Fluorescence probe studies upon microenvironment characteristics and aggregation properties of gemini sugar surfactants in an aquatic environment

The micellar aggregation behavior and micropolarity characteristics of a series of N,N′-bisalkyl-N,N′-bis[(3-d-aldonylamido)propyl]ethylenediamines referred to as gemini bis(CnX) surfactants (where Cn = n-butyl, n-octyl, n-decyl and n-dodecyl; X = gluconyl and lactobionyl, denoted as GA and LA, respectively) has been investigated by steady-state fluorescence, SSF (by emission spectra of pyrene) and time-resolved fluorescence quenching, TRFQ (using pyrene as the fluorescence probe and cetylpyridinium ion as the fluorescence quencher). Remarkably low magnitude cmc values were found for the studied aldonamide-type gemini surfactants, in a range from 4.5 × 10−6 M (bis(C12GA)) to 2.3 × 10−3 M (bis(C4LA)). The variation of cmc for bis(CnLA) (n = 4, 8, 10 and 12) follows a linear semi-logarithmic pattern. The obtained I1/I3,c > cmc ratios from 1.41 to 1.5 indicate a considerable high polarity sensed by pyrene in micelles of bis(CnX), dependent upon both the kind of aldonyl entity, and the length of hydrophobic tail. According to TRFQ, all studied systems display micellar growth (demonstrated by values of an average aggregation number of surfactant chains per micelle) with increasing surfactant concentration typically from 100 up to 1000 times more than the cmc. The aggregation numbers, Nagg (Ntail) and the “polydispersity index”, σ/Nω (less than ∼0.3), show that micelles of bis(CnLA)’s of up to ca. 500cmc, are nearly spherical and almost monodisperse. Their spheroidal micelle growth and enhanced micellar polydispersity is observed as the surfactant concentration increases. Such a behavior was found to be strictly related to geometric aspects of a given surfactant dimeric structure.