Highly-sensitive reflectometry setup capable of probing the electrical double layer on silica

This study describes an optical reflectometry setup capable of detecting adsorption at a liquid/solid interface. This setup has improved sensitivity over the previous reflectometry instruments discussed in the literature. This improvement is achieved by implementing a stabilized He–Ne laser and lock-in detection scheme. The high sensitivity of the present setup was demonstrated by probing the formation of the electrical double layer at the water–silica interface. Quantitative interpretation of the data was achieved with the basic Stern model. The determined value of the single-ion refractive index increment for Na+ was 7.1 ± 0.2 mL/mol. The detection limit of the present setup is <1 μg/m2, which is comparable to modern surface plasmon resonance instruments, and is 10 times better than currently achieved with quartz crystal microbalances and other optical surface sensitive techniques

Adsorption of monovalent and divalent cations on planar water-silica interfaces studied by optical reflectivity and Monte Carlo simulations

International audienceAdsorption on planar silica substrates of various monovalent and divalent cations from aqueous solution is studied by optical reflectivity. The adsorbed amount is extracted by means of a thin slab model. The experimental data are compared with grand canonical Monte Carlo titration simulations at the primitive model level. The surface excess of charge due to adsorbed cations is found to increase with pH and salt concentration as a result of the progressive dissociation of silanol groups. The simulations predict, in agreement with experiments, that the surface excess of charge from divalent ions is much larger than from monovalent ions. Ion-ion correlations explain quantitatively the enhancement of surface ionization by multivalent cations. On the other hand, the combination of experimental and simulation results strongly suggests the existence of a second ionizable site in the acidic region. Variation of the distance of closest approach between the ions and surface sites captures ion specificity of water-silica interfaces in an approximate fashion

Response of Adsorbed Polyelectrolyte Monolayers to Changes in Solution Composition

Reflectometry and quartz crystal microbalance are used to study the response of adsorbed polyelectrolyte monolayers to solutions of variable composition. These techniques respectively yield the dry and wet masses of the adsorbed layer, and by combing these results, one obtains the water content and the thickness of the polyelectrolyte films. The systems investigated are films of adsorbed poly(allyl amine) (PAH) and poly-l-lysine (PLL) on silica and films of poly(styrene sulfonate) (PSS) on amino-functionalized silica. When such films are adsorbed from concentrated polyelectrolyte solutions containing high levels of salt, they are found to swell reversibly up to a factor of 2 when incubated in solutions of low salt. This swelling is attributed to the strengthening of repulsive electrostatic interactions between the adsorbed polyelectrolyte chains. PAH films may also swell upon decrease of pH, and collapse upon a pH increase. This transition shows a marked hysteresis and can be rationalized by the competition of electrostatic repulsions between the chains and their attraction to the surface. The presently observed swelling phenomena are caused by a collective process driven by the electrostatic repulsion between the densely adsorbed polyelectrolyte chains. Such responsive layers are only obtained by adsorption from high polyelectrolyte and salt concentrations. Layers absorbed at low polyelectrolyte and salt concentrations show only minor swelling effects, since the adsorbed polyelectrolytes layers are dilute and the adsorbed polyelectrolyte chains interact only weakly