| Abstract
| - Electric field induced switching behaviors of a series of low-density ω-carboxyalkyl modified H−Si(111) and the mixed ω-carboxyalkyl/alkyl covered H−Si(111) have been simulated by using moleculardynamics (MD) simulation techniques. The external electric fields may drive surface-confined moleculesto reversibly change conformations between the all-trans (switching ‘on') and the mixed trans-gauche(switching ‘off') states. Such surfaces switch wettabilities between the hydrophilic state and the moderatelyhydrophobic state. It has been found in broad ranges of intensities of applied electric fields, −2.0 × 109V/m ≤ Edown ≤ 0 and 1.8 × 109 V/m ≤ Eup ≤ 7.3 × 109 V/m, both the low-density (11.1%−33.3%)ω-carboxyalkyl and the mixed ω-carboxyalkyl/alkyl (in mole fraction of 0.4 ≤ Ncarboxyalkyl : Nalkyl ≤ 3.0)monolayers covering H−Si(111) exhibit conformational switching in the aqueous medium. The criticalintensity of the electric field, Eup = 1.8 × 109 V/m, which is required to trigger the switches is observed byour MD simulations and further rationalized by a thermodynamical model. Some important factors in thecontrol of switching performances, such as the steric hindrances, the formation of the electric double layerat the monolayer/electrolyte solution interface, the hydration effects of carboxylate anions, the componentsof surrounding electrolyte solutions, as well as the rigidity of surface-confined chains are elucidated. Thelower ionic strength and additions of acetonitrile molecules in the surrounding aqueous solution can reducethe value of critical intensity of the electric field and hence facilitate the realization of switching. Somepractical considerations in construction and optimum design of switching surfaces are also suggested.
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