We report the formation of self-assembled monolayers (SAMs) on gold substra
tes by exposure to n-alkanethiols [CH3(CH2)(n-1)SH; n = 8, 10, 12, 16, and
18] in liquid and supercritical carbon dioxide. The results of this novel s
tudy show that an environmentally friendly solvent can be used to form high
ly crystalline SAMs with few gauche defects and that pressure as well as ex
posure time can be used to affect the structural and barrier properties of
the monolayer film. Reflectance infrared spectroscopy, electrochemical impe
dance spectroscopy, and wetting measurements were used to characterize the
SAMs. The effects of pressure (76-300 bar) and adsorption time (3-90min) on
the formation of the SAMs were explored. The overall chain density of thes
e SAMs was greater than that for SAMs formed in common organic solvents suc
h as ethanol. The properties of the SAMs were slightly affected by the pres
sure during formation. At 35 degreesC, as the carbon dioxide pressure incre
ased (from 76 to about 140 bar), the packing density and resistance of the
SAM increased. SAMs prepared at higher pressures ranging from about 140 to
300 bar exhibited similar resistances, capacitances, and canted structures.
There was also no significant difference in using liquid (25 degreesC and
103 bar) or supercritical (35 degreesC and 103 bar) carbon dioxide for SAM
formation. Supercritical carbon dioxide also enabled the formation of SAMs
using polar adsorbates (-OH- and -CO2H-terminated thiols) to prepare high-e
nergy surfaces that are wet by water.