Molecular Interactions of a Polyaromatic Surfactant C5Pe in Aqueous Solutions Studied by a Surface Forces Apparatus

Abstract

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.

Cellulosic Substrates for Removal of Pollutants from Aqueous Systems: A Review. 3. Spilled Oil and Emulsified Organic Liquids

“…Because asphaltenes are enriched at the surfaces of oil phases (Natarajan et al 2011), such a system is highly relevant to the collection of crude oil onto a sorbent material. As would be expected based on double layer interactions, the forces between the surfaces coated with the model asphaltene molecule were greatly affected by changes in pH, monovalent salt concentration, and especially by Ca 2+ ion addition (Wang et al 2012). These results are generally in agreement with those by Svitova et al (2002), as mentioned in the earlier discussion of aqueous environmental properties.…”

“…An electrostatic double-layer force arises when two surfaces come close enough for their diffuse layers overlap to an appreciable extent. Wang et al (2012) recently demonstrated the influence of the double layer in governing the forces between mineral (mica) surfaces in an aqueous system with the presence of a surfactant having characteristics similar to the asphaltene component of crude oils. Because asphaltenes are enriched at the surfaces of oil phases (Natarajan et al 2011), such a system is highly relevant to the collection of crude oil onto a sorbent material.…”

Model molecules mimicking asphaltenes

“…Wang et al in Edmonton, Canada applied this technique to measure the interactions between mica surfaces covered with the model compound C5Pe both in organic solvent (toluene and heptane) 115 and aqueous phase 116 .…”

“…To probe the role of charges on C5Pe model compound for the interfacial activity, Wang et al continued to measure the interactions between mica surfaces covered with the model compound C5Pe both in aqueous phases 116 . Repulsive forces were detected between two adsorbed C5Pe layers.…”

Surface forces in unconventional oil processing

“…Due to unknown molecular structures of asphaltenes that greatly hindered the progress on understanding the role of molecular forces in emulsion stabilization, surface forces were measured using asphaltene "model" compounds for a more nuanced understanding of surface forces at play. Using SFA recently for example, Wang et al 50 studied interaction forces for an asphaltene model compound known as C5Pe in aqueous solutions. C5Pe, also known as N-(1-hexylheptyl)-N′-(5-carboxylicpentyl) perylene-3,4,9,10-tetracarboxylic bisimide, consists of four aromatic rings fused together with three cyclic rings containing heteroatoms of O and N. 50 The interaction force (F/R) between C5Pe-coated surfaces was found to be repulsive, that is of steric and electrostatic origin.…”