Investigation of humate-cetyltrimethylammonium complexes by small-angle X-ray scattering

The structural examination of the complexes formed between humic acid and cationic surfactants has environmental implications. A humic acid (HA) dissolved in 0.1 M NaOH (5 g/L) was reacted with a cationic surfactant (hexadecyltrimethylammonium bromide or CTAB) at initial solution concentrations of 1, 5, 10, 20, 30, 40 and 50 mM. The HA precipitated at CTAB concentrations of 20, 30, and 50 mM but the complexes were soluble at 40 mM and below 20 mM. The charge neutralization between humic acid anions and CTAB micelles and the subsequent charge reversal due to hydrophobic interactions explain the behavior of the HA-CTAB complexes. The HA solution (5 g/L), reaction products (supernatants and precipitates), and pure cationic surfactant solutions were studied by the small-angle X-ray scattering (SAXS) technique in order to determine the structure of HA-CTAB complexes. The scattering intensity (I(q)I(q)) of various HA-CTAB systems were recorded over a range of scattering vectors (q=0.053–4.0 nm−1q=0.053–4.0 nm−1). HA forms networks in an alkaline solution with a characterization length of 7.8 nm or greater. The HA-CTAB precipitates and the 50-mM CTAB solution gave d100d100 and d110d110 reflections of a hexagonal structure. The hexagonal array of cylindrical CTAB micelles has a lattice parameter of 5.01 nm in pure solution, and the parameter decreases in the order: 4.96, 4.91, and 4.85 nm for the precipitates of HA-CTAB (50, 30, and 20 mM, respectively), indicating that the structure of CTAB micelles was disturbed by the addition of HA. The molecular properties and behavior of HA in solution were discussed.

Humic acid acts as a polyvalent anion neutralizing the positive charges on CTAB micelles in a manner similar to the reaction between charged polymers and surfactants. At high CTAB concentration the hexagonal array structure of CTAB micelles was preserved, though deformed by the interaction, suggesting that there is no regular repetition of structural units in humic acid because such structural regularity would have caused the formation of new CTAB structures.Figure optionsDownload as PowerPoint slide