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.

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.

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.

Dynamic interactions between microbubbles in water.

The interaction between moving bubbles, vapor voids in liquid, can arguably represent the simplest dynamical system in continuum mechanics as only a liquid and its vapor phase are involved. Surprisingly, and perhaps because of the ephemeral nature of bubbles, there has been no direct measurement of the time-dependent force between colliding bubbles which probes the effects of surface deformations and hydrodynamic flow on length scales down to nanometers. Using ultrasonically generated microbubbles (approximately 100 microm size) that have been accurately positioned in an atomic force microscope, we have made direct measurements of the force between two bubbles in water under controlled collision conditions that are similar to Brownian particles in solution. The experimental results together with detailed modeling reveal the nature of hydrodynamic boundary conditions at the air/water interface, the importance of the coupling of hydrodynamic flow, attractive van der Waals-Lifshitz forces, and bubble deformation in determining the conditions and mechanisms that lead to bubble coalescence. The observed behavior differs from intuitions gained from previous studies conducted using rigid particles. These direct force measurements reveal no specific ion effects at high ionic strengths or any special role of thermal fluctuations in film thickness in triggering the onset of bubble coalescence.

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.

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.

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.

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.

Sonochemical synthesis of magnetic Janus nanoparticles.

The sonochemical synthesis of nanosized surface-dissymmetrical (Janus) particles is described. The Janus particles were composed of silica and polystyrene, with the polystyrene portion loaded with nanosized magnetite particles. It is shown that the Janus particles can be used to form kinetically stable oil-in-water emulsions that can be spontaneously broken on application of an external magnetic field. The one-pot synthetic process used to prepare the Janus particles has several advantages over other conventional methods of producing such particles.

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.

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.

Precision AFM measurements of dynamic interactions between deformable drops in aqueous surfactant and surfactant-free solutions.

The atomic force microscope (AFM) has provided unprecedented opportunities to study velocity-dependent interactions between deformable drops and bubbles under a range of solution conditions. The challenge is to design an experimental system that enables accurate force spectroscopy of the interaction between deformable drops and thus the extraction of accurate quantitative information about the physically important force-separation relation. This step requires very precise control and knowledge of the interfacial properties of the interacting drops, the drive conditions of the force-sensing cantilever, the disposition of the interacting drops on the substrate and on the cantilever, and transducer calibrations of the instrument in order to quantify the effects of approach velocities and interfacial deformation. This article examines and quantifies in detail all experimental conditions that are necessary to facilitate accurate processing of dynamic force spectroscopy data from the AFM using the well-defined system of tetradecane drops in aqueous solutions under surfactant and surfactant-free conditions over a range of force magnitudes that has not been attained before. The ability of drops to deform and increase the effective area of interaction instead of decreasing the distance of closest approach when disjoining pressure exceeds the Laplace pressure means that the DLVO paradigm of colloidal stability as being determined by a balance of kinetic energy against the height of the primary maximum is no longer valid. The range of interfacially active species present in alkane-aqueous systems investigated provides insight into the applicability of the tangentially immobile boundary condition in colloidal interactions.

Sonochemical polymerization of miniemulsions in organic liquids/water mixtures.

The sonochemical oil-in-water miniemulsion polymerization of n-butyl methacrylate (BMA) has been studied in mixtures with a range of aliphatic and aromatic hydrocarbon liquids under ambient conditions. Measurements of monomer conversion percentage and molecular weights of the BMA polymers were performed to investigate the effect of the various organic liquids on the kinetics of the polymerization process and on the properties of the resultant polymers. Both the rates of polymerization and the molecular weights of the polymers formed were found to be dependent on the amount and type of the organic liquid present in the emulsion. The experimental results revealed that when the organic liquids were aliphatic, there were no significant changes in the rates of BMA polymerization whereas when the organic liquids were aromatic, the rates of polymerization were greatly reduced. Molecular weight data of the BMA polymers showed that in the presence of an organic liquid, the size of the polymer significantly decreased. The results have been interpreted in terms of the formation of a radical complex between the propagating radical and the organic liquid in the oil mixture, as well as chain transfer reactions that affect the kinetics of the polymerization process.

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.

Acoustic bubble sizes, coalescence, and sonochemical activity in aqueous electrolyte solutions saturated with different gases.

Acoustic bubble sizes, coalescence behavior, and sonochemical activity have been investigated in water in the presence of various electrolyte additives (KCl, HCl, and NaNO(3)) and saturating gases-helium, air, and argon. A strong correlation was identified between the bubble radius and the dissolved gas concentration in the cavitation medium. The extent of bubble coalescence for each gas was also studied in different electrolyte solutions. A causal relationship between coalescence and bubble size was inferred. Importantly, the effects of the different electrolytes could be completely attributed to their "salting out" effect on the dissolved gas, providing valuable insight into the contentious issue of ion-specific coalescence inhibition. Extrapolation of the bubble size data to conditions where bubble coalescence is minimal, i.e., zero gas concentration and zero ultrasound exposure time, yielded a bubble radius of 1.5 +/- 0.5 microm at an acoustic frequency of 515 kHz. In addition, the effects of electrolyte concentration and gas type on sonochemical activity were investigated. Sonochemical yields were increased by up to 1 order of magnitude at high electrolyte concentrations. This has been attributed to reduced gas and vapor content in the bubble core prior to collapse and a lower clustering density.

Lateral hydrodynamic interactions between an emulsion droplet and a flat surface evaluated by frictional force microscopy.

We introduce a lateral atomic force microscopy (AFM) method to measure the hydrodynamic drag force acting on a microscopic emulsion droplet moving parallel to a flat surface. A tetradecane oil droplet formed in an aqueous solution of sodium dodecylsulfate was attached to a V-shaped atomic force microscopy cantilever, and lateral hydrodynamic interactions between the droplet and a flat glass surface were measured using a range of scanning velocities. The droplet was positioned either far from the oscillating surface or was pressed to the surface under a constant applied load. These measurements demonstrate the feasibility of using AFM to study lateral hydrodynamic interactions and lubricity between soft matter materials relevant to a large number of applications in areas as diverse as flavor delivery in foods to the applications of emulsions or emollients in personal care products.

Viscosity effects on hydrodynamic drainage force measurements involving deformable bodies.

Dynamic force measurements have been made between an oil drop and a silica particle in surfactant and sucrose solutions with viscosities that range up to 50 times that of water. These conditions provide variations in the shear rate and the relative time scales of droplet deformation and hydrodynamic drainage in a soft matter system. The results obtained indicate that soft deformable boundaries have a natural response that limits the maximum shear rate that can be sustained in thin films compared to shear rates that can be attained in films bounded by rigid boundaries. In addition, to extend boundary slip studies on rigid surfaces, we use a smooth deformable droplet surface to probe the dependence of the boundary slip on fluid viscosity without the added complications of surface roughness or heterogeneity. Imposing a Navier slip model to characterize possible slip at the deformable oil-sucrose solution interface gives results that are consistent with a slip length of no larger than 10 nm over the range of solution viscosity studied, although an immobile (zero slip length) condition at the oil-sucrose solution interface is perfectly adequate. In high viscosity solutions, cantilever motion at high scan rates induces a significant cantilever deflection. A method has been developed to account for this effect in order to extract the correct dynamic force between the deformable drop and the particle.

Cavitation activation by dual-frequency ultrasound and shock waves.

High-speed photographic observations of cavitation occurring under a low-frequency (21 kHz) sonotrode tip in the presence of an additional, high-frequency (355 kHz) ultrasound source have been made in water and in dilute aqueous solute solutions. Acoustic emission spectra were measured to support the visual observations. It was seen that a nucleating effect of the high-frequency action on cavitation at the low-frequency sonotrode was highly power dependent, with cavitation being homogenous at low acoustic power and highly localised at high acoustic power. The presence of solutes was found to significantly affect the cavitation structures and the bubble fragmentation process. Both the fundamental high-frequency acoustic emission peak and the higher order low-frequency harmonics were significantly intensified in the dual-frequency mode in the presence of these solutes. Additionally, the application of a high-voltage induced acoustic shock-wave to two different ultrasound fields was investigated in water and surfactant solutions.

Kinetics and mechanism for the sonochemical degradation of a nonionic surfactant.

The sonolytic degradation of the nonionic surfactant, octaethylene glycol monododecyl ether (C(12)E(8)), has been studied at various initial concentrations below and above its critical micelle concentration (CMC). It has been observed that the degradation rate increases with an increase in the initial concentration of the surfactant until the CMC is reached. Above the CMC an almost constant degradation rate is observed, suggesting that the surfactant in its monomer form is involved in the degradation process. The degradation process of C(12)E(8) involves two distinct primary processes occurring at the bubble/solution interface: (a) hydroxylation/oxidation of the surfactant and (b) pyrolytic fragmentation of the surfactant. The oxidative cleavage of ethylene oxide units provides evidence for OH radical attack. Hydroxylation of the ethoxy chain gives rise to various short-chain carboxyalkyl-polyethylene glycol intermediates. The polyethylene glycol chain formed, due to the scission of the C(12)E(8) molecule, undergoes rapid hydroxylation/oxidation to yield simple compounds that have the potential to undergo further degradation. The detection of multiple intermediates indicates that several processes affect the complete degradation pathways of the surfactant molecule. TOC analysis, however, indicates that the sonolytic mineralization of the surfactant is difficult to achieve at reasonable rates due to the relatively low surface activity of the degradation products formed during sonolysis.