The improvement of the cationic/anionic surfactant interfacial activity via the selective host-guest recognition

Exploring the Recognition Mechanism of Surfactant-Cyclodextrin Complex Formation: Insights from SPR Studies on Temperature and Ionic Liquid Influence

This study examines the kinetics and thermodynamics of the inclusion complex (IC) formation between sodium dodecylbenzenesulfonate (SDBS) and amine-modified β-cyclodextrin (βCD-NH2) using surface plasmon resonance (SPR) and theoretical analysis. We determined a binding constant of 103 L mol-1 for the thermodynamically stable complex ([βCD-NH2/SDBS]°) within the temperature range of 285.2-301.2 K. The thermodynamic analysis revealed a transition from entropy-driven to enthalpy-driven behavior with increasing temperature. The rate constant for IC formation was approximately 102 M-1 s-1, with the residence time decreasing from 14.08 s at 285.2 K to 6.13 s at 301.2 K. We observed the formation of an activated complex ([βCD-NH2/SDBS]‡), with energetic parameters indicating temperature dependence. At 285.2 K, the activated enthalpy change was positive, while at 301.2 K, it was negative. The dissociation energetic parameters remained temperature-independent. Additionally, increasing concentrations of the ionic liquid 1-butyl-3-methylimidazolium chloride influenced the SDBS tail's conformation and penetration into the βCD-NH2 cavity at the activated state. These findings provide insights into the complexation mechanism and the effects of the temperature and ionic liquids on IC formation.

Interfacial Rheological and Emulsion Properties of Self-Assembled Cyclodextrin-Oil Inclusion Complexes

To investigate the effect of the molecular size of alkanes and the cavity size of cyclodextrins (CDs) on the formation of interfacial host-guest inclusion complexes, the interfacial tension (IFT) of CD (α-CD, β-CD, γ-CD) solutions against oils (hexadecane, dodecylbenzene) was determined by interfacial dilational rheology measurements. The results show that the "space compatibility" between CDs and oil molecules is crucial for the formation of interface host-guest inclusion complexes. Hexadecane with a smaller molecular size can form host-guest inclusion complexes with small cavities of α-CD and β-CD, dodecylbenzene with a larger molecular size can form interfacial aggregates with the medium-sized cavity of β-CD easily, and the polycyclic aromatic hydrocarbon molecules in kerosene can form inclusion complexes with the large cavity of γ-CD. The formation of interfacial inclusion complexes leads to lower IFT values, higher interfacial dilational modulus, nonlinear IFT responses to the interface area oscillating, and skin-like films at the oil-water interface. What's more, the phase behavior of Pickering emulsions formed by CDs with different oils is explored, and the phenomena in alkane-CD emulsions are in line with the results in dilatation rheology. The interfacial active host-guest structure in the kerosene-γ-CD system improves the stability of the Pickering emulsion, which results in smaller emulsion droplets. This unique space compatibility characteristic is of great significance for the application of CDs in selective host-guest recognition, sensors, enhanced oil recovery, food industries, and local drug delivery.