posted on 2012-12-21, 00:00authored byChihiro Urata, Benjamin Masheder, Dalton F. Cheng, Atsushi Hozumi
Alkylsilane-derived monolayer-covered surfaces generally
display
a reasonably good level of hydrophobicity but poor oleophobicity.
Here, we demonstrate that the physical attributes of alkylsilane-derived
surfaces (liquid-like or solid-like) are dependent on the alkyl chain
length and density, and these factors subsequently have significant
influence upon the dynamic dewetting behavior toward alkanes (CnH2n+2, where n = 7–16). In this study, we prepared and characterized
hybrid films through a simple sol–gel process based on the
cohydrolysis and co-condensation of a mixture of a range of alkyltriethoxysilanes
(CnH2n+1Si(OEt)3, where n = 3, 6, 8, 10, 12, 14, 16, and
18) and tetramethoxysilane (TMOS). Surprisingly, when the carbon number
(Cn) of alkyl chain was 10 and below,
the produced hybrid films were all smooth, highly transparent, and
showed negligible contact angle (CA) hysteresis. On these hybrid surfaces,
5 μL drops of alkanes (n-hexadecane, n-dodecane, and n-decane) could move easily
at low tilt angles (<5°) without pinning. On the other hand,
when the Cn exceeded 12, both transparency
and mobility of probe liquids significantly worsened. In the former
case, TMOS molecules played key roles in both forming continuous films
(as a binder) and improving flexibility of alkyl chains (as a molecular
spacer), resulting in the smooth liquid-like surfaces. Silylation
of the hybrid film and subsequent dynamic CA measurements proved the
presence of silanol groups on the outermost surfaces and demonstrated
that the dynamic dewettability of hybrid films worsened as packing
densities increased. Additionally, solvent effects (high affinity)
between the alkyl chains and alkane liquids imparted a more liquid-like
character to the surface. Thanks to these simple physical effects,
the resistance to the alkane droplet motion across tilted surfaces
was markedly reduced. With the longer carbon chains, the chain mobility
was strictly inhibited by mutual interactions between neighboring
alkyl chains even in the presence of TMOS molecules. The achieved
surfaces displayed a solid-like nature along with surface defects,
leading to inferior dynamic oleophobicity. Therefore, the critical
Cn of alkyl chain used for determining
final dynamic dewetting behavior against alkane liquids was 12. Furthermore,
our hybrid surfaces exhibited excellent antifingerprint properties,
particularly demonstrating low adhesion and easy removal from the
surface.