The hydrogel wound dressing based on polyvinyl alcohol （PVA） was prepared by the freezing-thawing cyclic method. The dehydration kinetics of prepared hydrogels was determined using the experimental method and mathem...The hydrogel wound dressing based on polyvinyl alcohol （PVA） was prepared by the freezing-thawing cyclic method. The dehydration kinetics of prepared hydrogels was determined using the experimental method and mathematical modeling based on diffusion mechanism. The results show that the dehydration rate of PVA hydrogel wound dressing inversely depends on the hydrogel thickness as well as water content of the wound. On the other hand, the initial water content of hydrogel and the atmospheric humidity have little direct effect on the dehydration rate. The good agreement between experimental and mathematical modeling results in early stages of dehydration process shows that the predominate factor determining the dehydration of these wound dressings is diffusion.展开更多
Powders of donepezil hydrochloride monohy- drate （Form I） underwent isomorphic dehydration, losing 3% w/w water between 90% and 10% relative humidity （RH） without changing its powder X-ray pattern. Below 10% RH, a...Powders of donepezil hydrochloride monohy- drate （Form I） underwent isomorphic dehydration, losing 3% w/w water between 90% and 10% relative humidity （RH） without changing its powder X-ray pattern. Below 10% RH, additional dehydration occurred in conjunction with a reversible phase transition between the monohy- drate state and a dehydrated state, with a 4.0% w/w loss to 0% RH. A combination of methods was used to understand the structural changes occurring during the desolvation process, including dynamic vapor sorption measurements, thermal analysis and powder X-ray diffraction. Form I showed the characteristics of the channel hydrate, whose non-isothermal dehydration behavior proceeds in two steps： （1） the loss of non-crystalline water adsorbed on the surface, and （2） the loss of one crystalline water in the channel. Dehydrated Form I is structurally similar to the monohydrate Form I. According to the heat of fusion and the crystal density criteria, the two crystal forms belonged to the univariant system, and the anhydrate （Form III） is stable. The dehydration kinetics was achieved from the TG-DTG curves by both the Achar method and the Coats- Redfern method with 15 frequently cited basic kinetic models. The dynamic dehydration processes for steps 1 and 2 were best expressed by the Zhuralev-Lesokin- Tempelman equation, suggesting a three-dimensional diffusion-controlled mechanism.展开更多
Factors influencing dehydration of xylose to furfural,such as catalyst and extract agents,were investigated.Results indicated that high-temperature water may substitute for solid and liquid acid as a catalyst,and ethy...Factors influencing dehydration of xylose to furfural,such as catalyst and extract agents,were investigated.Results indicated that high-temperature water may substitute for solid and liquid acid as a catalyst,and ethyl butyrate improved furfural yield for the high distribution coefficient.A furfural yield of 75 % was obtained at200 °C for 3 h in ethyl butyrate/water.The reaction kinetics of xylose dehydration to furfural was investigated and it was found that the reaction order was 0.5,and the activation energy was 68.5 k J/mol.The rate constant k showed a clear agreement with the Arrhenius law from160 to 200 °C.展开更多
文摘The hydrogel wound dressing based on polyvinyl alcohol （PVA） was prepared by the freezing-thawing cyclic method. The dehydration kinetics of prepared hydrogels was determined using the experimental method and mathematical modeling based on diffusion mechanism. The results show that the dehydration rate of PVA hydrogel wound dressing inversely depends on the hydrogel thickness as well as water content of the wound. On the other hand, the initial water content of hydrogel and the atmospheric humidity have little direct effect on the dehydration rate. The good agreement between experimental and mathematical modeling results in early stages of dehydration process shows that the predominate factor determining the dehydration of these wound dressings is diffusion.
文摘Powders of donepezil hydrochloride monohy- drate （Form I） underwent isomorphic dehydration, losing 3% w/w water between 90% and 10% relative humidity （RH） without changing its powder X-ray pattern. Below 10% RH, additional dehydration occurred in conjunction with a reversible phase transition between the monohy- drate state and a dehydrated state, with a 4.0% w/w loss to 0% RH. A combination of methods was used to understand the structural changes occurring during the desolvation process, including dynamic vapor sorption measurements, thermal analysis and powder X-ray diffraction. Form I showed the characteristics of the channel hydrate, whose non-isothermal dehydration behavior proceeds in two steps： （1） the loss of non-crystalline water adsorbed on the surface, and （2） the loss of one crystalline water in the channel. Dehydrated Form I is structurally similar to the monohydrate Form I. According to the heat of fusion and the crystal density criteria, the two crystal forms belonged to the univariant system, and the anhydrate （Form III） is stable. The dehydration kinetics was achieved from the TG-DTG curves by both the Achar method and the Coats- Redfern method with 15 frequently cited basic kinetic models. The dynamic dehydration processes for steps 1 and 2 were best expressed by the Zhuralev-Lesokin- Tempelman equation, suggesting a three-dimensional diffusion-controlled mechanism.
基金supported by the National Natural Science Foundation of China (No.21376136,No.21176142,No.21376140,No.21176142,and No.21466001)Program for Changjiang Scholars and Innovative Research Team in University (IRT13026)
文摘Factors influencing dehydration of xylose to furfural,such as catalyst and extract agents,were investigated.Results indicated that high-temperature water may substitute for solid and liquid acid as a catalyst,and ethyl butyrate improved furfural yield for the high distribution coefficient.A furfural yield of 75 % was obtained at200 °C for 3 h in ethyl butyrate/water.The reaction kinetics of xylose dehydration to furfural was investigated and it was found that the reaction order was 0.5,and the activation energy was 68.5 k J/mol.The rate constant k showed a clear agreement with the Arrhenius law from160 to 200 °C.
