E.F. de Souza

Interfacial aqueous solutions dielectric constant measurements using atomic force microscopy

Abstract The exchange of the volume of a region of the electric double layer of a mica surface immersed in aqueous solutions, with a dielectric constant ϵ DL , by a nanosized radius tip, with a dielectric constant ϵ Tip , is responsible for the repulsion at large distances from the surface (starting at ∼100 nm, diffuse layer) and followed by an attraction when the tip is immersed in the inner layer (∼10 nm). The calculated dielectric constant as a function of the distance to the charged interface obtained by fitting the force versus distance curves, allows the mapping of the inner layer dielectric constant profiles with a nanometer resolution.

Dielectric exchange force: a convenient technique for measuring the interfacial water relative permittivity profile

Water relative permittivity profiles perpendicular to mica surfaces have been measured by atomic force microscopy using the force acting on uncharged tips when immersed in the mica double-layer. This force is modelled by the gradient of the electrostatic energy variation (dielectric exchange force) involved in the immersion of the tip with a relative permittivity eTip in the double layer region with eDL. The measured variable permittivity profile starting at e≈4 at the interface and increasing to e≈80 about 10 nm from the surface suggests a reorientation of water molecule dipoles in the presence of mica interfacial charges. Changes in water polarization are therefore responsible for the hydration or structural forces acting on the tips immersed in the inner double layer. Corroboration for the proposed model (dielectric exchange force) is given by the observation of an attractive force when metal-coated tips (eTip≈∞) are immersed in the mica double layer. Support for the change in the water relative permittivity at the interface is given by measurements of only a repulsive force component when silicon nitride and silicon tips are immersed in solvent where there is no interaction between the mica surface and the solvent and, consequently, no solvent structuring at the interface.

Measurements of long-range attractive forces between hydrophobic surfaces and atomic force microscopy tips

Abstract We have measured the force acting on neutral tips as function of distance to hydrophobic surfaces in aqueous solutions. The unusually large magnitude of this force is attributed to an electrostatic response of the aqueous fluid structure (hydration layer). The exchange of a volume of this region with a dielectric permittivity ϵ int by the tip with a dielectric constant ϵ tip is responsible for the tip attraction when it is immersed in the hydration layer. Hydrophobic hydration layers, characterized by a variable dielectric permittivity profile, have measured widths of ∼4 and ∼8 nm for hydrophobic silicon and CTAB monolayer covering mica surfaces, respectively.

Superhydrophobic polyethylcyanoacrylate coatings. Contact area with water measured by Raman spectral images, contact angle and Cassie–Baxter model

Apolar fibers wired into a mesh-like microstructure forming a coating with a contact angle larger than 160° and fabricated by polycyanoacrylate polymerization are described. Interconnected fibers with diameters measuring approximately 5 μm are formed by texturized linear or folded nanowires. The structure forming the deposited film occupies ~1.5% of the coatings top geometric area. This value agrees with the water/coating contact area given by the Cassie-Baxter contact-angle model (~1.5%). The spatial distribution of the surface in contact with water was determined by Raman spectral imaging (~1.5%) using the polycyanoacrylate lines and by scanning electron microscopy (~2.0%).

Dielectric constant measurements of interfacial aqueous solutions using atomic force microscopy

Dielectric constant variation at aqueous solution/mica interfaces is shown to be responsible for the force acting on tips immersed in the double layer. The exchange of the volume of a region of the electric double layer of a mica surface immersed in aqueous solutions, with a dielectric constant, by the silicon nitrite tip, with a dielectric constant e Tip , is responsible for the repulsion at large distances from the surface (starting at ∼100 nm, diffuse layer) and followed by an attraction when the tip is immersed in the inner layer (≤10 nm). The force versus separation measured curves were fitted to the expression of the dielectric exchange force derived by using a continuum theory for a sharpened conical tip immersed in a spatially variable dielectric constant double layer electric field. The dielectric exchange effect gives a consistent description of the force acting on the tip by assuming a double layer region of water with e DL ≃80 at distances far away from surface (∼100 nm), followed by a region of lower dielectric constant at the inner layer. Support for the proposed model (dielectric exchange force) is given by the observation of an attractive force when metal (platinum) coated tips (e Tip ∞) are immersed in the mica double layer and the measurements of only repulsive force components when silicon nitride tips are immersed in solvent where there is no interaction between the mica surface and the solvent and consequently, no solvent structuring at the interface.

Ionic surfactant films imaged by atomic force microscopy

Forces acting on atomic force microscope (AFM) tips responsible for image formation are measured during scanning of films of ionic surfactant molecules adsorbed from aqueous solutions onto hydrophilic substrates. Near the critical micelle concentration mica substrate images show aggregate regions at the interface. Force versus distance measured curves show that patches form a thicker structure than the formed at partially covered regions, in agreement with the fact that at patches the adsorbates are perpendicularly oriented to the substrate plane. However, AFM topographic images registered at low scanning speed (5 μm/s) show that these patched regions appear as holes, forming inverted images. In AFM imaging of soft structures, as surfactants or biological material, inverted in contrast with images may be observed when there is a specific tip penetration through each scanned layer. This penetration is adjusted by changing the tip force set point, consequently different topographic profiles are obtained. The precise force set point to obtain the correct contrast in scanned images is obtained by the analysis of the force versus distance curves that show the normal to the scanned plane structure profile. Adsorption patterns as a function of time may be conveniently monitored and the adsorption rate may be determined.

Rupture force of adsorbed self-assembled surfactant layers: Effect of the dielectric exchange force

The tip applied force necessary to obtain tip/substrate contact, i.e., rupture force between adsorbed layers of self-assembled surfactant films and atomic force microscope (AFM) tips in water has been measured. A substantial contribution of this rupture force is due to the dielectric exchange force (DEF). The DEF model is in agreement with the observation that the surfactant layer rupture forces are smaller in the thickest layers, where the compactness of the adsorbed film results in the smallest values of the dielectric permittivity. Within experimental accuracy a dielectric permittivity value of ∼4 for bilayers and of ∼36 for monolayers is found.

Magnetic force images of nanomagnetic domains taken with platinum-coated tips

This article deals with magnetic force microscope images of nanosized domains in Co-coated films made by Pt-coated tips as well as micromagnetic images of data tracks written in recording media. Pt-coated tips have improved image delineation of the magnetic field distribution compared to images obtained by Co-coated hard magnetic tips. The force acting on Pt-coated tips in the magnetic field of the substrate was modeled assuming a paramagnetic tip. Due to the ferromagnetic nature of the interaction between the tip and substrate the spatial resolution of hard magnetic tips was shown to be inadequate to measure details of the features of nanosized domains. A comparison of the magnetic images made by Pt-coated tips with topographic images shows that magnetic domains resist thermal erasure at ambient temperature when they are formed of eight metallic grains.

Effect of magnetic coupling on non-radiative relaxation time of Fe3+ sites on LaAl1−xFexO3 pigments

Inorganic pigments of the system LaAl1–xFexO3 were prepared by the Pechini and the Solid State Reaction (SSR) methods. Magnetic interactions and non-radiative relaxation time were analyzed by means of phase-resolved photoacoustic spectroscopy and electron paramagnetic resonance (EPR) techniques. EPR results show a change in the magnetic behavior from paramagnetic (x = 0.2 and 0.4) to antiferromagnetic (x = 1.0), which is believed to be a result of the SSR preparation method. Trends in the optical absorption bands of the Fe3+ are attributed to their electronic transitions, and the increase in the bands intensity at 480 and 550 nm was assigned to the increase in the magnetic coupling between Fe–Fe. The phase-resolved method is capable of distinguishing between the two preparation methods, and it is possible to infer that SSR modifies the magnetic coupling of Fe–Fe with x.Inorganic pigments of the system LaAl1–xFexO3 were prepared by the Pechini and the Solid State Reaction (SSR) methods. Magnetic interactions and non-radiative relaxation time were analyzed by means of phase-resolved photoacoustic spectroscopy and electron paramagnetic resonance (EPR) techniques. EPR results show a change in the magnetic behavior from paramagnetic (x = 0.2 and 0.4) to antiferromagnetic (x = 1.0), which is believed to be a result of the SSR preparation method. Trends in the optical absorption bands of the Fe3+ are attributed to their electronic transitions, and the increase in the bands intensity at 480 and 550 nm was assigned to the increase in the magnetic coupling between Fe–Fe. The phase-resolved method is capable of distinguishing between the two preparation methods, and it is possible to infer that SSR modifies the magnetic coupling of Fe–Fe with x.