Tikhonov Aleksey M, Patel Harshit, Garde Shekhar, Schlossman Mark L
J Phys Chem B. 2006 Oct 5;110(39):19093-6. doi: 10.1021/jp064120q.
Interactions between surfactants, and the resultant ordering of surfactant assemblies, can be tuned by the appropriate choice of head- and tailgroups. Detailed studies of the ordering of monolayers of long-chain n-alkanoic and n-alkanol monolayers at the water-vapor interface have demonstrated that rigid-rod all-trans ordering of the tailgroups is maintained upon replacing the alcohol with a carboxylic acid headgroup. In contrast, at the water-hexane liquid-liquid interface, we demonstrate that substitution of the -CH(2)OH with the -COOH headgroup produces a major conformational change of the tailgroup from disordered to ordered. This is demonstrated by the electron density profiles of triacontanol (CH(3)(CH(2))(29)OH) and triacontanoic acid (CH(3)(CH(2))(28)COOH) monolayers at the water-hexane interface, as determined by X-ray reflectivity measurements. Molecular dynamics simulations illustrate the presence of hydrogen bonding between the triacontanoic acid headgroups that is likely responsible for the tail ordering. A simple free energy illustrates the interplay between the attractive hydrogen bonding and the ordering of the tailgroup.
表面活性剂之间的相互作用以及由此产生的表面活性剂聚集体的排列，可以通过对头基和尾基的适当选择来调节。对长链正烷酸和正烷醇单分子层在水汽界面排列的详细研究表明，在用羧酸头基取代醇后，尾基的刚性棒状全反式排列得以保持。相比之下，在水-己烷液-液界面，我们证明用-COOH头基取代-CH(2)OH会使尾基发生从无序到有序的主要构象变化。这通过水-己烷界面上三十烷醇（CH(3)(CH(2))(29)OH）和三十烷酸（CH(3)(CH(2))(28)COOH）单分子层的电子密度分布得到证明，该分布由X射线反射率测量确定。分子动力学模拟表明三十烷酸头基之间存在氢键，这可能是尾基有序排列的原因。一个简单的自由能说明了有吸引力的氢键和尾基排列之间的相互作用。
