Molecular Dynamics Simulation Study On The Gelation Process Of Nano-silica Sol And Its Influence Mechanism

The spontaneous gelation tendency of nanosilica sol(NSS)greatly affects its application in chemical production,biomedicine,soil pollution control and so on.Therefore,disclosing the gelation process and mechanism of NSS plays a vital role in effectively regulating the gelation process.At present,experimental research on the gelation process cannot figure out the gelation process and mechanism from a micro and mesoscopic perspective.In this essay,the molecular dynamics simulation was used to explore the gelation process of NSS and its mechanism,and the influence mechanism of several commonly used additives on the gelation process was further explored.Based on the simulation results,several additives were screened through experiments,and the effect of additives on the stability of the sol was verified.First,by constructing the interfacial gelation model of NSS,the crosslinking reaction of the NSS was simulated,and the gel mechanism of NSS was explored.The interface gelation model uses lamellar silica salbs to simulate two close nano silica particles,the model excludes the influence of the difference group distributions of the curved crystal and thermol motion on the gelation process.According to the simulation results,the gelation process mainly includes four stages:(1)Short silicon oxide chains(Si-O chains)are formed on the surface of nanosilica particles,this stage is the control step of the sol-gel transition process.(2)Si-O chains gradually grow into the solution.(3)Si-O chains are intertwined and connected to the surface of the nanosilica particles.The 3D network gel structure is formed by using Si-O chains as a skeleton.Based on the mechanism of the gelation process,the influence of silanol ionization on the gelation process was explored.With the ionization degree of silanol groups increases,the number of silanol groups on the particle surface available for reaction decreases.At the same time,preventing silicic acid from approaching to the surface,thereby inhibiting the gelation process,which is conducive to the stability of the sol.While the ionized product of silica acid,Si(OH)₃O^-,will promote the proximity of silica acid to nano-silica particles and promote the formation of Si-O chains.And when Si(OH)₃O^-condensation becomes part of the Si-O chain,rigidity of the Si-O chain is increased and the bifurcation growth of Si-O chain is reduced,leading to the quick extention for the Si-O chain,and accelerating the gelation rate of NSS.Therefore,properly increasing the ionization degree of the hydroxyl groups on the surface of the silica particle and reducing the ionization degree of silica acid can slow down the gelation process of NSS.Combined with the gelation process of NSS and its mechanism,the influence of salt,alcohol and surfactant on the first stage of the gelation process and the mechanism of infuence were explored.Salt mainly affects the gelation rate by changing the distribution of the components of the solution near the silica particles'surface.The effect of salt on the gelation rate is related to the types of anions and cations.The electric double layer formed by the adsorption of different ions on the silica surface attracts(or repels)Si(OH)₄ and Si(OH)₃O^-close to(or away from)the silica surface,and then accelerate(or slow down)the gelation rate.After the alcohol is added to the NSS,the hydroxyl groups of alcohol form H bonds with the hydroxyl groups on the silica surface,therefore,the alcohol adsorbs on the silica particles'surface,reducing concentration of Si(OH)₄ near silica surface,thereby significantly slowing the gelation rate of the NSS.Anionic surfactants cannot effectively approach the surface of silica,so their effect on the gelation rate is not significant.In addition,the influence of the structure of salt and alcohol on the gelation rate was also explored.The salt with stronger electrophilic cation,nucleophilic effect slightly stronger than Si(OH)₃O^-anion and n-propanol have a better effect on slowing down the gelation rate.The effect of compound additives of salt(Na Cl,Na NO₃)and n-propanol,salt(Na Cl,Na NO₃)and anionic surfactant(Sodium dodecyl sulfate,SDS),n-propanol and SDS on the gelation rate was further explored.N-propanol as a single additive can significantly reduce the concentration of Si(OH)₄ near the silica surface,but will slightly increase the concentration of Si(OH)₃O^-,while the anion NO₃^-can significantly reduce the concentration of Si(OH)₃O^-near the silica surface.Therefore,the combination of n-propanol and Na NO₃ can simultaneously reduce the concentration of Si(OH)₄ and Si(OH)₃O^-near the silica surface,thereby significantly reducing the gelation rate of NSS.The combination of SDS and Na NO₃ promotes the adsorption of SDS on the silica silica and reduces the concentration of Si(OH)₄ on the silica surface,thus slowing down the gelation tendency of NSS.The compounding of n-propanol and SDS has no obvious effect on the gelation rate of the sol.Finally,based on the simulation results,a variety of additives were screened.By measuring the changes in viscosity,zeta potential,and gel time after adding the additives to NSS,the influence of additives on the gelation process was further explored.The experimental results are basically consistent with the simulation predictions,that is,n-propanol and surfactants can effectively improve the stability of the sol,and the combined use of n-propanol and surfactants can further inhibit the transition from sol to gel.In this work,computer simulation,theoretical analysis and experimental research were employed to deeply explore the process and mechanism of the gelation process of NSS,as well as the influence mechanism of several commonly used additives and their composite additives on gelation process.The research results provide theoretical guidance for effectively improving the stability of nano-silica sol and screening suitable anti-gelling additives.