Lisardo Núñez

A kinetic analysis of the degradation of glucose by soil microorganisms studied by microcalorimetry

Abstract Microcalorimetry was used to study the microbial degradation of glucose in soil. Relationships between heat evolution and the viable cell counts permit the quantification of kinetic parameters for microbial growth in soil, such as Monods substrate constant, Ks = 1.62±0.08 mM, and the maximum microbial growth rate constant, μmax= 0.26±0.01 h−1.

Influence of water absorption on the mechanical properties of a DGEBA (n = 0)/1, 2 DCH epoxy system

The diffusive, calorimetric, and mechanical behavior of a system composed of a diglycidyl ether of bisphenol-A (DGEBA, n = 0) and 1, 2 diamine cyclohexane (1, 2 DCH) were investigated during water sorption at different temperatures (23, 47, 58, 77, and 100°C). Experimental results showed that the water absorption at these temperatures fitted well to Ficks law. The water moisture content at the equilibrium temperature and the water moisture content at the equilibrium-curing conditions dependences have been checked. The activation energy for diffusion was calculated obtaining a value 26.01 kJ/mol. Dynamic mechanical analysis of several samples immersed in water at 100°C during different periods of time showed no significant changes in the glass transition temperature, and a decrease in the storage modulus at 2% of water content was observed. Storage modulus remained essentialy constant above that water content. Values of glass transition temperature were corroborated by differential scanning calorimetric measurements.

Microcalorimetric measurements of the influence of glucose concentration on microbial activity in soils

Abstract Microcalorimetric techniques were used to investigate the kinetics of glucose uptake during exponential growth of soil microorganisms. Experiments were carried out using a humic cambisol soil amended with a nutrient solution containing different amounts of glucose (1.25–50.00 mg glucose g−1 soil). It was found that glucose uptake velocities increased up to a concentration corresponding to 15 mg of glucose in 0.2 ml of distilled water g−1 soil. Further increases of glucose gave similar μmax values (20 mg) or the velocity decreased (30–50 mg). Chemical analyses showed that, in all the cases the glucose was practically exhausted (>99%) at the peak time. Relevant kinetic parameters such as the Michaelis–Menten constant, Km=4.070±0.910 mM, maximum apparent growth rate μmax=0.069±0.003 h−1 and the inhibitory nutrient concentration were determined from the power–time curves recorded.

Microcalorimetric study of the effect of temperature on microbial activity in soils

Abstract The effect of temperature on the microbial degradation of glucose in soil was studied in a conduction-type microcalorimeter. The microbial growth constant, μ, was calculated from the heat evolution associated with the degradation of carbon source, and the power-time curves recorded at different temperatures. The activation energy was obtained from an Arrhenius-type equation and was found to be 39.57 kJ mol−1. Based on the fact that the pattern of bacterial growth is similar to the transition-state theory for chemical process, values of activation entropy, ΔS≠, and activation free energy, ΔG≠, were calculated. These data can be very useful when studying bacterial growth thermodynamic properties in soil.

Activation energies and rate constants for an epoxy/cure agent reaction

From the peak reaction temperatures as a function of heating rate, the activation energies were obtained for a system consisting of an epoxy resin (Badgen=0) and a curing agent (isophorone diamine), using a Perkin Elmer DSC7 operated in the dynamic mode. At the same time, the Arrhenius law was used to calculate rate constants.

Elastic Moduli and Activation Energies for an Epoxy/m-XDA System by DMA and DSC

The influence of the resin/diamine ratio on the properties of the system diglycidyl ether of bisphenol A (BADGE n=0/m-xylylenediamine) (m-XDA) was studied. Variation of this ratio resulted in significant effects on the cure kinetics and final dynamic mechanical properties of the product material.The study was made in terms of storage modulus (E′), vss modulus (E″) and molecular mass between cross-links (Mc) at different ratios. Two geometries (cylindrical and rectangular) were considered. The influence of temperature was studied through the activation energy (Ea>), which depends on the epoxy/amine ratio and the geometry of the samples. Glass transition temperatures (Tg>) and glass transition temperatures for thermosets with null degree of conversion (Tgo>) were determined by DSC. Tg> decreases when amounts of curing agent greatly in excess of the stoichiometric composition were used.

thermal stability of epoxy systems badge (n=0)/1,2-dch and badge (n=0)/ 1,2-dch/vinylcyclohexene dioxide

The thermal degradation of the epoxy system diglycidyl ether of bisphenol A (BADGE n=0)/1,2-diamine cyclohexane (DCH) containing different concentrations of an epoxy reactive diluent was studied by thermogravimetric analysis in order to determine the reaction mechanism of the degradation process and to compare it with the results for the same system without diluent. The value of the activation energy, necessary for this study, was calculated using various integral and differential methods. Values obtained using the different methods were compared to the value obtained by the Flynn-Wall-Ozawas method (between 193-240 kJ mol-1 depending on the diluent concentration) with does not require a knowledge of the nth order reaction mechanism. All the experimental results were compared to master curves in the range of Doyles approximation (20-35% of conversion). Analysis of the results suggests that the reaction mechanism could be F2, F3, or A2 type.

Kinetic study of an epoxy system badge (n=0)/1, 2 dch modified with an epoxy reactive diluent

The curing reaction of an epoxy system consisting of a diglycidyl ether of bisphenol A (n=0) and 1, 2 diaminecyclohexane (DCH) with an epoxy reactive diluent vinylcyclohexane dioxide was studied by temperature modulated differential scanning calorimetry (TMDSC). The models proposed by Kamal and by Horie et al. were employed in the kinetic study. From these studies reaction orders, rate constants, and activation energies were determined. The technique of TMDSC allows to include in the kinetic study the effect of diffusion by means of the mobility factor, calculated from the curves of the complex heat capacity registered during the curing isothermal experiments. The results were compared to those obtained for the same system employing the reaction rate data.

TTT Cure Diagram

Curing reactions of the epoxy system consisting of a diglycidyl ether of bisphenol A (BADGE n=0) and m-xylylenediamine (m-XDA) were studied to calculate time-temperature-transformation (TTT) isothermal cure diagram for this system. Gel times were measured as a function of temperature using solubility test. Differential scanning calorimetry (DSC) was used to calculate the vitrification times. DSC data show a one-to-one relationship between Tg and fractional conversion, a independent of cure temperature. As a consequence, Tg can be used as a measure of conversion. The activation energy for the polymerization overall reaction was calculated from the gel times obtained using the solubility test (41.5 kJ mol-1). This value is similar to the results obtained for other similar epoxy systems. Isoconversion contours were calculated by numerical integration of the best fitting kinetic model.

The influence of lixiviates on the thermal degradation of diglycidyl ether of bisphenol A n=0/1,2-diaminecyclohexane studied by dynamic mechanical analysis and thermogravimetry-Fourier transform infrared spectroscopy

The influence of the lixiviates originated in a municipal landfill on the thermal degradation of a polymeric system composed of a diglycidyl ether of bisphenol A (n = 0) and 1,2-diaminecyclohexane was studied by dynamic mechanical analysis. Storage modulus (E′), loss modulus (E″), and glass transition temperature were measured to make a comparative study between the samples before and after being exposed to the chemical compounds in the lixiviate agents. The different data obtained were analyzed to check the resistance of these materials to chemical attack and the possibility of their use as coating materials in plants where those reagents were present. Thermal stability of the system diglycidyl ether of bisphenol A/1,2-diaminecyclohexane exposed to the attack of lixiviates has also been studied by thermogravimetric analysis. A quantitative study of the gases originated during thermal degradation of the epoxy/diamine system made by infrared spectroscopy.