Interaction of cationic hydrophobic surfactants at negatively charged surfaces investigated by atomic force microscopy

Cathy E. Mcnamee, Hans Jürgen Butt, Ko Higashitani, Ivan U. Vakarelski, Michael Kappl

Research output: Contribution to journalArticlepeer-review

10 Scopus citations

Abstract

Atomic force microscopy was used to study the adsorption of the surfactant octadecyl trimethyl ammonium chloride (C18TAC) at a low concentration (0.03 mM) to negatively charged surfaces in water. Atomic force microscopy tips were functionalized with dimethyloctadecyl(3-tripropyl)ammonium chloride (C18TAC-si) or N-trimethoxysilylpropyl-N,N,N-trimethylammomium chloride (hydrophilpos-si) to facilitate imaging of the adsorbed surfactant without artifacts. Tapping mode images and force measurements revealed C 18TAC patches, identified as partial surfactant bilayers or hemimicelles. The forces controlling the adsorption process of the C 18TAC to a negatively charged surface were investigated by measuring the forces between a C18TAC-si or a hydrophilpos-si tip and a silica surface in the presence of varying concentrations of either NaCl or NaNO 3. Screening of forces with an increasing NaCl concentration was observed for the C18TAC-si and hydrophilpos-si tips, proving an electrostatic contribution. Screening was also observed for the hydrophilpos-si tip in NaNO3, whereas a long-range attraction was observed for the C18TAC-si tip for all NaNO3 concentrations. These results indicate that screening of the forces for the C18TAC-si tip depended on the type and/or size of the anion, possibly due to a different probability of the anions to enter the silane layers. The interaction of C18TAC patches with C18TAC-si tips in the presence of NaCl and the interaction of the patches with hydrophilpos-si tips in either NaCl or NaNO 3 were repulsive and independent of the number of force curves measured, indicating a stable, positively charged C18TAC patch. However, the forces measured between the patches and a C18TAC-si tip in NaNO3 depended on the number of force curves measured, indicating a change in patch structure induced by the first interaction.

Original languageEnglish (US)
Pages (from-to)11509-11515
Number of pages7
JournalLangmuir
Volume25
Issue number19
DOIs
StatePublished - Oct 6 2009
Externally publishedYes

ASJC Scopus subject areas

  • Electrochemistry
  • Condensed Matter Physics
  • Surfaces and Interfaces
  • Materials Science(all)
  • Spectroscopy

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