The arrangement of water molecules at charged aqueous interfaces is an important question in biology, electrochemistry, and geochemistry. Theoretical studies suggest that the molecules become arranged in several layers adjacent to a solid interface. Using atomic force microscopy we have measured the water dielectric-permittivity profile perpendicular to mica surfaces. The measured variable permittivity profile starting at epsilon approximately 4 at the interface and increasing to epsilon=80 about 10 nm from the surface suggests a reorientation of water molecule dipoles in the presence of the mica interfacial charge.
During the tip approach to hydrophobic surfaces like the water/air interface, the measured interaction force reveals a strong attraction with a range of approximately 250 nm at some points along the interface. The range of this force is approximately 100 times larger than the measured for gold (approximately 3 nm) and 10 times larger than the one for hydrophobic silicon surfaces (approximately 25 nm). At other points the interface exerts a medium range repulsive force growing stepwise as the tip approaches the interface plane, consequently the hydrophobic force is a strong function of position. To explain these results we propose a model where the force on the tip is associated with the exchange of a small volume of the interface with a dielectric permittivity epsilon(int) by the tip with a dielectric permittivity epsilon(tip). By assuming a oscillatory spatial dependence for the dielectric permittivity it is possible to fit the measured force profiles. This dielectric spatial variation was associated with the orientation of the water molecules arrangement in the interfacial region. Small nanosized hydrogen-bond connected cages of water molecules present in bulk water at the interface are oriented by the interfacial electric field generated by the water molecules broken bonds, one broken hydrogen bond out of every four. This interfacial field orients small clusters formed by approximately 100 water molecules into larger clusters (approximately 100 nm). In the limit of small (less than 5 nm thick) water molecule cages we have modeled the static dielectric permittivity (epsilon) as the average response of those cages. In these regions the dielectric permittivity for water/air interfaces decreases monotonically from the bulk value epsilon approximately 80 to approximately 2 at the interface. For regions filled with medium size cages, the tip senses the structure of each cage and the static dielectric permittivity is matched to the geometrical features of these cages sized approximately 25 to 40 nm. Interfacial electric energy density values were calculated using the electric field intensity and the dielectric permittivity obtained by the fitting of the experimental points. The integration of the electric energy density along the interfacial region gives a value of 0.072 J m(-2) for interfacial energy density for points where the hydrophobic force has a range of approximately 250 nm. Regions formed by various clusters result in lower values of the interfacial energy density.
Atomic force microscopy has enabled direct visualization of the liposome structure supported on mica surfaces in air and silanized mica surfaces in aqueous media. The images display distinct patterns of adhered liposomes: multiple and single vesicle liposomes and flat supported bilayers. The multiple vesicle liposome structure is not visible by optical microscopy since the vesicles forming the liposome have diameters as small as 20 nm. Molecularly resolved force versus distance curves displaying the organization of hydrocarbon chains (mono-or bilayers) corroborate the presence of distinct adsorbed structures observed by scanning the surface. The high resolution of the observed liposome images allows the visualization of the aggregation of the multiple vesicles forming liposomes which were shown to have their origin in the liposome formation process and not during adsorption. Since most of the observed liposomes are aggregated vesicles, this aggregated structure has a substantially larger stability during adsorption than the single vesicle structure and consequently a larger resistance in maintaining its shape and function as a carrier of cosmetics, food additives, and drugs. This observation also has some important consequences in the liposomes' selective permeability when they are used as carriers.
Duas amostras diferentes de partículas monodispersas de sílica de Stöber foram examinadas por microscopia eletrônica de transmissão analítica, utilizando-se diferentes tipos de imagens: campo-claro, campo escuro, imagens espectrais e mapas de distribuição elementar. As partículas (141 e 36 nm de diâmetro efetivo) contêm domínios de relação elementar O/Si e, portanto, de grau de hidratação muito variável, que coexistem com partículas medindo poucos nanômetros de diâmetro e com relação O/Si elevada, que aparecem dispersas no fundo das imagens. As imagens de campo claro e de perda de energia revelam que as partículas maiores possuem uma morfologia do tipo caroço-casca e as cascas das partículas possuem uma quantidade maior de domínios com razão O/Si maior, comparada com o interior da partícula, o que é atribuído ao acúmulo de domínios mais hidratados na casca, e também à presença de compostos de carbono, contaminantes da sílica. Por outro lado, as partículas menores (diâmetro efetivo = 36 nm) não são esféricas nem têm morfologia caroço-casca, embora também sejam formadas por domínios de composições químicas muito diferentes. Os vários mecanismos de formação de partículas apresentados na literatura são discutidos, considerando-se estes novos resultados.Two different samples of monodisperse Stöber silica particles were examined in the analytical transmission electron microscope, using different imaging modes: bright-field, dark-field, energyloss and elemental distribution maps. The particles (effective diameters = 141 and 36 nm) are formed by domains of variable O/Si ratio, which is consistent with a variable degree of hydration, and they coexist with particles with a high O/Si ratio measuring a few nanometers only, which appear dispersed in the picture background. Bright-field and energy-loss images of the larger particles show a core-and-shell morphology, and the shells have a higher amount of high-O/Si domains as well as contaminating carbon compounds. On the other hand, the smaller particles (effective diameter = 36 nm) are also formed by distinct domains, but their morphology is neither spherical or core-and-shell. The mechanisms for particle formation presented in the literature are discussed, considering the present findings.Keywords: colloidal silica, transmission electron microscopy, energy-filtered imaging, silica particle microchemistry J. Braz. Chem. Soc., Vol. 12, No. 4, 519-525, 2001. Printed in Brazil ©2001 Soc. Bras. Química 0103 -5053 $6.00+0.00 IntroductionUniform fine particles are interesting as model systems in the study of adsorption and catalysis, as well as in the study of size-dependent solid state properties such as quantum confinement 1 and superplasticity 2,3 . There have been significant achievements concerning the preparation of uniform colloid dispersions, both inorganic 4 and organic 5 . However, uniformity has often been considered only in relationship to particle size and shape, and little information is currently available concerning the uniformity of chemical composition of particl...
We have measured the force acting on neutral tips as a function of distance to hydrophobic surfaces in aqueous solutions. The unusually large magnitude of this force is attributed to the electrostatic response of the aqueous fluid structure (hydration layer) at the interface. The local electric field in an interfacial region is a manifestation of the distribution of surface polar residues, and we have assumed that the polarization (hydration) of the hydrophobic surface immersed in water is predominantly driven by the direct water binding. The simplest electrostatic description of the coupling between the interfacial polarization charges and the corresponding polarization charges of the solvent molecules is expressed here as the spatially variable dielectric permittivity int. The exchange of a volume of the interfacial region with int by a tip with a dielectric constant tip is responsible for the tip attraction. The variable dielectric permittivity profiles for the following interfaces were measured in order to clarify the origin of the long-range attractive forces: water/air, water/CTAB covered mica, and water/hydrophobic silicon.
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