Monte Carlo simulations of an amphiphilic polymer at a hydrophobic/hydrophilic interface

A simple and versatile route to amphiphilic polymethacrylates: catalytic chain transfer polymerisation (CCTP) coupled with post-polymerisation modifications

Amphiphilic polymers have become key figures in the fields of pharmacology, medicine, agriculture and cosmetics.

Adsorption and friction behavior of amphiphilic polymers on hydrophobic surfaces

The ability of amphiphilic polymers to self-assemble and form a gel or gel-like layer has been investigated by means of both experimental and theoretical studies on alkylated derivatives of poly(acrylic acid). Experiments were performed to determine the relationship between amphiphilic polymer chemistry, structure, water retention, and friction in the presence of hydrophobic substrates. The results indicate that the amphiphilic polymer forms a water-enriched, friction-reducing adsorbed layer on hydrophobic surfaces. The shear moduli and viscosities of the adsorbed layers, as determined by fitting the Voigt model to QCM-D data, were consistent with the presence of a gel. Computational studies on HPAA-12 were performed and are consistent with the presence of adsorbed conformations, in which the lowest free energy in the model corresponded to a partially adsorbed molecule, with a small fraction of hydrophobic side chains being compelled, for configurational reasons, to point into the bulk water. This would support the possibility of the formation of either a gel-like layer or surface aggregation. However, because the adsorption experiments showed no evidence of aggregation, this strongly suggests the formation of a gel.

Emerging Applications of Polymersomes in Delivery: from Molecular Dynamics to Shrinkage of Tumors

Polymersomes are self-assembled shells of amphiphilic block copolymers that are currently being developed by many groups for fundamental insights into the nature of self-assembled states as well as for a variety of potential applications. While recent reviews have highlighted distinctive properties - particularly stability - that are strongly influenced by both copolymer type and polymer molecular weight, here we first review some of the more recent developments in computational molecular dynamics (MD) schemes that lend insight into assembly. We then review polymersome loading, in vivo stealthiness, degradation-based disassembly for controlled release, and even tumor-shrinkage in vivo. Comparisons of polymersomes with viral capsids are shown to encompass and inspire many aspects of current designs.

Adsorption of Octyl Cyanide at the Free Water Surface as Studied by Monte Carlo Simulation

Monte Carlo simulations of the adsorption layer of octyl cyanide have been performed on the canonical (N, V, T) ensemble at 300 K. The systems simulated cover the range of octyl cyanide surface densities from 0.27 to 7.83 mumol/m2. The surface density value at which the saturation of the adsorption layer occurs is estimated to be 1.7 mumol/m2. At low surface densities, the main driving force of the adsorption is found to be the formation of hydrogen bonds between the water and octyl cyanide molecules, whereas at higher surface concentrations, the dipole-dipole attraction between the neighboring adsorbed octyl cyanide molecules becomes more important. At low surface concentrations, the water-octyl cyanide hydrogen bonds prefer tilted alignments relative to the interface; however, in the case of the saturated adsorption layer, the number of such hydrogen bonds is maximized, leading to the preference of these bonds for the orientation perpendicular to the interface. Contrary to nonionic surfactants of multiple hydrogen bonding abilities (e.g., 1-octanol, C8E3), the increasing surface concentration of octyl cyanide was not found to lead to considerable competition of the molecules for positions of optimal arrangement. As a consequence, the energy and geometry of the water-octyl cyanide hydrogen bonds are found to be insensitive to the octyl cyanide surface concentration.

Molecular dynamics simulations of amphiphilic graft copolymer molecules at a water/air interface

Fully atomistic molecular dynamics simulations of amphiphilic graft copolymer molecules have been performed at a range of surface concentrations at a water/air interface. These simulations are compared to experimental results from a corresponding system over a similar range of surface concentrations. Neutron reflectivity data calculated from the simulation trajectories agrees well with experimentally acquired profiles. In particular, excellent agreement in neutron reflectivity is found for lower surface concentration simulations. A simulation of a poly(ethylene oxide) (PEO) chain in aqueous solution has also been performed. This simulation allows the conformational behavior of the free PEO chain and those tethered to the interface in the previous simulations to be compared.

Structure of the nonionic surfactant triethoxy monooctylether C8E3 adsorbed at the free water surface, as seen from surface tension measurements and Monte Carlo simulations

The adsorption layer of the nonionic surfactant triethoxy monooctylether C8E3 has been investigated at the free water surface by means of both experimental and computer simulation methods. The surface tension of the aqueous solution of C8E3 has been measured by pendant drop shape analysis in the entire concentration range in which C8E3 is soluble in water. The data obtained from these measurements are used to derive the adsorption isotherm. The critical micellar concentration and the surface excess concentration of the saturated adsorption layer are found to be 7.48 mM and 4.03 micromol/m2, respectively, the latter value corresponding to the average area per molecule of 41 A2. In order to analyze the molecular level structure of the unsaturated adsorption layer, Monte Carlo simulations have been performed at four different surface concentration values, i.e., 0.68, 1.36, 2.04, and 2.72 micromol/m2, respectively. It has been found that the water surface is already almost fully covered at the lowest surface density value investigated, and the adsorbed molecules show a strong preference for lying parallel with the interface in elongated conformations. No sign of the penetration of the hydrophilic triethoxy headgroups into the aqueous phase to any extent has been observed. With increasing surface densities the preferential orientation of the apolar octyl tails gradually turns from lying parallel with the interface to pointing toward the vapor phase by their CH3 end, whereas the conformation of the adsorbed molecules becomes gradually less elongated. Both of these changes lead to the increase of the number of C8E3 molecules being in a direct contact (i.e., forming hydrogen bonds) with water. However, the increasing number of the C8E3 molecules hydrogen bonded to water is found to be accompanied by the weakening of this binding, i.e., the decrease of both the number of hydrogen bonds a bound C8E3 molecule forms with water and the magnitude of the average binding energy of the adsorbed C8E3 molecules.