Dewetting phenomenon: interfacial water structure at well-organized alkanethiol-modified gold-aqueous interface

Area of Science:

• Surface Science
• Physical Chemistry
• Materials Science

Background:

• Understanding water structure at hydrophobic interfaces is crucial for various chemical and biological processes.
• Short-chain alkanethiol monolayers on gold provide a model system for studying these interfaces.

Purpose of the Study:

• To probe interfacial properties at well-ordered short-chain alkanethiol monolayer-aqueous interfaces.
• To understand the structure of water near a hydrophobic surface.

Main Methods:

• Preparation of hexanethiol monolayers on gold substrates via adsorption.
• Characterization using reflection-absorption infrared spectroscopy (RAIRS), atomic force microscopy (AFM), quartz crystal microbalance (QCM), and electrochemical techniques.
• Interfacial capacitance measurements to probe solvent properties.

Main Results:

• Identification of a thin layer of solvent with reduced density/dielectric constant (drying transition) near the methyl groups of the monolayer.
• AFM measurements revealed variations in the solvent's dielectric constant as a function of distance from the monolayer.
• The transition layer (interphase) was determined to be approximately 2 nm thick.
• Similar drying transition phenomena were observed with formamide, suggesting it's not unique to water.

Conclusions:

• A distinct 'drying transition' layer exists at alkanethiol monolayer-aqueous interfaces.
• This layer, approximately 2 nm thick, exhibits reduced solvent density and dielectric properties.
• The findings provide insights into the structure of water and other solvents near hydrophobic surfaces.