Mayans, E.; Casanovas Salas, Jordi; Gil, A.; Jimenez, A.; Cativiela, C.; Puiggali, J.; Aleman, C. Langmuir Vol. 33, p. 1-13 DOI: 10.1021/acs.langmuir.7b00622 Data de publicació: 2017-04-25 Article en revista
Microstructures from small phenylalanine-based peptides have attracted great attention lately because these compounds are considered to be a new class of tunable materials. In spite of the extensive studies on uncapped diphenylalanine and tetraphenylalanine peptides, studies on the self-assembly of uncapped triphenylananine (FFF) are very scarce and nonsystematic. In this work, we demonstrate that FFF assemblies can organize in a wide number of well-defined supramolecular structures, which include laminated helical-ribbons, leaflike dendrimers, doughnut-, needle-, and flower-shapes. These organizations are produced by the attractive or repulsive interactions between already formed assemblies and therefore can be controlled through the choice of solvents used as the incubation medium. Thus, the formation of the desired supramolecular structures is regulated through the protonation/deprotonation of the terminal groups, the polarity of the incubation medium, which affects both peptide···solvent interactions and the cavity solvation energy (i.e., solvent···solvent interactions), and the steric interactions between own assemblies that act as building blocks. Finally, the ß-sheet disposition in the latter structural motifs has been examined using both theoretical calculations and Fourier transform infrared spectroscopy. Results indicate that FFF molecules can adopt both parallel and antiparallel ß-sheets. However, the former one is the most energetically favored because of the formation of p–p stacking interactions between the aromatic rings of hydrogen-bonded strands.
Reverse osmosis and nanofiltration (NF) employ composite membranes whose ultrathin barrier layers are significantly more permeable to water than to salts. Although solution-diffusion models of salt transport through barrier layers typically assume ubiquitous electroneutrality, in the case of ultrathin selective skins and low ion partition coefficients, space-charge regions may occupy a significant fraction of the membrane barrier layer. This work investigates the implications of these deviations from electroneutrality on salt transport. Both immobile external surface charge and unequal cation and anion solvation energies in the barrier layer lead to regions with excess mobile charge, and the size of these regions increases with decreasing values of either feed concentrations or ion partition coefficients. Moreover, the low concentration of the more excluded ion in the space-charge region can greatly increase resistance to salt transport to enhance salt rejection during NF. These effects are especially pronounced for membranes with a fixed external surface charge density whose sign is the same as that of the more excluded ion in a salt. Because of the space-charge regions, the barrier-layer resistance to salt transport initially rises rapidly with increasing barrier thickness and then plateaus or even declines within a certain thickness range. This trend in resistance implies that thin, defect-free barrier layers will exhibit higher salt rejections than thicker layers during NF at a fixed transmembrane pressure. Deviations from electroneutrality are consistent with both changes in NF salt rejections that occur upon changing the sign of the membrane fixed external surface charge, and CaCl2 rejections that in some cases may first decrease, then increase and then decrease again with increasing CaCl2 concentrations in NF feed solutions.
The development of suitable biomimetic scaffolds is a fundamental requirement of tissue engineering. Although electrospinning has emerged as an effective method for producing such scaffolds of nanometer-sized fibers, the influence of solution characteristics on the morphology of the resulting nanofibers depends on each polymer solution system. In this study, gelatin nanofibers and microfibers were prepared via electrospinning using mixtures of water and acetic acid at different ratios as solvents. The viscosities of gelatin solutions before electrospinning were analyzed and two different behaviors were found as a function of the solvent composition, taking into account classic models of polymer science. A power law relationship between viscosity and gelatin concentration was found for each solvent system, and an empirical model including the influence of acetic acid was obtained for aqueous systems. Moreover, a ternary diagram considering gelatin, water, and acetic acid mass fractions was constructed as a tool to establish the electrospinnability domains in terms of fiber occurrence and morphology. Also, the isodiametric curves were defined in the fibers region. Finally, in order to correlate the diameter of electrospun nanofibers and the electrospinnability zones, the Berry number was used. However, as its only allows the range of electrospinnability to be established for a fixed solvent composition, a new dimensionless parameter (Bemod) was suggested to take into account all the acetic acid aqueous solutions as a single solvent
Membranes composed of multilayer poly(4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) films on porous alumina supports exhibit high monovalent/divalent cation selectivities. Remarkably, the diffusion dialysis K(+)/Mg(2+) selectivity is >350. However, in nanofiltration this selectivity is only 16, suggesting some convective ion transport through film imperfections. Under MgCl(2) concentration gradients across either (PSS/PAH)(4)- or (PSS/PAH)(4)PSS-coated alumina, transmembrane potentials indicate Mg(2+) transference numbers approaching 0. The low Mg(2+) transference numbers with both polycation- and polyanion-terminated films likely stem from exclusion of Mg(2+) due to its large size or hydration energy. However, these high anion/cation selectivities decrease as the solution ionic strength increases. In nanofiltration, the high asymmetry of membrane permeabilities to Mg(2+) and Cl(-) creates transmembrane diffusion potentials that lead to negative rejections (the ion concentration in the permeate is larger than in the feed) as low as -200% for trace monovalent cations such as K(+) and Cs(+). Moreover, rejection becomes more negative as the mobility of the trace cation increases. Knowledge of single-ion permeabilities is vital for predicting the performance of polyelectrolyte films in the separation and purification of mixed salts.
Membranes composed of multilayer poly(4-styrenesul-
fonate) (PSS)/protonated poly(allylamine) (PAH)
lms on porous
alumina supports exhibit high monovalent/divalent cation selectivities.
Remarkably, the di
usion dialysis K
selectivity is >350.
However, in nano
ltration this selectivity is only 16, suggesting
some convective ion transport through
lm imperfections. Under
concentration gradients across either (PSS/PAH)
- or (PSS/
PSS-coated alumina, transmembrane potentials indicate Mg
transference numbers approaching 0. The low Mg
numbers with both polycation- and polyanion-terminated
stem from exclusion of Mg
due to its large size or hydration energy.
However, these high anion/cation selectivities decrease as the solution
ionic strength increases. In nano
ltration, the high asymmetry of
membrane permeabilities to Mg
creates transmembrane di
usion potentials that lead to negative rejections (the ion
concentration in the permeate is larger than in the feed) as low as
200% for trace monovalent cations such as K
Moreover, rejection becomes more negative as the mobility of the trace cation increases. Knowledge of single-ion permeabilities
is vital for predicting the performance of polyelectrolyte
lms in the separation and puri
cation of mixed salts.
Selective ion exclusion from charged nanopores in track-etched membranes allows separation of ions with different charges or mobilities. This study examines pressure-driven transport of dissolved ions through track-etched membranes modified by adsorption of poly(styrene sulfonate) (PSS)/protonated poly(allylamine) (PAH) films. For nominal 30 nm pores modified with a single layer of PSS, Br–/SO42– selectivities are ~3.4 with SO42– rejections around 85% due to selective electrostatic exclusion of the divalent anion from the negatively charged pore. Corresponding membranes containing an adsorbed PSS/PAH bilayer are positively charged and exhibit average K+/Mg2+ selectivities >10 at 8 mM ionic strength, and Mg2+ rejections are >97.5% at ionic strengths <5 mM. The high rejection of Mg2+ compared to SO42– likely results from both a smaller pore size after deposition of the PAH layer and higher surface charge because of Mg2+ adsorption. Simultaneous modeling of K+ and Mg2+ rejections using the nonlinearized Poisson–Boltzmann equation gives an average modified pore diameter of 8.4 ± 2.1 nm, which does not vary significantly with ionic strength. This diameter is smaller than that calculated from hydraulic permeabilities and estimated pore densities, suggesting that narrow regions near the pore entrance control ion transport. In addition to simple electrostatic exclusion, streaming potentials lead to differing rejections of Br– and acetate in PSS/PAH-modified pores, and of Li+ and Cs+ in PSS-modified pores. For these cases, electrical migration of ions toward the feed solution results in higher rejection of the more mobile ion
This manuscript describes the synthesis (based on the intermatrix
synthesis (IMS) method), optimization, and application to bacterial
disinfection of Ag@Co polymer metal nanocomposite materials with magnetic
and bactericidal properties. This material showed ideal bactericide
features for being applied to bacterial disinfection of water, particularly (1)
an enhanced bactericidal activity (when compared with other nanocomposites
only containing Ag or Co nanoparticles), with a cell viability close to 0% for
bacterial suspensions with an initial concentration below 105 colony forming
units per milliliter (CFU/mL) after a single pass through the material, (2)
capacity of killing a wide range of bacterial types (from coliforms to Grampositive
bacteria), and (3) a long performance-time, with an efficiency of
100% (0% viability) up to 1 h of operation and higher than 90% during the first
24 h of continuous operation. The nanocomposite also showed a good
performance when applied to water samples from natural sources with more complex matrices with efficiencies always higher
Mismatches in electrokinetic properties between micro- and nanochannels give rise to superposition of electroosmotic and pressure-driven flows in the microchannels. Parabolic
or similar flow profiles are known to cause the so-called hydrodynamic dispersion, which under certain conditions can be formally assimilated to an increase in the solute diffusivity (Taylor-Aris model). It is demonstrated theoretically that taking into account these phenomena modifies considerably
the pattern of current-induced concentration polarization of micro/nanointerfaces as compared to the classical model of unstirred
boundary layer. In particular, the hydrodynamic dispersion leads to disappearance of limiting current. At essentially "over-limiting" current densities, the time-dependent profiles of salt concentration in microchannels behave like sharp concentration "fronts" moving away from the interface until they reach the reservoir end of the microchannel. Under galvanostatic conditions postulated in
this study, these "fronts" move with practically constant speed directly proportional to the current density. The sharp transition from a low-concentration to a high-concentration zone can be useful for the analyte preconcentration via stacking. The pattern of moving sharp concentration "fronts" has been predicted for the first time for relatively broad microchannels with negligible surface conductance. The Taylor-Aris approach to the description of hydrodynamic dispersion is quantitatively applicable only to the analysis of sufficiently "slow" processes (as compared to the characteristic time of diffusion relaxation in the transversal direction). A
posteriori estimates reveal that the condition of "slow" processes is typically not satisfied close to current-polarized micro/nanointerfaces. Accordingly, to make the description quantitative, one needs to go beyond the Taylor-Aris approximation, which will be attempted in future studies. It is argued that doing so would make even stronger the dampening impact of hydrodynamic
dispersion on the current-induced concentration polarization of micro/nanointerfaces.
Experiments showing an increase in the wettability of a hydrophobic surface when using corona air ionization are shown. Photoluminiscence observations support the predictions of charge accumulation at the triple line and confirm previous experiments. In all of the experiments, the contact angle was in the saturation regime at a value smaller than that predicted by the
condition of a zero value for the solid-liquid surface tension. The PDMS did not show any deterioration due to the corona exposure under the experimental conditions used. The contact angle is shown to increase with humidity.
It is shown that in tangential electrokinetic measurements with porous films the porous structure makes contribution
not only to the cell electric conductance (as demonstrated previously) but also to the observed streaming current. Both of these contributions give rise to dependences of streaming-potential and streaming-current coefficients on the channel height. However, due to the combined contribution of two phenomena, the dependence of streaming-potential coefficient on the channel height may be rather complicated and not allow for simple extrapolation. At the same time,
the dependences of streaming-current coefficient and cell electric conductance on the channel height turn out linear and can be easily extrapolated to zero channel heights. This enables one to determine separately the contributions of external
surface of porous film and of its porous structure to the streaming current and of the channel and porous structure to the
cell electric conductance. This procedure is illustrated by the measurements of tangential electrokinetic phenomena and
electric conductance with Millipore mixed-cellulose membrane filters of various average pore sizes (from 0.025 to 5 μm)
in the so-called adjustable-gap cell of SurPASS electrokinetic instrument (Anton Paar GmbH). The design of this cell allows for easy and quasi-continuous variation of channel height as well as accurate determination of cell electric conductance, streaming-current coefficient, and channel height (from the cell hydraulic permeability). The quality of linear fits of experimental data has been found to be very good, and thus, the extrapolation procedures were quite reliable and accurate. Zeta-potentials could be determined of both external film and internal pore surfaces. It is demonstrated that the porous structures make considerable contributions to both streaming-current coefficient and cell electric conductance especially in the case of filters with larger pores. It is also found that, rather surprisingly, in filters with smaller pores the reduction in the filter electric conductivity turns out essentially stronger than could be expected proceeding from the filter porosity.