Planar Air-water Interface Research Articles

Stearic Acid as Polymerization Medium, Dopant and Hydrophobizer: Chemical Oxidative Polymerization of Pyrrole.

In recent years, fatty acids have garnered significant attention as a natural phase-change material and a hydrophobizer due to their biocompatibility and biodegradability. In this study, the utilization of fatty acid is proposed as a polymerization medium for the first time. As a specific reaction, chemical oxidative polymerizations of pyrrole is conducted using ferric chloride as an oxidant in a stearic acid medium. The polymerizations resulted in the production of micrometer-sized polypyrrole (PPy) grains, which are aggregates of atypical primary particles with submicrometer size. The PPy grains are doped with stearic acid, suggesting that the stearic acid functioned as a dopant and a hydrophobizing agent as well as a polymerization medium. The dried PPy grains can adsorb at the air-water interface and function as a liquid marble stabilizer with light-to-heat photothermal properties. The liquid marble can move on a planar air-water interface by Marangoni flow induced by NIR laser light irradiation.

An examination of the short-time approximation for mixed-controlled adsorption

A new linear approximation for a mixed-controlled adsorption process had been designed for estimating the surfactant diffusivity (Df) and adsorption rate constant (βf) by purely using the dynamic surface tension (ST) data (Moorkanikkara and Blankschtein, 2006). However, the uniqueness and applicability of this linear approximation are yet to be ascertained. Subsequently, a theoretical simulation was conducted to study the uniqueness and robustness of this linear approximation. An adsorption process of nonionic surfactant onto a freshly created planar air-water interface was considered and its mixed-controlled dynamic ST profiles were simulated using the Langmuir model. Linear regions in the simulated ST vs t1/2 profiles do exist, which could be described well by a straight line with an intercept >72 mN/m (representing the linear approximation). Values of Df and βf could then be estimated from the slope and the intercept of this straight line. While the applicability of this linear approximation was confirmed, the results did show a distinct possibility of obtaining multiple sets of Df and βf if different linear regions of the γ vs t1/2 profiles were selected for the fitting. Additionally, the applicability of this linear approximation on experimental dynamic ST data was tested.

Effect of geometric constraint caused by nearby particles on the detachment from the particle-laden interface

The bubble-particle detachment interaction is critical to the flotation of coarse (composite) particles which has attracted significant research interest in recent years. It is principally controlled by the capillary forces arising from the deformation of the air–water interface that has been quantified employing the single-particle models. However, these single-particle models can be far away from the detachment in the actual flotation process in which the bubble surface is covered by many attached particles. Here, we investigate the effect of the presence of nearby (attached) particles on the interfacial deformation, thereby influencing the particle detachment from the laden interface. Specifically, the detaching force and the deformed interface shape were simultaneously recorded for a floating sphere at the center of a hexagonal lattice unit that comprised six spheres fixed at an initially planar air–water interface. The 3-dimensional Young–Laplace equation was numerically solved to determine the interface deformation, quantify the detachment process, and validate the model prediction for the detaching force. The results show that the effect of nearby particles called the constraint effect on the interface deformation and the detaching force is significant. The nearby particles confine the interface deformation around the test particle, thereby restricting the movement of the three-phase contact line (TPCL) on the particle surface, leading to smaller changes in the TPCL radius and displacement into the liquid phase during the detachment interaction than those in the absence of nearby particles. However, the change in the contact angle and polar angle of TPCL on the test particle is insignificant. The constraint effect reduces the tenacity which is critical to the stability of bubble-particle aggregates and was not expected. The results of this work provide useful details about the detachment and floatability of particles at the particle-laden interface, which is relevant in many industrial applications, including flotation separation.

Motion of Particles in a Monolayer Induced by Coalescing of a Bubble with a Planar Air–Water Interface

The motion of particles in a monolayer induced by the coalescing of a bare bubble with a planar air-water interface was investigated in a modified Langmuir trough. Experiments were performed to understand the effect of particle hydrophobicity, subphase pH, packing density, the presence of a weak surfactant, and particle size distribution on the behavior of particle movement in the monolayer during the coalescence process. Video tracking software was used to track the particles and extract data based on the video footage. Visual inspection indicated that the coalescence of the bubble with the monolayer was a chaotic process which led the interface to oscillate to an extent that the particles underwent complete rearrangement. A simple analysis was carried out on the main forces involved in particle motion and rearrangement at the oscillating air-water interface. The motion characteristic of particles was evaluated by speed and mean-square displacement (MSD). The results showed that the butanol-treated particles had higher speed and MSD than the particles with a stronger affinity to the air-water interface. Similar results were also found at high subphase pH and low packing factor.

Selective separation of very fine particles at a planar air–water interface

Selective fine particle separation is a key unit operation in the mineral and related industries. In flotation, the capture of fine particles by bubbles is inefficient due to their low mass and momentum, which result in low particle–bubble collision efficiency. We demonstrate that it is possible to selectively separate a mixture of very fine hydrophobic graphite and hydrophilic quartz particles by direct contact with an air–water interface without a particle–bubble collision step involved. We demonstrate that it is possible to scale-up the process from a simple batch to a continuous process. Good selective separation of graphite from quartz gangue could be obtained under continuous conditions.

Adsorption of Submicrometer-Sized Cationic Sterically Stabilized Polystyrene Latex at the Air−Water Interface: Contact Angle Determination by Ellipsometry

Near-monodisperse, sterically stabilized cationic polystyrene latexes of either 122 or 310 nm diameter were prepared by aqueous emulsion polymerization using cheap, readily available reagents. At low pH, these latexes stabilized foams prepared by either hand-shaking or by using a foam column. SEM studies confirmed that the dried foam mainly comprised well-defined bilayers, which suggests that each air bubble is stabilized with a latex monolayer. Adsorption of the same latexes at the planar air-water interface was studied using the Langmuir-Blodgett trough technique. Surface pressure isotherms confirmed particle desorption from the interface on repeated compression of the latex monolayers. For the 122 nm latex at pH 2, ellipsometric analysis enabled a contact angle of approximately 43 degrees to be calculated from a simple two-layer model, which suggests that these particles have only moderate wettability. Similar results were obtained for the 310 nm latex, but the data were much less reliable in this case due to additional background particle scattering.

On the Biot slow S-wave

It is accepted widely that the Biot theory predicts only one shear wave representing the in-phase/unison shear motions of the solid and fluid constituent phases (fast S-wave). The Biot theory also contains a shear mode wherein the two constituent phases essentially undergo out-of-phase shear motions (slow S-wave). From the outset of the development of the Biot framework, the existence of this mode has remained unnoticed because of an oversight in decoupling its system of two coupled equations governing shear processes. Moreover, in the absence of the fluid strain-rate term in the Biot constitutive relation, the velocity of this mode is zero. Once the Biot constitutive relation is corrected for the missing fluid strain-rate term (i.e., fluid viscosity), this mode turns out to be, in the inertial regime, a diffusive process akin to a viscous wave in a Newtonian fluid. In the viscous regime, it degenerates to a process governed by a diffusion equation with a damping term. Although this mode is damped so heavily that it dies off rapidly near its source, overlooking its existence ignores a mechanism to draw energy from seismic waves (fast P- and S-waves) via mode conversion at interfaces and at other material discontinuities and inhomogeneities. To illustrate the consequence of generating this mode at an interface, I examine the case of a horizontally polarized fast S-wave normal incident upon a planar air-water interface in a porous medium. Contrary to the classical Biot framework, which suggests that the incident wave should be transmitted practically unchanged through such an interface, the viscosity-corrected Biot framework predicts a strong, fast S-wave reflection because of the slow S-wave generated at the interface.

Non-Contact to Contact Transition: Direct Measurements ofInteraction Forces between a Solid Probe and a Planar Air–WaterInterface

The interaction force between a solid probe and a planar air–waterinterface is measured by using an atomic force microscope. It isdemonstrated that during the approach of the probe to the air–waterinterface, the force curves decline all the time due to the van derWaals attraction and induces a stable profile of water surface raised.When the tip approaches very close to the water surface, force curvesjump suddenly, reflecting the complex behaviour of the unstable watersurface. With a theoretical analysis we conclude that before the tiptouches water surface, two water profiles appear, one stable and theother unstable. Then, with further approaching, the tip touches watersurface and the non-contact to contact transition occurs.

Effect of thickeners on the coalescence of protein-stabilized air bubbles undergoing a pressure drop

The effect of some common food thickeners on the coalescence stability of protein-stabilized air bubbles at a planar air–water interface has been studied, as the bubbles undergo expansion due to a pressure drop. The fraction of bubbles coalescing ( F c) has been measured as a function of bulk protein concentration ( C b) and rate of area expansion. The ranges of protein concentration and rate of area strain studied were 0.01–0.3 wt% and 10 −3 to 2.4 s −1, respectively. Pure β-lactoglobulin, a commercial whey protein isolate (WPI) consisting primarily of β-lactoglobulin, and pure ovalbumin were used. The thickeners used were sucrose (50 wt%), xanthan gum, guar gum, gelatine and pectin. In the absence of thickeners, F c was slightly lower for ovalbumin than for WPI but slightly higher than for pure β-lactoglobulin at the highest C b. It was found that stability with thickeners present was less dependent on expansion rate than for the proteins on their own. The effect of the concentration of thickeners on F c was therefore studied at a fixed area strain-rate of 0.13 s −1. Sucrose had a general stabilizing effect with all the proteins, though this stabilization was less marked at higher protein concentrations and expansion rates. Overall, gelatine had the most marked effect on F c, at concentrations as low as 0.01 wt%. For xanthan and guar gum, effects on F c were significant at concentrations between 0.025 and 0.1 wt%, but were variable, increasing or decreasing the value of F c, depending on the protein and its concentration. Pectin had a less noticeable effect than xanthan or guar gum, and then only at a considerably higher concentration, i.e., 0.25–1 wt%.

In vitro characterization of echogenic liposomes by acoustic scattering at 3.5–15.0 MHz

Echogenic liposomes (ELIP) are phospholipid vesicles that are being developed as ultrasound contrast agents and as vehicles for drug delivery. These particles, when conjugated with antibodies or peptides, can be used for targeted diagnostic imaging and therapy. Evaluating the acoustic properties of ELIP will allow for optimization of its utility as an ultrasound contrast agent. The diameter of the liposomes ranges from 0.25–10.0 μm. In order to measure the backscattering coefficient in vitro, the backscattered power was compared to that reflected from a perfect reflector, a planar air–water interface. The backscatter coefficient and attenuation of ELIP were evaluated as a function of concentration (0.8–9.9×108/ml) and frequency (3.5–15.0 MHz). By comparing the values of measured backscatter coefficient to a theoretical model that treats the gas within the liposomes as a free air bubble, a size estimate of the encapsulated gas is provided. Finally, calculation of the scattering to attenuation ratio (STAR) gives a value with which to evaluate the efficacy of ELIP as a contrast agent in cardiovascular ultrasound imaging. The backscatter coefficient shows promise as a sensitive method for determining whether the liposomes are left intact or if they are destroyed during imaging.

Coalescence stability of gas bubbles subjected to rapid pressure change at a planar air/water interface

We report the effects of rapid and slow bubble expansion, due to a reduction in air pressure, on bubble coalescence stability at a planar air–water interface. Bubbles stabilised by pure β-lactoglobulin (BLG), a commercial whey protein isolate (WPI) consisting primarily of BLG, and the non-ionic surfactant Tween 20, were examined. A semi-automatic image analysis routine has been developed to size and count touching bubbles in close-packed arrays. It was found that the bubble size distribution does not change significantly after expansion and coalescence, except for the lowest bulk protein/surfactant concentration ( C b). Bubbles stabilized by BLG were increasingly more stable against rapid expansion with increasing C b, compared with bubbles stabilised by WPI, suggesting significant differences between the adsorbed protein films. For both proteins the fraction of coalescence, ( F c) increased with increasing expansion rate, until a plateau in F c was reached. For Tween 20, F c decreased with increasing number of injected stable bubbles per unit area of planar interface. For bubbles formed in mixtures of BLG and Tween 20 (at high expansion rate) F c increased with increasing molar ratio of Tween 20:BLG. For proteins F c is probably determined by both adsorption kinetics and the interfacial rheology of the adsorbed films, whilst other factors, such has thin film hydrodynamics, bubble size, bubble packing and the relative mobility of the bubbles at the interface, probably have additional, but more minor influence on overall bubble stability.

In vitro characterization of liposomes and Optison by acoustic scattering at 3.5 MHz

Objective: To assess the potential of immunoliposomes as an ultrasound contrast agent relative to albumen-stabilized microbubbles, a quantitative assessment of echogenicity was performed. Methods: Liposomes are stabilized phospholipid vesicles, which can encapsulate both gas and fluid. With antibody conjugation, this new formulation of immunoliposomes can be targeted to atheroma and other pathologic components, and thus they are being developed as novel diagnostic ultrasound echo contrast agents to enhance imaging of atherosclerosis. The diameter of the liposomes ranges from 0.25 to 5.0 microns. To measure the backscattering coefficient of liposome suspensions of varying concentrations, the backscattered power at 3.5 MHz was compared to that reflected from a planar air-water interface. The backscattering coefficient was also estimated theoretically as a function of air volume fraction by modeling the encapsulated air as a free air bubble and assuming single bubble scattering. Results: For most of the concentrations of liposomes used (on the order 108/ml), the backscattering coefficient equals or exceeds that of Optison (Mallinckrodt, Inc.) at the human clinical dose concentration (104/ml). The backscattering coefficient at 3.5 MHz appears to be a linear function of liposome concentration, suggesting that single bubble acoustic scattering theory holds. Conclusions: This detection technique shows promise as a sensitive method for determining whether the immunoliposomes are left intact or if they are destroyed during imaging, thus helping to explore their potential as a vehicle for targeted drug delivery.

Disproportionation of clustered protein-stabilized bubbles at planar air–water interfaces

The rate of shrinkage of air bubbles, of initial radius from 50 to 200 μm, injected beneath a planar air–water interface has been measured. Bubbles were stabilized in solutions of 0.05 wt% gelatin or pure β-lactoglobulin. It has been observed that small size differences between two closely spaced or touching bubbles result in markedly divergent rates of shrinkage for the two. By studying a number of different initial bubble configurations, it is demonstrated that the overall change in bubble size distribution is strongly dependent on local, interbubble gas diffusion. In this respect, the strong tendency for the gelatin-stabilized bubbles to aggregate and shrink, while remaining in contact, produced patterns of disproportionation significantly different from those observed with β-lactoglobulin. In β-lactoglobulin solutions, it was usually found that bubbles initially in contact shrank away from each other with time, becoming increasingly isolated as a result. A theoretical approach that can exactly incorporate the perturbation of local diffusion fluxes due to the proximity of two bubbles is presented. This enables one to map a “stability diagram” that delineates regions where the larger bubble of a pair will either shrink or grow, according to the relative size of the bubbles and their separation. Theoretical calculations show that it is possible for a bubble to exhibit more complex shrinkage behavior than is predicted by a mean field approach or the Lifshitz, Slyozov, and Wagner (LSW) theory of Ostwald ripening for dilute systems. The inclusion of dilatational elasticity in the theoretical model introduces additional complications, which are also briefly discussed.

Kinetics of Disproportionation of Air Bubbles beneath a Planar Air–Water Interface Stabilized by Food Proteins

The rate of shrinkage of air bubbles of initial radii, r, from 50 to 150 μm injected beneath a planar air–water interface has been measured. Bubbles were stabilized by 0.05 wt% protein in approximately 0.1 mol dm −3 ionic strength buffer at pH 7.0 and at room temperature. Four proteins were studied: commercial whey protein isolate (WPI), sodium caseinate, gelatin, and pure β-lactoglobulin. Bubbles in all systems showed shrinkage due to diffusion of gas from the bubbles, which accelerated as the bubbles got smaller. Within approximately 1 h all bubbles had disappeared, having shrunk to below approximately 1 μm, so that in no cases was there evidence of stabilization via a surface rheological mechanism. The rates of shrinkage with the different proteins were not significantly different except in the case of gelatin, which at any given bubble size appeared to give a slightly higher rate, probably because the surface tension is higher for this system. A new theoretical analysis of the dissolution kinetics for the case of a bubble close to a planar interface has been developed. For caseinate and WPI a simple model incorporating a constant surface tension and a constant bubble–interface separation appears to account for the kinetics. Interestingly, the model predicts a linear dependence of r n versus time when n is closest to 3, in contrast to n=2 expected from previous work. For gelatin and pure β-lactoglobulin, the introduction of modest dilatational elasticities of approximately 2.3 and 7 mN m −1, respectively, gives good agreement between theory and experiment. This is particularly the case for β-lactoglobulin, where there is a noticeable slowing, but not cessation, of the shrinkage as the bubbles get smaller. In the light of these findings the practical significance of surface rheology with respect to stability to disproportionation is discussed. Finally, we present experimental evidence that a bubble stabilized by β-lactoglobulin shrinks to a nonspherical protein particle consisting of the completely collapsed protein film.

On relationships between molecular structure, interaction and surface behavior in mixture: small-molecule surfactant+protein

We report on the effect of distinct in nature small-molecule surfactants (model, a sodium salt of capric acid, Na-caprate; and commercially important, a citric acid ester of monoglyceride, CITREM; a sodium salt of stearol-lactoyl lactic acid, SSL (Na +); polyglycerol ester, PGE (080)) on molecular properties in a bulk and at the air–water interface of globular legumin and random-coiled micellar sodium caseinate. The role of the structure of both proteins and small-molecule surfactants in the effect studied has been elucidated by measurements in a bulk aqueous medium of the enthalpy of their interaction from mixing calorimetry, the change in value of weight average molecular weight of the proteins and the thermodynamics of the pair protein–protein interactions from laser static light scattering as well as, in addition, by measurements of the change in hydrodynamic radius for micellar sodium caseinate from laser dynamic light scattering. The effect of the small-molecule surfactants on the thermodynamics of the protein heat denaturation and thereby on the protein conformational stability has been studied by differential scanning calorimetry in the case of globular legumin. The interrelation between the effects of the small-molecule surfactants on the properties of the proteins in a bulk and at the planar air–water interface has been elucidated by tensiometry. The combined data of mixing calorimetry, differential scanning calorimetry and laser light scattering suggest some complex formation between the small-molecule surfactants and the proteins in a bulk aqueous medium. Predominantly hydrophobic interaction along with electrostatic and hydrogen bonding form the basis of the complex formation. The found effect of the small-molecule surfactants on the surface activity of their mixtures with proteins is governed primarily by both the extent of the protein association, resulting in specific hydrophobicity/hydrophilicity of the surface of the protein associates, and the specific protein conformational stability, for the globular protein, produced by the interaction between the proteins and the small-molecule surfactants.

Surface activity at the planar interface in relation to the thermodynamics of intermolecular interactions in the ternary system: maltodextrin–small-molecule surfactant-legumin

We report on the effect of potato maltodextrins with variable dextrose equivalent (Paselli SA-2, SA-6 and SA-10) on the surface behavior at the air–water interface of the mixture: legumin+small-molecule surfactant. Distinct in nature small-molecule surfactants (model: sodium salt of capric acid, Na-caprate; and commercially important: a citric acid ester of monoglyceride, CITREM) have been under our consideration. The role of the structure of both of the maltodextrins and the small-molecule surfactants in the effect studied has been elucidated by measurements in a bulk aqueous medium of the enthalpy of their interaction from mixing calorimetry, value of weight average molecular weight of the maltodextrins and the thermodynamics of the pair maltodextrin-solvent and maltodextrin-protein interactions from laser static light scattering. The combined data of mixing calorimetry and light scattering suggest some complex formation between the small-molecule surfactants and the maltodextrins. Predominantly hydrophobic interactions along with hydrogen bonding form the basis of the complexes. The effect of the maltodextrins on the thermodynamics of the protein heat denaturation and thereby on the protein conformational stability in the presence of the small-molecule surfactants has been studied by differential scanning calorimetry. The interrelation between the thermodynamics of intermolecular interactions in a bulk and the surface behavior at the planar air–water interface of the ternary systems (maltodextrin+legumin+small-molecule surfactant) has been elucidated by tensiometry. The effect of the maltodextrins on the surface activity of mixtures of legumin with the small-molecule surfactants is governed by the competitive in relation to the protein interactions with the small-molecule surfactants and a subsequent change in the thermodynamic properties of the both biopolymers, which are favorable to the ternary complex formation.

Neutron reflectivity study of the competitive adsorption of β-casein and water—soluble surfactant at the planar air-water interface

Application of the technique of specular neutron reflection to the study of adsorbed layer structure is illustrated for the case of β-casein at the air—water interface in the presence and absence of nonionic water—soluble surfactant C 12E 6. Guinier analysis of reflectivity data for a 5 × 10 −3 wt% solution of pure β-casein in air-contrast-matched water (8 vol% D 2O) at pH 7.0 gives a time-independent adsorbed amount of 2.05 ± 0.10 mg/m2 and an adsorbed layer thickness of 1.65 ± 0.07 nm; these values are found to increase quite substantially as the pH is reduced towards the protein's isoelectric point. In the presence of surfactant the loss of reflectivity is a direct measure of protein displacement from the interface because the hydrogenated surfactant is almost contrast matched to the aqueous phase. At a C 12E 6 concentration in the range 2–2.5 × 10 −4 wt% (surfactant: protein molar ratio, R ≈ 2.2), there is roughly half the protein lost from the interface but little change in adsorbed layer thickness as inferred from the slopes of the Guinier plots. Protein is effectively completely removed from the surface for R ≳ 10. These results are in semi-quantitative agreement with complementary competitive adsorption data for β-casein in soya oil-in-water emulsions.