Interaction between a Novel Gemini Surfactant and Cyclodextrin: NMR and Surface Tension Studies

Abstract

The interaction between cyclodextrins, hydroxypropyl-β-cyclodextrin (HPβCD), and hydroxypropyl-γ-cyclodextrin (HPγCD) and a novel type of nonionic surfactant synthesized from a fatty acid has been investigated. The so-called nonionic heterogemini surfactant (NIHG750) contains two hydrophobic groups and two hydrophilic groups, composed of one monomethyl ethylene glycol and one secondary OH group, CH3(CH2)7–CH[OH]–CH[O(CH2CH2O)16CH3]–(CH2)7CN. Surface tension studies indicate that micelles form in NIHG750 systems in both the presence and the absence of small quantities (molar ratio (HPβCD:NIHG750)≈2) of cyclodextrin (HPβCD or HPγCD). This gives NIHG surfactants an advantage compared to single-tailed nonionic surfactants, which generally lose their ability to micellize at much lower additions of cyclodextrins. However, the interaction between HPβCD and NIHG750 results in a disruption of the micellar aggregates at higher levels of cyclodextrin. In the dilute systems (CNIHG750<0.1% (w/w) aprox) prolate-shaped mixed aggregates (HPβCD and NIHG750) form, with a short and a long axis of the order of 8–9 and 17–20 Å, respectively. These gradually aggregate into micellar-like structures at higher concentrations. In the aqueous bulk phase HPβCD interacts mainly with the hydrophobic part of NIHG750, but both NMR and surface tension measurements indicate that an interaction with the hydrophilic part of NIHG750, as well, may exist. This interaction results in a better packing of NIHG750 at air-water interfaces. However, at elevated temperatures results from turbidity measurements indicate that NIHG750 and HPβCD interact mainly through the hydrophilic part of the surfactant; a decrease in the cloud point temperature is observed. The interaction of the larger cavity molecule, HPγCD, with NIHG750, on the other hand, seems to be relatively weak. The interaction, when present, most probably takes place through inclusion of the hydrophilic EO part of NIHG750. The results suggest that HPγCD in combination with NIHG750 is a better solubilizing system than with HPβCD.