Intermolecular hydrogen bonding: Two component anthraquinone monolayers

Single- and two-component monolayers of anthraquinone-2,7-disulfonic acid ( 2,7-AQDS) and 1-chloro-4-hydroxy-anthraquinone (1,4-AQClOH) have been forme d by spontaneous adsorption from low pH supporting electrolyte solutions (t ypically 1.0 M HClO4 in 80:20 H2O:DMF) and examined using cyclic voltammetr y. The voltammetric response for the single monolayers is well defined and the concentration dependence of the surface coverage is well described by t he Frumkin adsorption isotherm over the concentration range 1-30 mu M for b oth 2,7-AQDS and 1,4-AQClOH. The saturation coverages (Gamma(sat)) are (6.8 +/- 0.32) x 10(-11) and (1.9 +/- 0.15) x 10(-10) mol cm(-2), respectively, while the adsorption coefficients (beta) are (4.9 +/- 0.25) x 10(4) and (7 .5 +/- 0.5) x 104 M-1 Both monolayers show stabilizing lateral interactions . However, the 2,7-AQDS adsorbates interact more strongly with a Frumkin in teraction parameter (g) of -1.62 +/- 0.21 being observed compared to -0.21 +/- 0.02 for the 1,4-AQClOH monolayers. Mixed monolayers, formed from simul taneous adsorption of both anthraquinones also show lateral interactions wi th full widths at half-maximum of 56 +/- 2 mV being observed for both compo nents. Moreover, the formal potentials, saturation coverages, free energies of adsorption, and lateral interaction parameters of both adsorbates diffe r significantly when the adsorbates are immobilized within two-component ra ther than single-component monolayers. The relationship between the composi tion of the deposition solutions and the two-component monolayers suggests that mixed monolayers exist as a single phase in which the adsorbates do no t have a preference for being surrounded by their own kind. These stabilizi ng lateral interactions are consistent with intermolecular hydrogen bonding in which the 2,7-AQDS and 1,4-AQClOH adsorbates act as electron accepters and donors, respectively.