Nanosized Water Channels Associated with Hydrophobic and Hydrophilic Fibrillar Arrangements Formed on Nafion Surfaces in Confined Regions.

Herein, the origin of interfacial water nanosized channel distributions attached onto Nafion surfaces is investigated. The surface fibrillary hydrophilic and hydrophobic arrangements were observed on AFM images scanned on Nafion surfaces immersed in water. Then, by analyzing the force vs separation curves, it is possible to map arrays of interfacial water channels and their locations. Nafion surface profiles and the water interfacial patterns are then combined using this AFM technique. As there are no reported experimental techniques to measure water nanochannel cross sections, presented measurements report on their dimensions. Water nanochannels characterized by ε < 7 attached to hydrophilic fibrillary sections form aggregated water domains, a highly organized water structure compared with bulk water. Channels are attached to Nafion surface hydrophilic fibrillary domains in confined sites.

Fibrillary Arrangement of Elongated, Almost Parallel Aggregates of Hydrophobic and Hydrophilic Domains Forming the Nafion Surface Structure Improved Contrast Atomic Force Microscopy Images.

A significant improvement in spatial resolution is reported in Nafion surface maps when compared to previous atomic force microscopy images of the Nafion surface scanned in air. The technique ability is to generate maps showing approximately few nanometer (∼2-5 nm) patterns to the long fiber length (>2 µm). Atomic force microscopy force vs separation curve profiles registered in water are used to characterize the surface hydrophobic and hydrophilic domains. Initially, Nafion surfaces were imaged in air for comparison and then immersed in water. Nafion surfaces immersed in water display a matrix of hydrophilic and hydrophobic regions with fibrillary structure dimensions of ∼40 nm formed by fiber pairs. Ribbons formed by two pairs with diameters of ∼83 nm are separated by larger channels.

Variable Interfacial Water Nanosized Arrangements Measured by Atomic Force Microscopy.

While there seems to be broad agreement that cluster formation does exist near solid surfaces, its presence at the liquid/vapor interface is controversial. We report experimental studies we have carried out on interfacial water attached on hydrophobic and hydrophilic surfaces. Nanosized steps in the measured force vs distance to the surface curves characterize water cluster profiles. An expansion of the interfacial structure with time is observed; the initial profile extent is typically ∼1 nm, and for longer times expanded structures of ∼70 nm are observed. Our previous results showed that the interfacial water structure has a relative permittivity of ε ≈ 3 at the air/water interface homogeneously increasing to ε ≈ 80 at 300 nm inside the bulk, but here we have shown that the interfacial dielectric permittivity may have an oscillating profile describing the spatial steps in the force vs distance curves. This low dielectric permittivity arrangements of clusters extend the region with ε ≈ 3 inside bulk water and exhibit a behavior similar to that of water networks that expand in time.

Imaging Ion Pairs Forming Structural Arrangements in Interfacial Regions.

A technique to image ion pairs in solution is reported. We investigated structural and dynamic properties of ion-pair distributions deposited on highly oriented pyrolytic graphite (HOPG) surfaces in electrolyte solutions. Atomic force microscopy images of HOPG immersed in NaCl and KCl solutions display regular arrangements on top of the hexagonal carbon rings forming the HOPG atomic structure. These arrangements are the result of the low value of the aqueous interfacial dielectric constant (εr ≈ 3-11). The measured ion-pair radius is a function of the salt present in the solution; for KCl, the ion-pair radius is equal or smaller than 0.42 nm; for NaCl, the ion-pair radius is 0.36 nm. A comparison of these values with their crystalline lattice dimensions indicates that both KCl and NaCl ion pairs in solution at the HOPG/solution interfacial region exist as tight contact ion pairs in quasistationary distributions. The NaCl ion-pair distribution forms an aligned arrangement, and the KCl distribution is formed by intercalated pairs.

Hydrated excess protons and their local hydrogen bond transport network as measured by translational, librational, and vibrational frequencies.

A clear molecular description of excess hydrated protons and their local hydrogen bond transport network remains elusive. Here, the hydrogen bond network of excess hydrated protons in water bridges was probed by measuring their Raman spectra and comparing them to the spectra of protons in ice and water. The proton vibrational spectrum and the hydrogen bond network translational and librational spectra were recorded. The spectra of the water bridge and water exhibit clear differences, indicating the presence of a structure in water bridges when subjected to an electric field of ∼106 V/m that has not been previously reported. The intermolecular Raman spectrum of the floating water bridge exhibits a hydrogen bond stretching band at 150-250 cm-1, librational bands within the 300-1000 cm-1 spectral range, and a large band at 1500-3000 cm-1, which corresponds to the vibrational signature of excess hydrated protons in the water bridge structure. The excess protons are shown to move predominantly at the air/water interface, and the effect of this distribution is a measurable change in the air/water interfacial tension from ∼80 to ∼32 N/m. Therefore, hydrated protons must have a unique water arrangement that enables them to propagate without sinking into bulk water. This local polarized hydrogen bond network in the interfacial water region is characterized by a translational spectrum similar to that of ice V.

Hydrated Excess Proton Raman Spectral Densities Probed in Floating Water Bridges.

Excess proton structures in water remain unclear. The motion and nature of excess protons in water were probed using a supported water bridge structure in electric field (E) with an intensity of ∼106 V/m. The experimental setup generated protons that exhibit a long lifetime. The effect of excess protons in water induced a ∼3% variation in the pH for a 300 V overvoltage at the cathode. The current versus voltage curves show a current space-charge-limited operation. By measuring the space-charge distribution in both the cathode and anode and by adjusting the Mott-Gurney law to the measured excess hydrated proton current and the voltage drop in the cationic space-charge region, the protonic mobility was determined to be ∼200 × 10-8 m2/(V·s) (E ≈ 4 × 106 V/m). This measured mobility, which is typically five times larger than the reported mobility for protons in water, is in agreement with the mechanism outlined by Grotthuss in 1805. The measured mid-Raman spectrum covering 1000-3800 cm-1 range indicates the species character. The hydrated excess proton spectral response through the mid-Raman at 1760 and 3200 cm-1 was attributed to the Zundel complex and the region at ∼2000 to ∼2600 cm-1 response is attributed to the Eigen complex, indicating a core structure simultaneously with a Eigen-like and Zundel-like character, suggesting a rapid fluctuation between these two structures or a new specie.

Chiral Asymmetric Structures in Aspartic Acid and Valine Crystals Assessed by Atomic Force Microscopy.

Structures of crystallized deposits formed by the molecular self-assembly of aspartic acid and valine on silicon substrates were imaged by atomic force microscopy. Images of d- and l-aspartic acid crystal surfaces showing extended molecularly flat sheets or regions separated by single molecule thick steps are presented. Distinct orientation surfaces were imaged, which, combined with the single molecule step size, defines the geometry of the crystal. However, single molecule step growth also reveals the crystal chirality, i.e., growth orientations. The imaged ordered lattice of aspartic acid (asp) and valine (val) mostly revealed periodicities corresponding to bulk terminations, but a previously unreported molecular hexagonal lattice configuration was observed for both l-asp and l-val but not for d-asp or d-val. Atomic force microscopy can then be used to identify the different chiral forms of aspartic acid and valine crystals.

Physical properties of water near a gold surface: a nanorheological analysis.

Water at room temperature is not simply a medium for which uniform properties can always be assumed. Water close to solid hydrophobic or hydrophilic surfaces has elasticity, which is measured by monitoring the quartz crystal microbalance (QCM) resonant frequency and resistance. Small additions of salt are shown to modify this elasticity. Furthermore, near the hydrophobic QCM gold electrode, undersaturated aqueous NaCl solutions present a high concentration of ion pairs, which is confirmed by atomic force microscopy through force versus distance measurements.