The gemini surfactants, alkanediyl- α,ω-bis (dodecyldimethylammmonium) bromide, form aggregates, known as micelles, at the critical micelle concentration (CMC). These surfactants are conventionally labelled with 12-n-12 symbols to address the alkyl chains with a variable linker, n. Many gemini surfactants are known to readily undergo a sphere-to-rod transition above the CMC. This transition has been reported for 12-2-12 with a definite value of 4.2 mM. The transition for 12-3-12 has been roughly defined over a range of 20 to 60 mM and reports on the 12-4-12 surfactant indicate a transition well above 60 mM. This research investigated the sphere to rod transition of 12-3-12 at 25 °C applying two methods: conductivity and chemical trapping. The sphere-to-rod transition can be detected using chemical trapping where HPLC is utilized to quantify interfacial concentrations of water and the bromide counterion. Standards of the possible products were created and analyzed in order to calculate the relative product ratios of the chemical trapping experiments. Based on the relative water and bromide product ratios for micellar samples, the sphere-to-rod transition of the 12-3-12 occurs at 30 mM and no transition of the 12-4-12 occurs below 50 mM. Complementary conductivity measurements in the same concentration range yielded a change in the linear increase as a function of total surfactant concentration for the 12-3-12 yet show a simple linear behavior for the 12-4-12. The observed slope change for 12-3-12 was observed at 32 mM. Both methods support a change in aggregate shape in a narrowly defined range around 30 mM. Since it is known that the sphere to rod transition is sensitive to added electrolyte, it should be noted that this work was recorded with no added sodium bromide.

Chemical trapping and conductivity to determine sphere to rod transition of 12-3-12 surfactant

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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