Orientation and order of aqueous organic ions adsorbed to a solid surface

The adsorption and orientation of an aqueous organic ion with anisotropic shape (1,4-dimethylpyridinium, P⁺) at the surface of oxidized silicon carrying opposite charge (produced by conditions of high pH) were studied using polarized infrared spectroscopy in attenuated total reflection (FTIR-ATR). Orientation relative to the surface was quantified from the dichroic ratio of in-plane skeletal vibrations of the pyridinium ring (1643 and 1523 cm^-1), and the adsorbed amount was inferred from the intensity of these bands. The sticking energy of the organic ion was slightly larger than that of small inorganic ions of the same charge (Li⁺, Na⁺, Cs⁺). From relative quantities adsorbed in competitive adsorption, the relative sticking energy was quantified (∼7k_BT relative to Na⁺ at pH = 9.2 and varying in the order Cs⁺ > Na⁺ > Li⁺ by the total amount of 0.6k_BT). At low ionic strength (no inorganic ions present except those in the buffer solution), P⁺ stood preferably parallel to the surface when the surface coverage was low but more nearly upright both as its surface coverage increased and as the concentration of coadsorbed small ions increased. This shows the influence of steric packing on the orientation of this ion of asymmetric shape. The larger the hydrated diameter of the coadsorbed ion, the more the P⁺ ion tilted away from the surface (H⁺ < Li⁺, Na⁺, Cs⁺ < Mg²⁺). Furthermore, if the mass adsorbed exceeded a critical level, both the tilt and the amount adsorbed jumped in response to increasing P⁺ concentration in bulk solution, with hysteresis upon dilution. This jump, together with the measured ellipsometric thickness and contact angle, suggests that the discontinuity involved structural change within a single monolayer. The organic ion thus behaved at the surface as an embryonic amphiphile, although in the bulk, micelle formation has not been reported.