Anatol M. Brodsky

Evanescent fiber-optic chemical sensor for monitoring volatile organic compounds in water.

The transport of trichloroethylene, 1,1,1-trichloroethane, and toluene in aqueous solutions through a polydimethylsiloxane film was modeled using a Fickian diffusion model to fit data obtained from an evanescent fiber-optic chemical sensor (EFOCS). The resultant diffusion coefficients for these analytes were respectively 3 × 10(-)(7), 5 × 10(-)(7), and 1 × 10(-)(7) cm(2)/s. Inclusion of an interfacial conductance term, defined as the ratio of the mass transport coefficient across the polymer surface and the analyte diffusion coefficient in the polymer, was required to accurately model the data. It was determined that the interfacial conductance terms were generally of the same order of magnitude for the analytes examined, suggesting a constant transport mechanism for the analytes. Linear chemometric algorithms were used to model the EFOCS response to aqueous mixtures of the three analytes with individual analyte concentrations between 20 and 300 ppm. Both partial least-squares and principal component regression algorithms performed comparably on the calibration sets, with cross-validated root-mean-squared errors of prediction for trichloroethylene, 1,1,1-trichloroethane, and toluene of approximately 26, 29, and 22 ppm, respectively. The resultant prediction model was then used to determine analyte concentrations in an independent data set with comparable precision.

Is there predictive value in water computer simulations

Abstract There are hardly more important and less understood liquids than liquid water and aqueous solutions. The description of the thermodynamics and kinetics of uniform and, especially, non-uniform aqueous systems is the central problem of contemporary liquid state chemical physics with important applications in biochemistry, electrochemistry, industrial chemistry and geophysics. The importance of water and aqueous solutions explains the interest in their description by computer simulation methods such as Monte Carlo and molecular dynamics. We discuss the actual question of the limits of applicability of currently popular computational models. It is stressed that hydrogen bonding, one of the most important interactions in nature, cannot be quantitatively described in the framework of empirical potential models.

STUDY OF ANALYTE DIFFUSION INTO A SILICONE-CLAD FIBER-OPTIC CHEMICAL SENSOR BY EVANESCENT WAVE SPECTROSCOPY

The diffusion rates of various polar and nonpolar analytes in dimethylsiloxane were examined with the use of a commercially available 200-μm silica-core/300-μm silicone-clad fiber as the optical element for evanescent wave spectroscopy in the near-infrared spectral region. An analytical solution to Ficks second law was used to model the time-dependent analyte concentration at the core/cladding interface. Successful fit of the analytical solutions to infrared data verifies the assumption of constant diffusion coefficients that is necessary to solve the equation. Transport rates of polar analytes in silicone can be estimated with the use of a single-parameter model that results in diffusion coefficients of 3.2 × 10−1, 1.6 × 10−1, 8.1 × 10−7, and 3.9 × 10−7 cm2/s for methanol, ethanol, 2-propanol, and n-butanol, respectively. Estimating the transport of larger nonpolar analytes in the silicone cladding requires a two-parameter model that includes a diffusion coefficient and an interfacial conductance term. For pentane, hexane, heptane, and cyclohexane the resultant diffusion coefficients and interfacial conductance parameters are 6.9 × 10−7, 4.6 × 10−7, 4.4 × 10−7, and 2.3 × 10−7 cm2/s and 2500, 2000, 2000, and 600 μm−1, respectively.

GRATING LIGHT REFLECTION SPECTROSCOPY

R ecent advances in holographic techniques for fabricating high-quality diffraction gratings on dielectric substrates have stimulated their use for spectroscopic studies of surfaces and properties of media in contact with the gratingcovered interface. There are a number of examples in which surface gratings with periods on the order of visible light wavelengths have served as a coupler between light waves and surface plasmons and polaritons. Also, Sainov and co-worker, using a relatively old idea ® rst proposed by Rytov and Fabelynskii, constructed a binary metal grating device on glass, working in total internal re ̄ ection mode, as a sensor for the determination of absorbance in liquid samples. We have developed a new optical diagnostic method, grating light rē ection spectroscopy (GLRS), based on measurements near critical points of intensity and phase in waves rē ected from a transmission diffraction grating in contact with a diagnostic sample. The theory and experimental studies of the method are described in three previous articles

Optical Low‐Coherence Reflectometry for Nondestructive Process Measurements

Optical Low‐Coherence Reflectometry (OLCR) is a white‐light interference technique which can be used as a sensor in a number of processing applications. Research at the Center for Process Analytical Chemistry focuses on analysis of both transparent and highly‐scattering materials. OLCR has been applied to monitor the thickness of polymer films and clear coatings on scattering matrices. Additional process applications include fermentation monitoring and non‐invasive thickness determination of highly scattering coatings on both conducting and non‐conducting substrates.

Using Ultrasonic Diffraction Grating Spectroscopy to Characterize Fluids and Slurries

In ultrasonic diffraction grating spectroscopy, the grating surface is in contact with the liquid or slurry. The ultrasonic beam, traveling in the solid, strikes the back of the grating and produces a transmitted m = 1 beam in the liquid. The angle of this beam in the liquid changes with frequency and the so‐called critical frequency occurs when the angle is 90°. At this point, the signal of the reflected m = 0 wave—the signal observed in the experiment—increases and this increase is used to characterize the liquid or slurry.

Casimir free energy of a spherical cavity in a dielectric medium

An expression is derived for the Casimir free energy of a spherical cavity in a polar dielectric medium at finite temperature. In the process of the derivation the general problem of infinities and their renormalization in calculations of Casimir forces is analyzed. It is shown that the renormalized Casimir free energy has a minimum at a finite mesoscopic value of the cavity radius R=RMinCas, with the repulsion for R<RMinCas and the attraction for R⩾RMinCas. The implications of this result for the explanation of cavitation effects are discussed.

Closed-form approximate expressions for the electrochemical response of microelectrodes in the zero range limit

We show that the electrochemical response of one or more microelectrodes can be described, in the limit of small size, by closed-form solutions of diffusion-kinetic equations, which, under appropriate and well-defined conditions, should give a good approximation to the actual response. The predictions of the analytical solutions for the diffusion-kinetic model are compared with microelectrode array experiments of Bard et al. and show excellent agreement. The analytical results make clear the various scaling properties of the solution, as a function of electrode size and spacings, as well as indicating the range of validity of the diffusion-kinetic model itself.

Observing Effects of Particle Size for a Slurry Using Ultrasonic Diffraction Grating Spectroscopy

The ultrasonic diffraction grating is formed by machining triangular grooves, 240 microns apart, on the flat surface of a stainless steel (SS) half‐cylinder. Ultrasound from a send transducer travels through the SS and strikes the back of the grating where it is reflected to a receive transducer. A peak in the receive signal is used to determine the velocity of sound and observe effects of particle size.

Control of phase transition dynamics in media with nanoscale nonuniformities by coherence loss spectroscopy

The optical nondestructive characterization of chemical transformation dynamics and diffusion kinetics, including phase transitions, in heterogeneous media with a random distribution of nanoparticles (nano-nonuniformities), is of great theoretical and practical importance. Such characterization, with the help of coherence loss spectroscopy, considered in this paper can be applied for the control of a number of industrial processes dynamics, environmental monitoring, and medical diagnostics and therapy. As a specific example, the growth of crystal nuclei (embrions) as a result of the diffusion to them of a substance from the surrounding supersaturated solution is considered.