Synthesis and properties of a novel class of anionic gemini surfactants with polyoxyethylene spacers

A series of anionic gemini surfactants called (oligooxa)-α,ω-bis(m-alkylbenzene sulfonate) with different length in spacer and in hydrophobic chain, C8ExC8 (x = 1, 2, 3, 4, 8) and CnE4Cn (n = 8, 10, 12), have been synthesized from 4-n-alkylphenol and their basic physicochemical properties are investigated. The results indicate that they are different from cationic gemini surfactants called (oligooxa)alkanediyl-α,ω-bis(dimethyl-dodecylammonium bromide), 12-CH2CH2(EO)s-12, in the literature. As the number of oxyethylene segments increases, the cmc (critical micelle concentration) decreases initially and then increases, and the surface area per molecule decreases initially and then tends to a constant value of approximate 1.30 nm2. Both of the breakpoints appear at number 4 of oxyethylene segments in the spacer. It is confirmed that the hydrophilic spacer with oxyethylene moieties is not fully extended at the air–water interface. With increasing spacer length, the spacer becomes sufficiently flexible to adopt a particular conformation with a loop at the water side of the air–water interface. The fluorescence data show that the micropolarity increases little from C8E1C8 to C8E4C8 and decreases obviously when x changes from 4 to 8. In addition, the plot of the logarithm of the cmc against the alkyl chain length for CnE4Cn shows a linear decrease with the increase in chain length. The micropolarity of the gemini surfactants (CnE4Cn) decreases with an increase in the alkyl chain length (n from 8 to 12).