AFM surface force measurements conducted with silica in CnTACl solutions: Effect of chain length on hydrophobic force

Jinhong Zhang, Roe Hoan Yoon, Jan Christer Eriksson

Research output: Contribution to journalArticle

28 Scopus citations

Abstract

Surface forces were measured using an AFM with silica surfaces immersed in CnTACl (n = 12-18) solutions in the absence of added salt. The results showed long-range attractive forces that cannot be explained by the DLVO theory. The long-range attractions increased with increasing surfactant concentration, reaching a maximum at the point of charge neutralization (p.c.n.) and then decreased. The long-range forces decayed exponentially, with the decay lengths increasing from 3 to 32 nm as the chain length of the surfactants increased from C-12 to C-18. The measured forces can be fitted to the charged-patch model of Miklavic et al. [S.J. Miklavic, D.Y.C. Chan, L.R. White, T.W. Healy, J. Phys. Chem. 98 (1994) 9022-9032] by assuming patch sizes that are much larger than the values reported in the literature. It was found that the decay length decreases linearly with the effective concentration of the CH2/CH3 groups of the CnTACl homologues raised to the power of -1/2, which is in line with the Eriksson et al.'s hydrophobic force model derived using a mean-field approach. It appears, therefore, that the long-range attractions observed in the present work are hydrophobic forces originating from changes in water structure across the thin surfactant solution film between the silica surfaces. It is conceivable that hydrocarbon chains in solution disrupt the surface-induced water structure and cause a decrease in hydrophobic force. This observation may also provide an explanation for the very long-range forces observed with silylated, LB-deposited, and thiol-coated surfaces.

Original languageEnglish (US)
Pages (from-to)335-345
Number of pages11
JournalColloids and Surfaces A: Physicochemical and Engineering Aspects
Volume300
Issue number3 SPEC. ISS.
DOIs
StatePublished - Jun 15 2007
Externally publishedYes

Keywords

  • CTACl homologues
  • Charged patch
  • Film tension
  • Hydrophobic force
  • Long-range attraction
  • Point of charge neutralization
  • Water structure

ASJC Scopus subject areas

  • Surfaces and Interfaces
  • Physical and Theoretical Chemistry
  • Colloid and Surface Chemistry

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