We investigated the dielectric properties of various organic solvents and binary solvent mixtures at 21.4 °C over the frequency range of 200 MHz−13.5 GHz. These solvent mixturesnitrobenzene−N,N-dimethylformamide, 1-butanol−formamide, nitrobenzene−toluene, ethanol−1-butanol, and nitrobenzene−chlorobenzeneas well as the pure components display a Debye or near Debye dispersion. Their frequency-dependent dielectric properties can be summarized by three parameters in the Debye equation:  a static dielectric constant, a high-frequency limiting dielectric constant, and a dielectric relaxation time constant. On the basis of electrostatics, rate theory, and solution thermodynamics, we have developed dielectric “mixing rules” that describe the frequency-dependent dielectric properties of the solvent mixtures based on solution composition and the dielectric parameters for the solution components. These “mixing rules” yield good agreement between the predicted and experimental dielectric properties for the binary solvent mixtures over all solution compositions.

The temperature-sensitive nature of molecular fluoresence provides the basis for designing optical detection systems whereby changes in fluorescent intensity, peak position, or other spectral attributes can provide a local measurement of temperature.This review details the underlying photophysics responsible for the effects of temperature, compares their relative utilities for temperature sensing, and provides an overview of the instrumentational requirements for performing multi-dimensional temperature sensing. The requisite integration of chemistry and optics for this application helps define the desired properties for the fluorescent probe. In particular, bichromophoric fluorophores offer notable advantages by providing an internal reference for fluorometric temperature sensing. The review focuses its description on the operation and properties of this class of fluorescent compounds and summarizes the reported probes and their operating ranges. A model one-dimensional system for measuring spatial and temporal changes in temperatures using a bipyrenyl fluorophore is presented as demonstration of the ability to perform remote detection using a bichromophoric fluorescent probe. The selection of light source and detector are highlighted as are specific designs employing lasers and CCD cameras for expanding the ability of fluorometric sensing to produce three-dimensional profiles of temperature.

We report two fluorophores, N-(1-pyrenylmethyl)-1-pyrenebutanamide and N-(1-pyrenylmethyl)-1-pyreneacetamide, that exhibit temperature-dependent emission spectra over the temperature range of 20-100°C. The fluorophores are readily synthesized via a one-pot process and can be used in dilute form (<1 µM) for measuring the temperature of various organic solvents nonintrusively. The fluorophores allow measurement of temperatures within a precision of (1°C and provide an internal reference signal that allows factors such as fluorophore concentration, optical path, and excitation light intensity to be ignored.Fluorescence-based temperature sensing is receiving increasing interest as it finds unique applications in monitoring the temperature within micro-sized domains (e.g., a biological cell) or hostile environments (e.g., a microwave-irradiated system). 1-3 The measurement can be made externally to the region of interest and provides useful flexibility in defining the location of temperature measurement and in determining spatial variations in temperature within a region. This method has the advantage over physical probes of temperature (thermometers, thermocouples, etc.) for fluid systems and systems under electromagnetic irradiation in that flow patterns and field lines are not disturbed by the presence of these molecular probes.A number of fluorescence-based optical thermometers have relied on changes in the intensity or wavelength of the fluorescence maximum. Limitations of these systems include the inherent fluctuations in the intensity of the excitation source and observed shifts in wavelength on the order of <0.1 nm/°C. 3,4 Strategies incorporating two fluorescent moieties within one molecule have provided an internal reference source that obviates factors that would affect measurements based on the intensity of a single emission process. Among this class of molecular probes, those containing two pyrenyl units offer high quantum yield and good sensitivity. Limitations for these fluorophores include difficulties associated with their multistep syntheses 5 and their restricted temperature range of operation. Presently, fluorophores of this type have been reported only for measuring temperatures in organic solvents above ∼110°C and below ∼20°C. 2,6,7 Here we describe the synthesis and characterization of a series of easily prepared bipyrenyl fluorophores that are suitable for fluorescence thermometry over the temperature range of 20-100°C, i.e., from room temperature to the boiling point of most common organic solvents.

We report million-atom reactive molecular dynamic simulations of shock initiation of β-cyclotetramethylene tetranitramine (β-HMX) single crystals containing nanometer-scale spherical voids. Shock induced void collapse and subsequent hot spot formation as well as chemical reaction initiation are observed which depend on the void size and impact strength. For an impact velocity of 1 km s(-1) and a void radius of 4 nm, the void collapse process includes three stages; the dominant mechanism is the convergence of upstream molecules toward the centerline and the downstream surface of the void forming flowing molecules. Hot spot formation also undergoes three stages, and the principal mechanism is kinetic energy transforming to thermal energy due to the collision of flowing molecules on the downstream surface. The high temperature of the hot spot initiates a local chemical reaction, and the breakage of the N-NO2 bond plays the key role in the initial reaction mechanism. The impact strength and void size have noticeable effects on the shock dynamical process, resulting in a variation of the predominant mechanisms leading to void collapse and hot spot formation. Larger voids or stronger shocks result in more intense hot spots and, thus, more violent chemical reactions, promoting more reaction channels and generating more reaction products in a shorter duration. The reaction products are mainly concentrated in the developed hot spot, indicating that the chemical reactivity of the hmx crystal is greatly enhanced by void collapse. The detailed information derived from this study can aid a thorough understanding of the role of void collapse in hot spot formation and the chemical reaction initiation of explosives.

The microwave absorption characteristics of xylene as a model nonpolar solvent are dramatically increased by the incorporation of dispersed cobalt and magnetite nanoparticles. The addition of 1−2 vol % of these colloids to xylene can produce heating rates by microwaves at 2.45 GHz that approach those for water. The particles have diameters of 5−20 nm and contain a coating on their surface to avoid their aggregation and precipitation from solution. The small particle sizes are compatible with a rapid process of heat transfer to the surrounding xylene, thereby minimizing the generation of large temperature gradients around the particles. Cobalt particles are more effective than magnetite particles for enhancing the heating rates of xylene by microwaves, with nanoparticles of cobalt with diameters less than 10 nm exhibiting greater levels of microwave absorption enhancement than nanoparticles of larger diameters.