On the Potential of Using Dual-Function Hydrogels for Brackish Water Desalination.

Although current desalination technologies are mature enough and advanced, the shortage of freshwater is still considered as one of the most pressing global issues. Therefore, there is a strong incentive to explore and investigate new potential methods with low energy consumption. We have previously reported that reversible thermally induced sorption/desorption process using polymeric hydrogels hold promise for water desalination with further development. In order to develop a more effective hydrogels architecture, polyelectrolyte moieties were introduced in this work as pendent chains and a thermally responsive polymer as network backbone using reversible addition-fragmentation chain transfer (RAFT) polymerisation. The ability of the comb-type polymeric hydrogels to desalinate water was evaluated. These hydrogels were proved to absorb water with low salinity from brine solution of 2 g L - 1 NaCl and release the absorbed water at relatively low temperature conditions of 50 ∘ C. The fraction of the grafted polyacrylic acid and the comb-chain length were varied to understand their influence on the swelling/deswelling behaviour for these hydrogels. The ionic fraction in the hydrogels and the resulting hydrophilic/hydrophobic balance are crucial for the proposed desalination process. In contrast, the comb-chain length impacted the swelling behaviour of hydrogels but showed relatively little influence on the dewatering process.

Design of Thermally Responsive Polymeric Hydrogels for Brackish Water Desalination: Effect of Architecture on Swelling, Deswelling, and Salt Rejection.

In this work, we explore the ability of utilizing hydrogels synthesized from a temperature-sensitive polymer and a polyelectrolyte to desalinate salt water by means of reversible thermally induced absorption and desorption. Thus, the influence of the macromolecular architecture on the swelling/deswelling behavior for such hydrogels was investigated by tailor-made network structures. To this end, a series of chemically cross-linked polymeric hydrogels were synthesized via free radical-initiated copolymerization of sodium acrylate (SA) with the thermoresponsive comonomer N-isopropylacrylamide (NIPAAm) by realizing different structural types. In particular, two different polyNIPAAm macromonomers, either with one acrylate function at the chain end or with additional acrylate functions as side groups were synthesized by controlled polymerization and subsequent polymer-analogous reaction and then used as building blocks. The rheological behaviors of hydrogels and their estimated mesh sizes are discussed. The performance of the hydrogels in terms of swelling and deswelling in both deionized water (DI) and brackish water (2 g/L NaCl) was measured as a function of cross-linking degree and particle size. The salt content could be reduced by 23% in one cycle by using the best performing material.

Effect of the degree of dissociation of molecules in a monolayer at an air/water interface on the force between the monolayer and a like-charged particle in the subphase.

We used the monolayer particle interaction apparatus to measure the force between a monolayer of stearic acid or octadecanol at the air/water interface and a colloidal silica sphere. The silica sphere approached the monolayer from the aqueous subphase. The aim was to analyze how the magnitude of the charge of a deformable interface affects the interaction between that interface and a like-charged hard particle. The charge density of the stearic acid monolayer was controlled by adjusting the pH (5.8-9) and the surface pressure. The octadecanol monolayer acted as a reference; the alcohol headgroup did not dissociate between pH 5.8-9.0. Stable monolayers of dissociated stearic acid molecules were formed at the air/water interface by dissolving stearic acid into the subphase to give a saturated concentration at each pH value studied. The approach force curve showed that the electrostatic repulsion increased with an increasing degree of dissociation and therefore the charge of the monolayer. The strength of the repulsion corresponded to that measured between two like-charged hard surfaces, but the apparent range of the repulsion was larger for a deformable interface. Retracting force curves displayed a significant adhesion, whose magnitude and range depended on the surface pressure and subphase pH.

Interfacial forces between a silica particle and phosphatidylcholine monolayers at the air-water interface.

Interfacial forces between a silica or borosilicate particle in water and phospholipids at the air-water interface were studied using the Monolayer Particle Interaction Apparatus. This instrument allowed the forces to be measured as the colloidal probe approached the monolayer from the liquid phase. The proper working principle of this setup was demonstrated by measuring the forces between a particle and a mica plate in 0.1 mM NaCl. The effect of the alkyl chain length on the adhesion between the particle and the monolayer was investigated using four different 1,2-dialkyl-sn-glycero-3-phosphocholines (DMPC, DPPC, DSPC, and DBPC), which had 14, 16, 18, and 22 carbon atoms per alkyl chain, respectively. The adhesion force increased with the square of the particle radius. The lipids in the liquid-expanded (LE) phase showed an attraction to the particle, explained by an electrostatic attraction and/or the formation of a three-phase contact line that lead to a capillary force. All monolayers showed an adhesion in their retract force curve, which decreased with an increased chain length and surface pressure. Interfacial stiffness was generally seen to increase with the phospholipid chain length and to decrease with surface pressure, explained by an increase in the intermolecular van der Waals interaction and a decrease in the interfacial tension, respectively. The adhesion between the particle and monolayer was concluded to be controlled by the contact area between the particle and monolayer, and therefore the monolayer stiffness and the electrostatic interactions.

Evaporation dynamics of microdroplets on self-assembled monolayers of dialkyl disulfides.

We present a study of the static wettability and evaporation dynamics of sessile microdroplets of water on self-assembled monolayers (SAMs) prepared with unsymmetric dialkyl disulfides CH(3)-(CH(2))(11+m)-S-S-(CH(2))(11)-OH (m = 0, +/- 2, +/- 4, +/- 6) on gold-covered mica. The advancing and receding contact angles decrease linearly with increasing hydrophilicity of the SAM. The latter was changed either via the molar ratio or via the chain length of the hydroxyl-terminated alkyl chains in the monolayer. In contrast to SAMs made of thiols, the contact angle hysteresis was 10 degrees for all disulfides, irrespective of their chain lengths. During evaporation of single droplets, a transition from pinning to constant contact angle mode was observed. The transition time between the modes increases with the surface hydrophilicity, leading to longer pinning. This way, the time for complete droplet evaporation decreases by approximately 30% owing to the fact that during pinning the overall droplet area stays large for a longer time. For single droplets the measured total evaporation times agree well with the calculated ones, showing the validity of the standard evaporation model for both evaporation modes. In contrast to the results for single droplets, many droplets with different initial volumes show a power-law dependence on the total evaporation time with an exponent different from 1.5 as expected from the standard model. For disulfides with m not equal 0, the exponent is in the range of 1.40-1.47 increasing with the surface hydrophilicity. For the SAMs with m = 0 the exponent increases up to 1.61 for the most hydrophilic surface. We explain this deviation from the standard evaporation model with the presence of a liquid precursor film around the droplet, which either enhances or decelerates evaporation. Our results suggest that SAMs of dialkyl disulfides offer the possibility to tune the wettability of gold surfaces in a more controlled way than thiols do.

Microstructures formation by deposition of toluene drops on polystyrene surface.

Here we develop a new approach for producing diverse microstructures by deposition of nano-litre solvent droplets onto polymer surface, which is based on a syringe system coupling with an adjustable substrate stage. Two basic procedures, contact mode and non-contact mode, are used for providing either the sessile drop or the pendent drop. In the contact mode, the influences of process parameters and intrinsic parameters are extensively investigated. By varying the process parameters such as the substrate-approaching and retraction speeds and the delay between these two movements, the microstructures can be tuned from concave to convex, whereas by varying intrinsic parameters such as the initial drop volume,V-shaped and U-shaped concave structures can typically be generated. The drying of these microstructures can partially remove entrapped solvent from the swollen polymer material, and the removed solvent volume is found to correspond to the width of microstructures. However, the shapes of these microstructures show no changes before and after drying. Ripples perpendicular to the stretch direction appear in the center of the microstructures for the stretched polymer substrate and become much more regular with increasing stretch ratio. As a sessile solvent drop is replaced by a pendent solvent drop, a non-contact mode results. Without direct contact between the solvent and the polymer substrate, solvent vapor diffuses from the droplet surface into the polymer matrix and eventually provides concave structures. The shape of the concave structures is dependent on the exposure time of the solvent vapor.

Determination of cross-link density in ion-irradiated polystyrene surfaces from rippling.

The irradiation of polymer surfaces with ion beams leads to pronounced chemical and physical modifications when the ions are scattered at the atoms in the polymer chain. In this way, different products of decomposition occur. Here we show that by changing the ion fluence and the mass of the ion the local mechanical properties as Young's modulus of a polystyrene surface layer can be tailored. By annealing prestretched irradiated PS near the glass transition, surface rippling occurs in the irradiated areas only, which can be described with an elastic model. The moduli obtained from rippling periodicities and elastic model assumptions are in the range between 8 and 800 MPa at the glass transition and characterize the irradiated PS as rubberlike. From these values the network density and the molar mass of entanglement are quantified. The obtained network density equals the density of hydrogen vacancies generated through the scattered ions, as confirmed by simulations of the atomic scattering and displacement processes. The obtained molar mass of entanglement reveals that the PS locally was densely cross-linked. Our results show that even for nondiscrete layered polymer systems relevant polymer parameters can be derived from the well-known surface rippling without the need for costly chemical analysis.

Microstructuring of polystyrene surfaces with nonsolvent sessile droplets.

Herein, we study the microstructuring of toluene-vapor-softened polystyrene surfaces with nonsolvent sessile droplets. Arrays of microvessels are obtained by depositing non-evaporating droplets of ethylene glycol/water on the original polystyrene surfaces and subsequently exposing them to saturated toluene vapor. The droplets act as a mask on the polymer, thereby impeding the toluene vapor to diffuse and soften the polystyrene surface below them. Alternatively, the formation of microcraters at random positions--with an average depth-to-width aspect ratio of 0.5 and a diameter as small as 1.5 microm--is achieved by condensing water droplets on a softened polystyrene surface. The cross-sections of the microvessels and the contact angle of the sessile water droplets suggest that the structures are formed by the combined action of the Laplace pressure at the bottom of the droplet and the surface tension acting at the three-phase contact line of the droplets. As a support, the rim height and the depth of the microvessels are fitted with an elastic theory to provide Young's modulus of the softened polystyrene surface.

Protein diffusion across the interface in aqueous two-phase systems.

We present a detailed study of the diffusive transport of proteins across a fluid phase boundary within aqueous two-phase systems. The aim of the work is to investigate whether local effects at the phase boundary cause a retardation of the diffusive transport between the phases. Possible modifications of interfacial mass transfer could be due to protein adsorption at the phase boundary or local electric fields from electric double layers. Experiments with a microfluidic system have been performed in which protein diffusion (bovine serum albumin and ovalbumin) within a bilaminated configuration of two phases containing polyethylene glycol and dextran is analyzed. A one-dimensional model incorporating phase-specific diffusion constants and the difference in chemical potential between the phases has been formulated. A comparison of experimental and simulation data shows a good overall agreement and suggests that a potential local influence of the phase boundary on protein transport is insignificant for the systems under investigation.

Surface stress, thickness, and mass of the first few layers of polyelectrolyte.

The effects of surface stress and mass loading upon the adsorption of polyelectrolytes onto flexible silicon micromechanical cantilever sensors (MCSs) were studied in situ. A self-assembled monolayer of 2-mercaptoethylamine chloride (2-MEA) on gold was used to achieve single-side adsorption on the MCS. Such a preparation gave a positive surface potential, whereas a bare SiOx surface gave a negative surface potential. Wide scan X-ray photoelectron spectroscopy confirmed that the adsorption of polystyrenesulfonate (PSS) and polyallylamine hydrochloride (PAH) followed the general rule expected from the electrostatic interaction between the substrate and the polyelectrolyte, whereas the adsorption polyethyleneimine (PEI) did not. The adsorption of PAH on SiO(x) from a 3 mM water solution containing 1 M NaCl was associated with a deflection of the MCS toward the polyelectrolyte monolayer (tensile surface stress) owing to the hydrogen bonding between neighboring amino groups. Here, a surface stress change of 1.4 +/- 0.1 N/m was estimated. The adsorption of PSS from a 3 mM water solution containing 1 M NaCl on a 2-MEA surface induced a deflection of the MCS away from the polyelectrolyte layer (compressive stress), toward the SiO(x) side. Here, a surface stress change of 3.1 +/- 0.3 N/m was determined. The formation of a PAH layer on top of the PSS layer resulted in a deflection of the MCS toward the PAH layer. This indicated that the adjacent PSS layer was deswelling, corresponding to a surface stress change of 0.5 +/- 0.1 N/m.

Microstructures by solvent drop evaporation on polymer surfaces: dependence on molar mass.

When a solvent drop evaporates from a polymer surface, it leaves behind a characteristic structure, typically a crater. We deposited toluene drops with a microsyringe onto planar polystyrene (PS) surfaces and analyzed the surface topography after drying. For low molar mass PS (Mw = 20.9-24.3 kDa) dotlike protrusions with a ridge at the periphery formed on the polymer surface. With increasing molar mass the central region decreased in height. At Mw = 29.6-643 kDa a craterlike structure with a depression in the center and a ridge was observed. At even higher molar mass, irregular structures without rotational symmetry occurred. We explain the observed dependence on the molar mass with a different degree of entanglement, leading to different dissolution rates and different diffusion constants.

Probing cooperative interactions of tailor-made nucleation surfaces and macromolecules: a bioinspired route to hollow micrometer-sized calcium carbonate particles.

It is well known that the formation of biominerals by living organisms is governed by the cooperation of soluble and insoluble macromolecules with peculiar interfacial properties. To date, most of the studies on mineralization processes involve model systems that account only for the existence of one organic matrix and thus disregard the interaction between the soluble and insoluble organic components that is crucial for a better understanding of the processes taking place at the inorganic-organic interface. We have set up a model system composed of a matrix surface, which is composed of a self-assembled monolayer (SAM) and a soluble component, poly(aspartic acid). It could be demonstrated that the phase selection of calcium carbonate and the morphology of the resulting particles are determined by the stabilization of amorphous precursor particles by the polymer and the interaction between polymer and SAM. The morphology of the hollow vaterite microspheres are reminiscent to a 3D analogue of the so-called "coffee-stain effect", where the transformation from a voluminous hydrated, amorphous material to a more dense crystalline material leads to the formation of hollow spheres from massive spherical microparticles.

N,N'-dimethyl-2,3-dialkylpyrazinium salts as redox-switchable surfactants? Redox, spectral, EPR and surfactant properties.

N,N'-dimethyl-2,3-dialkylpyrazinium salts show reversible transitions between all redox stages and a monolayer formation at the air-water interface that suggests their use as redox-switchable surfactants.

Nanostructuring effect of plasma and solvent treatment on polystyrene.

Plasma treatment of polymer surfaces is used to control the generation of topological surface structures: stripes, starlike morphologies, and pinnacles in the range from 100 nm up to several micrometers. These protrusions arise when the plasma-treated polymer surface is exposed to an organic solvent (liquid or vapor phase). The distribution density and the height of the observed structures on the surface are functions of the power density of the plasma reactor and the exposure time to the plasma, the duration of the development process, the type of the polymer, and its manufacturing. We suggest that the structures are generated by selective swelling of less cross-linked areas within the polymer surface and not by rearrangement or dissolution of polymer chain fragments created by plasma, or by amphiphilic moieties due to oxidation as a consequence of plasma treatment.

Role of lipid interactions in autoimmune demyelination.

A morphological transformation involving loss of adhesion between myelin lamellae and formation of myelin vesicles has been described as a mechanism for demyelination in multiple sclerosis and marmoset experimental allergic encephalomyelitis (EAE). Although protein interactions are involved in maintaining normal myelin structure, we describe here how lipids contribute to myelin stability and how lipid changes in EAE, including increases in lipid polyunsaturation and negatively charged phosphatidylserine (PS), promote demyelination. Three physico-chemical techniques were used to identify these changes: (1) Langmuir monolayer isotherms indicated that EAE white matter lipids were significantly more "expanded" (fluid) than controls. (2) NMR spectroscopy indicated that EAE myelin lipids were more polyunsaturated than controls. (3) High-performance liquid chromatography (HPLC) with an evaporative light scattering detector indicated increased PS in EAE compared to controls, while sphingomyelin (SM), sulfatides and phosphatidylcholine (PC) were decreased. We present a physical model considering electrostatic, van der Waals and undulation forces to quantify the effect of these changes on myelin adhesion at the extracellular interface. Taken together, the isotherm, NMR, HPLC and modeling results support a mechanism for autoimmune demyelination whereby the composition of myelin lipids is altered in a manner that increases myelin fluidity, decreases myelin adhesion, increases membrane curvature, and promotes vesiculation.

Structure of hydroxylated galactocerebrosides from myelin at the air-water interface.

Hydroxy-galactocerebrosides (mixed chain length, constituent of myelin membranes) from bovine brain are investigated as monolayers at the air-water interface with isotherms, fluorescence microscopy, x-ray reflectivity and grazing incidence diffraction. With grazing incidence diffraction a monoclinic tilted chain lattice is found in the condensed phase. According to x-ray reflectivity, the longest chains protrude above the chain lattice and roughen the lipid/air interface. On compressing the chain lattice, the correlation length increases by approximately 65%; obviously, the sugar headgroups are flexible enough to allow for lattice deformation. With fluorescence experiments, small coexisting fluid and ordered domains are observed, and there is lipid dissolution into the subphase as well. The dissolved hydroxy-galactocerebroside molecules reenter on monolayer expansion. The electron density profiles derived from x-ray reflectometry (coherent superposition) show that the chain-ordering transition causes the molecules to grow into the subphase.