A. V. Shkirin

Nanoscale structure of dissolved air bubbles in water as studied by measuring the elements of the scattering matrix

Results of experiments with laser modulation interference microscopy and the Mueller-matrix scatterometry show that macroscopic scatterers of light waves are present in doubly distilled water free of external solid impurities. The experimental data can be interpreted using a computational model of micron-scale clusters composed of polydisperse air bubbles having effective radii of 70-90 nm. The fractal dimension of such clusters was evaluated as 2.4-2.8 and their concentration appeared to be approximately 10(6) cm(-3).

Structure of the nanobubble clusters of dissolved air in liquid media.

A qualitative model of the nucleation of stable bubbles in water at room temperature is suggested. This model is completely based on the property of the affinity of water at the nanometer scale; it is shown that under certain conditions the extent of disorder in a liquid starts growing, which results in a spontaneous decrease of the local density of the liquid and in the formation of nanometer-sized voids. These voids can serve as nuclei for the following generation of the so-called bubstons (the abbreviation for bubbles, stabilized by ions). The model of charging the bubstons by the ions, which are capable of adsorption, and the screening by a cloud of counter-ions, which are incapable of adsorption, is analyzed. It was shown that, subject to the charge of bubston, two regimes of such screening can be realized. At low charge of bubston the screening is described in the framework of the known linearized Debye–Huckel approach, when the sign of the counter-ion cloud preserves its sign everywhere in the liquid surrounding the bubston, whereas at large charge this sign is changed at some distance from the bubston surface. This effect provides the mechanism of the emergence of two types of compound particles having the opposite polarity, which leads to the aggregation of such compound particles by a ballistic kinetics.

Long-living nanobubbles of dissolved gas in aqueous solutions of salts and erythrocyte suspensions

Results of experiments combining laser modulation interference microscopy and Mueller matrix scatterometry show that macroscopic scatterers of light are present in liquids free of external solid impurities. Experimental data on distilled water and aqueous NaCl solutions of various concentrations as well as physiological saline solution are reported. The experimental data can be interpreted by using a model of micron-scale clusters composed of polydisperse air nanobubbles having effective radii of 70-100 nm. Their concentration increases with the growth of ionic content. We hypothesize that under certain conditions those clusters of nanobubbles can affect the erythrocyte structure.

Nanobubble clusters of dissolved gas in aqueous solutions of electrolyte. I. Experimental proof

Results of experiments with dynamic light scattering, phase microscopy, and polarimetric scatterometry allow us to claim that long-living gas nanobubbles and the clusters composed of such nanobubbles are generated spontaneously in an aqueous solution of salt, saturated with dissolved gas (say, atmospheric air). The characteristic sizes of both nanobubbles and their clusters are found by solving the inverse problem of optical wave scattering in ionic solutions. These experimental results develop our earlier study reported by Bunkin et al. [J. Chem. Phys. 130, 134308 (2009)] and can be treated as evidence for the special role of ions in the generation and stabilization of gas nanobubbles.

Laser scattering in water and aqueous solutions of salts

An automated laser polarimetric scatterometer, operating at a wavelength of 532nm, has been developed for measuring the total scattering matrix of liquid samples in a cylindrical geometry. The optical scheme of the polarimeter combines an electro-optic modulator with a rotary quarter-wave plate. The results of experiments involving distilled water and NaCl aqueous solutions are reported. The measured angular dependencies of the elements of the scattering matrix are compared with numerical computations based on the T-matrix method. The experimental scattering matrix data can be interpreted as a scattering on an ensemble of stochastic micron-scale clusters composed of polydisperse air bubbles having effective radii of 70-100 nm. The fractal dimension of such clusters was evaluated as 2.5-2.8. Their concentration increases with salt addition from 103 cm-3 in distilled water to 106 cm-3 in 0.8 M aqueous solution of NaCl.

Laser diagnostics of the Bubston phase in the bulk of aqueous salt solutions

Stable gas nanobubbles in the bulk of NaCl aqueous solutions and clusters of these nanobubbles have been investigated at different ion concentrations by four independent laser techniques (phase microscopy, dynamic light scattering, optical breakdown, and measurement of angular dependences of the light scattering matrix). The results obtained by these radically different techniques are in good agreement. It is found that the nanobubble size is practically constant and amounts to approximately 100nm in the range of ion concentrations 10−6<C <1M. It is shown that a necessary condition for nanobubble nucleation is the saturation of solution with dissolved air. It is revealed that nanobubble clusters form a thermodynamically nonequilibrium phase with a lifetime of several months.

Phase states of water near the surface of a polymer membrane. Phase microscopy and luminescence spectroscopy experiments

Phase microscopy is used to show that the refractive index in the near-surface layer of water at the surface of a polymer Nafion membrane increases by a factor of 1.1 as compared to bulk water. Moreover, this layer exhibits birefringence. Experiments on UV irradiation of dry (anhydrous) and water-soaked Nafion are performed in grazing-incidence geometry to study their stimulated luminescence spectra. These spectra are found to be identical in both cases. For dry Nafion, luminescence can only be excited if probing radiation illuminates the polymer surface. The luminescence of water-soaked Nafion can also be excited if the distance between the optical axis and the surface is several hundred micrometers.

Refraction coefficient of water and aqueous solutions in the optical frequency range in the vicinity of Nafion

Within the present experimental working it has been shown that in the vicinity of Nafion (ionexchange membrane) the water refraction coefficient grows approximately by a factor of 1.1 in comparison with its value in bulk liquid. The refractive coefficient changes at a distance of about 50 microns. The effect of changing the refraction coefficient is determined by pH value and the temperature of the liquid. In the experimental study of the «glycerin/water» mixture at different concentrations it has been found that the refractive coefficient may increase because Nafion surface attracts the dipole of water clusters. This effect occurs due to the swelling of Nafion in water and its surface takes a charge.

Dynamics of Nafion membrane swelling in H2O/D2O mixtures as studied using FTIR technique

Experiments with Fourier transform spectrometry of Nafion, a water-swollen polymeric membrane, are described. The transmittance spectra of liquid samples and Nafion, soaked in these samples, were studied, depending on the deuterium content in water in the spectral range 1.8-2.15 μm. The experiments were carried out using two protocols: in the first protocol we studied the dynamics of Nafion swelling in H2O + D2O mixtures for the deuterium concentrations 3 < C < 104 ppm, and in the second protocol we studied the dynamics of swelling in pure heavy water (C = 106 ppm). For liquid mixtures in the concentration range 3 < C < 104 ppm, the transmittance spectra are the same, but for Nafion soaked in these fluids, the corresponding spectra are different. It is shown that, in the range of deuterium contents C = 90-500 ppm, the behavior of transmittance of the polymer membrane is non-monotonic. In experiments using the second protocol, the dynamics of diffusion replacement of residual water, which is always present in the bulk of the polymer membrane inside closed cavities (i.e., without access to atmospheric air), were studied. The experimentally estimated diffusion coefficient for this process is ≈6·10-11 cm2/s.

Time dependence of the luminescence from a polymer membrane swollen in water: Concentration and isotopic effects

The effect of UV irradiation of the surface of a Nafion polymer electrolyte membrane swollen in water in the pump grazing incidence geometry has been experimentally investigated. The photoluminescence from the Nafion surface has been measured in the spectral range characteristic of this polymer. The photoluminescence signal from a polymer with a variable isotopic composition is found to be sensitive to swelling in water. The spectral absorption lines of dry and water-swollen Nafion samples are characterized. It is shown that the luminescence centers in the polymer are sulfonic acid groups located on the ends of perfluorovinyl ether groups, which form the teflon base. Measurements of the temporal dynamics of the luminescence of these groups have revealed an informationally important and significant dependence of the luminescence parameters on the degree of Nafion swelling. A pronounced and nontrivial dependence of these parameters on the content of heavy isotope D2O in water is also found.