Reversibly adsorbed monolayers on microelectrodes: Effect of potential on the adsorption thermodynamics
Reversibly adsorbed monolayers on microelectrodes: Effect of potential on the adsorption thermodynamics
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DOI：
10.1021/ac9602884
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发表时间：
1996-09-15
影响因子：
7.4
通讯作者：
Forster, RJ
中科院分区：
文献类型：
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作者：
Forster, RJ
通过在汞微电极上的可逆吸附，制备了2-羟基蒽醌（2OH-AQ）单层膜。这些单层膜与HC10(4)作为支撑电解质接触时的循环伏安法几乎是理想的，并且与通过双电子，双质子转移机制进行的还原过程相一致。循环伏安法表明，在2 × 10(-8) ~ 2 × 10(-6) M的浓度范围内，Langmuir等温线准确地描述了表面覆盖率γ与2OH-AQ体积浓度的关系，观察到极限表面覆盖率γ (s)为（1.0 +/- 0.08）× 10(-10) mol cm(-2)，吸附系数β为（6.3 +/- 0.6）× 10(6) M（-1）。由于吸附在该系统中是可逆的，改变单层形成时的电位对表面覆盖的影响，或吸附热力学，不能通过在空白电解质溶液中进行非原位测量来研究。为了解决这个问题，微秒时间尺度的计时电流法被用于时间分辨双层充电和向吸附的蒽醌部分的非均相电子转移。这种方法允许在单层氧化还原活性的电位下测量双层电容C-dl，从而允许探测电位和体积2OH-AQ浓度对C-dl的影响。当溶液中2OH-AQ的浓度系统变化时，在形式电位E度下测量C-dl时，得到的Gamma(s)和beta值与伏安法测量的值相同。这一观察结果表明，C-dl至少在E度上与γ呈线性关系。当沉积电位从-0.400到+0.300 V系统变化时，电容数据被用来确定吸附等温线，其中单层分别被完全还原和氧化。这些数据表明，单分子膜的还原形式，三羟基蒽，可能比氧化的蒽醌膜更致密。此外，吸附自由能δ G（ads）的大小发生了显著变化，完全还原和氧化单层膜的吸附自由能δ G（ads）分别为-41.4 +/- 2.7和-26.5 +/- 0.3 kJ mol（-1）。这些自由吸附能的差异可能是由于氧化膜和还原膜分子间氢键的程度不同而引起的。
Monolayers of 2-hydroxyanthraquinone (2OH-AQ) have been formed by reversible adsorption onto mercury microelectrodes. Cyclic voltammetry of these monolayers in contact with HC10(4) as supporting electrolyte is nearly ideal and is consistent with reduction proceeding by a two-electron, two-proton transfer mechanism. Cyclic voltammetry reveals that the dependence of the surface coverage Gamma on the bulk concentration of 2OH-AQ is accurately described by the Langmuir isotherm over the concentration range from 2 x 10(-8) to 2 x 10(-6) M, A limiting surface coverage Gamma(s) of (1.0 +/- 0.08) x 10(-10) mol cm(-2) and an adsorption coefficient beta of (6.3 +/- 0.6) x 10(6) M(-1) are observed. Since adsorption is reversible in this system, the effect of changing the potential at which the monolayer is formed on the surface coverage, or the adsorption thermodynamics, cannot be investigated by ex situ measurement of Gamma in a blank electrolyte solution. To address this issue, microsecond time scale chronoamperometry has been used to time-resolve double-layer charging and heterogeneous electron transfer to the adsorbed anthraquinone moieties, This approach allows the double-layer capacitance C-dl to be measured even at potentials where the monolayer is redox active, thus allowing the effect of potential and bulk 2OH-AQ concentration on C-dl to be probed, Significantly, when C-dl is measured at the formal potential E degrees' as the concentration of 2OH-AQ in solution is systematically varied, the values of Gamma(s) and beta obtained are identical to those measured voltammetrically. This observation suggests that C-dl depends linearly on Gamma, at least at E degrees'. Capacitance data have been used to determine the adsorption isotherms as the deposition potential was systematically varied from -0.400 to +0.300 V, where the monolayer is fully reduced and oxidized, respectively. These data suggest that the reduced form of the monolayer, trihydroxyanthracene, may be more compact than the oxidized anthraquinone films. Moreover, the magnitude of the free energy of adsorption Delta G(ads) changes significantly, with values of -41.4 +/- 2.7 and -26.5 +/- 0.3 kJ mol(-1) being observed for fully reduced and oxidized monolayers, respectively. These differences in the free energies of adsorption may arise because of different extents of intermolecular hydrogen bonding in the oxidized and reduced films.