Adsorption of ionic surfactants at the air-solution interface

Neutron reflection (NR) and surface tension methods were compared for accessing the equilibrium adsorption isotherm of various ionic surfactants at the air-water interface. Four custom-synthesized anionics we,re investigated in detail: sodium dihexyl sulfosuccinate (di-C6SS), bis(1H,1H-perfluoro-n-pentyl)sodium sulfosuccinate (di-CF4), bis(1H,1H,5H-octafluoro-n-pentyl)sodium sulfosuccinate (di-HCF4), and bis(1H, 3H,5H-octafluoropentyl)-2-sulfoglutaconate (di-HCF4GLU). Commercial n-alkyltrimethylammonium bromide (Cn-TAB) cationic surfactants with C12, C14, and C16 chain lengths were also studied. The experiments examined the validity of the Gibbs equation, and the prefactor 2, for these seven compounds. Effects of contaminants, trace levels of polyvalent metal ions, and hydrophobic impurities were assessed for the anionics. When added at low levels, tetrasodium ethylenediaminetetraacetate (EDTA) was effective for eliminating effects of metallic impurities, and foam fractionation was used to remove hydrophobic contaminants. The effects of these treatments on the apparent surface excess and procedures for obtaining agreement between the neutronic and tensiometric isotherms are described. Finally, it was confirmed that the Gibbs prefactor of 2 applies for all these 1:1 ionic surfactants.