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

The adsorption and orientation of an aqueous organic ion with anisotropic s hape (1,4-dimethylpyridinium, P+) at the surface of oxidized silicon carryi ng opposite charge (produced by conditions of high pH) were studied using p olarized infrared spectroscopy in attenuated total reflection (FTIR-ATR). O rientation relative to the surface was quantified from the dichroic ratio o f 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 sma ll inorganic ions of the same charge (Li+,Na+, Cs+). From relative quantiti es adsorbed in competitive adsorption, the relative sticking energy was qua ntified (similar to 7k(B)T relative to Na+ at pH = 9.2 and varying in the o rder Cs+ > Na+ > Li+ by the total amount of 0.6k(B)T). At low ionic strengt h (no inorganic ions present except those in the buffer solution), pf stood preferably parallel to the surface when the surface coverage was low but m ore nearly upright both as its surface coverage increased and as the concen tration of coadsorbed small ions increased. This shows the influence of ste ric 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 awa y from the surface (H+ < Li+, Na+, Cs+ < Mg2+). Furthermore, if the mass ad sorbed exceeded a critical level, both the tilt and the amount adsorbed jum ped in response to increasing P+ concentration in bulk solution, with hyste resis upon dilution. This jump, together with the measured ellipsometric th ickness and contact angle, suggests that the discontinuity involved structu ral change within a single monolayer. The organic ion thus behaved at the s urface as an embryonic amphiphile, although in the bulk, micelle formation has not been reported.