Measurement of the interaction between adsorbed polymer layers: the steric effect

Preparation of monoethyl fumarate-based molecularly imprinted polymers and their application as a solid-phase extraction sorbent for the separation of scopolamine from tropane alkaloids

Molecularly imprinted polymers (MIPs) prepared using conventional functional monomers exhibit poor specific extraction of scopolamine from tropane alkaloids, which hinders their application in separation and purification. In this paper, a novel molecularly imprinted polymer (MIP) was prepared by precipitation polymerization using scopolamine as the template, monoethyl fumarate (MFMA) as a functional monomer, and ethylene dimethacrylate (EGDMA) as a cross-linker. The advantages of the supercritical fluid technology for the removal of the template were verified by comparing the efficiency of the swelling method and the Soxhlet extraction method. The prepared MFMA-based MIPs (MFMA-MIPs) showed a high adsorption capacity (49.75 mg g⁻¹) and high selectivity toward scopolamine with a selectivity coefficient of 3.5. ¹H NMR spectroscopy was performed to demonstrate the interactions between the two functional groups of the functional monomer and the template. Lastly, MFMA-MIPs were used as solid phase extraction (SPE) sorbents for scopolamine analysis. It was found that 97.0–107.0% of the template had been extracted using the SPE column from the complex of scopolamine, atropine and anisodamine. The mean recoveries of scopolamine from plant samples were 96.0–106.0% using the established method, which showed a good linearity in the range of 8.0–4.0 × 10⁴ μg L⁻¹. The results showed that MFMA-MIPs could be applied for the separation of scopolamine from tropane alkaloids.

Monoethyl fumarate with two functional groups was introduced to prepare a MIP for the separation of scopolamine from tropane alkaloids.

Tribological Behavior of Grafted Nanoparticle on Polymer-Brushed Walls: A Dissipative Particle Dynamics Study

Two contacting surfaces grafted with polymer brushes have potential applications due to their extraordinary lubricating behavior. However, the polymer brushes may have poor mechanical stability under high normal and shear stresses, which is a challenge for practical usage of polymer brush systems. In this study, we propose the use of grafted nanoparticles as nanobearings on polymer-brush-coated surfaces to alleviate the harsh working conditions of polymer brushes and to improve their mechanical stability. We have performed dissipative particle dynamics (DPD) simulations to investigate the tribological interaction between grafted nanoparticle and parallel walls with noncharged polymer brushes in the presence of explicit solvent. The influences of several parameters (solvent quality, brush miscibility, etc.) on the tribological behavior of the system are investigated. The grafted nanoparticle obviously acts as a nanobearing that partially replaces the sliding contact between two brushed walls with rolling contact between the grafted nanoparticle and two brushed walls and reduces the number of DPD particles withstanding high force. Although the introduction of the grafted nanoparticle into polymer-brushed walls increases the friction coefficient by 20-30%, it does not greatly decrease lubrication of the brushed walls, while still helping in stabilizing the system of polymer brushes to be used with liquids with low viscosity, such as water. The DPD simulation results and analysis performed in this study would be beneficial in designing systems with polymer-brushed surfaces and grafted nanoparticles.

Dynamic forces between emulsified water drops coated with Poly-Glycerol-Poly-Ricinoleate (PGPR) in canola oil

The dynamic collision of emulsified water drops in the presence of non-ionic surfactants plays a crucial role in many practical applications. Interaction force between water drops coated with non-ionic food grade surfactants is expected to exhibit rich dynamic behavior that is not yet explored. The collision forces between immobilized water drops in canola oil in the presence of a well-known food grade surfactant polyglycerol polyricinoleate (PGPR) are measured at concentrations well below typically used to form stable emulsions. An extension or kink, attributed to a short-range attractive interaction due to PGPR bridging between the drops, was observed in the retract portion of the force curves at higher applied forces or slower collision velocities. The Stokes-Reynolds-Young-Laplace (SRYL) model was used to calculate theoretical force curves. For higher collisions velocities, the agreement between the calculated and experiment data was acceptable, but the SRYL model failed to describe the extension or kink feature observed at slower velocities below. Both the AFM data and the comparison to the model calculation indicated the presence of a short-range attractive force, not of a hydrodynamic origin, attributed to the bridging and extension of PGPR molecules on the surface of water drops below saturation of the interface.

Metal Nanoparticles in Ionic Liquids

During the last years ionic liquids (ILs) were increasingly used and investigated as reaction media, hydrogen sources, catalysts, templating agents and stabilizers for the synthesis of (monometallic and bimetallic) metal nanoparticles (M-NPs). Especially ILs with 1,3-dialkyl-imidazolium cations featured prominently in the formation and stabilization of M-NPs. This chapter summarizes studies which focused on the interdependencies of the IL with the metal nanoparticle and tried to elucidate, for example, influences of the IL-cation, -anion and alkyl chain length. Qualitatively, the size of M-NPs was found to increase with the size of the IL-anion. The influence of the size of imidazolium-cation is less clear. The M-NP size was both found to increase and to decrease with increasing chain lengths of the 1,3-dialkyl-imidazolium cation. It is evident from such reports on cation and anion effects of ILs that the interaction between an IL and a (growing) metal nanoparticle is far from understood. Factors like IL-viscosity, hydrogen-bonding capability and the relative ratio of polar and non-polar domains of ILs may also influence the stability of nanoparticles in ionic liquids and an improved understanding of the IL-nanoparticle interaction would be needed for a more rational design of nanomaterials in ILs. Furthermore, thiol-, ether-, carboxylic acid-, amino- and hydroxyl-functionalized ILs add to the complexity by acting also as coordinating capping ligands. In addition imidazolium cations are precursors to N-heterocyclic carbenes, NHCs which form from imidazolium-based ionic liquids by in situ deprotonation at the acidic C2-H ring position as intermediate species during the nanoparticle seeding and growth process or as surface coordinating ligand for the stabilization of the metal nanoparticle.

Nanoparticles in ionic liquids: interactions and organization

Interactions between nanoparticles and ionic liquids can lead to a variety of organized structures.

Poly(ethylene glycol) micro-patterns as environmentally sensitive template for selective or non-selective adsorption

Poly(ethylene glycol) (PEG), a hydrophilic and repulsive polymer to non-specific adsorption, was stamped onto carboxylic acid-enriched polymer surfaces using the micro-contact printing technique. The patterns are stabilized via hydrogen bonds. Areas printed with PEG were then shown to be non-adsorbed with fluorescein isothiocyanate (FITC)-labeled dextran, while the poly(methacrylic acid) (PMAA) regions could via hydrogen bonding. Due to this contrast, well defined dextran patterns were obtained. Tuned with pH and temperature, the PEG molecules could be detached from the surfaces, erasing the template. Moreover, ionization of PMAA at higher pH induced an abrupt transition to an extended conformation, weakening the interactions between PMAA and dextran. Not only the dextran patterns lose their spatial selectivity, but also the overall adsorption amount is much lower. The pH sensitivity was in a quite narrow range, i.e. around pH 5. As the hydrogen bonds are also temperature sensitive, the attach points of PEG molecules on the surfaces disappeared at higher temperature. For poly(acrylic acid) (PAA) photografted surfaces, the pH sensitivity was more complicated due to the formation of the compact complexes of PEG and PAA molecules.

Influence of DNA adsorption and DNA/cationic surfactant coadsorption on the interaction forces between hydrophobic surfaces

The forces between hydrophobic surfaces with physisorbed DNA are markedly and irreversibly altered by exposure to DNA/cetyltrimethylammonium bromide (CTAB) mixtures. In this colloidal probe atomic force microscopy study of the interactions between a hydrophobic polystyrene particle and an octadecyltrimethylethoxysilane-modified mica surface in sodium bromide solutions, we measure distinct changes in colloidal forces depending on the existence and state of an adsorbed layer of DNA or CTAB-DNA complexes. For bare hydrophobic surfaces, a monotonically attractive approach curve and very large adhesion are observed. When DNA is adsorbed at low bulk concentrations, a long-range repulsive force dominates the approach, but on retraction some adhesion persists and DNA bridging is clearly observed. When the DNA solution is replaced with a CTAB-DNA mixture at relative low CTAB concentration, the length scale of the repulsive force decreases, the adhesion due to hydrophobic interactions greatly decreases, and bridging events disappear. Finally, when the surface is rinsed with NaBr solution, the length scale of the repulsive interaction increases modestly, and only a very tiny adhesion remains. These pronounced changes in the force behavior are consistent with CTAB-induced DNA compaction accompanied by increased DNA adsorption, both of which are partially irreversible.

Measuring forces with the AFM: polymeric surfaces in liquids

This review links together for the first time both the practicalities of force measurement and the work carried out to date on force detection between polymeric surfaces in liquids using the atomic force microscope (AFM). Also included is some of the recent work that has been carried out between surfactant surfaces and biologically coated surfaces with the AFM. The emphasis in this review is on the practical issues involved with force measurement between these types of surfaces, and the similarities and irregularities between the observed types of forces measured. Comparison is made between AFM and surface force apparatus (SFA) measurements, as there is a much longer history of work with the latter. Results indicate that forces between the surfaces reviewed here are a complicated mixture of steric-type repulsion, conformational behaviour on separation and long-range attraction, which is often ascribed to 'hydrophobic' forces. The origin of this latter force remains uncertain, despite its almost ubiquitous appearance in force measurements with these types of surfaces.

Normal Forces between Cellulose Surfaces Measured with Colloidal Probe Microscopy

Colloidal probe microscopy was employed to study interactions between cellulose surfaces in aqueous solutions. Hydrodynamic forces must be accounted for in data analysis. Long-range interactions betweeen cellulose surfaces are governed by double-layer forces and, once surfaces contact, by osmotic repulsive forces and viscoelasticity. Increasing the ionic strength decreases surface potentials and increases adhesive forces. Polyelectrolytes cause strong steric repulsion at high surface coverage, where interactions are sensitive to probe velocity. Polymer bridging occurs at low coverage. The conformation of adsorbed polyelectrolytes depends on the polymer concentration. Copyright 2000 Academic Press.

Study of a Solvated Adsorbed Gelatin Layer Using a Modified Force Microscope

The interaction energies between gelatin-coated surfaces at various electrolyte and pH conditions are reported. The surfaces are of glass and are negatively charged under all conditions used here. Gelatin is a polyampholyte, with an isoelectric pH (IEP) of approximately 4.9. At low pH the gelatin molecules have a net positive charge, and thus the polyampholyte tends to adsorb with a relatively flat conformation. As the pH is increased the strong attractive interaction between the surface and the polyampholyte decreases as more negative charges and then fewer positive charges appear on the polyampholyte, and so the gelatin extends away from the surface. On changing electrolyte concentrations after adsorption no effect was seen at the IEP, but the layer was observed to swell at more alkaline pHs. This is consistent with the net minimum charge situation on the polymer under these conditions. Changing the adsorption conditions was seen to have an effect, and this is attributed to the different affinities of the gelatin chain to the surface depending on the solution chemistry. Results obtained when the gelatin was initially adsorbed on one surface or two were similar, suggesting that the gelatin transfers rapidly from one surface to another. The importance of adsorption conditions as well as current conditions is discussed. Copyright 1999 Academic Press.

Surface modification of polymeric biomaterials with poly(ethylene oxide), albumin, and heparin for reduced thrombogenicity

Appropriate surface modification has significantly improved the blood compatibility of polymeric biomaterials. This article reviews methods of surface modification with water-soluble polymers, such as polyethylene oxide (PEO), albumin, and heparin. PEO is a synthetic, neutral, water-soluble polymer, while albumin and heparin are a natural globular protein and an anionic polysaccharide, respectively. When grafted onto the surface, all three macromolecules share a common feature to reduce thrombogenicity of biomaterials. The reduced thrombogenicity is due to the unique hydrodynamic properties of the grafted macromolecules. In aqueous medium, surface-bound water-soluble polymers are expected to be highly flexible and extend into the bulk solution. Biomaterials grafted with either PEO, albumin, or heparin are able to resist plasma protein adsorption and platelet adhesion predominantly by a steric repulsion mechanism.