Document Detail

Odd-Even Effect in the Hydrophobicity of n-Alkanethiolate Self-Assembled Monolayers Depends on the Roughness of the Substrate and the Orientation of the Terminal Moiety.
MedLine Citation:
PMID:  25233063     Owner:  NLM     Status:  Publisher    
The origin of the odd-even effect in properties of self-assembled monolayers (SAMs), and/or in technologies derived from them, is poorly understood. We report that hydrophobicity, and hence surface wetting of SAM, is dominated by the nature of the substrate (surface roughness and identity) and SAM tilt angle, which influences surface dipoles/orientation of the terminal moiety. We measured static contact angles (θs) made by water droplets on n-alkanethiolate SAMs with odd (SAM(O)) or even (SAM(E)) number of carbons (ave. θs range: 105.8° - 112.1°). When SAMs were fabricated on smooth 'template-stripped' metal (M(TS)) surfaces (rms roughness = 0.36±0.01 nm for Au(TS) and 0.60±0.04 nm for Ag(TS)), the odd-even effect, characterized by a zig-zag oscillation in values of θs was observed. We, however, did not observe the same effect with rougher - 'as deposited' (M(AD)) - surfaces (rms roughness = 2.27±0.16 nm for Au(AD) and 5.13±0.22 nm for Ag(AD)). The odd-even effect in hydrophobicity inverts when the substrate changes from Au(TS) (higher θs for SAM(E) than SAM(O) with average Δθs |n-(n+1)| ≈ 3°) to Ag(TS) (higher θs for SAM(O) than SAME with average Δθs |n-(n+1)| ≈ 2°). Comparison of hydrophobicity across Ag(TS) and Au(TS) showed statistically significant difference (student t-test) between SAM(E) (Δθs |Ag evens - Au evens| ≈ 5°, p < 0.01) but failed to show statistically significant differences on SAM(O) (Δθs |Ag odds - Au odds| ≈ 1°, p>0.1 From these results we deduce that the roughness of the metal substrate (from comparison of M(AD) vs M(TS)) and orientation of the terminal -CH2CH3 (by comparing SAM(E) and SAM(O) on Au(TS) vs Ag(TS)) plays a major role in the hydrophobicity, and by extension general wetting properties, of n-alkanethiolate SAMs.
Lucas B Newcomb; Ian D Tevis; Manza B J Atkinson; Symon Gathiaka; Rafael E Luna; Martin Thuo
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Publication Detail:
Type:  JOURNAL ARTICLE     Date:  2014-9-18
Journal Detail:
Title:  Langmuir : the ACS journal of surfaces and colloids     Volume:  -     ISSN:  1520-5827     ISO Abbreviation:  Langmuir     Publication Date:  2014 Sep 
Date Detail:
Created Date:  2014-9-18     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  9882736     Medline TA:  Langmuir     Country:  -    
Other Details:
Languages:  ENG     Pagination:  -     Citation Subset:  -    
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