Site Energy Distribution Function Research Articles

Preparation of atenolol imprinted polysaccharide based biomaterials for a transdermal drug delivery system

The main objective of this study was to prepare atenolol (ATN) imprinted polysaccharide based biomaterials using mungbean starch (MBS), polyvinyl alcohol (PVA), and plasticizers under UV curing and to evaluate their recognition and drug release behavior. The prepared ATN imprinted biomaterials were characterized by FT-IR and SEM. In addition, the ATN recognition of biomaterials were interpreted by the adsorption isotherm models and binding site energy distribution. The Freundlich and Sips equations showed better correlation with the experimental data than the Langmuir equation. The binding site energy distribution function was also useful to compare the recognition between target molecule and biomaterials. The results of ATN release indicate that the release at pH 4.0 was higher than that at pH 10.0. Results of the artificial skin test verified that ATN was released constantly for 14 days. In addition, the ATN release of biomaterial patches followed the Fickian diffusion mechanism, but followed the non-Fickian diffusion behavior with artificial skin.

Synthesis, characterization and application of surface-modified biochar synthesized from rice husk, an agro-industrial waste for the removal of hexavalent chromium from drinking water at near-neutral pH

This study reports the synthesis, characterization and applicability of surface-modified biochar, an eco-friendly and economic adsorbent for the removal of highly toxic hexavalent chromium from drinking water at a pH suitable for drinking water. The acid modification of the pristine biochar prepared by the pyrolysis of rice husk substantially enhanced the carbon content percentage by 25.29% and active sites by 10 times in the acid-modified biochar and produced a BET surface area of 142.78 ± 2.57 m2 g−1. Furthermore, the alteration of the surface of pristine biochar resulted in the substantial increase in C=C, C=O, C–O, phenolic and alcoholic –OH bonds on the surface of the biochar which further facilitated the adsorption of Cr. The batch adsorption study was carried out to comprehend the process of adsorption. The Freundlich isotherm model has been found to be describing the adsorption process most precisely. The maximum adsorption capacity has been found to be 4109 μg g−1 at 30 °C and 4536 μg g−1 at 50 °C from the analysis of approximate adsorption site energy distribution function. The analysis of approximate adsorption site energy distribution function further showed the availability of more energetic sites to the adsorbate Cr(VI) oxyanions at higher temperatures. Further, the endothermic nature of the adsorption apart from the experimental outcomes has been evinced from the D–R isotherm model and from the values of ∆G° and ∆H°. The negative values of ∆G° at all the experimental temperatures indicate the spontaneity of the adsorption process. Furthermore, the values of ∆G° more than − 20 kJ mole−1 indicate the physisorption of Cr(VI) oxyanions on the adsorbent surface.

Adsorption behavior of norfloxacin and site energy distribution based on the Dubinin-Astakhov isotherm

Concerns about the water resources contaminated by fluoroquinolone antibiotics have prompted research on effective and efficient treatment technologies. In this work, adsorbents based on barley straw were characterized on morphology, surface functional groups, and charge states for the adsorption of norfloxacin, a representative of fluoroquinolone antibiotics, from aqueous solutions. The effects of solution pH were studied, and high norfloxacin adsorption capacities of pretreated barley straw were achieved in a wide pH range (2.90–10.50), which were much higher than those of raw barley straw. The adsorbent was also able to remove norfloxacin from low to high concentration range, demonstrating its capability for norfloxacin removal from water bodies. The electron-donor-acceptor interactions were proposed as one of the main adsorption mechanisms. The adsorption kinetic data achieved at a range of concentrations were well described by the pseudo-second-order kinetic model. The adsorption equilibrium data were reasonably well-fitted by the Dubinin-Astakhov model, and a site energy distribution function based on the Dubinin-Astakhov model was determined. With higher site energies, the pretreated barley straw demonstrated a much stronger adsorption affinity for norfloxacin than raw barley straw.

Spectral dependence of the localization degree in the one-dimensional disordered Lloyd model

We calculate the Anderson criterion and the spectral dependence of the degree of localization in the first nonvanishing approximation with respect to disorder for one-dimensional diagonally disordered models with a site energy distribution function that has no finite even moments higher than the zeroth. For this class of models (for which the usual perturbation theory is inapplicable), we show that the perturbation theory can be consistently constructed for the joint statistics of advanced and retarded Green’s functions. Calculations for the Lloyd model show that the Anderson criterion in this case is a linear (not quadratic as usual) function of the disorder degree. We illustrate the calculations with computer experiments.

A site energy distribution function for the characterization of the continuous distribution of binding sites for gases on a heterogeneous surface

A binding site energy distribution function for a Jensen-Seaton isotherm and its limiting cases was proposed and successfully applied to the adsorption of several gas molecules on different essentially microporous carbons. According to the proposed model the studied carbon materials exhibit two energetic states where the site energy is exponentially or unimodaly distributed depending on the adsorption pressure. Carbons with a larger contribution from micropores seem to be in general more heterogeneous, with a wider binding site energy distribution, than less microporous carbons.

Site Energy Distribution Function for the Sips Isotherm by the Condensation Approximation Method and Its Application to Characterization of Porous Materials

A site energy distribution function is proposed that can rapidly generate the binding energy distribution of any adsorbent for any target molecules using only the Sips isotherm parameters, the advantages of which are discussed. The proposed model is successfully applied to determine the site energy distribution of a series of activated carbons, activated carbon fiber, and single-wall carbon nanotubes (SWCNT) for the adsorption of several gas molecules. The proposed model revealed the fact that the sorption capacity of the activated carbons for the target molecule depends on the shape and the size of the binding site energies. The proposed model is very simple to use and does not need any complicated or sophisticated computer programs to determine the distribution of binding energies on the adsorbent surface.

A site energy distribution function from Toth isotherm for adsorption of gases on heterogeneous surfaces

A site energy distribution function based on a condensation approximation method is proposed for gas-phase adsorption systems following the Toth isotherm. The proposed model is successfully applied to estimate the site energy distribution of three pitch-based activated carbons (PA, PFeA and PBA) developed in our laboratory and also for other common adsorbent materials for different gas molecules. According to the proposed model the site energy distribution curves of the activated carbons are found to be exponential for hydrogen at 77 K. The site energy distribution of some of the activated carbon fibers, ambersorb, Dowex optipore, 13X Zeolite for different adsorbate molecules represents a quasi-Gaussian curve with a widened left hand side, indicating that most sites have adsorption energies lower than a statistical mean value.

A continuous site energy distribution function from Redlich–Peterson isotherm for adsorption on heterogeneous surfaces

A site energy distribution function for carbon adsorbents is proposed for the adsorption equilibrium data following a Redlich–Peterson isotherm. The proposed model is successfully applied to determine the site energy distribution of two pitch-based activated carbons (PA and PBA) developed in our laboratory and also for other common adsorbent materials for different gas molecules. According to the proposed model the site energy distribution curve for carbons PA and PBA are found to be unimodal for N 2 gas molecules at 77 K. The proposed model successfully predicts the site energy parameters of both homogeneous and heterogeneous adsorbents.

Characterisation of Surface Activity of Carbon Black and its Relation to Polymer‐Filler Interaction

AbstractThe surface activity of commercial and experimental carbon blacks varying in particle size and primary aggregate structure was investigated with regard to surface roughness and energetic surface structure of primary particles. The energetic surface structure was described by the site energy distribution function f(Q), which was determined mainly from the gas adsorption isotherms of ethene. It was found that the surface of carbon black was energetically very heterogeneous. It consisted of at least four different adsorption sites (I: Q ≈ 16 kJ · mol−1; II: Q ≈ 20 kJ · mol−1; III: Q ≈ 25 kJ · mol−1; IV: Q ≈ 30 kJ · mol−1). The fraction of the sites I–IV depended on the production process of the carbon black grades and the particle size. For the furnace blacks, the fraction of high‐energy sites decreased significantly with particle size and disappeared almost completely during graphitisation. This indicates that the reinforcing potential of carbon black is closely related to the amount of highly energetic sites that can be well quantified by the applied gas adsorption technique. The surface roughness was characterised by the surface fractal dimension, Ds, which was determined by two different techniques: the yardstick‐method and the extended Frenkel‐Halsey‐Hill‐theory (fractal FHH‐theory). It is found that the furnace blacks have an almost equal roughness with a surface fractal dimension of Ds ≈ 2.6 beyond a length scale z ≈ 6 nm. This result is shown to be in fair agreement with analytical models and computer simulations of surface growth of carbon black in a furnace reactor.magnified image

Resonance Fluorescence Spectra of Zn-Substituted Myoglobin

Laser-induced fluorescence spectra, including narrow resonance lines, were measured for Znsubstituted myoglobin (ZnMb) in the lowest optical absorption band at 4 K by using a combination of short light pulses from a CW mode-locked laser for the excitation and a time-correlated single-photon counting method for the detection. The site-energy distribution function, i.e., the distribution of the number of chromophore at various sites as a function of the zero-phonon transition energy, was determined from the excitation profile of the narrow resonance fluorescence line. The single-site fluorescence spectrum was determined by use of the saturation eect of laser-induced fluorescence. By comparing the laser-induced fluorescence spectra and the absorption spectrum with the spectra calculated by using the experimentally determined site-energy distribution and single-site fluorescence spectra, we found that the coupling between the electrons of the chromophore and the vibrations of the polypeptide was quite weak. Furthermore, the density of states of vibrational modes of myoglobin weighted by the coupling strength between the chromophore and the polypeptide chain was determined from the single-site fluorescence spectrum by solving an integral equation numerically.

A didactic model of solvent shifts, inhomogeneous broadening, and linear and quadratic electron–phonon coupling

A simplified model of optical spectra of chromophores embedded in disordered solids is proposed. Using the repulsive–dispersive potentials of the ground state and the excited state in Lennard–Jones form the expressions are obtained for inhomogeneous site energy distribution function, and the linear and quadratic coupling strength of electronic transition to local phonons. Experimentally, the shift of spectral holes was studied for chlorin in polymer hosts at temperatures between 5 and 40 K and by applying hydrostatic He gas pressure up to 200 bar. The opposite frequency dependencies of line shifts induced by temperature and pressure are in agreement with the model, if the potential minimum of the excited state is shifted to a shorter distance.

Glass transition of Zn-substituted myoglobin probed by absorption and site-selective fluorescence spectroscopies

The temperature dependence of absorption and laser-induced fluorescence spectra including the resonance line has been measured for Zn-substituted myoglobin (ZnMb) in the temperature range between 4 and 300 K. The spectral shape observed below 180 K has been found to be well explained by assuming that the site-energy distribution function is temperature independent and is equal to that determined at 4 K. This may indicate that ZnMb is glass-like below 180 K. The spectral shape of the absorption spectrum at temperatures higher than 180 K has been explained by using a configuration-coordinate model, assuming the conformational substates are distributed thermally. This means that ZnMb is liquid-like in this temperature region. As a result, it is concluded that ZnMb undergoes a liquid-glass transition around 180 K.

Virial analysis of low-coverage physical adsorption data on heterogeneous surfaces

The general treatment of the McMillan–Mayer theory of solutions is applied to low-coverage adsorption on heterogeneous surfaces. Expressions are derived for the second and third gas/solid virial coefficients for adsorption on model heterogeneous surfaces. The gaussian, log-normal, exponential and Maxwell–Boltzmann functions are examined as possible models for the site energy distribution function and it is found that, of these, only the gaussian gives a suitable expression. A method of analysis is given that can be used to fit experimental second gas/solid virial coefficients to a model with a gaussian site energy distribution. Equations for the analysis of the third gas/solid virial coefficient are obtained for a model that incorporates the gaussian site energy distribution with either a random or a patchwise pair distribution function. The methods of analysis described here are applicable over the range of temperature for which the second and third gas/solid virial coefficients are normally determined either by adsorption or gas-chromatographic measurements (3 ⩽Iµ*/kT⩽ 8). The approximations inherent in the analysis are less than the usual experimental uncertainty in the virial coefficients. The limited available data are analyzed and the potential of the method for the characterization of heterogeneous surfaces is discussed.

Adsorption on heterogeneous surfaces: A dubinin-radushkevich equation

A site energy distribution function for a ‘generalized form’ of the Dubinin-Radushkevich equation has been obtained by the Stieltjes transform method of Sips. The distribution function is identical in form to the one empirically obtained by Hobson and Armstrong on the basis of the potential theory of adsorption. The Langmuir adsorption isotherm is presumed to be valid for the individual ‘homotattic’ sites and, therefore, the generalized Dubinin-Radushkevich equation and hence the distribution function are only valid within the domain of conditions imposed on the Langmuir isotherm.