Neutron reflectivity studies of the surface excess of gemini surfactants at the air-water interface

Neutron reflection has been used to determine the surface coverage of a series of cationic gemini surfactants of the general formula [CmH2m+1N(CH3)(2)-(CH2)(s)-N(CH3)(2)CmH2m+1]B-2 designated C-n-C-m-C-n, where n and m denote the number of carbon atoms in the chains of the free alkyl chain and the spacer, respectively. The compounds studied were C-12-C-3-C-12, C-12-C-4-C-12, C-12-C-6-C-12, C-12-C-12-C-12 and C-12-xylyl-C-12, where the spacer is the xylene group, -CH2-phi-CH2, in the last of these compounds. By the use of partially deuterated forms of the surfactants in null reflecting water, neutron reflection was used to measure directly values of the area per molecule at the critical micelle concentration (cmc) of 66, 82, 95, 140, and 97 +/- 3 Angstrom(2), respectively. Comparison with the results from surface tension measurements shows that the appropriate prefactor P in the Gibbs equation, d gamma =-PRT Gamma d ln c, is approximately 2 for all except the compound with the xylyl spacer, for which P is about 3. A tentative explanation of the unexpected value of 2 is that the dicationic surfactant ions are all in the form of a 1:1 complex with the Br- ions in the bulk solution. Some support for this comes from the behavior of the C-12-C-6-C-12 compound, for which P changes to 3 at concentrations much lower than the cmc, consistent with dissociation of the complex.