Effect of solvent quality and chain density on normal and frictional forces between electrostatically anchored thermoresponsive diblock copolymer layers

Equilibration in adsorbing polymer systems can be very slow, leading to different physical properties at a given condition depending on the pathway that was used to reach this state. Here we explore this phenomenon using a diblock copolymer consisting of a cationic anchor block and a thermoresponsive block of poly(2-isopropyl-2-oxazoline), PIPOZ. We find that at a given temperature different polymer chain densities at the silica surface are achieved depending on the previous temperature history. We explore how this affects surface and friction forces between such layers using the atomic force microscope colloidal probe technique. The surface forces are purely repulsive at temperaturesPeer reviewe

Surface Deposition and Phase Behavior of Oppositely Charged Polyion–Surfactant Ion Complexes. Delivery of Silicone Oil Emulsions to Hydrophobic and Hydrophilic Surfaces

The adsorption from mixed polyelectrolyte-surfactant solutions at hydrophobized silica surfaces was investigated by in situ null-ellipsometry, and compared to similar measurements for hydrophilic silica surfaces. Three synthetic cationic copolymers of varying hydrophobicity and one cationic hydroxyethyl cellulose were compared in mixtures with the anionic surfactant sodium dodecylsulfate (SDS) in the absence or presence of a dilute silicone oil emulsion. The adsorption behavior was mapped while stepwise increasing the concentration of SDS to a polyelectrolyte solution of constant concentration. The effect on the deposition of dilution of the bulk solution in contact with the surface was also investigated by gradual replacement of the bulk solution with 1 mM aqueous NaCl. An adsorbed layer remained after complete exchange of the polyelectrolyte/surfactant solution for aqueous NaCl. In most cases, there was a codeposition of silicone oil droplets, if such droplets were present in the formulation before dilution. The overall features of the deposition were similar at hydrophobic and hydrophilic surfaces, but there were also notable differences. SDS molecules adsorbed selectively at the hydrophobized silica surface, but not at the hydrophilic silica, which influenced the coadsorption of the cationic polymers. The largest amount of deposited material after dilution was found for hydrophilic silica and for the least-hydrophobic cationic polymers. For the least-hydrophobic polyions, no significant codeposition of silicone oil was detected at hydrophobized silica after dilution if the initial SDS concentration was high

Albumin-hyaluronan interactions : influence of ionic composition probed by molecular dynamics

The lubrication mechanism in synovial fluid and joints is not yet fully understood. Nevertheless, intermolecular interactions between various neutral and ionic species including large macromolecular systems and simple inorganic ions are the key to understanding the excellent lubrication performance. An important tool for characterizing the intermolecular forces and their structural consequences is molecular dynamics. Albumin is one of the major components in synovial fluid. Its electrostatic properties, including the ability to form molecular complexes, are closely related to pH, solvation, and the presence of ions. In the context of synovial fluid, it is relevant to describe the possible interactions between albumin and hyaluronate, taking into account solution composition effects. In this study, the influence of Na+, Mg2+, and Ca2+ ions on human serum albumin–hyaluronan interactions were examined using molecular dynamics tools. It was established that the presence of divalent cations, and especially Ca2+, contributes mostly to the increase of the affinity between hyaluronan and albumin, which is associated with charge compensation in negatively charged hyaluronan and albumin. Furthermore, the most probable binding sites were structurally and energetically characterized. The indicated moieties exhibit a locally positive charge which enables hyaluronate binding (direct and water mediated)

A cartilage-inspired lubrication system

Articular cartilage is an example of a highly efficacious water-based, natural lubrication system that is optimized to provide low friction and wear protection at both low and high loads and sliding velocities. One of the secrets of cartilage\u27s superior tribology comes from a unique, multimodal lubrication strategy consisting of both a fluid pressurization mediated lubrication mechanism and a boundary lubrication mechanism supported by surface bound macromolecules. Using a reconstituted network of highly interconnected cellulose fibers and simple modification through the immobilization of polyelectrolytes, we have recreated many of the mechanical and chemical properties of cartilage and the cartilage lubrication system to produce a purely synthetic material system that exhibits some of the same lubrication mechanisms, time dependent friction response, and high wear resistance as natural cartilage tissue. Friction and wear studies demonstrate how the properties of the cellulose fiber network can be used to control and optimize the lubrication and wear resistance of the material surfaces and highlight what key features of cartilage should be duplicated in order to produce a cartilage-mimetic lubrication system

Human saliva and model saliva at bulk to adsorbed phases – similarities and differences

Human saliva, a seemingly simple aqueous fluid, is, in fact, an extraordinarily complex biocolloid that is not fully understood, despite many decades of study. Salivary lubrication is widely believed to be a signature of good oral health and is also crucial for speech, food oral processing and swallowing. However, saliva has been often neglected in food colloid research, primarily due to its high intra- to inter-individual variability and altering material properties upon collection and storage, when used as an ex vivo research material. In the last decade, colloid scientists have attempted designing model (i.e. ‘saliva mimicking fluid’) saliva formulations to understand saliva-food colloid interactions in an in vitro set up and its contribution on microstructural aspects, lubrication properties and sensory perception. In this Review, we critically examine the current state of knowledge on bulk and interfacial properties of model saliva in comparison to real human saliva and highlight how far such model salivary formulations can match the properties of real human saliva. Many, if not most, of these model saliva formulations share similarities with real human saliva in terms of biochemical compositions, including electrolytes, pH and concentrations of salivary proteins, such as α-amylase and highly glycosylated mucins. This, together with similarities between model and real saliva in terms of surface charge, has led to significant advancement in decoding colloidal interactions (bridging, depletion) of charged emulsion droplets and associated sensory perception in the oral phase. However, model saliva represents significant dissimilarity to real saliva in the lubricating properties. Based on in-depth examination of properties of mucins from animal sources (e.g. pig gastric mucins (PGM) or bovine submaxillary mucin (BSM)), we can recommend that BSM is currently the most optimal mucin source when attempting to replicate saliva based on surface adsorption and lubrication properties. Even though purification via dialysis or chromatographic techniques may influence various physicochemical properties of BSM, such as structure and surface adsorption, the lubricating properties of model saliva formulations based on BSM are generally superior and more reliable than PGM counterpart at orally relevant pH. Comparison of mucin-containing model saliva with ex vivo human salivary conditioning films suggests that mucin alone cannot replicate the lubricity of real human salivary pellicle. Mucin-based multi-layers containing mucin and oppositely charged polyelectrolytes may offer promising avenues in the future for engineering biomimetic salivary pellicle, however, this has not been explored in oral tribology experiments to date. Hence, there is a strong need for systematic studies with employment of model saliva formulations containing mucins with and without polycationic additives before a consensus on a standardized model saliva formulation can be achieved. Overall, this review provides a comprehensive framework on simulating saliva for a particular bulk or surface property when doing food oral processing experiments

Surface force studies of association phenomena at solid-liquid interfaces

The main topics of this thesis, association phenomena inbulk and at solid-liquid interfaces in polar and non-polarmedia, were studied by a range of methods. Direct forcemeasurements employing the interferometric surface forceapparatus (SFA) was the chief technique. In addition, atomicforce microscopy (AFM), X-ray photoelectron spectroscopy (XPS),small angle neutron scattering (SANS), dynamic light scattering(DLS), turbidimetric and electrophoretic mobility measurementswere also applied. These techniques give complementaryinformation, and together they can provide a rather detailedpicture of the fairly complex systems studied. The first system studied was designed to explore particleinteractions in non-polar media. It consisted of polar or,alternatively, non-polar surfaces immersed in a non-polarmedium, triolein, in some cases containing additives such asphospholipids, polyglycerol polyricinoleate (PGPR), and traceamounts of water. It was investigated how triolein mediates thesurface interactions and how these interactions are affected bythe presence of additives. Triolein adsorbs onto mica surfacesthus producing a barrier against flocculation of the particles.The additives mentioned interact with the surfaces and witheach other, thus altering the surface interactions. Water, forinstance, being strongly polar, preferentially adsorbs ontomica and disturbs the triolein ordering at the mica surfacescausing the barrier against flocculation to vanish. Owing tocapillary condensation of water, a strong adhesion between thesurfaces is instead developed. On the other hand, it could beshown that in the presence of phospholipids, the effect ofwater was opposite: long-range repulsive forces develop due toweakly adsorbed reversed phospholipid aggregates. The second type of system studied contained cationicpolyelectrolytes and oppositely charged surfactants. Suchsystems occur in numerous technological processes: wastewatertreatment and ore recovery as well as laundry and body care.Polyelectrolytes and surfactants associate in bulk solution,and they also adsorb on surfaces. Thus, the relation betweenthe bulk properties of polyelectrolyte-surfactant mixtures andtheir properties at the solid-liquid interface is of greatscientific and industrial interest. The bulk properties ofpolyelectrolyte-surfactant aggregates were characterised byDLS, SANS, turbidimetry and electrophoretic mobilitymeasurements. It was concluded that to a certain extent theinterfacial properties of polyelectrolyte-surfactant aggregatescan be rationalised by considering their bulk properties.However, it was also shown that the presence of a surfaceaffects the association between polyelectrolytes andsurfactants. The chemical composition of adsorbed aggregates isdifferent from that of aggregates in solution, and, moreover,the structure of surface-bound polyelectrolyte-surfactantaggregates changes slowly with time. Keywords:Triolein, phosphatidiylethanolamine,phospholipid, lecithin, capillary condensation, polyglycerolpolyricinoleate, aggregation, adsorption, surface forces,structural forces, mica, polar surface, non-polar surface,polyelectrolyte, surfactant, sodium dodecyl sulphate,polyelectrolyte-surfactant association, turbidity,electrophoretic mobility, AFM, SANS, DLS, SFA.NR 2014080