Quantifying the Relationship between Surface Hydrophobicity and Depletion Layer Thickness

When water comes into contact with an extended hydrophobic surface a uniform region of low density forms, called the depletion layer. This phenomenon has only been experimentally verified on surfaces with contact angles \textgreater 100$^{\circ}$, but understanding how the thickness of the depletion layer changes with the hydrophobicity at intermediate contact angles is one of the underlying mechanisms behind several biological systems including colloidal self-assembly, protein folding, and fluid flow across membranes. We aim to quantify this relationship by using self-assembled monolayers of 1-octadecanthiol and 11-mercaptoundecanoic acid on gold to produce surfaces with contact angles between 55$^{\circ}$ and 107$^{\circ}$. We then use surface plasmon resonance spectroscopy to determine the thickness of the depletion layer formed for each surface.