Sodium Atom Emission from Aqueous Surfactant Solutions Exposed to Ultrasound.

Emission from electronically excited sodium atoms (Na*) was observed when argon saturated aqueous solutions of the anionic surfactants, sodium dodecyl sulfate, sodium octyl sulfate, sodium 1-pentanesulfonate, and sodium 1-octanesulfonate were sonicated using 358 kHz ultrasound. The same emission band, centered at about 590 nm, was also obtained in aqueous NaCl solutions, although a ∼100-fold higher concentration than that used for the surfactant solutions was required to obtain an emission of comparable intensity. The results have been interpreted in terms of the surfactant adsorbing at the gas-solution interface of the bubbles generated by the ultrasound, generating an electrostatic surface potential, and attracting Na+ counterions to the bubble surface. It is reasoned that Na+ ions are simultaneously reduced and electronically excited at the bubble-solution interface during the final stages of the collapse phase of the acoustically driven bubble. It is proposed that sodium ion bound water molecules reduce interfacial Na+ under the extreme, perhaps supercritical, conditions the interface experiences on bubble implosion.

Formation of Amino Acids on the Sonolysis of Aqueous Solutions Containing Acetic Acid, Methane, or Carbon Dioxide, in the Presence of Nitrogen Gas.

The sonolysis of aqueous solutions containing acetic acid, methane, or carbon dioxide in the presence of nitrogen gas was found to produce a number of different amino acids at a rate of ∼1 to 100 nM/min, using ultrasound at an operating power of 70 W and 355 kHz. Gas-phase elementary reactions are suggested, and discussed, to account for the formation of the complex biomolecules from the low molar mass solutes used. On the basis of the results, a new hypothesis is presented to explain the formation of amino acids under primitive atmospheric conditions and how their formation may be linked to the eventual abiotic genesis of life on Earth.

Direct AFM force measurements between air bubbles in aqueous monodisperse sodium poly(styrene sulfonate) solutions.

Structural forces play an important role in the rheology, processing and stability of colloidal systems and complex fluids, with polyelectrolytes representing a key class of structuring colloids. Here, we explore the interactions between soft colloids, in the form of air bubbles, in solutions of monodisperse sodium poly(styrene sulfonate) as a model polyelectrolyte. It is found that by self-consistently modelling the force oscillations due to structuring of the polymer chains along with deformation of the bubbles, it is possible to precisely predict the interaction potential between approaching bubbles. In line with polyelectrolyte scaling theory, two distinct regimes of behaviour are seen, corresponding to dilute and semi-dilute polymer solutions. It is also seen that by blending monodisperse systems to give a bidisperse sample, the interaction forces between soft colloids can be controlled with a high degree of precision. At increasing bubble collision velocity, it is revealed that hydrodynamic flow overwhelms oscillatory structural interactions, showing the important disparity between equilibrium behaviour and dynamic interactions.

Direct AFM force measurements between air bubbles in aqueous polydisperse sodium poly(styrene sulfonate) solutions: effect of collision speed, polyelectrolyte concentration and molar mass.

Interactions between colliding air bubbles in aqueous solutions of polydisperse sodium poly(styrene sulfonate) (NaPSS) using direct force measurements were studied. The forces measured with deformable interfaces were shown to be more sensitive to the presence of the polyelectrolytes when compared to similar measurements using rigid interfaces. The experimental factors that were examined were NaPSS concentration, bubble collision velocity and polyelectrolyte molar mass. These measurements were then compared with an analytical model based on polyelectrolyte scaling theory in order to explain the effects of concentration and bubble deformation on the interaction between bubbles. Typically structural forces from the presence of monodisperse polyelectrolyte between interacting surfaces may be expected, however, it was found that the polydispersity in molar mass resulted in the structural forces to be smoothed and only a depletion interaction was able to be measured between interacting bubbles. It was found that an increase in number density of NaPSS molecules resulted in an increase in the magnitude of the depletion interaction. Conversely this interaction was overwhelmed by an increase in the fluid flow in the system at higher bubble collision velocities. Polymer molar mass dispersity plays a significant role in the interactions present between the bubbles and has implications that also affect the polyelectrolyte overlap concentration of the solution. Further understanding of these implications can be expected to play a role in the improvement in operations in such fields as water treatment and mineral processing where polyelectrolytes are used extensively.

The hydrophobic force: measurements and methods.

The hydrophobic force describes the attraction between water-hating molecules (and surfaces) that draws them together, causing aggregation, phase separation, protein folding and many other inherent physical phenomena. Attempts have been made to isolate the range and magnitude of this interaction between extended surfaces for more than four decades, with wildly varying results. In this perspective, we critically analyse the application of common force-measuring techniques to the hydrophobic force conundrum. In doing so, we highlight possible interferences to these measurements and provide physical rationalisation where possible. By analysing the most recent measurements, new approaches to establishing the form of this force become apparent, and we suggest potential future directions to further refine our understanding of this vital, physical force.

Multibubble sonoluminescence in ethylene glycol/water mixtures.

The multibubble sonoluminescence (MBSL) signals generated by 3.5 ms pulses of 515 kHz ultrasound in air-saturated ethylene glycol, water, and ethylene glycol/water mixtures were examined in the absence and presence of a range of solutes, including aliphatic alcohols of various chain lengths (C3-C6) and ionic and zwitterionic surfactants. In general, the alcohols quenched the SL in most solvent mixtures and the surfactants enhanced the sonoluminescence signal. However, in some solvent mixtures complex effects were observed in the presence of the solutes. The discussion presented rationalizes the varied behavior of the solutes on the MBSL observed in terms of their influence on inter- and intrabubble effects experienced by bubbles in an ultrasound field.

On the generation of the hydrated electron during the sonolysis of aqueous solutions.

The formation of the hydrated electron through the secondary reaction, H + OH(­) → H(2)O + e(aq), has been examined in the sonolysis of argon-saturated aqueous solutions at an ultrasound frequency of 355 kHz. The detection of the hydrated electron was achieved by measuring its reaction with the one-electron acceptors Fe(CN)(6)(3­) and methyl viologen. The results obtained indicate that hydrated electrons are produced predominately at the bubble/aqueous solution interface at comparatively high local concentrations, estimated to be >>1.5 × 10(­3) M. The half life of the hydrated electron under such conditions is estimated to be <<60 ns.

Sonoluminescence quenching and cavitation bubble temperature measurements in an ionic liquid.

A comparison between the temperatures within imploding acoustic cavitation bubbles and the extent of sonoluminescence (SL) quenching by C(1)-C(5) aliphatic alcohols in 1-ethyl-3-methylimidazolium ethylsulfate ([EMIM][EtSO(4)], a well known imidazolium based room temperature ionic liquid (RTIL)), has been made at an ultrasound frequency of 213 kHz. The temperatures obtained ranged from 3500 ± 200K, in neat [EMIM][EtSO(4)], to about 3200 ± 200K in RTIL-alcohol containing solutions. It was also found that the SL intensity decreased with increasing concentration (up to 1M) of the alcohols to a greater extent compared with the relative changes to the bubble temperatures. Both the extent of the reduction in the bubble temperatures and the SL quenching were much smaller than those obtained in comparable aqueous solutions containing aliphatic alcohols. Possible reasons for the differences in the observed trends between water/alcohol and [EMIM][EtSO(4)]/alcohol systems under sonication at 213 kHz are discussed.

Measurement of the Hydrophobic Force in a Soft Matter System.

The hydrophobic attraction describes the well-known tendency for nonpolar molecules and surfaces to agglomerate in water, controlled by the reorganization of intervening water molecules to minimize disruption to their hydrogen bonding network. Measurements of the attraction between chemically hydrophobised solid surfaces have reported ranges varying from tens to hundreds of nanometers, all attributed to hydrophobic forces. Here, by studying the interaction between two hydrophobic oil drops in water under well-controlled conditions where all known surface forces are suppressed, we observe only a strong, short-ranged attraction with an exponential decay length of 0.30 ± 0.03 nm-comparable to molecular correlations of water molecules. This attraction is implicated in a range of fundamental phenomena from self-assembled monolayer formation to the action of membrane proteins and nonstick surface coatings.

The behavior of acoustic bubbles in aqueous solutions containing soluble polymers.

The effects of the water-soluble polymer polyvinylpyrrolidone (PVP) on the multibubble sonoluminescence (MBSL) intensity generated in aqueous solutions exposed to ultrasound at the two ultrasound frequencies of 20 and 363 kHz have been examined. In both cases, the presence of PVP, at concentrations of up to 2 g/100 mL, was found to enhance the MBSL intensity emitted from the solutions. On the basis of the intensity behavior of the SL observed from aqueous solutions containing PVP/surfactant and PVP/alcohol mixtures, it is suggested that PVP enhances MBSL by increasing the number of active bubbles in the system by hindering bubble-bubble coalescence processes and probably also by changing the structure of the bubble "clouds" formed at the acoustic antinodes in solution. The influence of PVP on bubble-bubble coalescence rates was also measured to support the interpretation of the MBSL emission experiments.

Reaction of ferricyanide and methyl viologen with free radicals produced by ultrasound in aqueous solutions.

The redox reactions of organic radicals, with Fe(CN)63− and methyl viologen, generated from the sonochemical decomposition of aliphatic alcohols in aqueous solutions, have been studied. The number of radicals produced was found to relate to the amount of adsorbed alcohol molecules (Gibbs surface excess) at the gas−aqueous solution interface for any bulk solution concentration of the alcohol. The majority of the radicals produced stem from the thermal degradation of the alcohol molecules that have entered imploding cavitation bubbles. The maximum rate of reduction at 355 kHz, of Fe(CN)63−, was 2.6 ± 0.3 µM min−1, whereas for methyl viologen, it was 1.2 ± 0.3 µM min−1 under the conditions used.The difference in the rates is attributed to the reaction of various pyrolytically produced organic radicals with the methyl viologen radical cation. The possible reactions occurring in the sonolysis of alcohol/water systems are discussed in detail.

Polymeric stabilized emulsions: steric effects and deformation in soft systems.

Polymeric stabilizers are used in a broad range of processes and products, from pharmaceuticals and engine lubricants to formulated foods and shampoos. In rigid particulate systems, the stabilization mechanism is attributed to the repulsive force that arises from the compression of the polymer coating or "steric brush" on the interacting particles. This mechanism has dictated polymer design and selection for more than thirty years. Here we show, through direct measurement of the repulsive interactions between immobilized drops with adsorbed polymers layers in aqueous electrolyte solutions, that the interaction is a result of both steric stabilization and drop deformation. Drops driven together at slow collision speeds, where hydrodynamic drainage effects are negligible, show a strong dependence on drop deformation instead of brush compression. When drops are driven together at higher collision speeds where hydrodynamic drainage affects the interaction force, simple continuum modeling suggests that the film drainage is sensitive to flow through the polymer brush. These data suggest, for drop sizes where drop deformation is appreciable, that the stability of emulsion drops is less sensitive to the molecular weight or size of the adsorbed polymer layer than for rigid particulate systems.

Anomalous pull-off forces between surfactant-free emulsion drops in different aqueous electrolytes.

A systematic study of collisions between surfactant-free organic drops in aqueous electrolyte solutions reveals the threshold at which continuum models provide a complete description of thin-film interactions. For collision velocities above ~1 µm/s, continuum models of hydrodynamics and surface forces provide a complete description of the interaction, despite the absence of surfactant. This includes accurate prediction of coalescence at high salt concentration (500 mM). In electrolyte solutions at intermediate salt concentration (50 mM), drop-drop collisions at lower velocity (<1 µm) or extended time of forced drop-drop interaction exhibit a strong pull-off force of systematically varying magnitude. The observations have implications on the effects of ion-specificity and time-dependence in drop-drop interactions where kinetic stability is marginal.

Measurement and analysis of forces in bubble and droplet systems using AFM.

The use of atomic force microscopy to measure and understand the interactions between deformable colloids - particularly bubbles and drops - has grown to prominence over the last decade. Insight into surface and structural forces, hydrodynamic drainage and coalescence events has been obtained, aiding in the understanding of emulsions, foams and other soft matter systems. This article provides information on experimental techniques and considerations unique to performing such measurements. The theoretical modelling frameworks which have proven crucial to quantitative analysis are presented briefly, along with a summary of the most significant results from drop and bubble AFM measurements. The advantages and limitations of such measurements are noted in the context of other experimental force measurement techniques.

Bubble coalescence during acoustic cavitation in aqueous electrolyte solutions.

Bubble coalescence behavior in aqueous electrolyte (MgSO(4), NaCl, KCl, HCl, H(2)SO(4)) solutions exposed to an ultrasound field (213 kHz) has been examined. The extent of coalescence was found to be dependent on electrolyte type and concentration, and could be directly linked to the amount of solubilized gas (He, Ar, air) in solution for the conditions used. No evidence of specific ion effects in acoustic bubble coalescence was found. The results have been compared with several previous coalescence studies on bubbles in aqueous electrolyte and aliphatic alcohol solutions in the absence of an ultrasound field. It is concluded that the impedance of bubble coalescence by electrolytes observed in a number of studies is the result of dynamic processes involving several key steps. First, ions (or more likely, ion-pairs) are required to adsorb at the gas/solution interface, a process that takes longer than 0.5 ms and probably fractions of a second. At a sufficient interfacial loading (estimated to be less than 1-2% monolayer coverage) of the adsorbed species, the hydrodynamic boundary condition at the bubble/solution interface switches from tangentially mobile (with zero shear stress) to tangentially immobile, commensurate with that of a solid-liquid interface. This condition is the result of spatially nonuniform coverage of the surface by solute molecules and the ensuing generation of surface tension gradients. This change reduces the film drainage rate between interacting bubbles, thereby reducing the relative rate of bubble coalescence. We have identified this point of immobilization of tangential interfacial fluid flow with the "critical transition concentration" that has been widely observed for electrolytes and nonelectrolytes. We also present arguments to support the speculation that in aqueous electrolyte solutions the adsorbed surface species responsible for the immobilization of the interface is an ion-pair complex.

Kinetics and mechanism for the sonophotocatalytic degradation of p-chlorobenzoic acid.

The advanced oxidation processes of sonolysis (213 kHz), photocatalysis, and a hybrid combination of both (sonophotocatalysis) have been used in the degradation of a representative aromatic carboxylic acid, p-chlorobenzoic acid (PCBA), in ambient air saturated aqueous solutions. The formation of degradation products were monitored quantitatively and qualitatively using HPLC and MS/MS. A kinetic model was used to account for the degradation of the PCBA in the presence of intermediate degradation products and also their formation and subsequent degradation. Under certain experimental conditions a small (20% enhancement) synergistic effect in the degradation rate was evident in the combined process compared with the sum of the individual processes.

The mechanism of sonophotocatalytic degradation of methyl orange and its products in aqueous solutions.

In this study, it was found that a hybrid technique, sonophotocatalysis, is able to degrade a parent organic pollutant (methyl orange) as well as its by-products. The analysis of products formed during the whole degradation has demonstrated that the pH or the selection of oxidation process (sonolysis/photocatalysis/sonophotocatalysis) is able to control the degradation pathway. It was shown in the by-products analysis that the solution pH can alter the amount of each product generated during the sonophotocatalytic degradation. It was revealed that the different degradation rates of methyl orange and its products result from the solution pH and the nature of the organic products. Furthermore, a comparison of the data obtained from the oxidation processes on the degradation of the reaction intermediates identified the advantages of the combined system. It is concluded that sonophotocatalysis is capable of yielding a more complete and faster mineralization of organic pollutants than the individual processes. However, as in the degradation of the parent compound, the overall mineralization is lower than an additive effect (negative synergistic effect).

Effect of gold oxide in measurements of colloidal force.

Atomic force microscopy, contact-angle, and spectroscopic ellipsometry measurements were employed to investigate the presence and properties of gold oxide on the surface of gold metal. It was found that, in agreement with available literature, unoxidized gold surfaces were hydrophobic, whereas oxidation rendered the surface highly hydrophilic. The oxide could be removed with ethanol or base but appeared to be stable over long periods in water or salt solutions between pH 3 and 7. After oxidation, the oxide layer thickness, determined using ellipsometry, was consistent with an approximate monolayer of Au-O bonds at the gold surface. The presence of gold oxide was found to alter significantly the electrical double-layer characteristics of the gold surface below pH 6 and may explain the apparent inconsistencies in observed force behavior where gold is employed as well as aiding in design of future microfluidic systems which incorporate gold as a coating.

Repulsive van der Waals forces in soft matter: why bubbles do not stick to walls.

Measurements of nonequilibrium hydrodynamic interactions between bubbles and solid surfaces in water provide direct evidence that repulsive van der Waals forces of quantum origin control the behavior of liquid films on solids in air. In addition to being the simplest and most universal 3-phase system, the deformable air-water interface greatly enhances the sensitivity of force measurements compared with rigid systems. The strength of the repulsive interaction, controlled by the choice of solid, is sufficient to prevent coalescence (sticking) on separation due to hydrodynamic interactions.