Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 2V4 Canada.
J Phys Chem B. 2012 Sep 13;116(36):11187-96. doi: 10.1021/jp304444d. Epub 2012 Aug 31.
Studies on molecular mechanisms of polyaromatic surfactants in stabilizing water-in-oil (W/O) or oil-in-water (O/W) emulsions are of great scientific and practical importance. A polyaromatic surfactant N-(1-hexylheptyl)-N'-(5-carboxylicpentyl) perylene-3,4,9,10-tetracarboxylic bisimide (C5Pe) with well-defined molecular structure containing fused aromatic rings and heteroatoms similar to asphaltene molecules, was used in this study in an attempt to understand molecular interaction mechanisms of heavy oil components in aqueous solutions. A surface forces apparatus (SFA) was used to directly measure the molecular interactions of C5Pe. Solution pH, salt concentration and Ca(2+) addition showed a strong impact on molecular interactions between C5Pe adsorbed on mica surfaces. The repulsion observed between the two adsorbed C5Pe molecular layers was shown to have a steric and electrosteric origin. The force-distance profiles at short separation distances under high compression force were well fitted with the Alexander-de Gennes (AdG) model. At pH ≥ 4, the repulsive forces measured over a long separation distance under low compression force were shown to deviate from the AdG model but could be fitted with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, indicating an electrostatic origin of the observed repulsion due to ionization of -COOH groups. Adhesion between two C5Pe surfaces was shown to decrease sharply with increasing solution pH and salt concentration, being attributed to the decrease in surface hydrophobicity and hence hydrophobic attraction. Addition of Ca(2+) ions induced the formation of large C5Pe aggregates due to strong bonding of Ca(2+) with -COOH groups, leading to a longer range steric repulsion. Our results provide a new insight into the molecular interactions of polyaromatic surfactants at oil-water interfaces and in complex aqueous solutions.
关于多环芳烃表面活性剂在稳定水包油（W/O）或油包水（O/W）乳液中的分子机制的研究具有重要的科学和实际意义。本研究中使用了一种具有明确分子结构的多环芳烃表面活性剂 N-(1-己基庚基）-N'-(5-羧基戊基）苝-3,4,9,10-四羧酸二酰亚胺（C5Pe），它含有融合的芳环和与沥青质分子相似的杂原子，试图了解重油成分在水溶液中的分子相互作用机制。表面力仪（SFA）用于直接测量 C5Pe 的分子相互作用。溶液 pH 值、盐浓度和 Ca(2+) 加人对 C5Pe 吸附在云母表面的分子相互作用有很强的影响。观察到的两个吸附 C5Pe 分子层之间的排斥作用具有位阻和静电位阻的起源。在高压缩力下的短分离距离下的力-距离曲线很好地符合亚历山大-德加内斯（AdG）模型。在 pH 值≥4 时，在低压缩力下测量的长分离距离下的排斥力偏离 AdG 模型，但可以用德杰纳-朗道-范威克（DLVO）理论拟合，表明观察到的排斥作用是由于 -COOH 基团的电离而具有静电起源。两个 C5Pe 表面之间的附着力随着溶液 pH 值和盐浓度的增加而急剧下降，这归因于表面疏水性的降低，从而导致疏水力的降低。加入 Ca(2+) 离子会由于 Ca(2+)与 -COOH 基团的强键合而导致 C5Pe 形成大的聚集体，从而导致更长程的位阻排斥。我们的结果为多环芳烃表面活性剂在油水界面和复杂水溶液中的分子相互作用提供了新的见解。
