An. Parikh et al., CORRELATION OF MOLECULAR-ORGANIZATION AND SUBSTRATE WETTABILITY IN THE SELF-ASSEMBLY OF N-ALKYLSILOXANE MONOLAYERS, Journal of physical chemistry, 99(24), 1995, pp. 9996-10008
Monolayers of n-octadecylsiloxane (CH3(CH2)(17)SiOxHy;ODS) were self-a
ssembled from n-octadecyltrichlorosilane solutions onto a series of OH
- and CH3-containing surfaces prepared from the self-assembly of contr
olled composition mixtures of HO(CH2)(16)SH and H3C(CH2)(15)SH on gold
(RS/Au). Using null ellipsometry, infrared spectroscopy, and hexadeca
ne contact angles; the coverages, chain structures, and surface wettin
g of the formed ODS assemblies were determined as a function of the OH
fraction, f(OH) = [OH]/[CH3 + OH], in the starting RS assembly. Three
distinct ODS adsorption regimes were observed: (1) on pure CH3 surfac
es no stable adsorbed layer forms; (2) for 0.1 less than or similar to
f(OH) less than or similar to 0.8, the coverage is incomplete and mon
otonically increases with f(OH) and the ODS structures consist of a ra
nge of coexisting domains of nearly all-trans chains and disordered, l
iquid-like components with maximum disorder content, estimated as > 80
%, arising near f(OH) similar to 0.5; and (3) for f(OH) > 0.8, a high
coverage, close-packed monolayer is formed with predominantly all-tran
s chains tilted at 8-12 degrees from the surface normal, a distinctly
different tilt than the known value of 26-30 degrees for the RS underl
ayer and an indication of strong structural decoupling (incommensurabi
lity) between the two highly organized layers. The f(OH)-dependence of
the structures is explained on the basis of a previously proposed hyp
othesis that a continuous preadsorbed, substrate-bound water film is r
equired for achieving maximum organization during n-alkylsiloxane self
-assembly and that, in the present case of the OH/CH3 surfaces, the re
quired water film structure at the preparation solution/substrate inte
rface is not reached until high f(OH) values.