Effects of surfactant concentration on alkyl chain arrangements in dry and swollen organic montmorillonite

• The d-spacings of dry and swollen OMt are affected by surfactant concentration.
• The surfactant polar head overturns at a low concentration for the dry OMt.
• The maximum d-spacing of dry OMt larger than previous reported is founded.
• Alkyl chains form a paraffin-type monolayer in NMP with various tilting angles.
• Solvent polarity, surfactant loading and molecular length control OMt swelling.

To investigate the effects of surfactant concentration on alkyl chain arrangements in dry and swollen organic montmorillonite (OMt), Na+-montmorillonite (Na+-Mt) was modified with cationic surfactants with different alkyl chain lengths, including tetradecyl-trimethylammonium bromide (TTAB), cetyltrimethylammonium bromide (CTAB) and steartrimonium bromide (STAB), at a series of concentrations. The properties of dry and swollen OMt were characterized by thermal analysis and X-ray diffraction. Two kinds of novel arrangement of alkyl chains were found for dry CTAB-Mt. The surfactant polar head showed two arrangements when the CTAB concentration increased from 0.23 to 0.45 cation exchange capacity (CEC). The arrangement of the paraffin-type bilayer had a larger tilting angle at 1.93 CEC than that of reported research, leading to a maximum d-spacing of 5.81 nm. For swollen CTAB-Mt in the solvent, the d-spacing first increased and then remained constant with increase in the amount of added CTAB. The swelling behavior was proposed, including three distinct stages: solvent-controlling stage, surfactant concentration-controlling stage, and alkyl chain length-controlling stage. The alkyl chain arrangements of dry and swollen OMt were further confirmed by TTAB-Mt and STAB-Mt.

The alkyl chain arrangements in the interlayer.Figure optionsDownload as PowerPoint slide