Chain-chain interaction between surfactant monolayers and alkanes or alcohols at solid/liquid interfaces

Sum-frequency surface vibrational spectroscopy was used to study chain conformation of self-assembled surfactant monolayers at various quartz/alkane and quartz/alcohol interfaces as a function of surfactant surface density and chain lengths of both surfactant and alkane or alcohol molecules. We found that, in comparison with fully packed monolayers such as octadecyltrichlorosilane (OTS) on glass or alkanethiols on gold, the chain conformation of loosely packed monolayers is considerably more sensitive to their environment. With sufficiently high surfactant surface density and sufficiently long chain lengths of both surfactant and alkane, the chain-chain interaction between surfactant and alkane molecules can effectively eliminate all the gauche defects initially present in the surfactant chains. With alkanes replaced by alcohols the hydrophobic effect appears to be dominant, as the alcohol molecules like to form a hydrogen-bonding network at an interface with the hydroxyl groups facing the surfactant monolayer. This causes the surfactant chains to curl up. However, if the long-chain alcohols are diluted in a nonpolar liquid, the hydrogen-bonding network is disrupted and the alcohol can now adsorb individually at the interface. Then again, the chain-chain interaction dominates and if it is sufficiently strong, the chains become all-trans. Our results are relevant for proper interpretation of wetting phenomena on self-assembled monolayers and provide a microscopic understanding of the interplay between interfacial structure and intermolecular interactions between monolayers and the surrounding liquid.