A molecular-thermodynamic model for Gibbs monolayers formed from redox-active surfactants at the surfaces of aqueous solutions: Redox-induced changes in surface tension
A molecular-thermodynamic model for Gibbs monolayers formed from redox-active surfactants at the surfaces of aqueous solutions: Redox-induced changes in surface tension
复制标题
DOI：
10.1021/la9807208
复制
发表时间：
1999-02-02
期刊：
影响因子：
3.9
通讯作者：
Abbott, NL
中科院分区：
文献类型：
--
作者：
Aydogan, N;Gallardo, BS;Abbott, NL
我们报告的吉布斯单分子膜形成的氧化还原活性表面活性剂（11-ferrocenylundecyl）三甲基溴化铵（II+），或氧化的II+（II 2+），在水溶液的表面的分子热力学模型的发展。该模型提供了过去实验测量的说明（Gallardo，B. S.的; Metcalfe，K. L.的; Abbott，N. L. Langmuir 1996，12，4116-4124），其证明了II+到II 2+的电化学氧化导致该氧化还原活性表面活性剂的水溶液的过量表面浓度和表面张力的大的和可逆的变化。该模型的结果使我们得出结论，II+假定在水溶液的表面环状构象。这种环状构象降低了II+水溶液的表面张力，在85埃（2）/分子（在0.1 M Li 2SO 4中）的极限表面积下，表面张力接近49 mN/m。表面张力降低的根本原因不是静电对表面压力的贡献（如经典离子表面活性剂的情况），而是由于表面活性剂在溶液表面的约束（环状）构型（链堆积）引起的熵贡献。在II+（0.1 mM）的临界胶束浓度（CMC）附近的浓度下，II+氧化成II 2+导致表面活性剂从溶液表面解吸，并且表面张力从49 mN/m增加到72 mN/m。脱附的过程是由氧化引起的表面活性剂自缔合的疏水驱动力的降低以及吸附的表面活性剂之间的静电排斥驱动的。相反，在II+的浓度大大超过其CMC，II+到II 2+的氧化驱动胶束的单体在本体溶液中的破坏，从而增加了化学势和表面活性剂的过量表面浓度：氧化诱导的表面活性剂的过量表面浓度的增加导致表面张力的降低。这些结果结合起来，氧化还原活性表面活性剂的设计提供了原则。
We report the development of a molecular-thermodynamic model for Gibbs monolayers formed from the redox-active surfactant (11-ferrocenylundecyl)trimethylammonium bromide (II+), or oxidized II+ (II2+), at the surfaces of aqueous solutions. This model provides an account of past experimental measurements (Gallardo, B. S.; Metcalfe, K. L.; Abbott, N. L. Langmuir 1996, 12, 4116-4124) which demonstrated electrochemical oxidation of II+ to II2+ to lead to large and reversible changes in the excess surface concentrations and surface tensions of aqueous solutions of this redox-active surfactant. The results of the model lead us to conclude that II+ assumes a looped conformation at the surfaces of aqueous solutions. This looped conformation lowers the surface tensions of aqueous solutions of II+ to similar to 49 mN/m at a limiting surface area of 85 Angstrom(2)/molecule (in 0.1 M Li2SO4). The underlying cause of the reduction in surface tension is not an electrostatic contribution to the surface pressure (as is the case with classical ionic surfactants) but rather an entropic contribution due to the constrained (looped) configuration of the surfactant at the surface of the solution (chain packing). At concentrations around the critical micelle concentration (CMC) of II+ (0.1 mM), oxidation of II+ to II2+ results in the desorption of surfactant from the surface of the solution and an increase in surface tension from 49 to 72 mN/m. The process of desorption is driven by an oxidation-induced decrease in the hydrophobic driving force for self-association of the surfactants as well as an electrostatic repulsion between adsorbed surfactants. In contrast, at concentrations of II+ that substantially exceed its CMC, oxidation of II+ to II2+ drives the disruption of micelles to monomers in the bulk solution, thus increasing the chemical potential and excess surface concentration of surfactant: the oxidation-induced increase in excess surface concentration of surfactant leads to a decrease in surface tension. These results, when combined, provide principles for the design of redox-active surfactants.