Location of Polyelectrolytes in Swollen Lipid Oligobilayers

Osteoarthritis is caused by degeneration of the cartilage, which covers the bone ends of the joints and is decorated with an oligolamellar phospholipid (PL) bilayer. The gap between the bone ends is filled with synovial fluid mainly containing hyaluronic acid (HA). HA and PLs are supposed to reduce friction and protect the cartilage from wear in joint movement. However, a detailed understanding of the molecular mechanisms of joint lubrication is still missing. Previously, we found that aqueous solutions of HA and poly(allylamine hydrochloride) (PAH), the latter serving as a polymeric analogue to HA, adsorb onto the headgroups of surface-bound 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) oligobilayers and significantly enhance their stability with respect to shear forces, typically occurring in joint movement. We now investigated the precise location of PAH chains across the lipid films in neutron reflectivity measurements, as bridging of the oligobilayers by polyelectrolytes (PEs) might be the cause for their improved mechanical stability. In a first set of experiments, we used hydrogenated PAH and chain-deuterated DMPC (DMPC-d54) to improve the contrast between the lipids and potentially intruding PAH. However, due to difficulties in distinguishing between incorporation of water and PAH, penetration into the lipid chain region could hardly be proven quantitatively. Therefore, we designed a more elaborate experiment based on mixed films of DMPC-d54 and hydrogenated DMPC, which is insensitive to water penetration into the films. Beside facilitating a detailed structural characterization of the oligolamellar system, this elaborate approach showed that PAH adsorbs to the DMPC heads and penetrates the lipid tail strata. No PAH was found in the lipid head strata, which excludes bridging of several lipid bilayers by the PE chains. The data are consistent with the assumption that PAH bridges are formed between the headgroups of two adjacent bilayers and contribute to the enhanced mechanical stability.

Microscopic Analysis of the Water/Glycerol/EO30PS System in Bulk and on a Solid Substrate

Surfactant systems are often employed in cosmetic formulations where they dry on skin as a surface, thereby becoming increasingly concentrated systems. To better understand this drying process, we focused on the difference of self-assembled structures of the water/glycerol/polyoxyethylene (30) phytosteryl ether (EO30PS) system in bulk and on a solid substrate because the interaction between the substrate and the surfactant may have a substantial effect on the self-assembly, which may be related to the bulk structure but in detail may also differ strongly from the bulk situation. In bulk, small-angle neutron scattering (SANS) experiments showed that with increasing loss of water, the degree of ordering increases but changes of the aggregate structure are rather small. The results indicate that ellipsoidal micelles of EO30PS are densely packed and simply become more ordered in bulk during the drying process. On the other hand, neutron reflectometry revealed that EO30PS molecules adsorb onto a Si surface in the form of bilayers and analysis indicates that at a high concentration (c = 20 wt %), there are on average two bilayers (a double bilayer) on the Si substrate. The adsorbed membrane structure of EO30PS is rather thin with respect to its hydrophobic part, indicating tilted molecules, containing only some solvent, and being not highly ordered. These experimental results then allow for a much deeper understanding of the structural properties of practical formulations as they are applied, for instance, in cosmetic lotions.

Phase separation in a ternary DPPC/DOPC/POPC system with reducing hydration

The maintenance of plasma membrane structure is vital for the viability of cells. Disruption of this structure can lead to cell death. One important example is the macroscopic phase separation observed during dehydration associated with desiccation and freezing, often leading to loss of permeability and cell death. It has previously been shown that the hybrid lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) can act as a line-active component in ternary lipid systems, inhibiting macroscopic phase separation and stabilising membrane microdomains in lipid vesicles [1]. The domain size is found to decrease with increasing POPC concentration until complete mixing is observed. However, no such studies have been carried out at reduced hydration. To examine if this phase separation is unique to vesicles in excess water, we have conducted studies on several binary and ternary model membrane systems at both reduced hydration ("powder" type samples and oriented membrane stacks) and in excess water (supported lipid bilayers) at 0.2 mol fraction POPC, in the range where microdomain stabilisation is reported. Differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) are used to map phase transition temperatures, with X-ray and neutron scattering providing details of the changes in lipid packing and phase information within these boundaries. Atomic force microscopy (AFM) is used to image bilayers on a substrate in excess water. In all cases, macroscopic phase separation was observed rather than microdomain formation at this molar ratio. Thus POPC does not stabilise microdomains under these conditions, regardless of the type of model membrane, hydration or temperature. Thus we conclude that the driving force for separation under these conditions overcomes any linactant effects of the hybrid lipid.

Lithium insertion into silicon electrodes studied by cyclic voltammetry and operando neutron reflectometry

Operando neutron reflectometry measurements were carried out to study the insertion of lithium into amorphous silicon film electrodes during cyclic voltammetry (CV) experiments at a scan rate of 0.01 mV s-1. The experiments allow mapping of regions where significant amounts of Li are incorporated/released from the electrode and correlation of the results to modifications of characteristic peaks in the CV curve. High volume changes up to 390% accompanied by corresponding modifications of the neutron scattering length density (which is a measure of the average Li fraction present in the electrode) are observed during electrochemical cycling for potentials below 0.3 V (lithiation) and above 0.2 V (delithiation), leading to a hysteretic behaviour. This is attributed to result from mechanical stress as suggested in the literature. Formation and modification of a surface layer associated with the solid electrolyte interphase (SEI) were observed during cycling. Within the first lithiation cycle the SEI grows to 120 Å for potentials below 0.5 V. Afterwards a reversible and stable modification of the SEI between 70 Å (delithiated state) and 120 Å (lithiated state) takes place.

Drastic Swelling of Lipid Oligobilayers by Polyelectrolytes: A Potential Molecular Model for the Internal Structure of Lubricating Films in Mammalian Joints

Osteoarthritis is the most common arthropathy in western civilization. It is primarily caused by the degeneration of lipid-coated cartilage, leading to increased friction in joints. Hyaluronic acid (HA), a negatively charged polysaccharide and the main component of the synovial fluid, is held responsible for joint lubrication. It is believed that HA, adsorbed to the lipid-coated cartilage, forms a protective layer against wear. Studies have shown that the concentration and molecular weight (MW) of HA are reduced in joints suffering from osteoarthritis. On the basis of these observations, local joint injections of HA or mixtures of HA and surface-active phospholipids (SAPLs) have been applied as medical cures to restore the functionality of the joints in a procedure called viscosupplementation. However, this cure is still disputed, and no consensus has been reached with respect to optimum HA concentration and MW. To provide detailed insight in the structural rearrangement of lipid films upon contact with HA or polymeric analogues, we studied the interaction of the polyelectrolyte poly(allylamine hydrochloride) (PAH) with surface-bound oligobilayers of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) by neutron reflectivity (NR) and ellipsometry. Using this model system, we found a drastic swelling of the lipid films as a function of PAH concentration, whose strength compares to that in previous studies on HA incubation. In contrast, no significant dependence of film thickness on PAH MW was observed. A detailed picture of the film architecture was developed which inter alia shows that charged PAH is adsorbed to the lipid headgroups, leading to electrostatic repulsion. The swelling behavior is well explained by the equilibrium of Coulomb and van der Waals interactions in a DLVO-based model. Our detailed structural analysis of the PAH/lipid interfacial layer may help to elucidate the mechanisms of viscosupplementation and derive a structure-function relationship for the lubricating interface in mammalian joints.

BioRef II-Neutron reflectometry with relaxed resolution for fast, kinetic measurements at HZB

We present an upgrade to the time-of-flight neutron reflectometer BioRef at the research reactor BER II of the Helmholtz-Zentrum Berlin für Materialien und Energie (HZB). Through the integration of an additional chopper into the existing setup, the available wavelength resolution is significantly extended. Now two distinct operation modes can be used: a high resolution mode with Δλ/λ ranging from 1% to 5%, which allows for the investigation of thick films up to 4000 Å, and a high flux mode with Δλ/λ = 7%-11%. In the high flux mode, reflectivity curves from 0.007 Å^-1 to 0.2 Å^-1 with three angular settings can be recorded in 7 min. For a single angular setting and its respective window in Q-space, a time resolution of even less than 4 min is reached. The different configurations are documented by respective measurements (a) on a Ni-Ti multilayer and (b) the swelling kinetics of a solid-supported phospholipid coating upon incubation in a polyelectrolyte solution.

Lithiation of Crystalline Silicon As Analyzed by Operando Neutron Reflectivity

We present an operando neutron reflectometry study on the electrochemical incorporation of lithium into crystalline silicon for battery applications. Neutron reflectivity is measured from the ⟨100⟩ surface of a silicon single crystal which is used as a negative electrode in an electrochemical cell. The strong scattering contrast between Si and Li due to the negative scattering length of Li leads to a precise depth profile of Li within the Si anode as a function of time. The operando cell can be used to study the uptake and the release of Li over several cycles. Lithiation starts with the formation of a lithium enrichment zone during the first charge step. The uptake of Li can be divided into a highly lithiated zone at the surface (skin region) (x ∼ 2.5 in LixSi) and a much less lithiated zone deep into the crystal (growth region) (x ∼ 0.1 in LixSi). The total depth of penetration was less than 100 nm in all experiments. The thickness of the highly lithiated zone is the same for the first and second cycle, whereas the thickness of the less lithiated zone is larger for the second lithiation. A surface layer of lithium (x ∼ 1.1) remains in the silicon electrode after delithiation. Moreover, a solid electrolyte interface is formed and dissolved during the entire cycling. The operando analysis presented here demonstrates that neutron reflectivity allows the tracking of the kinetics of lithiation and delithiation of silicon with high spatial and temporal resolution.

Preparation of a New Oligolamellar Stratum Corneum Lipid Model

In this study, we present a preparation method for a new stratum corneum (SC) model system, which is closer to natural SC than the commonly used multilayer models. The complex setup of the native SC lipid matrix was mimicked by a ternary lipid mixture of ceramide [AP], cholesterol, and stearic acid. A spin coating procedure was applied to realize oligo-layered samples. The influence of lipid concentration, rotation speed, polyethylenimine, methanol content, cholesterol fraction, and annealing on the molecular arrangement of the new SC model was investigated by X-ray reflectivity measurements. The new oligo-SC model is closer to native SC in the total number of lipid membranes found between corneocytes. The reduction in thickness provides the opportunity to study the effects of drugs and/or hydrophilic penetration enhancers on the structure of SC in full detail by X-ray or neutron reflectivity. In addition, the oligo-lamellar systems allows one to infer not only the lamellar spacing, but also the total thickness of the oligo-SC model and changes thereof can be monitored. This improvement is most helpful for the understanding of transdermal drug administration on the nanoscale. The results are compared to the commonly used multilamellar lipid model systems and advantages and disadvantages of both models are discussed.

Comparative Adsorption of Saturated and Unsaturated Fatty Acids at the Iron Oxide/Oil Interface

A detailed comparison of the adsorption behavior of long straight chain saturated and unsaturated fatty acids at the iron oxide/oil interface has been considered using a combination of surface study techniques. Both depletion isotherms and polarized neutron reflectometry (PNR) show that the extent of adsorption decreases as the number of double bonds in the alkyl chains increases. Sum frequency generation spectroscopic measurements demonstrate that there is also an increase in chain disorder within the adsorbed layer as the unsaturation increases. However, for the unsaturated analogues, a decrease in peak intensity is seen for the double bond peak upon heating, which is thought to arise from isomerization in the surface-bound layer. The PNR study of oleic acid adsorption indicates chemisorbed monolayer adsorption, with a further diffuse reversible adsorbed layer formed at higher concentrations.

Volume profile of α-chymotrypsin during adsorption and enzymatic reaction on a poly(acrylic acid) brush

The catalytic rate of α-chymotrypsin that is adsorbed on a poly(acrylic acid) brush can be enhanced by pressure.

Polymer-Induced Swelling of Solid-Supported Lipid Membranes

In this paper, we study the interaction of charged polymers with solid-supported 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) membranes by in-situ neutron reflectivity. We observe an enormous swelling of the oligolamellar lipid bilayer stacks after incubation in solutions of poly(allylamine hydrochloride) (PAH) in D₂O. The positively charged polyelectrolyte molecules interact with the lipid bilayers and induce a drastic increase in their d-spacing by a factor of ~4. Temperature, time, and pH influence the swollen interfacial lipid linings. From our study, we conclude that electrostatic interactions introduced by the adsorbed PAH are the main cause for the drastic swelling of the lipid coatings. The DMPC membrane stacks do not detach from their solid support at T > T_m. Steric interactions, also introduced by the PAH molecules, are held responsible for the stabilizing effect. We believe that this novel system offers great potential for fundamental studies of biomembrane properties, keeping the membrane’s natural fluidity and freedom, decoupled from a solid support at physiological conditions.

Surface-Active Lipid Linings under Shear Load--A Combined in-Situ Neutron Reflectivity and ATR-FTIR Study

We study shear effects in solid-supported lipid membrane stacks by simultaneous combined in-situ neutron reflectivity (NR) and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The stacks mimic the terminal surface-active phospholipid (SAPL) coatings on cartilage in mammalian joints. Piles of 11 bilayer membranes of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) are immobilized at the interface of the solid silicon support and the liquid D2O backing phase. We replace the natural hyaluronic acid (HA) component of synovial fluid by a synthetic substitute, namely, poly(allylamine hydrochloride) (PAH), at identical concentration. We find the oligolamellar DMPC bilayer films strongly interacting with PAH resulting in a drastic increase of the membranes d spacing (by a factor of ∼5). Onset of shear causes a buckling-like deformation of the DMPC bilayers perpendicular to the applied shear field. With increasing shear rate we observe substantially enhanced water fractions in the membrane slabs which we attribute to increasing fragmentation caused by Kelvin-Helmholtz-like instabilities parallel to the applied shear field. Both effects are in line with recent theoretical predictions on shear-induced instabilities of lipid bilayer membranes in water (Hanasaki, I.; Walther, J. H.; Kawano, S.; Koumoutsakos, P. Phys. Rev. E 2010, 82, 051602). With the applied shear the interfacial lipid linings transform from their gel state Pβ' to their fluid state Lα. Although in chain-molten state with reduced bending rigidity the lipid layers do not detach from their solid support. We hold steric bridging of the fragmented lipid bilayer membranes by PAH molecules responsible for the unexpected mechanical stability of the DMPC linings.

Substrate-stress-induced magnetic and nonmagnetic structural correlations in Fe/Si multilayers

Application of a bias voltage can influence the growth kinetics and thereby the stress in a magnetic multilayer. The inherent structural correlations in turn can influence the magnetic domain structures and thereby the overall device performance. Here, prototypical Fe/Si supermirrors are subjected to stress relaxation during the growth of sequential layers by applying a sufficient substrate bias voltage. A change in the coercivity associated with the grain size variation upon biasing is found. Most interestingly, using polarized neutron scattering, it is possible to identify that the conformal roughness becomes nonconformal with the relaxation of stress within the multilayers. The magnetic domains, on the other hand, always remain nonconformal (independent of the structural change) as they undergo spatial fluctuations around a mean magnetization. This study underscores the importance of the substrate biasing in affecting the structural correlation, which is detrimental to the resultant optical (e.g.supermirror) quality.

Poly-acrylic Acid Brushes and Adsorbed Proteins

Planar polyelectrolyte brushes are prepared by Langmuir–Schaefer based grafting of perdeuterated (styrene)49-b-(acrylic acid)222 block copolymer (dPS-b-PAA) to dPS pre-coated silicon supports with grafting density σ PAA from 0.07 to 0.11 nm –2. The structure of the solvent-swollen brushes, i. e. the volume fraction profile of polymer segments, ϕ PAA, as a function of altitude z from the grafting plane into the liquid phase is extracted from neutron reflectivity measurements. We find that for all cases investigated ϕ PAA(z) resembles a Gaussian profile. Although very condensed, the PAA brushes can be loaded with bovine serum albumin (BSA). The integral amount of adsorbed BSA scales linearly with grafting density. We compare our z-resolved volume fraction profile ϕ BSA(z) of BSA on PAA brushes with existing literature on that system. It is found that a cross-over takes place in the adsorption scheme from ternary compressive, where proteins can approach the grafting surface only by compressing the brush, to ternary insertive, where proteins enter the brush with only local perturbation of the concentration profile, as a function of R P/H max, where R P is the Stokes-Radius of the protein, and H max is the experimentally determined maximum height of the brush.

Bilayer undulation dynamics in unilamellar phospholipid vesicles: effect of temperature, cholesterol and trehalose

We report a combined dynamic light scattering (DLS) and neutron spin-echo (NSE) study on the local bilayer undulation dynamics of phospholipid vesicles composed of 1,2-dimyristoyl-glycero-3-phosphatidylcholine (DMPC) under the influence of temperature and the additives cholesterol and trehalose. The additives affect vesicle size and self-diffusion. Mechanical properties of the membrane and corresponding bilayer undulations are tuned by changing lipid headgroup or acyl chain properties through temperature or composition. On the local length scale, changes at the lipid headgroup influence the bilayer bending rigidity κ less than changes at the lipid acyl chain: We observe a bilayer softening around the main phase transition temperature Tm of the single lipid system, and stiffening when more cholesterol is added, in concordance with literature. Surprisingly, no effect on the mechanical properties of the vesicles is observed upon the addition of trehalose.

About different types of water in swollen polyelectrolyte multilayers

The review addresses swelling of polyelectrolyte multilayers in water. Different models for the determination of the water content are compared. It is clearly shown that voids under dry conditions present cavities for water which contribute to the water content of the multilayer in the swollen state. This so-called "void water" does not lead to any changes in thickness but in scattering length density during swelling. The "swelling water" leads to both changes in scattering length density and in thickness. Depending on the preparation conditions like the type polymers, polymer charge density, ionic strength and type of salt the ratio of "void water" differs between 1 and 15 vol.% while the amount of "swelling water" is of several ten's of vol.%.

Pressure-induced protein adsorption at aqueous-solid interfaces

There seems to be a general relation between the standard Gibbs energy change of unfolding, ΔG°unf, of a protein and its affinity to aqueous-solid interfaces. So-called "hard" proteins (ΔG°unf is large) are found to adsorb less strongly to such interfaces than "soft" proteins (ΔG°unf is small). Here, we provide direct support for this rule by using high pressure to modulate the folding stability of a protein. We have performed high-pressure total internal reflection fluorescence (HP-TIRF) spectroscopy and high-pressure neutron reflectometry (HP-NR) to measure the degree of adsorption and the structure of lysozyme on planar solid surfaces as a function of pressure for the first time. By carrying out these experiments at hydrophilic and hydrophobic surfaces with varying concentrations of glycerol, we have found strong evidence that ΔG°unf has indeed a direct influence. At high pressures, there is a larger degree of lysozyme adsorption, probably because lysozyme becomes a "soft" protein under these conditions. The results of this study demonstrate that high pressure is a very useful tool to explore thermodynamics of protein-interface interactions.

Reduced protein adsorption by osmolytes

Osmolytes are substances that affect osmosis and are used by cells to adapt to environmental stress. Here, we report a neutron reflectivity study on the influence of some osmolytes on protein adsorption at solid-liquid interfaces. Bovine ribonuclease A (RNase) and bovine insulin were used as model proteins adsorbing at a hydrophilic silica and at a hydrophobic polystyrene surface. From the neutron reflectivity data, the adsorbed protein layers were characterized in terms of layer thickness, protein packing density, and adsorbed protein mass in the absence and presence of urea, trehalose, sucrose, and glycerol. All data point to the clear effect of these nonionic cosolvents on the degree of protein adsorption. For example, 1 M sucrose leads to a reduction of the adsorbed amount of RNase by 39% on a silica surface and by 71% on a polystyrene surface. Trehalose was found to exhibit activity similar to that of sucrose. The changes in adsorbed protein mass can be attributed to a decreased packing density of the proteins in the adsorbed layers. Moreover, we investigated insulin adsorption at a hydrophobic surface in the absence and presence of glycerol. The degree of insulin adsorption is decreased by even 80% in the presence of 4 M of glycerol. The results of this study demonstrate that nonionic cosolvents can be used to tune and control nonspecific protein adsorption at aqueous-solid interfaces, which might be relevant for biomedical applications.

Surfactant adsorption at the metal-oil interface

The structure of the adsorbed palmitic acid at the iron oxide/oil interface has been investigated using polarized neutron reflectometry. The palmitic acid was found to be strongly adsorbed at the oxide/oil interface resulting in a monolayer of thickness 16 ± 4 Å for 150 and 500 ppm palmitic acid concentrations (16 ± 5 Å for the 1000 ppm solution). These layer thicknesses suggest tilt for the palmitic acid molecules with respect to the interface. The model also requires a second diffuse layer extending in the bulk oil. The thickness of this diffuse layer was 35 ± 17 Å for the 150 ppm solution and 45 ± 22 Å for 500 and 1000 ppm solution. The composition profiles at the interface suggest a depletion of the oil in the vicinity of the interface as the concentration of palmitic acid increases.

BioRef: a versatile time-of-flight reflectometer for soft matter applications at Helmholtz-Zentrum Berlin

BioRef is a versatile novel time-of-flight reflectometer featuring a sample environment for in situ infrared spectroscopy at the reactor neutron source BER II of the Helmholtz Zentrum Berlin für Materialien und Energie (HZB). After two years of design and construction phase the instrument has recently undergone commissioning and is now available for specular and off-specular neutron reflectivity measurements. BioRef is especially dedicated to the investigation of soft matter systems and studies at the solid-liquid interface. Due to flexible resolution modes and variable addressable wavelength bands that allow for focusing onto a selected scattering vector range, BioRef enables a broad range of surface and interface investigations and even kinetic studies with subsecond time resolution. The instrumental settings can be tailored to the specific requirements of a wide range of applications. The performance is demonstrated by several reference measurements, and the unique option of in situ on-board infrared spectroscopy is illustrated by the example of a phase transition study in a lipid multilayer film.

Simultaneous polarized neutron reflectometry and anisotropic magnetoresistance measurements

A novel experimental facility to carry out simultaneous polarized neutron reflectometry (PNR) and anisotropic magnetoresistance (AMR) measurements is presented. Performing both techniques at the same time increases their strength considerably. The proof of concept of this method is demonstrated on a CoO/Co bilayer exchange bias system. Although information on the same phenomena, such as the coercivity or the reversal mechanism, can be separately obtained from either of these techniques, the simultaneous application optimizes the consistency between both. In this way, possible differences in experimental conditions, such as applied magnetic field amplitude and orientation, sample temperature, magnetic history, etc., can be ruled out. Consequently, only differences in the fundamental sensitivities of the techniques can cause discrepancies in the interpretation between the two. The almost instantaneous information obtained from AMR can be used to reveal time-dependent effects during the PNR acquisition. Moreover, the information inferred from the AMR measurements can be used for optimizing the experimental conditions for the PNR measurements in a more efficient way than with the PNR measurements alone.

High pressure cell for neutron reflectivity measurements up to 2500 bar

The design of a high pressure (HP) cell for neutron reflectivity experiments is described. The cell can be used to study solid-liquid interfaces under pressures up to 2500 bar (250 MPa). The sample interface is based on a thick silicon block with an area of about 14 cm(2). This area is in contact with the sample solution which has a volume of only 6 cm(3). The sample solution is separated from the pressure transmitting medium, water, by a thin flexible polymer membrane. In addition, the HP cell can be temperature-controlled by a water bath in the range 5-75°C. By using an aluminum alloy as window material, the assembled HP cell provides a neutron transmission as high as 41%. The maximum angle of incidence that can be used in reflectivity experiments is 7.5°. The large accessible pressure range and the low required volume of the sample solution make this HP cell highly suitable for studying pressure-induced structural changes of interfacial proteins, supported lipid membranes, and, in general, biomolecular systems that are available in small quantities, only. To illustrate the performance of the HP cell, we present neutron reflectivity data of a protein adsorbate under high pressure and a lipid film which undergoes several phase transitions upon pressurization.

Pressure cell for investigations of solid-liquid interfaces by neutron reflectivity

We describe an apparatus for measuring scattering length density and structure of molecular layers at planar solid-liquid interfaces under high hydrostatic pressure conditions. The device is designed for in situ characterizations utilizing neutron reflectometry in the pressure range 0.1-100 MPa at temperatures between 5 and 60 °C. The pressure cell is constructed such that stratified molecular layers on crystalline substrates of silicon, quartz, or sapphire with a surface area of 28 cm(2) can be investigated against noncorrosive liquid phases. The large substrate surface area enables reflectivity to be measured down to 10(-5) (without background correction) and thus facilitates determination of the scattering length density profile across the interface as a function of applied load. Our current interest is on the stability of oligolamellar lipid coatings on silicon surfaces against aqueous phases as a function of applied hydrostatic pressure and temperature but the device can also be employed to probe the structure of any other solid-liquid interface.

Interaction of IAPP and insulin with model interfaces studied using neutron reflectometry

The islet amyloid polypeptide (IAPP) and insulin are coproduced by the beta-cells of the pancreatic islets of Langerhans. Both peptides can interact with negatively charged lipid membranes. The positively charged islet amyloid polypeptide partially inserts into these membranes and subsequently forms amyloid fibrils. The amyloid fibril formation of insulin is also accelerated by the presence of negatively charged lipids, although insulin has a negative net charge at neutral pH-values. We used water-polymer model interfaces to differentiate between the hydrophobic and electrostatic interactions that can drive these peptides to adsorb at an interface. By applying neutron reflectometry, the scattering-length density profiles of IAPP and insulin, as adsorbed at three different water-polymer interfaces, were determined. The islet amyloid polypeptide most strongly adsorbed at a hydrophobic poly-(styrene) surface, whereas at a hydrophilic, negatively charged poly-(styrene sulfonate) interface, the degree of adsorption was reduced by 50%. Almost no IAPP adsorption was evident at this negatively charged interface when we added 100 mM NaCl. On the other hand, negatively charged insulin was most strongly attracted to a hydrophilic, negatively charged interface. Our results suggest that IAPP is strongly attracted to a hydrophobic surface, whereas the few positive charges of IAPP cannot warrant a permanent immobilization of IAPP at a hydrophilic, negatively charged surface at an ionic strength of 100 mM. Furthermore, the interfacial accumulation of insulin at a hydrophilic, negatively charged surface may represent a favorable precondition for nucleus formation and fibril formation.

Shear induced relaxation of polymer micelles at the solid-liquid interface

A 20% aqueous solution of (ethylene oxide) 99-(propylene oxide) 65-(ethylene oxide) 99, F127, was investigated by combining rheology in a cone/plate-geometry and surface-sensitive grazing incident neutron scattering. The crystalline structure formed by the polymer micelles becomes less pronounced for low shear rates, but correlations increase for higher shear rates. After stopping shear a slow relaxation of the micelles is found in the vicinity (50 mum thick layer) of a hydrophilic silicon wall (strong micelle-wall interaction), while a fast relaxation is observed in the boundary layer against the hydrophobic silicon wall (weak micelle-wall interaction). The results show that in the vicinity of the interface wall-particle interactions compete heavily with the shear force acting on the liquid.

Internal structure of a thin film of mixed polymeric micelles on a solid/liquid interface

The adsorption of mixed micelles of poly(4-(2-amino hydrochloride-ethylthio)-butylene)- block-poly(ethylene oxide), PAETB 49- b-PEO 212 and poly(4-(2-sodium carboxylate-ethylthio)-butylene)- block-poly(ethylene oxide), PCETB 47- b-PEO 212 on solid/liquid interfaces has been studied with light, X-ray, and neutron reflectometry. The structure of the adsorbed layer can be described with a two-layer model consisting of an inner layer formed by the coacervate of the polyelectrolyte blocks PAETB 49 and PCETB 47 ( approximately 1 nm) and an outer layer of PEO 212 blocks ( approximately 6 nm). The micelles unfold upon adsorption forming a rather homogeneous flat layer that exposes its polyethylene oxide chains into the solution, thus rendering the surface antifouling after modification with the micelles.

Laminar order within Langmuir-Blodgett multilayers from phospholipid and myelin basic protein: a neutron reflectivity study

Multilayers consisting of negatively charged phospholipid DMPA and myelin basic protein (MBP) were assembled by Langmuir-Blodgett deposition of floating Langmuir monolayers from the air/water interface to solid substrates. Protein/lipid samples were obtained by binding MBP from the aqueous subphase to the phospholipid monolayers before deposition. The vertical organization of these model membranes (i.e., with organization perpendicular to the substrate surface) was investigated in detail by neutron reflectivity measurements, and the internal distribution of water molecules was determined from the change of contrast after in-situ H2O/D2O exchange. The multilayers were well ordered, with repeating lipid bilayers as fundamental structural unit. MBP was inserted in between adjacent lipid headgroups, such as in the natural myelin membrane. Water molecules in the multilayers were present mainly in the lipid headgroup and protein slab. On exposition of the pure lipid multilayers to a dry atmosphere, a reduction of the bilayer spacing was determined, whereas the global lamellar order was not affected. In contrast, drying of the protein/lipid multilayers induced degradation of the laminar order. The data demonstrate that ordered Langmuir-Blodgett multilayers are versatile model systems for studying how competing interactions between lipid, protein, water, and ions affect the global organization of such multilamellar lipid/protein assemblies. Here, the water molecules were found to be a necessary mediator to maintain the laminar order in a multilayer from DMPA and myelin basic protein.

The Influence of Secondary Interactions during the Formation of Polyelectrolyte Multilayers: Layer Thickness, Bound Water and Layer Interpenetration

With X-ray and neutron reflectivity, the structure and composition of polyelectrolyte multilayers from poly(allyl amine) (PAH) and poly(styrene sulfonate) (PSS) are studied as function of preparation conditions (salt concentration and solution temperature, T). The onset of a temperature effect occurs at 0.05 M NaCl (Debye length approximately 1 nm). At 1 M salt, the film thickness increases by a factor of 3 on heating the deposition solution from 5 to 60 degrees C. The PAH/PSS bilayer thickness is independent of the kind of salt (NaCl or KCl), yet its composition is different (more bound water for NaCl). At low T, the internal roughness is 33% of the bilayer thickness; it increases to 60% at high T. The roughening is accompanied by a total loss of bound water. At which temperature the roughening starts is a function of the kind of salt (50 degrees C for NaCl and 35 degrees C for KCl). The strong temperature dependence and the eventual loss of bound water molecules may be attributed to the hydrophobic force; however, there is an isotope effect, since the loss of bound water is less pronounced in the deuterated layers.

Density depletion at solid-liquid interfaces: a neutron reflectivity study

Neutron reflectivity experiments conducted on self-assembled monolayers (SAMs) against polar (water) and nonpolar (organic) liquid phases reveal further evidence for a density reduction at hydrophobic-hydrophilic interfaces. The density depletion is found at the interface between hydrophobic dodecanethiol (C12) and hexadecanethiol (C16) SAMs and water and also between hydrophilic SAMs (C12/C11OH) and nonpolar fluids. The results show that the density deficit of a fluid in the boundary layer is not unique to aqueous solid-liquid interfaces but is more general and correlated with the affinity of the liquid to the solid surface. In water the variation of pH has only minor influence, while different electrolytes taken from the Hofmeister series seem to increase the depletion. On hydrophobic SAMs an increase in density depletion with temperature was observed, in agreement with Monte Carlo simulations performed on corresponding model systems. The increase in the water density depletion layer is governed by two effects: the surface energy difference between water and the substrate and the chemical potential of the aqueous phase.

Thermal stability of poly(o-methoxyaniline) layer-by-layer films investigated by neutron reflectivity and UV-VIS spectroscopy

Neutron reflectivity measurements were used to investigate the thermal stability of layer-by-layer (LBL) films of poly(o-methoxyaniline) (POMA), which was probed by increasing the temperature up to 80 degrees C of a D2O solution in contact with the LBL films. The study was made possible by adsorbing POMA layers on a PEI/(PSS/PAH)5/PSS LBL film template, leading to less rough POMA layers in comparison with the POMA/poly(vinylsulfonic acid sodium salt) (PVS) LBL films adsorbed directly on glass and silicon substrates. While the latter yielded almost fringeless neutron reflectivity curves due to the large roughness, the fitting of the data for POMA films adsorbed onto the template film and UV-vis measurements indicated that the topmost layer is affected for films heated in solution up to 80 degrees C. This is essentially the same thermal stability of LBL films from the template films made with conventional polyelectrolytes. A decrease in thickness of approximately 10 A was inferred when the solution temperature increased from 25 degrees C to 80 degrees C, which was maintained when the sample was cooled back to 25 degrees C. This decrease, observed for solutions of pH 3 and pH 8, is consistent with thermally-stimulated desorption and was corroborated by UV-VIS absorption experiments. The unexpected stability of the POMA layer at pH 8 is attributed to the layer-by-layer structure of the films that allows POMA to remain doped, in its salt emeraldine form, even at high pH.

Reversible Activation of Diblock Copolymer Monolayers at the Interface by pH Modulation, 1: Lateral Chain Density and Conformation

This study focuses on the design of chemically regulated surfaces that allow for reversible control of the interactions between biological matter (cells and proteins) and planar substrates. As a tunable interlayer, we use a monolayer of a near-monodisperse poly[2-(dimethylamino)ethyl methacrylate-block-methyl methacrylate] (PDMAEMA-PMMA) diblock copolymer. Owing to the relatively large fraction (50%) of the hydrophobic PMMA block, this copolymer forms a stable Langmuir monolayer at the air/water interface. Both in situ and ex situ film balance experiments suggest that the hydrophilic PDMAEMA block adsorbs to the air/water interface in its uncharged state (pH 8.5), but stretches into the subphase in its charged state (pH 5.5). Optimization of the preparation protocols enables us to fabricate stable, homogeneous diblock copolymer films on hydrophobized substrates via Langmuir-Schaefer transfer at well-defined lateral chain densities. Ellipsometry and X-ray reflectivity studies of the transferred films confirm that the film thickness can be systematically regulated by the lateral chain densities. The transferred copolymer films remain stable in water for about a week, suggesting that they are promising materials for the creation of pH-controlled solid substrates for the support of biological matter such as proteins and cells.

Reversible Activation of Diblock Copolymer Monolayers at the Interface by pH Modulation, 2: Membrane Interactions at the Solid/Liquid Interface

A monolayer of the pH-responsive poly[2-(dimethylamino)ethyl methacrylate-block-methyl methacrylate] diblock copolymer [PDMAEMA-PMMA] was transferred from the air/water interface to a silicon substrate for evaluation as a tunable interlayer between biological material and solid substrates. Specular neutron reflectivity experiments revealed that the weak polyelectrolyte PDMAEMA chains at the solid/liquid interface can be reversibly activated by pH modulation. The thickness, scattering length density, and surface roughness of the polymer film can be systematically controlled by pH titration. As a simple model of plasma membranes, a lipid bilayer was deposited onto the polymer film. The membrane-substrate interaction was characterized by neutron reflectivity experiments, demonstrating that the membrane-substrate distance could be reversibly regulated by pH titration. These results confirm the potential of stimuli-responsive polymers for precise control of cell-surface interactions.

Neutron reflectivity studies of critical adsorption: behavior of the surface scaling function

Neutron reflectometry has been employed to examine the nature of the critical adsorption surface scaling function for a near-critical mixture of hexane-d14+perfluorohexane adsorbing to a solid substrate from the liquid one-phase region. The analysis method of Dietrich and Schack has been applied to examine the nature of the power-law part of the critical adsorption surface scaling function, which has been found to behave as m(z) approximately P0z(-mu) as the critical point is approached. Values of mu = 0.514+/-0.018 and P0 = 0.90+/-0.04 have been obtained. These values are consistent with theoretical expectations (mu(th) = 0.516+/-0.004; P0(th) = 0.94+/-0.05), the value determined from Monte Carlo simulations (P(MC)0 = 0.866, and other experimental determinations (P(ex)0 = 0.955=/-0.08).