Micellar structures of dimeric surfactants with phosphate head groups and wettable spacers: A small-angle neutron scattering study

Dimeric or gemini surfactants consist of two hydrophobic chains and two hyd rophilic head groups covalently connected by a hydrophobic or hydrophilic s pacer. This paper reports the small-angle neutron scattering (SANS) measure ments from aqueous micellar solutions of two different recently developed t ypes of dimeric surfactants: (i) bis-anionic C16H33PO4--(CH2)(m)-PO4-C16H33 ,2Na(+) dimeric surfactants composed of phosphate head groups and a hydroph obic polymethylene spacer, referred to as 16-m-16,2Na(+), for spacer length s m = 2, 4, 6, and 10, (ii) bis-cationic C16H33N+(CH3)(2)-CH2-(CH2-O-CH2)(p )-CH2-N+ (CH3)(2)C16H33,2Br(-) dimeric surfactants composed of dimethylammo nium head groups and a wettable polyethylene oxide spacer, referred to as 1 6-CH2-p-CH2-16,2Br(-), for spacer lengths p = 1 - 3. The micellar structure s of these surfactants are compared with the earlier studied bis-cationic C 16H33N+ (CH3)(2)-(CH2)(m)-N+ (CH3)(2)C16H33,2Br(-) dimeric surfactants comp osed of dimethylammonium head groups and a hydrophobic polymethylene spacer , referred to as 16-m-16,2Br(-). It is found that 16-m-16,2Na(+), similar t o 16-m-16,2Br(-), form various micellar structures depending on the spacer length. Micelles an disklike for rn = 2, rodlike for m = 4, and prolate ell ipsoidal fur m = 6 and 10. The micelles of 16-CH2-p-CH2-16,2Br(-) are prola te ellipsoidal for all the values of p = 1 - 3. It is also found that micel les of 16-m-16,2Na(+) and 16-CH2-p-CH2-16,2Br(-) are large in comparison to those of 16-in-16,2Br(-) for similar spacer lengths. This is connected wit h the fact that both in 16-m-16,2Na(+) and 16-CH2-p-CH2-16,2Br(-), the head group or the spacer is more hydrated as compared to that in the 16-m-16,2B r(-). An increase in the hydration of the spacer or the head group increase s the screening of the Coulomb repulsion between the charged head groups. T his effect has been found to be more pronounced in the dimeric surfactants having wettable spacers. [S1063-651X(99)00303-7].