Monte Carlo Simulation Of Charge Regulation In Linear Weak Polyelectrolyte

We studied the charge regulation mechanism of linear weak polyelectrolyte system using Monte Carlo molecular simulation method in order to further understand the stimulation-response principle of the system.The effects of pH value,salt concentration and valence on the titration process and conformational properties of weak polyelectrolyte system were mainly studied.Firstly,we combined mathematical modeling and Monte Carlo algorithm to model the charge regulation process of weak polyelectrolyte and developed a set of complete Monte Carlo molecular simulation program for this model.In this program,the counterions and salt particles are explicitly characterized and the electrostatic interactions between charged groups in the system are calculated by using the Ewald summation method.In addition,this program not only includes several standard Metropolis algorithms,but we also propose two configuration-bias sampling algorithms,segment regrowing algorithm and local rotating cluster algorithm,according to the characteristics of the system.The simulation results show that in the salt-free system,the ionization degree of the system can be controlled by adjusting the pH to induce the transformation of the weak polyelectrolyte from the folded conformation to the extended conformation.The statistical results show that the rotation radius increases linearly with the increase of ionization degree.The existence of counterions obviously promotes the deprotonation process of weak polyelectrolyte and at the same time causes counterion condensation effect,which makes the simulation more difficult.The charge and counterions are not uniformly distributed along the weak polyelectrolyte chain.Charges tend to gather at both ends of the chain while counterions tend to gather in the chain.According to our observation,this phenomenon is universal in the linear weak poly electrolyte system.In the salt system,the monovalent salt has almost no effect on the titration process of weak polyelectrolyte,while the trivalent salt promotes the deprotonation process of weak polyelectrolyte more than the divalent salt,making the titration process in the trivalent salt system more closer to the ideal titration curve.The conformation of the weak polyelectrolyte is regulated by the high valence salt to a certain extent.The flexibility of the weak polyelectrolyte is increased by the divalent salt,which has the effect that the conformation expansion is lower than that of the monovalent salt or salt-free system.As the degree of ionization increases,the weak polyelectrolyte in the trivalent salt system first forms a stable folded structure,and then the chain conformation gradually expands to the extended structure.The higher the concentration of trivalent salt in the system,the less easily the stable folded structure will be destroyed by the pH value of the system.For the high valence salt system,the degree of ion condensation is higher.When the degree of ionization is low,the high valence ions in the system will gather around the charged groups on the chain and form a relatively stable structure.