3种大黄饮片的水分吸附及热力学特性分析

Analysis of Water Adsorption and Thermodynamic Properties of Three Rhei Radix et Rhizoma Decoction Pieces

目的:考察生大黄、酒大黄及大黄炭的水分吸附和热力学特性,为其干燥和储存提供指导。方法:通过静态称重法测定生大黄、酒大黄及大黄炭在25,35,45℃下的吸附等温线,采用7种常用的水分吸附模型拟合试验数据,确定最佳模型,研究3种饮片的吸附热力学参数。结果:生大黄、酒大黄与大黄炭的吸附最佳模型均为GAB模型,在25,35,45℃条件下大黄炭的绝对安全含水量分别为7.43%,6.79%,6.20%;酒大黄分别为8.68%,8.17%,7.03%;生大黄分别为9.88%,9.36%,7.77%;相对安全含水量大黄炭分别为9.46%,8.63%,8.21%;酒大黄分别为11.49%,11.03%,9.74%;生大黄分别为13.49%,12.66%,11.14%。3种饮片净等量吸附热(Qst)和微分熵(Sd)都随着平衡含水量的升高而降低,Qst和Sd符合熵-焓互补理论,生大黄、酒大黄、大黄炭的等速温度分别为386.66,391.15,394.34 K(单位换算为1 K=-272.15℃),吉布斯自由能分别为0.372 2,0.406 0,0.372 2 kJ·mol-1,吸附过程都是由焓驱动的非自发过程。结论:3种饮片的平衡含水量,单分子层含水量,Qst和Sd的大小排序均为生大黄>酒大黄>大黄炭,饮片吸湿能力排序为生大黄>酒大黄>大黄炭,炒炙方法明显影响3种大黄饮片的吸湿性与热力学性质,造成这种差异的原因可能是炒炙的温度较高致使生大黄中的吸湿性强的基团减少,疏水性强的物质增加,以及饮片质构的变化。通过对3种大黄饮片水分吸附特性的研究,可为其储存条件和干燥工艺等方面的选择提供参考。

Objective:To investigate the moisture adsorption and thermodynamic characteristics of raw products,wine-processed products and fried charcoal products of Rhei Radix et Rhizoma,in order to guide their drying and storage. Method: Static isotherm weighing method was used to determine the adsorption isotherm curves of three Rhei Radix et Rhizoma decoction pieces at 25,35,45 ℃,and the test data were fitted with 7 commonly used water adsorption models to determine the best model for studying the adsorption thermodynamic parameters of these decoction pieces. Result: The best adsorption models of these three decoction pieces were all GAB model. At 25,35,45 ℃,the absolute safe moisture content of fried charcoal products was 7.43%,6.79% and 6.20%,of wine-processed products was 8.68%,8.17% and 7.03%,of raw products was 9.88%,9.36% and 7.77%,respectively. At 25,35,45 ℃,the relative safe moisture content of fried charcoal products was 9.46%,8.63% and 8.21%,of wine-processed products was 11.49%,11.03% and 9.74%,of raw products was 13.49%,12.66% and 11.14%,respectively. The net equivalent heat of adsorption(Qst)and differential entropy(Sd)of these three kinds of decoction pieces all decreased with the increase of equilibrium moisture content,Qstand Sdwere in accordance with the entropy-enthalpy complementary theory. The constant velocity temperatures of raw products,wine-processed products and fried charcoal products of Rhei Radix et Rhizoma were 386.66,391.15,394.34 K(unit conversion of 1 K=-272.15 ℃ ),their Gibbs free energies were 0.372 2,0.406 0,0.372 2 kJ·mol-1,respectively. Their adsorption processes were an unspontaneous process driven by enthalpy. Conclusion: The orders of equilibrium moisture content,monomolecular layer moisture content,Qst and Sdof three Rhei Radix et Rhizoma decoction pieces are all raw products>wine-processed products>fried charcoal products. The moisture absorption capacity of the decoction pieces is ranked as raw products>wineprocessed products>fried charcoal products. The frying and roasting process significantly affects the hygroscopicity and thermodynamic properties of the three decoction pieces,the reason for this difference may be that the high temperature of the stir-frying results in the decrease of the hygroscopic groups and the increase of the hydrophobic materials in raw products,and the change in the texture of the decoction pieces. The research on the water adsorption characteristics of three Rhei Radix et Rhizoma decoction pieces can provide reference for selecting their storage conditions and drying process.