Molecular dynamics study of wetting behavior of grafted thermo-responsive PNIPAAm brushes

Influence of Temperature-Guided SAM Growth on Wetting and Its Mass Transfer Models

The formation and growth of self-assembled monolayers (SAMs) composed of amphiphiles have garnered significant attention due to their diverse technical applications. This article reports the findings of molecular dynamics simulations aimed at elucidating the intricate relationship between the wetting behavior of amphiphiles, specifically n-alkanols, and the growth of their SAMs on a mica surface under varying temperature conditions. The investigation quantifies the structural characteristics of the formed SAMs, including density profiles, in-plane radial distribution functions, order parameters, and end-to-end length distributions of n-alkanol molecules within the SAM. Thermodynamic properties, such as the second virial coefficient and excess entropy, are examined in relation to temperature and time. The growth of the SAM is assessed by analyzing characteristic time scales at different temperatures and in-plane diffusion of n-alkanol molecules and utilizing classical theories of mass transfer to quantify the growth rate as a function of temperature. These results are then correlated with changes in the contact angle and spreading coefficient of n-alkanol droplets on the mica surface over time, providing insights into the impact of SAM growth on the wetting behavior and the mass transfer model of such systems.

Evolution, Stability, and Applicability of Surfactant Aggregates in Targeted Delivery

Self-assembly/self-aggregation of surfactant molecules in bulk and the vicinity of a surface has been a topic of interest for decades because of its utilization in numerous modern technical applications. In this article, the results of molecular dynamics simulations are reported to investigate the self-aggregation of sodium dodecyl sulfate (SDS) at an interface of mica and water. SDS molecules starting from lower to higher surface concentrations tend to create distinct aggregated structures in the vicinity of a mica surface. The structural properties, such as density profiles, radial distribution functions, and thermodynamic properties like excess entropy and second virial coefficient, are calculated to address the bits and pieces of the self-aggregation. The change in the free energy for aggregates of varied sizes approaching the surface from the bulk aqueous solution, along with the change in their shapes during the process in terms of change in the radius of gyration and its components, is reported respectively to model a generic pathway for a surfactant-based targeted delivery system.

Critical Analysis of Adhesion Work Measurements from AFM-based Techniques for Soft Contact

HYPOTHESIS

The work of adhesion is a thermodynamic quantity that is frequently measured by atomic force microscopy (AFM). Determination of the work of adhesion requires quasi-equilibrium measurements, where we address the question of to what extent atomic force microscopy qualifies for quasi-equilibrium measurements.

EXPERIMENT

To measure the work of adhesion, we combined soft colloidal probe AFM (SCP AFM) with reflection interference contrast microscopy (RICM). This allowed us to extract the work of adhesion either from the pull-off force or from the contact radius. With these methods, we investigated the adhesion behavior of poly(N-isopropylacrylamide) (PNIPAM) polymer brushes in the swollen and solvent-induced collapsed state by systematically analyzing contact radii and adhesive forces.

FINDINGS

In the swollen state, the adhesion to the PNIPAM brush was fivefold larger and exhibited significant time dependencies when measured with SCP AFM. A strong rate dependence of the pull-off force method was indicative of a non-equilibrium process. In order to reliably determine the equilibrium work of adhesion, the contact radius method was found to be the better because it is not rate dependent. Our work highlights the important benefits of using optical measurements to determine the contact radius when deriving the works of adhesion between colloidal probes and polymer brush surfaces.

Temperature-Responsive Polymer Brush Coatings for Advanced Biomedical Applications

Modern biomedical technologies predict the application of materials and devices that not only can comply effectively with specific requirements, but also enable remote control of their functions. One of the most prospective materials for these advanced biomedical applications are materials based on temperature-responsive polymer brush coatings (TRPBCs). In this review, methods for the fabrication and characterization of TRPBCs are summarized, and possibilities for their application, as well as the advantages and disadvantages of the TRPBCs, are presented in detail. Special attention is paid to the mechanisms of thermo-responsibility of the TRPBCs. Applications of TRPBCs for temperature-switchable bacteria killing, temperature-controlled protein adsorption, cell culture, and temperature-controlled adhesion/detachment of cells and tissues are considered. The specific criteria required for the desired biomedical applications of TRPBCs are presented and discussed.

The structure of polymer brushes: the transition from dilute to dense systems: a computer simulation study

Monodisperse polymer brushes were studied by means of Monte Carlo simulations. A coarse-grained model of a polymer brush was designed and the Cooperative Motion Algorithm was employed to model the polymerization process 'grafted from' and to study the structure of a brush immersed in a good solvent. The structure of brushes was determined as a function of the chain length and the grafting density. The influence of these parameters on the scaling properties of the brush was presented and discussed. A thorough analysis of the distribution of concentrations of the polymer segments and the distribution of chain free ends was also carried out. The analysis of the depth of penetration of the low molecular weight solvent into the brush area showed that the main factor determining the penetration is the grafting density. Good agreement between the simulation results and theoretical predictions is observed, especially for longer chains and higher grafting density. The origin of small quantitative differences between the simulation and theoretical results is discussed.

Memory effects in polymer brushes showing co-nonsolvency effects

Densely packed polymer chains grafted to a substrate, especially polymer brushes, have been studied intensively. Of special interest are systems that react to changes in external conditions or"remember" previous conditions. With this focus, we explore the properties of PNiPAAm brushes and relate published work to own results. The co-nonsolvency effect leads to a collapse of a PNiPAAm brush for a certain mixing ratio of ethanol in water. This also influences the wetting behavior of PNiPAAm brushes. We show that through prewetting of a brush with different liquids (water and ethanol), the contact angle of subsequent water drops changes significantly. To explain this change, the swelling of the brush was investigated with spectroscopic ellipsometry and the orientation of the molecules at the surface with sum-frequency generation (SFG). Only little change in swelling was found. The SFG measurements reveal in the ethanol prewetted case a well ordered hydrophobic methyl layer at the interface, which is consistent with the contact angle measurement.

Nanotube Active Water Pump Driven by Alternating Hydrophobicity

Water transport must be efficiency controlled for the future sustainability of life. Various water transport systems using carbon nanotubes have been proposed in recent years. Although these systems are more permeable than aquaporins, their water transport is passive. In this study, we successfully demonstrate an active water pump driven by simple hydrophobic interaction through computer simulation. Even in the absence of a pressure- or density-gradient, the proposed pump can actively transport water molecules by alternately switching the hydrophobicity of the pump surface. The water transport rate can be easily controlled by varying the time interval of switching. The pump with optimized switching time exhibits prominent water permeance. The results obtained herein can be applied in various water transport technologies because of the simple mechanics. The proposed water pump has the potential to realize an effective device such as a low-energy artificial purification system.

Molecular Structure Inhibiting Synergism in Charged Surfactant Mixtures: An Atomistic Molecular Dynamics Simulation Study

Synergistic and nonsynergistic surfactant-water mixtures of sodium dodecyl sulfate (SDS), lauryl betaine (C12B), and cocoamidopropyl betaine (CAPB) systems are studied using molecular simulation to understand the role of interactions among headgroups, tailgroups, and water on structural and thermodynamic properties at the air-water interface. SDS is an anionic surfactant, while C12B and CAPB are zwitterionic; CAPB differs from C12B by an amide group in the tail. While the lowest surface tensions at high surface concentrations in the SDS-C12B synergistic system could not be reproduced by simulation, estimated partitioning between surface and bulk shows trends consistent with synergism. Structural analysis shows the influence of the SDS headgroup pulling C12B to the surface, resulting in closely packed structures compared to their respective homomolecular-surfactant systems. The SDS-CAPB system, on the other hand, is nonsynergistic when the surfactants are mixed on account of the tilted structure of the CAPB tail. The translational excess entropy due to the tailgroup interactions discriminates between the synergistic and nonsynergistic systems. The implications of such interactions on surfactant effects in complex, multicomponent atmospheric aerosols are discussed.

Tunable assembly and disassembly of responsive supramolecular polymer brushes

Tunable assembly and disassembly of stimuli-responsive supramolecular polymer brushes are induced by using host–guest interactions between β-cyclodextrin and azobenzene groups.