Kinetics and selectivity of asphaltene hydrocracking

Thermal hydrocracking and catalytic hydrocracking over NiMo/γ-Al₂O₃ of a pentane-insoluble asphaltene were conducted in a microbatch reactor at 430 °C. The experimental data of asphaltene conversion fit second-order kinetics adequately, to give the apparent rate constants of 2.435 × 10⁻² and 9.360 × 10⁻² wt frac⁻¹ min⁻¹ for the two processes respectively. A three-lump kinetic model is proposed to evaluate rate constants of parallel reactions from asphaltenes to liquid oil (k₁) and to gas + coke (k₃), and consecutive reaction from liquid to gas + coke (k₂). The evaluated k₁ is 2.430 × 10⁻² and 9.355 × 10⁻² wt frac⁻¹ min⁻¹, k₂ is 2.426 × 10⁻² and 6.347 × 10⁻³ min⁻¹, and k₃ is 5.416 × 10⁻⁵ and 4.803 × 10⁻⁵ wt frac⁻¹ min⁻¹ for asphaltenes hydrocracking in the presence or absence of the catalyst, respectively. Analysis of selectivity shows that the catalytic hydrocracking process promotes liquid production and inhibits coke formation effectively.     

Fluorescence studies of associative behavior of cationic surfactant moieties covalently linked to poly(acrylamide) at the surfactant head or tail

Association properties of hydrophobically modified water-soluble polymers bearing the same (or similar) hydrophobic and charged groups at different relative positions (with respect to the polymer main chain) were compared. Two types of surfactant monomers (surfmers) were synthesized; a cationic surfactant was linked to the p-position of styrene at the surfactant charged head (dimethyldodecyl-4-vinylbenzylammonium chloride (St-QC₁₂)) or at the tail end (13-(4-vinylphenyl)tridecyltrimethylammonium bromide (St-C₁₃Q)). These surfmers were copolymerized with acrylamide (AAm), and the association behavior of the copolymers in water was characterized using different fluorescence techniques. The surfmer contents in the AAm/St-QC₁₂ and AAm/St-C₁₃Q copolymers were fixed to more or less 10 mol%. The surfmer units in the copolymers self-associate to form multipolymer aggregates. An apparent critical micelle concentration (cmc) (i.e. a polymer concentration for the onset of interpolymer association of the surfmer units) estimated from excitation spectra of pyrene probes for AAm/St-QC₁₂ (cmc=2.6×10⁻⁴ g/l) was several times lower than that for AAm/St-C₁₃Q (cmc=1.3×10⁻³ g/l), surfmer units in AAm/St-QC₁₂ showing a stronger tendency for interpolymer association than those in AAm/St-C₁₃Q. The presence of excimer emissions in steady-state fluorescence spectra for the p-substituted styrene residues (phenylene groups) in AAm/St-C₁₃Q indicated that the benzene rings are in close proximity to one another in the polymer aggregates. Steady-state fluorescence spectra and fluorescence quenching by thallium nitrate and 1-nitrohexane indicated that the phenylene groups in AAm/St-C₁₃Q were incorporated in hydrophobic microdomains formed from surfactant hydrocarbon chains, whereas the phenylene groups in AAm/St-QC₁₂ are located near the surface of polymer aggregates in a way that they are sufficiently apart from each other and hence no excimer is formed.     

Free-radical propagation and termination kinetics of the butyl acrylate dimer studied by pulsed laser polymerization techniques

The propagation and termination rate coefficients for bulk polymerization of the butyl acrylate dimer (BA dimer) are determined by pulsed laser techniques. The rate coefficient for propagation, k_p, is deduced for temperatures from 20 to 90 °C via the pulsed laser polymerization-size exclusion chromatography (PLP-SEC) method at pulse repetition rates between 1 and 10 Hz. The Arrhenius parameters were found to be: E_A(k_p) = (34.2 ± 1.0) kJ mol⁻¹ and A(k_p)/L mol⁻¹ s⁻¹ = (1.08 ± 0.49) × 10⁷ L mol⁻¹ s⁻¹. The termination rate coefficient, k_t, has been measured via SP-PLP-ESR, single pulse-pulsed laser polymerization in conjunction with time-resolved electron spin resonance detection of radical concentration. The resulting Arrhenius parameters as deduced from the temperature range −15 to +30 °C are: E_A(〈k_t〉) = (22.8 ± 3.7) kJ mol⁻¹ and log(A/L mol⁻¹ s⁻¹) = 10.6 ± 1. The chain-length dependence of k_t was studied at 30 °C. For short chains a significant dependence was found which may be represented by an exponent α = 0.79 in the power-law expression k_t(i) = k_t⁰i^−α.     

Kinetics of asphaltene thermal cracking and catalytic hydrocracking

A pentane-insoluble asphaltene was processed by thermal cracking and catalytic hydrocracking over NiMo/γ-Al₂O₃ in a microbatch reactor at 430 °C. Kinetic analysis shows that the first-order kinetics fits the data of conversion in reaction times ≤ 30 min approximately, but deviates from the data of times over 30 min significantly; whereas the second-order kinetics fits the data of the reaction times up to 60 min adequately, to give the apparent rate constants of 1.704 × 10⁻² and 9.360 × 10⁻² wt frac⁻¹min⁻¹ for the two cracking processes. Furthermore, a three-lump kinetic model is proposed to include parallel reactions of asphaltenes to produce liquid oil (k₁) and gas + coke (k₃), and consecutive reaction from liquid to gas + coke (k₂). The evaluated value of k₁ is 1.697 × 10⁻² and 9.355 × 10⁻² wt frac⁻¹min⁻¹, k₂ is 3.605 × 10⁻² and 6.347 × 10⁻³ min⁻¹ , and k₃ is 6.934 × 10⁻⁵ and 4.803 × 10⁻⁵ wt frac⁻¹min⁻¹ for asphaltenes thermal cracking and catalytic hydrocracking, respectively. Selectivity analysis shows that the catalytic hydrocracking process promotes liquid production and inhibits coke formation effectively.     

A new fast methodology to measure total diene content in gasoline

The rate constants for the quenching of biacetyl phosphorescence by a series of conjugated dienes were measured. 1,3-cyclohexadiene (k_qP = 2.94 × 10⁹ s⁻¹ mol⁻¹ L), 2,5-dimethyl-2,4-hexadiene (k_qP = 1.91 × 10⁹ s⁻¹ mol⁻¹ L), 2,4-dimethyl-1,3-pentadiene (k_qP = 1.78 × 10⁸ s⁻¹ mol⁻¹ L), 3-methyl-1,3-pentadiene (k_qP = 1.22 × 10⁸ s⁻¹ mol⁻¹ L), 2,4-hexadiene (k_qP = 1.35 × 10⁸ s⁻¹ mol⁻¹ L) and trans-2-methyl-1,3-pentadiene (k_qP = 3.84 × 10⁸ s⁻¹ mol⁻¹ L). Cyclooctene also quenched biacetyl phosphorescence but with a lower rate (k_qP = 1.97 × 10⁷ s⁻¹ mol⁻¹ L). Quenching was not observed with 1-methylnaphthalene. Since conjugated dienes quench biacetyl phosphorescence preferentially, this method was studied using gasoline samples with known diene composition. A good correlation was found between the rate of quenching of biacetyl by the gasoline samples and the quantity of conjugated dienes present.     

Molecular characteristics of poly(1-trimethylsilyl-1-propyne) in dilute solutions

Samples and fractions of a membrane-forming polymer, poly(1-trimethylsilyl-1-propyne) (PTMSP), were studied by methods of molecular hydrodynamics (velocity sedimentation, translational isothermal diffusion and viscometry) in cyclohexane in the molecular mass range 60<M×10⁻³ g mol⁻¹<430. The following molecular-mass dependencies of the hydrodynamic characteristics (intrinsic viscosity [η] (cm³ g⁻¹), sedimentation coefficient s₀(s) and translational diffusion coefficient D₀ (cm² s⁻¹)) were established: [η]=0.198 M^0.50±0.06; s₀=8.66×10⁻¹⁶M^0.50±0.04; D₀=9.30×10⁻⁵M^{−0.50±0.04}. On the basis of the hydrodynamics data the equilibrium rigidity and hydrodynamic diameter of PTMSP chains were evaluated. The equilibrium properties of the different disubstituted polyacetylenes molecules are compared on the base of the normalised scaling plots.     

Modeling RAFT polymerization kinetics via Monte Carlo methods: cumyl dithiobenzoate mediated methyl acrylate polymerization

Cumyl dithiobenzoate (CDB) mediated methyl acrylate (MA) bulk polymerizations at 80 °C, using CDB concentrations between 1.5×10⁻² and 5.0×10⁻² mol L⁻¹, were modeled via a novel Monte Carlo simulation procedure with respect to experimental time-dependent conversions, X, number average molecular weights, M_n, and weight average molecular weights, M_w. The simulations were based upon individual treatment of 5×10⁸ discrete molecules in accordance to their actual reaction pathways. The kinetic scheme employed includes termination reactions of intermediate RAFT radicals with propagating radicals and reaction steps of the RAFT pre-equilibrium, which are different from those of the RAFT main equilibrium. The equilibrium constant of the main equilibrium of the CDB/MA system at 80 °C was found to be K=1.2×10⁴ L mol⁻¹, indicating a relatively stable intermediate radical. The concentration of the intermediate RAFT radical, although not employed as experimental input data for the modeling, was calculated by using the obtained set of kinetic parameters as being in excellent agreement with experimental electron spin resonance spectroscopic data.     

The effect of cytosine on the two-step electroreduction of Zn ions in acetic buffers

Cytosine plays an important role in many biological processes since it constitutes the buildings blocks of DNA and RNA. A two-step reduction of Zn²⁺ ions at the dropping mercury electrode in acetic buffers at pH 4 and 5 in the presence of cytosine was examined. The measurements were performed using an impedance method in a wide potential and frequency ranges.The values of the standard rate constants k_s in the both studied system decrease from 3.8 × 10⁻³ to 2 × 10⁻³ cm s⁻¹ at pH 4 and from 5.1 × 10⁻³ to 2.5 × 10⁻³ cm s⁻¹ at pH 5. The values of the standard rate constants k_s1 characterizing the stage of the first electron transfer decrease similarly. However, the values of the standard rate constants k_s2 characterizing the stage of the second electron exchange decrease more markedly in the buffer at pH 4 than in the buffer at pH 5.     

Electrocatalytic reduction of nitrite using ferricyanide; Application for its simple and selective determination

The electrocatalytic reduction of nitrite has been studied by ferricyanide at the surface of carbon paste electrode. Cyclic voltammetry and chronoamperometry techniques were used to investigate the suitability of ferricyanide as a mediator for the electrocatalytic nitrite reduction in aqueous solution with various pH. Results showed that pH 0.00 is the most suitable for this purpose. In the optimum pH, the electrocatalytic ability about 700 mV can be seen and the homogeneous second-order rate constant (k_s) for nitrite coupled catalytically to ferricyanide was calculated 2.75 × 10³ M⁻¹ s⁻¹ by Nicholson-Shain method. Also, electron transfer coefficients (α) for ferricyanide was determined by using various electrochemical approaches such as Tafel plot in the absence and presence of nitrite 0.556 and 0.760, respectively. The catalytic reduction peak current was linearly dependent on the nitrite concentration and the linearity range obtained was 5.00 × 10⁻⁵ to 1.00 × 10⁻³ M. Detection limit has been found to be 2.63 × 10⁻⁵ M (2σ). This method has been applied as a selective, simple and precise method for determination of nitrite in real sample.     

Electrocatalytic oxidation of ascorbic acid by [Fe(CN)6] redox couple electrostatically trapped in cationic N,N-dimethylaniline polymer film electropolymerized on diamond electrode

Multinegatively charged metal complex, hexacyanoferrate ([Fe(CN)₆]⁴⁻), was electrostatically trapped in the cationic polymer film of N,N-dimethylaniline (PDMA) which was electrochemically deposited on the boron-doped diamond (BDD) electrode by controlled-potential electro-oxidation of the monomer. This ferrocyanide-trapped PDMA film was used to catalyze the oxidation of ascorbic acid (AA). Increase in the oxidation current response with a negative shift of the anodic peak potential was observed at the cationic PDMA film-coated BDD (PDMA|BDD) electrode, compared with that at the bare BDD electrode. A more drastic enhancement in the oxidation peak current as well as more negative shift of oxidation potential was found at the ferrocyanide-trapped PDMA film-coated BDD ([Fe(CN)₆]^3−/4−|PDMA|BDD) electrode. This [Fe(CN)₆]^3−/4−|PDMA|BDD electrode can be used as an amperometric sensor of AA. Ferrocyanide, electrostatically trapped in the polymer film shows more electrocatalytic activity than that coordinatively attached to the polymer film or dissolved in the solution phase. The electrocatalytic current depends on the surface coverage of ferricyanide, Γ_Fe, within the polymer film. Diffusion coefficient (D) of AA in the solution was estimated by rotating disk electrode voltammetry: D = (5.8 ± 0.3) × 10⁻⁶ cm² s⁻¹. The second-order rate constant for the catalytic oxidation of AA by ferricyanide was also estimated to be 9.0 × 10⁴ M⁻¹ s⁻¹. In the hydrodynamic amperometry using the [Fe(CN)₆]^3−/4−|PDMA|BDD electrode, a successive addition of 1 μM AA caused the successive increase in current response with equal amplitude and the sensitivity was calculated as 0.233 μA cm⁻² μM⁻¹.     

Catalytic reduction of hydrogen peroxide at metal hexacyanoferrate composite electrodes and applications in enzymatic analysis

The electrocatalytic activity of various metal hexacyanoferrates (Mhcfs) (i) immobilized on graphite electrodes, and (ii) as components of a composite electrode was investigated with respect to the reduction of hydrogen peroxide. The flow-through working electrode was a thin layer consisting of a composite of Mhcf, graphite, and polymethylmetacrylate (PMMA) as a binder, sandwiched between two Plexiglas plates. Among the pure Mhcfs immobilized on a graphite electrode, iron(III) hexacyanoferrate (Prussian blue) exhibits the highest electrocatalytic effect, whereas in the composite electrodes chromium(III) hexacyanoferrate (Crhcf) shows the highest activity and best performance and reproducibility for the electrochemical reduction of H₂O₂. The Crhcf electrode provides a linear dependence on H₂O₂ concentration in the range 2.5 × 10⁻⁶ mol L⁻¹ (LOD) to 1 × 10⁻⁴ mol L⁻¹ (phosphate buffer, pH 7). The sensor was applied for the detection of H₂O₂ enzymatically produced by glucose oxidase. The optimal conditions for the peroxide injection were 2 min after the beginning of the reaction and 25 °C with a detection limit of 7.0 × 10⁻⁶ mol L⁻¹ for glucose.     

Kinetics of nitroxide-controlled radical polymerization during the non-stationary state

The activation-deactivation equilibrium of nitroxide-controlled radical polymerization of styrene at 123 °C was investigated. For this purpose the reaction solution was examined time dependently during the initial phase of the polymerization by using an SEC column combination providing a very good separation of the low-molecular weight species. By time-dependent measurement of the alkoxyamine concentration the activation rate of the alkoxyamines PhEt-TIPNO (N-tert-butyl-N-(2-methyl-1-phenyl-propyl)-O-(1-phenyl-ethyl)-hydroxylamine) k_act = 3.2 × 10⁻³ s⁻¹ and PhEt-BIPNO (N-tert-butyl-N-(1-isopropyl-2-methyl-propyl)-O-(1-phenyl-ethyl)-hydroxylamine) k_act = 6.4 × 10⁻³ s⁻¹ can be determined directly.Considering the Persistent Radical Effect theory, the measurement of the free nitroxide concentration allows to determine the pseudo-equilibrium constant of dissociation/combination between dormant and active species for polystyryl-TIPNO and polystyryl-BIPNO, K = 7.5 × 10⁻⁹ mol/L and 1.08 × 10⁻⁸ mol/L, respectively.     

Effect of nitrobenzene on initiator-fragment incorporation radical polymerization of divinylbenzene with dimethyl 2,2′-azobisisobutyrate

The polymerization of divinylbenzene (DVB) with dimethyl 2,2′-azobisisobutyrate (MAIB) was conducted at 70 and 80 °C in benzene in the presence of nitrobenzene (NB) as a retarder. When the concentrations of DVB, MAIB, and NB were 0.45, 0.50, and 0.50 mol/l, respectively, the polymerization proceeded without any gelation to yield soluble polymers. The polymer yield (up to 65%) and the molecular weight (M_n=1.5-4.2×l0⁴ at 70 °C and 1.3-3.9×l0⁴ at 80 °C) increased with time. The polymer formed in the polymerization at 80 °C for 4 h consisted of the DVB units with (4 mol%) and without double bond (41 mol%), methoxycarbonylpropyl group as MAIB-fragment (48 mol%), and NB unit (7 mol%). Incorporation of such a large number of the initiator-fragments as terminal groups in a polymer molecule indicates that the polymer is of a hyperbranched structure. The polymer showed an upper critical solution temperature (40 °C on cooling) in an acetone-water [14:1 (v/v)] mixture. The results of MALLS and viscometric measurements and TEM observation supported that the polymers formed in the present polymerization have a hyperbranched structure. The polymerization system at 70 °C involved an ESR-observable nitroxide radical formed by the addition of polymer radical to the nitro group of NB. The polymerization was kinetically investigated in dioxane. The initial polymerization rate (R_p) at 70 °C was expressed by R_p=k[MAIB]^0.5[DVB]^0.9[NB]^−0.4. The kinetic results were explained on the basis of the reversible addition of polymer radical to NB and the termination between the polymer radical and the nitroxide radical. The overall activation energy of the polymerization was 27.8 kcal/mol.     

Studies on the electrochemical behavior of 3-nitrobenzaldehyde thiosemicarbazone at glass carbon electrode modified with nano-γ-Al2O3

Nano-γ-Al₂O₃ is dispersed onto the glass carbon electrode (GCE) by polishing. This nanostructured modified GCE exhibits a great enhancement to the redox responses of 3-nitrobenzaldehyde thiosemicarbazone (3-NBT). In comparison with bare GCE, 3-NBT gives a more sensitive voltammetric response because of the nanoparticle’s unique properties. The lowest detectable concentration (3σ) of 3-NBT is estimated to be 1.18 × 10⁻⁶ M (accumulation for 4 min). The linear relationship between peak current and concentration of 3-NBT holds in the range 1.0 × 10⁻⁵ M to 1.0 × 10⁻⁴ M (r = 0.9981). The electrochemical properties of 3-NBT on this modified electrode have been investigated with various electrochemical methods. The results indicate that the transference of one electron and one proton involves electrode radical reaction processes I and II, respectively. The coverage value (Γ) of 1.62 × 10⁻⁹ mol cm⁻² was calculated and the electrochemical parameters, diffusion coefficient D (2.54 × 10⁻³ cm² s⁻¹, 2.03 × 10⁻³ cm² s⁻¹) and reaction rate constant k_s (5.9573 s⁻¹, 7.15 × 10⁻² cm s⁻¹) were obtained for quasi-reversible system I and irreversible system II, respectively.     

Synthesis of star polymer by means of the living polymerization of [(o-trifluoromethyl)phenyl]acetylene using a MoOCl4-based catalyst and the linking method

A star polymer was synthesized by addition of 1,4-diethynyl-2,5-dimethylbenzene as linking agent (30 °C, 24 h) after living polymerization of [(o-trifluoromethyl)phenyl]acetylene (o-CF₃PA) with MoOCl₄-n-Bu₄Sn-EtOH catalyst (in anisole, 30 °C, 20 min; [Mo]=10 mM, [P^∗]/[Mo]=40%, [o-CF₃PA]₀=200 mM). The M_n values of the living and star polymers were 8.1×10³ and 5.3×10⁴, respectively, according to gel permeation chromatography, while these values determined by multi-angle laser light scattering (MALLS) were 7.8×10³ and 2.5×10⁵. The M_w/M_n and arm number of the star polymer were 1.04 and 29, respectively, according to MALLS. The molecular weight and arm number of star polymer increased with increasing linking agent concentration and polymerization temperature.     

Revisiting of dimensional scaling of linear chains in dilute solutions

The dimensions of linear polymer chains are scaled to their molar mass (M) as R = kM^α with α = 1/2 and 3/5 in a theta and an athermal solvent, respectively. In a good solvent, both k and α are a function of the solvent quality and chain length range. A high-temperature laser light-scattering spectrometer was used to measure the average radius of gyration (〈R_g〉) and hydrodynamic radius (〈R_h〉) of a set of narrowly distributed linear polystyrene chains in decalin over a wide temperature range. k and α in the scaling experimentally varying with T over a chain length range was analyzed. The results reveal that for 〈R_g〉, α = 0.59 − 0.09exp(−τ/0.066) and k = 0.60τ^2α−1, reasonably agreeing with the thermal blob theory. For 〈R_h〉, α = 0.59 − 0.09exp(−τ/0.106), but k deviates from the relationship of k ∝ τ^2α−1, reflecting that the hydrodynamic interaction and chain draining are not considered in the thermal blob theory.     

Poly(phenylacetylene) with bulky chiral germyl groups: synthesis and effects of measuring solvents and temperature on chiroptical properties

(+)-p-[{Methyl(1-naphthyl)phenyl}germyl]phenylacetylene, an acetylene with a bulky chiral germyl group, was polymerized with [(nbd)RhCl]₂-Et₃N to give a high-molecular-weight polymer in good yield. The CD spectrum of the polymer exhibited very large molar ellipticities [θ] in the UV region in non-aromatic solvents (e.g. THF and CHCl₃). In contrast, the CD signals of the polymer in aromatic solvents (e.g. toluene, tetralin, and benzene) became appreciably smaller: [θ]_max=6.4×10⁴ (330 nm) and −4.7×10⁴° cm² dmol⁻¹ (370 nm) in CHCl₃; [θ]_max=1.1×10⁴ (330 nm) and −0.7×10⁴° cm² dmol⁻¹ (370 nm) in toluene. The [θ]_max values of the polymer in aromatic solvents increased when the solutions were heated, which is attributed to decreased π-π interaction between the solvents and side groups.     

Electrochemical characteristics of dopamine oxidation at palladium hexacyanoferrate film, electroless plated on aluminum electrode

A novel electrode material was obtained at an aluminum electrode (Al) by a simple electroless method including two consecutive procedures: (i) the electroless deposition of metallic palladium on the Al electrode surface from PdCl₂ + 25% ammonia solution and (ii) the chemical transformation of deposited palladium to the palladium hexacyanoferrate (PdHCF) films in a solution containing 0.5 M K₃[Fe(CN)₆]. The modified Al electrode demonstrated a well-behaved redox couple due to the redox reaction of the PdHCF film. The PdHCF film showed an excellent electrocatalytic activity toward the oxidation of dopamine (DA). The effect of solution pH on the voltammetric response of DA has been investigated. A linear calibration graph was obtained over the DA concentration range 2-51 mM. The rate constant k and transfer coefficient α for the catalytic reaction and the diffusion coefficient of DA in the solution D, were found to be 4.67 × 10² M⁻¹ s⁻¹, 0.63 and 2.5 × 10⁻⁶ cm² s⁻¹, respectively. The interference of ascorbic acid was investigated and greatly reduced using a thin film of Nafion on the modified electrode. The modified electrode indicated reproducible behavior and a high level stability during electrochemical experiments, making it particularly suitable for the analytical purposes.     

Increasing ionic conductivity and mechanical strength of a plastic electrolyte by inclusion of a polymer

In this contribution we present a soft matter solid electrolyte which was obtained by inclusion of a polymer (polyacrylonitrile, PAN) in LiClO₄/LiTFSI-succinonitrile (SN), a semi-solid organic plastic electrolyte. Addition of the polymer resulted in considerable enhancement in ionic conductivity as well as mechanical strength of LiX-SN (X = ClO₄, TFSI) plastic electrolyte. Ionic conductivity of 92.5%-[1 M LiClO₄-SN]:7.5%-PAN (PAN amount as per SN weight) composite at 25 °C recorded a remarkably high value of 7 × 10⁻³ Ω⁻¹ cm⁻¹, higher by few tens of order in magnitude compared to 1 M LiClO₄-SN. Composite conductivity at sub-ambient temperature is also quite high. At −20 °C, the ionic conductivity of (100 − x)%-[1 M LiClO₄-SN]:x%-PAN composites are in the range 3 × 10⁻⁵-4.5 × 10⁻⁴ Ω⁻¹ cm⁻¹, approximately one to two orders of magnitude higher with respect to 1 M LiClO₄-SN electrolyte conductivity. Addition of PAN resulted in an increase of the Young's modulus (Y) from Y → 0 for LiClO₄-SN to a maximum of 0.4 MPa for the composites. Microstructural studies based on X-ray diffraction, differential scanning calorimetry and Fourier transform infrared spectroscopy suggest that enhancement in composite ionic conductivity is a combined effect of decrease in crystallinity and enhanced trans conformer concentration.     

Diffusional mass transfer model for the adsorption of food dyes on chitosan films

The adsorption kinetics of erythrosine B and indigo carmine on chitosan films was studied by a diffusional mass transfer model. The experimental curves were obtained in batch system under different conditions of stirring rate (80–200 rpm) and initial dye concentration (20–100 mg L⁻¹). For the model development, external mass transfer and intraparticle diffusion steps were considered and the specific simplifications were based on the system characteristics. The proposed diffusional mass transfer model agreed very well with the experimental curves, indicating that the surface diffusion was the rate limiting step. The external mass transfer coefficient (k_f) was dependent of the operating conditions and ranged from 1.32 × 10⁻⁴ to 2.17 × 10⁻⁴ m s⁻¹. The values of surface diffusion coefficient (D_s) increased with the initial dye concentration and were in the range from 0.41 × 10⁻¹⁴ to 22.90 × 10⁻¹⁴ m² s⁻¹. The Biot number ranged from 17.0 to 478.5, confirming that the intraparticle diffusion due to surface diffusion was the rate limiting step in the adsorption of erythrosine B and indigo carmine on chitosan films.