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.     

The pressure and temperature dependence of the rate constant for methyl radical recombination over the temperature range 296–577 K

The rate constant for methyl radical recombination has been measured over the temperature range 296–577 K and at pressures between 5 and 500 Torr using laser flash photolysis, coupled with absorption spectroscopy at 216.36 nm. Analysis of the fall-off curves gives k_∞ = (2.78 ± 0.18) × 10^?11 exp(154 ± 22 K/T) cm³ molecule^?1 s^?1 and k₀ = (6.0 ± 3.3) × 10^?29 exp(1680 ± 300 K/T) cm⁶ molecule^?2 s^?1. The quoted errors (two standard deviations) do not include the present uncertainty in the absorption cross section, which is a major source of error (± 30%).     

High-temperature shock tube measurements of methyl radical decomposition

We have studied the two-channel thermal decomposition of methyl radicals in argon, involving the reactions CH3 + Ar --> CH + H2 + Ar (1a) and CH3 + Ar --> CH2 + H + Ar (1b), in shock tube experiments over the 2253-3527 K temperature range, at pressures between 0.7 and 4.2 atm. CH was monitored by continuous-wave, narrow-line-width laser absorption at 431.1311 nm. The collision-broadening coefficient for CH in argon, 2gamma(CH-Ar), was measured via repeated single-frequency experiments in the ethane pyrolysis system behind reflected shock waves. The measured 2gamma(CH-Ar) value and updated spectroscopic and molecular parameters were used to calculate the CH absorption coefficient at 431.1311 nm (23194.80 cm(-1)), which was then used to convert raw traces of fractional transmission to quantitative CH concentration time histories in the methyl decomposition experiments. The rate coefficient of reaction 1a was measured by monitoring CH radicals generated upon shock-heating highly dilute mixtures of ethane, C2H6, or methyl iodide, CH3I, in an argon bath. A detailed chemical kinetic mechanism was used to model the measured CH time histories. Within experimental uncertainty and scatter, no pressure dependence could be discerned in the rate coefficient of reaction 1a in the 0.7-4.2 atm pressure range. A least-squares, two-parameter fit of the current measurements, applicable between 2706 and 3527 K, gives k(1a) (cm(3) mol(-1) s(-1)) = 3.09 x 1015 exp[-40700/T (K)]. The rate coefficient of reaction 1b was determined by shock-heating dilute mixtures of C2H6 or CH3I and excess O2 in argon. During the course of reaction, OH radicals were monitored using the well-characterized R(1)(5) line of the OH A-X (0,0) band at 306.6871 nm (32606.52 cm(-1)). H atoms generated via reaction 1b rapidly react with O2, which is present in excess, forming OH. The OH traces are primarily sensitive to reaction 1b, reaction 9 (H + O2 --> OH + O) and reaction 10 (CH3 + O2 --> products), where the rate coefficients of reactions 9 and 10 are relatively well-established. No pressure dependence could be discerned for reaction 1b between 1.1 and 3.9 atm. A two-parameter, least-squares fit of the current data, valid over the 2253-2975 K temperature range, yields the rate expression k(1b) (cm(3) mol(-1) s(-1)) = 2.24 x 10(15) exp[-41600/T (K)]. Theoretical calculations carried out using a master equation/RRKM analysis fit the measurements reasonably well.     

A test of the maximum bubble pressure method for electrocapillary measurements

The reliability of the maximum bubble pressure method for electrocapillary measurements has been tested in aqueous and non-aqueous solutions. The results suggest that, although contact angle problems are actually eliminated by using this method, serious systematic errors may be observed. These errors are attributed to changes of the effective radius of the capillary tip, due to changes of the thickness of a wetting film formed between the mercury and the capillary wall. A detailed examination of these errors when we use siliconized and non-siliconized capillaries is presented and discussed.     

Ionization of methyl orange in aqueous sodium chloride solutions

Ionization constants of sodium 4′-dimethylaminoazobenzene-4-sulphonate (methyl orange) were determined by means of spectrophotometric measurements in water and in aqueous sodium chloride solutions with molalities up to 2 mol·kg⁻¹ at temperatures between 278.15 K and 333.15 K. The temperature dependence of the thermodynamic acidity constant shows a slight curvature in accordance with published data. The influence of sodium chloride on the methyl orange deprotonation was assessed by the measurement of stoichiometric acidity constants in this ionic medium. The Pitzer theory, widely used in the evaluation of the excess free energy of non-ideal electrolyte solutions, was applied to the computation of the activity coefficients of the chemical species involved in the equilibria and a good fit of those equations to the experimental data was observed, at all temperatures under consideration.     

A spectroscopic determination of the methyl radical recombination rate constant in shock waves

The reactions CH₃ +

and CD₃ +

have been studied in shoch waves at 1200–1500 K and densities of 2 × 10^?6 ?2 × 10^?4 mol cm^?3 using UV absorption near 216 nm. The rate constants at the highest densities: k_H = (1.7 ± 0.6) × 10^?11 cm³ s^?1 and k_D = (2.2 ± 0.9) × 10^?11 cm³ s^?1 are close to the second order limit. At the lowest densities the rates are lower by a factor of 5. The experimental results agree well with theoretical predictions based on the statistical adiabatic channel model but differ from those of conventional RRKM calculations. A direct observation of the equilibrium C₂H₆ ? 2CH₃ favours the “high” value for ΔH⁰₀ (87.76 kcal/mol).     

On the decay of the ozonide radical in aqueous alkaline solutions

In flash photolysis of an oxygenated aqueous potassium persulphate solution at pH 12.5 the decay of the ozonide radical has been found to follow $\text{3}\text{2}$ order kinetics which has been explained by reactions O^?₃ + O^? ? 2 O^?₂ and O^?₃ + HO₂ → 2 O₂ + OH^?     

The effects of chemically induced polarisation in the cage recombination products of the methyl radical

A study of the CIDNP effects on methyl protons was carried out for the cage recombination products formed during the thermal decomposition reactions of peracetyls and peracetylcarbonates. The increase of both the cage recombination product yield and the enhancement of CIDNP on the CH₃ protons is shown to be connected with the RCO₂ radical lifetime, where R is an alkyl or alkoxy radical. A general theory of CIDNP is presented and the calculated data prove to be in satisfactory agreement with the experimental results.     

Properties of aqueous solutions of methyl cellulose-polyvinyl alcohol blends

Rheological properties of dilute and moderately concentrated aqueous solutions of methyl cellulose-polyvinyl alcohol blends, as well as the conditions of gelation in these solutions were studied. The compatibility of the polymers was examined by the solvent vapor sorption method; the range of the compositions corresponding to thermodynamic compatibility of methyl cellulose and polyvinyl alcohol was identified.     

Interfacial tension measurements at the interface between two highly immiscible electrolyte solutions: the trapped air bubble method

A dynamic method for the measurement of interfacial tension at a liquid–liquid interface under controlled interfacial potential difference is described. The interface was formed as a drop suspended at the tip of a liquid filled glass micro-syringe into which a trapped air bubble of known volume had been introduced. Changes in volume of the air bubble allow changes in the surface tension to be calculated. Application to measurement of the variation in interfacial tension with cyclic variation in interfacial potential difference (‘cyclic volttensiometry’) is demonstrated.     

Bound water measurements for aqueous protein solutions and food gels

The role of aqueous media in the stabilization of globular proteins and formation of gels was studied by absorption millimeter spectroscopy. This method allowed to measure bound water, the fraction of water which had decreased rotational mobility owing to the presence of solute. Hydration data for globular proteins were compared with data obtained previously for low-weight molecules and groups. It was found that rotational mobility of water molecules in the hydration shells of various kinds of solutes (groups) decreased in the following order: water structure breaking compounds>polar groups>unfolded proteins>globular proteins>non-polar groups. Time courses of the storage modulus were determined for the chemical acidification by glucono-δ-lactone (GDL) of milk samples prepared from skimmed milk powder (SMP). Gelation of unheated milk was a monotonous process that started at pH 4.9. Heat-treated milk from SMP (16 and 14 g per 100 ml) acidified by GDL (3 g per 100 ml) at 43 °C gave non-monotonous kinetics of gelation with two phases corresponding to the mechanisms induced by denatured whey proteins at pH>5 and by casein–casein interactions at pH 4.8–4.9. For heat-treated milk, measurement of bound water gave two stages of decrease in water mobility. Additional hydration of SMP during acidification gave 0.15–0.2 g and 0.8 g bound H₂O per gram of SMP for unheated and heat-treated milk, respectively.     

Hydroxyl radical,sulfate radical and nitrate radical reactivity towards crown ethers in aqueous solutions

Reaction rate constants of crown ethers (12-crown-4, 15-crown-5, 18-crown-6) and their analogs 1,4-dioxane (6C2) with some important oxidative radicals, hydroxyl radical (OH), sulfate radical (SO₄^?) and nitrate radical (NO₃), were determined in various aqueous solutions by pulse radiolysis and laser photolysis techniques. The reaction rate constants for 6C2 and crown ethers with OH and SO₄^? increase with the number of hydrogen atoms in the ethers, indicating that the hydrogen-atom abstraction is a dominant reaction between crown ethers and these two radicals. The presence of cations in solution has negligible effect on the rate constants of crown ether towards OH and SO₄^?. However, for the NO₃, the rate constants are not proportional to the number of hydrogen atoms in ethers, and 12-crown-4 (12C4) is the most reactive compared with other crown ethers. Except 12C4 and 6C2, the cations in the aqueous solution affect the reactivities of 15-crown-5 (15C5) and 18-crown-6 (18C6). The cations with high binding stability for crown ether would improve the reactivity of 15C5. For the studied crown ethers, the reaction rate constants of these oxidative radicals have the order OH>SO₄^?>NO₃. Furthermore, the formation of radicals after the reaction of crown ethers with sulfate radical could be observed in the range of 260–280 nm using laser photolysis and pulse radiolysis. This is the first report on the kinetic behavior of crown ethers with NO₃, and it would be helpful for the understanding of stability of crown ethers in the processing of spent nuclear fuel.     

Thermoreversible gelation and phase separation in aqueous methyl cellulose solutions

We derived typical phase diagrams for aqueous solutions of methyl cellulose (MC) of different molecular weights via micro‐differential scanning calorimetry, small‐angle X‐ray scattering, and visual inspection. The phase diagrams showed the cooccurrence of gelation and phase separation and qualitatively agreed with the theoretically calculated diagrams. The sol–gel transition line and phase separation line of a lower critical solution point type shifted toward lower temperatures and lower concentrations with an increase in the MC molecular weight. The sol–gel transition line intersected at a temperature higher than the critical point of the phase separation; therefore, both sol–gel phase separation and gel–gel phase separation were possible, depending on the temperature. Specifically, through visual inspection of a high molecular weight MC sample in the critical temperature region, we observed phase separation into two coexisting gels with different polymer concentrations. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 91–100, 2001     

Estimated ionicB-coefficients from NMR measurements in aqueous electrolyte solutions

¹⁷O-NMR spin-lattice relaxation timesT ₁ of D₂O molecules were measured at 5–85°C in D₂O solutions of alkali metal halides (LiClCsCl, KBr, and KI), DCl, KOD, Ph₄PCl, NaPh₄B, and tetraalkylammonium bromides (Me₄NBrAm₄NBr) in the concentration range 0.1–1.4 mol-kg^–1 TheB-coefficients of the electrolytes obtained from the concentration dependence of relaxation ratesR ₁=1/T₁ were divided into the ionicB-coefficients by three methods: (i) the assumption ofB (K⁺)=B(Cl^–), (ii) the assumption ofB(Ph₄P⁺)=B(Ph₄B^–), and (iii) the use ofB(Br^–) obtained from a series ofB(R₄NBr). It was found that Methods (ii) and (iii) resulted in an abnormal temperature dependence of theB-coefficients of alkali metal ions and a negative values of rotational correlation times _c at lower temperatures for hydroxide and halide ions. These results suggest that the methods based on the van der Waals volume are not adequate for the ionic separation of NMRB-coefficients. From the analysis using the assumption ofB(K⁺)=B(Cl^–), it was found that D₃O⁺, OD^–, and Me₄N⁺ ions are the intermediates between structure makers and breakers, and that the hydrophobicity of phenyl groups is weaker than that of alkyl groups due to the interactions between water molecules and -electrons in phenyl groups.     

Cholic acid-based fluorescent sensor for mercuric and methyl mercuric ion in aqueous solutions

Cholic acid-based fluorescent PET sensor probe 1, bearing a pair of dithiocarbamate pendants as the receptive site and anthracene moiety as the signal displaying unit, was designed and synthesized. The sensor probe not only shows high selectivity and sensitivity to Hg²⁺ in aqueous acetonitrile solution, but also responds moderately to MeHg⁺. A distinctive OFF-ON type signaling of up to 10-fold enhancement was observed for this new sensor probe toward Hg²⁺.     

Dynamic surface tension measurements of surfactant solutions using the maximum bubble pressure method – limits of applicability

One of the essential differences in the design of bubble pressure tensiometers consists in the geometry of the measuring capillaries. To reach extremely short adsorption times of milliseconds and below, the so-called deadtime of the capillaries must be of the order of some 10 ms. In particular, for concentrated surfactant solutions, such as micellar solutions, short deadtimes are needed to minimize the initial surfactant load of the generated bubbles. A theoretical model is derived and confirmed by experiments performed for a wide range of experimental conditions, mainly in respect to variations in deadtime and bubble volume.     

Acoustic emission spectra from 515 kHz cavitation in aqueous solutions containing surface-active solutes

The effect of adding surface-active solutes to water being insonated at 515 kHz has been investigated by monitoring the acoustic emission from the solutions. At low concentrations (<3 mM), sodium dodecyl sulfate causes marked changes to the acoustic emission spectrum which can be interpreted in terms of preventing bubble coalescence and declustering of bubbles within a cavitating bubble cloud. By conducting experiments in the presence of background electrolytes and also using non-ionic surfactants, the importance of electrostatic effects has been revealed. The results provide further mechanistic evidence for the interpretation of the effect of surface-active solutes on acoustic cavitation and hence on the mechanism of sonochemistry. The work will be valuable to many researchers in allowing them to optimize reaction and process conditions in sonochemical systems.     

Molecular clusters in aqueous solutions of pyridine and its methyl derivatives

Small-angle neutron scattering proved that molecules in aqueous solutions of pyridine, 2-methylpyridine and 2,6-dimethylpyridine form clusters. The clusters are dynamic aggregates consisting of hydrogen-bonded water-amine complexes. Strengthening of the hydrogen bonds between water and amine molecules due to the methyl groups in the ortho position in the pyridine ring makes the structures more stable, as was evidenced by relatively long times of the structural relaxation. The strong intermolecular forces affect the thermal expansion of the systems. No aggregates similar to those in aqueous systems are present in the methanolic ones. That points to the crucial role of water in the molecular clustering. A molecule of methanol, although capable of hydrogen bonding with the amines, cannot participate in larger structures because of the lack of protons that could form the enhanced network. Thus, even if the amine-methanol complexes occur, they are incapable of further association. It was shown that the co-operative nature of hydrogen bonds and the propensity of water to association are the main factors that determine the properties of aqueous systems.