnetic field, emulsion droplets form 1D array that diffract visible light. A large blue-shift in the diffracted light is observed in the presence of interacting Ag(+) because of the reduction in the interdroplet spacing within the 1D array. The in-situ equilibrium force-distance measurement results show that the onset of repulsions and magnitude of repulsive forces are strongly influenced by the presence of Ag(+) in ppb levels. This suggests that the Ag(+) ions screen the surface charges through the formation of both Stern and diffuse electric double layer and produces a dramatic blue-shift in surfactant-stabilized emulsion, whereas a dramatic conformational change in the adsorbed polymer layer causes a reduction in the 1D array spacing in the diblock polymer stabilized emulsion. The force-distance results are compared with the predictions of electrical double-layer and repulsive steric forces. The droplet array shows an excellent selectivity to Ag(+) due to the strong interaction of Ag(+) with the stabilizing moieties at the oil-water interface. The possible mechanisms of interaction of Ag(+) with surfactant and polymer are discussed. The dramatic decrease in the 1D array spacing in the presence of Ag(+) may find promising practical applications in the development of optical sensors for selective detection of cations with ultrahigh sensitivity.

Sensing of biologically important cations such as Na(+), K(+), Ca(2+), Cu(2+), and Fe(3+) using magnetic nanoemulsions

We report a simple approach to the ultrasensitive detection of biologically important metal ions using a magnetic nanoemulsion. The nanoemulsion used in our study was an oil-in-water emulsion droplet of average size ∼190 nm containing ferrimagnetic iron oxide nanoparticles of average size ∼10 nm. In a static magnetic field, the emulsion droplets self-assemble into a nanoarray with distinct interdroplet spacing. In the presence of cations in the solution, the nanofluid array shows a large blue shift in the diffracted Bragg peak and a visually perceivable color change due to changes in the electrical double layer upon the diffusion of cations. The colloidal force-distance measurements in the presence of cations show large variations at the onset of repulsion in the presence of cations. The sensor shows good selectivity to Na(+), K(+), Ca(2+), Cu(2+), and Fe(3+) ions and offers a rapid response compared to conventional techniques. This approach can be useful for the recognition of biologically important cations.

Measuring colloidal forces with the magnetic chaining technique

In 1994 Leal Calderon et al. (Phys. Rev. Lett. 72, 2959 (1994)) introduced the magnetic chaining technique to directly probe the force-distance profile between colloidal particles. In this paper, we revisit this approach in two ways. First, we describe a new experimental design which allows us to utilize sample volumes as low as a few microliters, involving femtomoles of surface active macromolecules. Secondly, we extensively describe the characterization and preparation of the magnetic colloids, and we give a quantitative evaluation of performance and resolution of the technique in terms of force and interparticle separation.

Oil-in-oil microencapsulation technique with an external perfluorohexane phase

Commonly, the microencapsulation of a lipophilic drug in a polymeric matrix via an ordinary oil/oil emulsification allows entrapping limited drug amounts due to its loss into the external phase. In this present paper, a new microencapsultion method describes the use of perfluorohexane as an external oil phase in order to produce microparticles of polyvinylpyrrolidon/vinylacetate (copovidone) and Eudragit RS. Due to its highly non-solvent properties to most compounds, very limited miscibility with organic solvents, and very low toxicity, perfluorohexane (PFH) represents an excellent liquid for an external phase of the emulsion. Copovidone and Eudragit RS microparticles were prepared by an oil/PFH method trapping ibuprofen as a lipophilic model drug and compared to results from conventional methods (oil/water and oil/oil). Morphological analyses of the obtained particles underlined the general matrix structure. The particle size varied between 75microm (oil/oil) and 400microm (oil/PFH) largely influenced by the stirring speed. Although drug release kinetics were principally similar for all preparation methods, it was generally found that encapsulation rates of oil/water and oil/PFH systems (oil/water: 74+/-9%; oil/PFH: 86+/-10%) were superior to ordinary oil/oil emulsification (3+/-1%). The use of PFH was found to be a new promising tool for the preparation of microparticles. This modified emulsification method allowed the entrapment of lipophilic drugs into hydrophilic or lipophilic polymers in the absence of an aqueous phase.

Forces between two oil drops in aqueous solution measured by AFM

The surface and hydrodynamic forces between individual oil droplets in solution can provide insight into both emulsion stability and processes such as drop coalescence in liquid-liquid extraction. We present the first measurements of the interaction forces between alkane droplets in aqueous solution using atomic force microscopy. The radii of the droplets were well below the capillary lengths for the system, thus gravity effects are negligible, and interfacial tension and interaction forces governed the system behavior. The effects of modulating electrostatic double-layer interactions and interfacial tension through the presence of an anionic surfactant are demonstrated. Challenges in interpretation of the force data due to drop deformation are also discussed. A range of drop approach and retract speeds was used to determine the regime where hydrodynamic drainage effects had significant impact on the measurement.

Stretching and collapse of neutral polymer layers under association with ionic surfactants

We provide experimental evidence for stretching and collapse of neutral polymer layers, already adsorbed at an oil-in-water interface, due to its interaction with surfactants. Upon stretching, the first interaction length (2L(0)) follows a power law dependence on surfactant concentration ( proportional, variant C(x)(s), where x approximately 0.5 for cationic surfactants) and collapses in the presence of salt, as a relatively weak power law (C(-y)(s), where y=0.17), in good agreement with brush length decay for polyelectrolyte brushes.