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Zwitterionic materials have showntheir excellent performance inmany biological and chemical applications. Zwitterionic materialspossess moieties that own both cationic and anionic groups. The associationsamong zwitterionic moieties through electrostatic interactions playan important role in properties of zwitterionic materials. However,the relationship between the molecular structures and associationsof zwitterionic moieties are still not well understood. This workcompared thermal- and salt-responsive behaviors of sulfobetaine andcarboxybetaine polymers by examining their rheological propertiesas a function of temperature and their hydrodynamic sizes as a functionof salt concentration. Results showed that carboxybetaine polymersdo not exhibit stimuli responses as expected from the antipolyelectrolytebehavior of zwitterionic polymers as observed in sulfobetaine polymers.We studied and compared the associations among zwitterionic moietiesin these two zwitterionic polymers using molecular dynamic simulations.Simulation results show that the charge-density difference betweencationic and anionic groups determines the associations among zwitterionicmoieties, which are responsible for different stimuli responses ofcarboxybetaine and sulfobetaine polymers. [ABSTRACT FROM AUTHOR]

Zwitterionic carboxybetaines (CBs) are ubiquitous innature andconsidered promising materials for biological and chemical applications.A thorough understanding of the effect of carbon spacer length (CSL)on molecular properties is important. In this work, using moleculardynamics simulation and quantum chemical calculation, we investigatedthe effect of CSL on the molecular properties of CB molecules. Thehydration number, structure, and dynamics of carboxylic and trimethylammonium groups were investigated and found to present different behaviorsin regards to the variation of CSL. The simulation results with partialcharges developed from quantum chemical calculations were comparedwith those with partial charges from the OPLS all atom (OPLSAA) forcefield. The hydration free energy of CB molecules and CB–Na+association was also studied as a function of CSL. [ABSTRACT FROM AUTHOR]

In this work, molecular dynamics simulations were performed to study a carboxybetaine methacrylate (CBMA) hydrogel under various swelling states. The water content in this study ranged from 28% to 91% of the total weight of the hydrogel. Three transitions of the CBMA hydrogel were observed as the water content increased. The first transition occurs when the water content increases from 33% to 37%. The observed kink in the self-diffusion coefficient of water indicates that the hydration of the polymer network of the hydrogel is saturated; the further added water is in a less confined state. The second transition was found to be related to the physical cross-links of the polymer network. As the water content rises to above 62%, the lifetime of the physical cross-links decreases significantly. This abrupt change in the lifetime indicates that the transition represents the equilibrium swelling state of the hydrogel. Finally, the third transition was observed when the water content goes above 81%. The significant increases in the bond and angle energies of the polymer network indicate that the hydrogel reaches its upper limit swelling state at this transition. These results are comparable to previously published experimental studies of similar zwitterionic hydrogels. [ABSTRACT FROM AUTHOR]

Using molecular dynamics simulations, we investigated the associations between two zwitterions (carboxybetaine and sulfobetaine) and four types of cations (Li, Na, K, and Cs) in aqueous solutions. We studied the number and lifetime of various zwitterion–cation associations and observed that both carboxybetaine and sulfobetaine have the same order of association number and lifetime: Li> Na> K> Cs. Simulation results showed that the association variation as a function of cation types for these two zwitterions is significantly different. The effect of anion type on the order was also investigated by varying the type of anions from Cl–to Br–and F–. In order to further investigate zwitterion–cation association, we simulated the systems either with one type of zwitterion and two types of cations or with one type of cation and both carboxybetaine and sulfobetaine presented. This allowed direct competition between the solutes, and the observed association number and lifetime validated the order. Simulation results further demonstrated that, although CB associates stronger with Li橷 Na똖 SB, the latter is associated preferentially by K橷 Cs. [ABSTRACT FROM AUTHOR]