Self-assembling characteristics of a new nonionic gemini surfactant

Abstract

A new class of unusual nonionic gemini surfactant, viz. the bis-amide p-phenylenediamine Boc-bis-glycamide, was synthesized for the first time in our laboratory. The bis-amide is insoluble in water. However, the evidence for micelle formation of this bis-amide in chloroform has been obtained from surface tension, Langmuir film balance, and UV-visible, IR, and fluorescence spectroscopy. The critical micelle concentration (cmc) of this bis-amide, obtained by these techniques, correlates well. Tensiometry gave the cmc value, surface activity, and Gibbs molecular surface area. Film studies on the bis-amide gemini gave limiting areas and collapse pressures. The results of film studies also suggest that the bis-amide forms a stable monolayer film even at low concentrations and the limiting areas obtained at high concentrations (above the cmc value) are in good agreement with the contact area estimated from the theoretically calculated Connolly surface treatments. Both theoretical and experimental results suggest that the bis-amide molecules at high concentrations (above the cmc value) prefer slightly tilted conformations rather than a fully extended conformation at the air/water interface. The aggregation numbers of the bis-amide have also been determined by using both steady-state and time-resolved fluorescence methods, and they are in good agreement with each other. The concentration dependence aggregation number for bis-amide micelles has also been observed, and the results suggest the formation of only small micelles, despite the potential to grow; this is similar to the case for many micellar geminis. Computer simulation techniques show that about 11 molecules of bis-amide can aggregate to form a cluster. The IR spectra of the bis-amide in pre- and postmicellar regions were analyzed; there is no significant change in the intensity of the intermolecular hydrogen-bonding pattern for the bis-amide in the monomeric and micellar states. However, the intensity of the solvent-exposed -N-H stretching band increased as a function of bis-amide concentration after the cmc value was obtained. The fluorescence intensity of ANS in chloroform increases on interaction with bis-amide micelles, suggesting that, even in apolar media, ANS binds to the bis-amide in a region of lower polarity than does the chloroform alone. The pyrene emission intensity ratio of the first vibronic band to the third band as a function of bis-amide concentration suggested the hydrophobic character of bis-amide micelles in chloroform. Since there is no trace amount of water present in the system and the aggregate formation of the bis-amide is in the presence of pure chloroform solvent only, the species may be considered an inverted micelle. Steady-state fluorescence results suggest that 66% of the total pyrene emission is accessible to quenching in bis-amide micelles in chloroform. On the basis of lifetime measurements, the bimolecular quenching rate constant of the pyrene by CPC quencher in bis-amide micelles was found to be (1.4 ± 0.3) × 10⁷ M^-1 s^-1, which is due to collisional quenching by strongly partitioning quenchers.