Role of Naphthenic Acids in Controlling Self-Aggregation of a Polyaromatic Compound in Toluene

Molecular dynamics simulation on water/oil interface with model asphaltene subjected to electric field

HYPOTHESIS

The droplet-medium interfaces of petroleum emulsions are often stabilized by the indigenous surface-active compounds (e.g., asphaltenes), causing undesired issues. While demulsification by electric field is a promising technique, fundamental study on the droplet-medium interface influenced by electric field is limited. Molecular dynamics (MD) simulations are expected to provide microscopic insights into the nano-scaled water/oil interface.

METHODS

MD simulations are conducted to study the adsorption of model asphaltene molecules (represented by N-(1-hexylheptyl)-N'-(5-carboxylicpentyl) perylene-3,4,9,10-tetracarboxylic bisimide (C5Pe)) on a water-toluene interface under various strengths of electric field. The adsorption amount and structural feature of C5Pe molecules at water-toluene interface are investigated, and the effects of electric field and salt are discussed.

FINDINGS

C5Pe molecules tend to adsorb on the water-oil interface. As the electric field strength increases, the adsorption amount first slightly increases (or remains constant) and then decreases. The electric field disrupts the compact π-π stacking between C5Pe molecules and increases their mobility, causing a dispersed distribution of the molecules with a wide range of orientations relative to the interface. Within the studied range, the addition of salt ions appears to stabilize the interface at high electric field. These results provide useful insights into the mechanism and feasibility of demulsification under electric field.

Molecular Basis of the Reinforced Effect of Berberine against Cutinase from Colletotrichum capsisi by Supplying Sodium Stearate as Dispersant

Berberine (BBR) is a promising botanical pesticide that can reduce the enzyme activity of secreted cutinase from fungal pathogens. However, only less than 15% of total activity was prohibited. Herein we researched BBR's self-aggregation in water via molecular dynamics simulations, and further investigated the effect of dispersant on blocking the aggregation together with the impact on cutinase. Strong hydrophobic interactions were found between adjacent BBR molecules, and these molecules formed clustered conformations at different BBR concentrations. Interestingly, one of the tested dispersants, sodium stearate (ST), is able to insert into BBR clusters and form stable interaction until the end of simulation, resulting in decreased hydrophobic strength in the BBR-ST cluster. More importantly, supply of ST with BBR resulted in BBR's reinforced hydrophobic interactions between BBR and the catalytic center of cutinase, which led to the inactivated mode of cutinase. Finally, wet experiments demonstrated that combined application of BBR and ST indeed resulted in a synergy-like effect on reducing the activity of cutinase. Overall, our findings revealed the mechanism of the reinforced effect of BBR against cutinase when supplying ST as dispersant, suggesting an undiscovered role of ST in enhancing the efficiency of this botanical pesticide.

Competitive Adsorption of Naphthenic Acids and Polyaromatic Molecules at a Toluene-Water Interface

The early-stage competitive co-adsorption of interfacially active naphthenic acids (NAs) and polyaromatic (PA) molecules to a toluene-water interface from the bulk toluene phase was studied using molecular dynamics (MD) simulation. The NA molecules studied had the same polar functional group but different cycloaliphatic nonpolar tails, and a perylene bisimide (PBI)-based molecule was used as a representative PA compound. The results from our simulations suggest that the size and structural features of NA molecules greatly influence the interfacial activity of PA molecules and partitioning of NA molecules at the toluene-water interface. At low concentrations of PA (∼2.3 wt %) and NA (∼0.4 wt %) molecules, NA molecules containing large cycloaliphatic rings (e.g., four rings) or with a very long aliphatic tail (e.g., carbon chain length of 14) were observed to impede the migration of PA molecules to the interface, whereas small NA molecules containing two cycloaliphatic rings had little effect on the adsorption of PA molecules at the toluene-water interface. At high NA concentrations, the adsorption of PA molecules (∼5.75-17.25 wt %) was greatly hindered by the presence of small NA molecules (∼1.6-4.8 wt %) due to the solvation of PA nanoaggregates in the bulk. Adsorption mechanisms of PA and NA molecules at toluene-water interfaces were clarified through a detailed analysis on the interactions among different species in the system. The results obtained from this work provide insights into designing appropriate chemical demulsifiers or co-demulsifiers for breaking water-in-oil emulsions of great industrial applications.