Mechanism And Simulation Of Impurities Flocculation In Water Extract Of Traditional Chinese Medicine

Flocculation technology is much valuable for purifying the water extract of traditional Chinese medicine(WTCM),but the interaction between flocculant and impurities and the flocculation mechanism are not clear due to the components in WTCM are eurypalynous and complex,resulting in the flocculation effect and impurity removal is difficult to be improved significantly.In this dissertation,the experimental exploration and quantum chemistry simulations on starch and tannic acid removal by flocculation are carried out.The powers driving impurities flocculation by flocculant,the active sites of the interaction between impurities and flocculants and the contributions of different functional groups to these interactions are revealed at the molecular level,which can provide theoretical and practical guidance for flocculation improvement of WTCM and developing efficient flocculants.In the flocculation experiment of impurities of WTCM,the removal effects of starches flocculated by three kinds of polyacrylamide with different charge densities and tannic acid treated by three kinds of chitosan and their derivatives are studied,respectively.The influence of flocculant type,dosage and charge density on flocculation effect is investigated,and the optimum process conditions are determined too.The flocculation mechanisms of impurities are analyzed according to the characterization results of flocculants,impurities and their flocs.The results show that the appropriate charge density is useful to achieve good flocculation effect,the basic structure of starch and tannic acid are not changed in their flocs,and the main powers driving impurities flocculation with flocculatns are hydrogen bond,charge neutralization and bridging.On the simulation of impurities flocculation by quantum chemistry methods and molecular modeling technology,five simulation systems of starch flocculation with nonionic polyacrylamide and cationic polyacrylamide,as well as tannic acid with chitosan,protonated chitosan and hydroxypropylated chitosan are established.The simulation calculations are carried out by means of molecular dynamics annealing,semi-empirical method and density functional theory.It is concluded that Van der Waals force(including induction,dispersion,etc.)and hydrogen bonding are the main driving powers for flocs formation by the thermodynamic parameters and floc complex configuration analysis.Boltzmann distribution of the geometric structure of flocs complexes indicates that the flocs mainly are composed of one kind of most stable complex structure and the coexistence of other complexes.By quantitatively analyzing the van der Waals surface of impurity and flocculant molecules,the electrostatic potential and its extreme points distribution on the molecule surface are given.The minimum points of electrostatic potential of starch dimer are mainly near the oxygen atoms,the maximum points of electrostatic potential are mainly near the hydrogen atoms in the hydroxyl group,and hydroxyl groups have different electrostatic activities.The electrostatic potential extreme points distributions of nonionic polyacrylamide pentamers vary with their structure and present periodic change.The group CONH₂ in nonionic polyacrylamide pentamers is the electrostatic active point.The nucleophilic activity of the oxygen atom in this group is greater than that of the nitrogen atom,and the electrostatic potential of the hydrogen atom that is close to the oxygen atom is smaller than that of the hydrogen atom on the other side of molecule.The area of positive electrostatic potential on the surface of cationic polyacrylamide pentamers account for 89.9%of the total molecular surface area,and the maximum electrostatic potentials are obviously higher than those of nonionic polyacrylamide.The hydrogen atoms in three hydroxyls of tannic acid monomer molecule have different electrostatic activity,the maximum electrostatic potential is around H15 of C5-OH,and the minimum value appears near the oxygen atom in the carbonyl group.The electrostatic activity of hydroxyl groups in chitosan dimer is higher than that of amino groups.The area of the electrostatic potential positive region for the protonated chitosan dimer accounts for 99.5%of the molecular van der Waals surface.The electrostatic potential distribution of hydroxypropyl chitosan dimer is similar to that of chitosan dimer,but the active hydroxyl group is added.The analysis results of floc complex structures,reduced density gradient(RDG)and independent gradient model(IGM)of the most stable floc complex in each flocculation simulation system show that the formation of floc complex is involved in more than one electrostatic potential extreme points on the molecules of impurity and flocculant.The interactions between the starch and polyacrylamide are dominated by hydrogen bond,van der Waals and steric hindrance interaction,while the interactions between tannic acid and chitosan are dominated by hydrogen bond,T-type of?-?stacking,van der Waals and steric hindrance interaction.Both atoms and pairs with larger contributions are involved in intermolecular hydrogen bonding and van der Waals interaction.The more the number of hydrogen bonds,the stronger the strength,and the more conducive to form the most stable floc complex.The symmetry adapted perturbation theory(SAPT)energy decomposition of the most stable floc complexes in the five flocculation systems is carried out.The interaction between impurity and flocculant is deconstructed as electrostatic interaction,exchange,mutual exclusion,induction and dispersion,of which electrostatic interaction is the most important one.The proportion of electrostatic in total attractive energy of the above five systems is 53.21?49.58?52.77?54.48 and 57.50%,respectively.Induction and dispersion interactions are also non-negligence.