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 ser ies 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 s pacer, respectively. The compounds studied were C-12-C-3-C-12, C-12-C-4-C-1 2, C-12-C-6-C-12, C-12-C-12-C-12 and C-12-xylyl-C-12, where the spacer is t he xylene group, -CH2-phi-CH2, in the last of these compounds. By the use o f partially deuterated forms of the surfactants in null reflecting water, n eutron reflection was used to measure directly values of the area per molec ule at the critical micelle concentration (cmc) of 66, 82, 95, 140, and 97 +/- 3 Angstrom(2), respectively. Comparison with the results from surface t ension measurements shows that the appropriate prefactor P in the Gibbs equ ation, d gamma =-PRT Gamma d ln c, is approximately 2 for all except the co mpound with the xylyl spacer, for which P is about 3. A tentative explanati on 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. So me 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, consist ent with dissociation of the complex.