Role of Spacer Chain Length in Dimeric Micellar Organization. Small Angle Neutron Scattering and Fluorescence Studies

Abstract

Micelles of different dimeric amphiphiles Br -, n-C 16 H 33 NMe 2 + -(CH 2 ) m -N + Me 2 -n-C 16 H 33 , Br -(where m ) 3, 4, 5, 6, 8, 10, and 12) adopt different morphologies and internal packing arrangements in aqueous media depending on their spacer chain length (m). Detailed measurements of small angle neutron scattering (SANS) cross sections from different bis-cationic, dimeric surfactant micelles in aqueous media (D 2 O) are reported. The data have been analyzed using the Hayter and Penfold model for macro ion solution to compute the interparticle structure factor S(Q) taking into account the screened Coulomb interactions between the dimeric micelles. The SANS analysis clearly indicated that the extent of aggregate growth and the variations of shapes of the dimeric micelles depend primarily on the spacer chain length. With spacer chain length, m e 4, the propensity of micellar growth was particularly pronounced. The effects of the variation of the concentration of dimeric surfactants with m ) 5 and 10 on the SANS spectra and the effects of the temperature variation for the micellar system with m ) 10 were also examined. The critical micelle concentrations (cmc) and their microenvironmental feature, namely, the microviscosities that the dimeric micellar aggregates offer to a solubilized, extrinsic fluorescence probe, 1,6-diphenyl-1,3,5-hexatriene, were also determined. The changes of cmcs and microviscosities as a function of spacer chain length have been explained in terms of conformational variations and progressive looping of the spacer in micellar core upon increasing m values.

A fluorescence probe study of gemini surfactants in aqueous solution: a comparison between n-2-n and n-6-n series of the alkanediyl-a,w-bis (dimethylalkylammonium bromides)

“…The ionization degrees of the n-6-n surfactants are on average twice as high as those obtained by the n-2-n series (See Table 1). These results agree with those reported in the literature, which showed a significant increase in the micelle ionization degree upon increasing spacer carbon number for dimerics with a hydrophobic spacer: 10-s-10 [27-29], 12-s-12 [18], 16-s-16 [30,31]. In general the a values for both n-2-n and n-6-n series are higher than those reported for common alkyltrimethylammonium.…”

Dimeric (Gemini) Surfactants: Effect of the Spacer Group on the Association Behavior in Aqueous Solution

“…7) (12,62,63). This value of s is also that for which the melting temperature of the m-s-m surfactants is a maximum.…”

Micelle formation and CMC of gemini surfactants: a thermodynamic model

“…CTAB (cetyl trimethyl ammonium bromide) can be compared with the C 16 series of gemini surfactants. The CMC for CTAB is 10 − 3 M [10], while for 16-3-16,2 Br − , the CMC is nearly two orders of magnitude lower (2.50-2.55× 10 − 5 M) [8,11]. Further, gemini surfactants with short spacers form large, threadlike aggregates with interesting rheological properties, whereas the corresponding monomeric surfactant forms only small, spherical aggregates [12].…”