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R Sivaramakrishnan

Publications and source records attributed to R Sivaramakrishnan.

9 recordsLinked to original sources
PubMed2006

High pressure pyrolysis of toluene. 1. Experiments and modeling of toluene decomposition.

The pyrolysis of toluene, the simplest methyl-substituted aromatic molecule, has been studied behind reflected shock waves using a single pulse shock tube. Experiments were performed at nominal high pressures of 27 and 45 bar and spanning a wide temperature range from 1200 to 1900 K. A variety of stable species, ranging from small hydrocarbons to single ring aromatics (principal soot precursors such as phenylacetylene and indene) were sampled from the shock tube and analyzed using standard gas chromatographic techniques. A detailed chemical kinetic model with 262 reactions and 87 species was assembled to simulate the stable species profiles (specifically toluene, benzene and methane) from the current high-pressure pyrolysis data sets and shock tube-atomic resonance absorption spectrometry (ARAS) H atom profiles obtained from prior toluene pyrolysis experiments performed under similar high-temperature conditions and lower pressures from 1.5 to 8 bar. The primary steps in toluene pyrolysis represent the most sensitive and dominant reactions in the model. Consequently, in the absence of unambiguous direct experimental measurements, we have utilized recent high level theoretical estimates of the barrierless association rate coefficients for these primary reactions, C6H5 + CH3 --> C6H5CH3 (1a) and C6H5CH2 + H --> C6H5CH3 (1b) in the detailed chemical kinetic model. The available data sets can be successfully reconciled with revised values for deltaH0f(298K)(C6H5CH2) = 51.5 +/- 1.0 kcal/mol and deltaH0f(298K)(C6H5) = 78.6 +/- 1.0 kcal/mol that translate to primary dissociation rate constants, reverse of 1a and 1b, represented by k(-1a,infinity) = (4.62 x 10(25))T(-2.53)exp[-104.5 x 10(3)/RT] s(-1) and k(-1b,infinity) = (1.524 x 10(16))T(-0.04)exp[-93.5 x 10(3)/RT] s(-1) (R in units of cal/(mol K)). These high-pressure limiting rate constants suggest high-temperature branching ratios for the primary steps that vary from 0.39 to 0.52 over the temperature range 1200-1800 K.

Journal Article↗
PubMed2006

High pressure pyrolysis of toluene. 2. Modeling benzyl decomposition and formation of soot precursors.

The pyrolysis of toluene, the simplest methyl-substituted aromatic molecule, has been studied behind reflected shock waves using a single pulse shock tube. Part 1 in this two-part series focused on the high-pressure experimental results and the high-pressure limiting rate coefficients for the primary steps in toluene decomposition. The present work focuses on the modeling of benzyl decomposition and the growth of key soot precursors (C2H2, C4H2, C8H6, and indene) from toluene pyrolysis with 81 among the 262 reactions in the detailed toluene model representing the chemistry that describes the formation and decomposition of these species. Feasible pathways for benzyl decomposition as well as phenylacetylene and indene formation have been tested. The simulations also show very good agreement with the single pulse shock tube profiles for the growth of key soot precursors such as C2H2, C4H2, C8H6, and indene.

Journal Article↗
PubMed2006

The microwave absorption of emulsions containing aqueous micro- and nanodroplets: a means to optimize microwave heating.

The microwave absorption at frequencies between 10 MHz and 4 GHz is measured for aqueous brine droplets dispersed in a dielectric medium (epsilon(')=2.0). By varying the size of the droplets, ion type and ion concentration, it is found that the microwave absorption goes through a maximum which depends on the type of ions and their concentration. The absorption process is attributed to the polarization of the microdroplets through surface charges. Means to optimize microwave heating in emulsions is discussed.

Electric Capacitance↗
PubMed2006

Organ mapping using parelectric spectroscopy.

Whenever physical methods are used in the field of diagnostics, it is necessary to find an unambiguous mapping of the properties of the tested tissues (e.g. normal or pathologic) to their answer to the respective analysis tool such as nuclear magnetic resonance (NMR), ultrasound, x-rays or the relatively new method of parelectric spectroscopy (PS). The well-established non-invasive NMR method has, by now, a sufficiently wide-spread atlas of such mappings. This has to be contrasted to the situation of the PS method where first experiments showed the fulfillment of conditions necessary for any reliable diagnosis, namely the uncertainties of the results being small compared to the differences between the normal and pathologic state of the tissues under test. To help close this gap, we present here results of the behaviour of 12 different organs of mice, taken 20 min after excision and give the dependence of the two most essential PS parameters, the dipole density Delta epsilon and the mobility f(0), on the type of healthy organs. To be able to use tumorous tissues preserved in formaldehyde after excision for comparison purposes, we have been measuring the changes of some organs between the fresh state and the preserved state under formaldehyde for over 180 min each.

Animals↗
PubMed2005

Lipid nanoparticles for skin penetration enhancement-correlation to drug localization within the particle matrix as determined by fluorescence and parelectric spectroscopy.

With topical treatment of skin diseases, the requirement of a high and reproducible drug uptake often still is not met. Moreover, drug targeting to specific skin strata may improve the use of agents which are prone to cause local unwanted effects. Recent investigations have indicated that improved uptake and skin targeting may become feasible by means of nanoparticular systems such as solid lipid nanoparticles (SLN), nanostructured lipid carriers (NLC) and nanoemulsions (NE). Here we describe techniques to characterize drug loading to carrier systems and skin penetration profiles by using the lipophilic dye nile red as a model agent. Since the mode of drug association with the particle matrix may strongly influence the efficiency of skin targeting, parelectric spectroscopy (PS) was used to differentiate between matrix incorporation and attachment to the particle surface and fluorescence spectroscopy (FS) to solve dye distribution within NLC particles. Nile red was incorporated into the lipid matrix or the covering tensed shell, respectively, of SLN and NLC with all the lipids studied (Compritol, Precirol, oleic acid, Miglyol). In NLC, the dye was enriched in the liquid phase. Next, nile red concentrations were followed by image analysis of vertical sections of pigskin treated with dye-loaded nanoparticular dispersions and an oil-in-water cream for 4 and 8 h in vitro. Following the SLN dispersions, dye penetration increased about fourfold over the uptake obtained following the cream. NLC turned out less potent (<threefold increase) and penetration appeared even reduced when applying a NE. In contrast to previous studies with glucocorticoids attached to the surface of SLN, a targeting effect was not detected here. Therefore, drug targeting appears to be more strictly related to the mode of interaction of drug and particle than penetration enhancement.

Administration, Topical↗
PubMed2005

Ring conserved isodesmic reactions: A new method for estimating the heats of formation of aromatics and PAHs.

Density functional theory (DFT) has been used along with isodesmic reaction schemes to estimate heats of formation for aromatics and polynuclear aromatic hydrocarbons (PAHs). Calculations have been performed for 42 molecules, 12 of which have uncertain or unknown experimental values, using the B3-LYP functional with the small 6-31G(d) basis set. Heats of formation for the group of test molecules were estimated using both conventional bond separation (BS) isodesmic reactions as well as a new technique of ring conserved (RC) isodesmic reactions which is able to correct systematic errors in B3-LYP calculations. When a ring conserved isodesmic reaction based on delocalization energies is used, the estimated heat of formation is more accurate than that obtained by the bond separation technique. The methodology for creating and using appropriate ring conserved isodesmic reactions is discussed. The present scheme also compares favorably against a recently developed bond centered group additivity scheme that was tested against a large number of PAH molecules.

Journal Article↗
PubMed2004

Parelectric spectroscopy of drug-carrier-systems--distribution of carrier masses or activation energies.

The answer of a high-frequency electromagnetic wave to a sample as termination of an open-ended coaxial line gives the mobility and the density of permanent electric dipole moments in the substance under test. As long as these dipoles are attached to carrier molecules of well defined masses, both parameters can be extracted from the reflected wave in a quick manner giving unambiguous results. The corresponding algorithm has been applied to solid lipid nanoparticles with glucocorticoid molecules attached to or incorporated in the carrier molecules. The results from measurements in the frequency region (0.1-100) MHz have recently been published. As soon as we have to envisage a distribution in carrier masses and/or in activation energies of the attached molecules, we have to apply a more sophisticated evaluation algorithm. The need for a more generalised algorithm is clear as well, when we have to deal with more than one dipole-carrying constituent in the samples. All these evaluation algorithms shall be presented together with the mathematical basis in a short but exact form.

Drug Carriers↗
PubMed2004

Glucocorticoid entrapment into lipid carriers--characterisation by parelectric spectroscopy and influence on dermal uptake.

Topical glucocorticoids such as betamethasone 17-valerate (BMV) and prednicarbate (PC) are an important therapeutic option in atopic eczema. To reduce the risk of dermal atrophy, we aimed at BMV incorporation into solid lipid nanoparticles (SLN) for epidermal targeting using various lipids and emulsifiers corresponding to previous work on PC. Cutaneous absorption into excised human skin was compared to the one with a cream. While Compritol-based particles increased BMV uptake about fourfold we failed, however, to obtain epidermal targeting. To obtain insight into the location of active substance relative to the carrier, we used the recently optimised method of parelectric spectroscopy (PS). In fact, we were able to study electric dipole movements in the broad field of a frequency span from 0.1 to 100 MHz demonstrating that glucocorticoids are attached to the particle surface but are not incorporated into the lipid matrix. With BMV, the loading capacity of the particle surface lies clearly below the usual concentration of 0.1% which is not the case with PC. An adequate association of drug and carrier is essential for epidermal targeting. Parelectric spectroscopy provides insight into the interaction between drug and lipidic carrier.

Adult↗