Department of Chemical Informatics, Faculty of Education, University of Szeged, Boldogasszony sgt. 6. H-6725 Szeged, Hungary.
Phys Chem Chem Phys. 2010 May 14;12(18):4604-16. doi: 10.1039/b923382b. Epub 2010 Mar 16.
Grand canonical Monte Carlo simulations are used to determine water adsorption on prototypical organic surfaces as a function of relative humidity at 300 K. Three model surfaces formed by well-ordered self-assembled monolayers (SAMs) of alkanethiolate chains on gold are investigated: (i) a smooth hydrophobic surface of methyl-terminated C(7)-CH(3) SAM; (ii) a rough hydrophobic surface of randomly mixed two-component SAM, composed of equal fractions of C(5)-CH(3) and C(7)-CH(3) chains (C(5)/C(7)-CH(3) SAM); and (iii) a smooth hydrophilic surface of carboxyl-terminated C(7)-COOH SAM. The all atom CHARMM22 force field is used for the SAM chains together with the SPC/E model for water. No noticeable water adsorption is observed on the smooth hydrophobic surface up to saturation. The mild surface roughness introduced by the uneven chain length of the two components constituting the C(5)/C(7)-CH(3) SAM has no significant effect on the surface hydrophobicity, and the rough hydrophobic surface also remains dry up to the point when water condensation occurs. In contrast, water readily adsorbs onto the hydrophilic surface by forming hydrogen bonds with the COOH groups of the substrate. In addition, hydrogen bonding with pre-adsorbed water molecules contributes to the mechanism of water uptake. Under low humidity conditions, water is present on the hydrophilic surface as individual molecules or small water clusters and, with increasing relative humidity, the surface coverage grows continuously beyond a monolayer formation. The adsorbed water film is observed to be rather inhomogeneous with patches of bare surface exposed. The amount of water constituting a stable adsorption layer prior to condensation is estimated to consist of about 2-5 molecular layers. Detailed analysis of the simulation results is used to obtain important insights into the structure and energetics of water adsorbed on highly oxidized organic surfaces exposed to ambient air of increasing relative humidity.
我们使用巨正则蒙特卡罗模拟方法，在 300 K 下，测定了相对湿度对典型有机表面上水吸附的影响。我们研究了三种由金上有序自组装单分子层（SAM）形成的模型表面：（i）由甲基末端的 C（7）-CH（3）SAM 组成的光滑疏水性表面；（ii）由 C（5）-CH（3）和 C（7）-CH（3）链（C（5）/C（7）-CH（3）SAM）以相等分数随机混合而成的粗糙疏水性表面；（iii）由羧基末端的 C（7）-COOH SAM 组成的光滑亲水性表面。我们使用全原子 CHARMM22 力场来模拟 SAM 链，同时使用 SPC/E 模型来模拟水。在光滑疏水性表面上，直到饱和，我们都没有观察到明显的水吸附。由构成 C（5）/C（7）-CH（3）SAM 的两种组分的不均匀链长引入的轻微表面粗糙度对表面疏水性没有显著影响，粗糙疏水性表面在水发生冷凝之前也保持干燥。相比之下，水很容易通过与基底的 COOH 基团形成氢键而吸附在亲水性表面上。此外，与预吸附水分子的氢键作用有助于水的吸收机制。在低湿度条件下，水以单个分子或小的水分子簇的形式存在于亲水性表面上，随着相对湿度的增加，表面覆盖率不断增加，超过单层形成。我们观察到吸附的水膜相当不均匀，有裸露的表面斑块。在冷凝之前形成稳定吸附层的水的量估计由大约 2-5 个分子层组成。通过对模拟结果的详细分析，我们获得了关于在不断增加的相对湿度的环境空气中暴露的高度氧化的有机表面上水吸附的结构和能量学的重要见解。
