Coarse-grained molecular modelling of amphiphilic polymers at a water/air interface

Previous atomic-level simulations have been shown to provide invaluable insight into the adsorption behaviour of amphiphilic polymers at a water-air interface. Neutron reflectivity profiles generated from these simulations showed good agreement with experiment, particularly at low surface concentrations. Unfortunately, previous detailed atomistic simulations have failed to produce adequate results at high surface concentrations due to crowded configurations, which could not relax within the simulation times available. To tackle this problem, a coarse-graining (CG) technique, where the structure of the simulated molecule is simplified to a chain of beads, has been employed in this study. This provides for the simulation of larger time and length scales allowing for a more detailed study of the capture of polymer chains by a surface and the structure of surface layers. The work presented in this thesis involves development of coarse-grained models for water and for poly(ethylene oxide) (PEO)/water systems, with the aim of reproducing the properties of key importance for the bulk and liquid/vapour interfacial states. These models are then used in the coarse-grained simulation studies of di-and trifluoro dendritic end-capped PEO at an air-water interface; the amphiphilic polymers that have been studied recently by neutron reflectivity experiments. It is shown in this study that simulation of very large polymer chains comparable to that used in real experiments, is achievable using coarse-grained molecular dynamics. Neutron reflectivity profiles generated from simulations of di- and trifluoro dendritic end-capped PEO materials at low polymer concentrations are in good agreement with experiment data. Simulations at high polymer concentrations showed no evidence of a stretched brush structure, in accordance with experimental findings. It is shown from these simulations that there are polymers adsorbed to the interface by a combination of fluorocarbon ends and ethylene oxide segments,resulting in a rather at layer structure. At high surface concentrations of polymers, it proved possible to see the formation of polymer micelles in bulk water. The process of micelle capture by the surface and incorporation of the micelle contents into the surface, were also observed

A coarse-grained model for polyethylene glycol in bulk water and at a water/air interface

A coarse-grained model for polyethylene glycol (PEG) in water has been developed using a combination of the iterative Boltzmann inversion (IBI) methodology and a suitable coarse-grained water potential. The combined coarse-grained model is shown to be effective in reproducing the properties of single chains in bulk water and multiple chains across a series of chain lengths and concentrations, and is transferable to PEG chains at a water/air interface. Good agreement is achieved with both experiment and reference atomistic simulations in an explicit solvent. Simulations of a single chain in aqueous solution yield a molecular weight (MW)-radius of gyration (Rg) relation that compares favourably with the reported scaling law from experiment. Simulations of multiple chains across a wide concentration range show no concentration dependence of Rg, in agreement with previous atomistic simulations. The model we develop is shown to be transferable between polymer in bulk water and at a water/air interface. For interfacial simulations, PEG chains are found to spontaneously migrate to the surface and adsorb to form a thin surface layer, which thickens with increasing surface concentration. The point at which the surface is fully saturated with polymer, and the polymer layer thicknesses obtained from simulations, are both in good agreement with experimental findings. At high surface concentrations, when the surface is fully saturated with polymer, ethylene oxide (EO) segments are found to extend into the water subphase as loop and tail conformations, with this extension increasing with further increases in the surface concentration. The coarse-grained model is noted to provide very large increases in simulation speed, with equilibration times of <1000× the reference atomistic models. We also consider a number of different coarse-grained models for water in this study, showing that the CSJ model adopted in this work [Chiu et al., J. Chem. Theory Comput., 2010, 6, 851] is far superior for studying water at a water/air surface, than many of the previous coarse-grained models of water

Zinc-Silver Doped Mesoporous Hydroxyapatite Synthesized via Ultrasonic in Combination with Sol-Gel Method for Increased Antibacterial Activity

Bone materials are mainly composed of an inorganic constituent called hydroxyapatite (HA). In the current study, mesoporous Zn2+/Ag+ doped hydroxyapatite nanoparticles (Zn-Ag doped HA) with high antibacterial activity were synthesized through ultrasonic coupled sol-gel techniques under calcination temperatures of 600 °C for 4 h and 1100 °C for 1 h. The variance in the molar ratio of Zn2+/Ag+ in Ca9.0Zn1.0−xAgx(PO4)6(OH)2 (x = 0.0, 0.25 to 1.0) and its effects on the chemical and physical properties of the powdered samples were investigated. The results show that the hexagonal framework of HA incorporated both the Zn2+ and Ag+ ions and the rhombohedral structure of β-TCP. The main functional groups of HA and Zn-Ag doped HA samples were hydroxyl and phosphate. All samples have mesoporous characteristics with a Type IV isotherm. The agar well diffusion process was used to examine antibacterial activity against E. coli, P. aeruginosa, S. aureus, B. cereus and B. subtilis. Effective antibacterial activity was displayed by Zn-Ag doped HA. Excellent antibacterial performance was shown by Ca9.0Zn0.75Ag0.25(PO4)6(OH)2 against all tested bacterial strains, except P. aeruginosa. This material showed inhibition zones ranging from 7 to 11 mm, implying that it is a suitable material with an antibacterial action for environmental applications, specifically for water purification