Surface and aggregation behavior of aqueous solutions of Ru(II) metallosurfactants.: 3.: Effect of chain number and orientation on the structure of adsorbed films of [Ru(bipy)2(bipy′)]Cl2 complexes

The surface behavior of a range of surfactant [Ru(biPY)(2)(p,p'-dialkyl-2,2'-bipy)]Cl-2 complexes, which we express as (RuqCn)-C-p where n is the alkyl chain. length, p refers to the substitution position on the bipyridine ligand (= 4 or 5), and q (= 1 or 2) is the number of substituted alkyl chains, has been examined using neutron reflectometry. The adsorption of the single-chain (Ru1C19)-C-4 and (Ru1C19)-C-5 surfactants is strongly time-dependent, taking in excess of 10 h to form an equilibrium film. It is suggested that the slow adsorption rate is related to the alkyl chain length rather than the low monomer concentration present in the solutions. At concentrations below the critical micelle concentration (cmc) of (Ru1C19)-C-4, the film of (Ru1C19)-C-5 is denser than that of (RU1C19)-C-4 at comparable concentration, consistent with the mass densities of the bulk solids, whereas at concentrations close to and greater than this cmc the converse pertains. Close to the cmc, the adsorbed films possess an average area per molecule significantly less than the nominal headgroup area of the surfactants (similar to30 Angstrom(2) compared with similar to100 Angstrom(2)). This fact together with consideration of the thickness and density of the adsorbed films leads to the conjecture that surface aggregates may be the adsorbing units. The adsorption of the double-chain surfactant Ru C-4(2)19, in contrast to the behavior of the (Ru1C19)-C-p surfactants, is weak and independent of time. This behavior is attributed to the alkyl chain orientation. The adsorption behavior of a racemic mixture of the Delta and Lambda isomers of (Ru2C19)-C-4 has been compared with that of the Delta isomer. It is found that the film of racemic material is more closely packed than that of the resolved complex.