An interplay between spacer nature and alkyl chain length on aqueous micellar properties of cationic Gemini surfactants: A multi-technique approach

Abstract

Micellar parameters (critical micelle concentration (cmc), degree of micellar ionization (α), Krafft temperature (Tk), micro-polarity, aggregation number (Nagg) and dielectric constant (εa)) of aqueous cationic geminis (denoted as m-s-m; where, m = 12 or 14 and s = polymethylene, diester, diamido or diester isosorbate) have been studied by multi-technique approach (conductometry, fluorescence and 1H NMR) and compared with alkyltrimethylammonium bromide (CmTAB). Geminis show lower cmc and Tk than CmTAB. Among the geminis, hydrophilic spacer containing geminis show even lower cmc and Tk than the polymethylene spacer geminis. Temperature dependence of cmc shows a decrease, constancy followed by an increase (U-shaped behaviour) as s or m changes. Thermodynamic parameters have been obtained by temperature dependence of cmc. Intra-micellar monomeric arrangement and micellar size information have been acquired by 2D NOESY/COSY 1H NMR and dynamic light scattering (DLS), respectively. Data suggest that the hydrophilic spacer based geminis produce compact micelles with higher polarity compared to polymethylene spacer geminis (or CmTAB). The approach might be employed to tune the micellar interior environment/size, by changing the nature of the spacer or m.