How Do We Measure Hydrophobicity

There are a number of techniques that can be used to measure the degree of hydrophobicity of a surface. Direct techniques such as water uptake measurements and sessile drop contact angles are often supplemented with spectroscopic studies using FTIR and NMR to provide evidence of structural groups present in aerogel samples and to quantitate the relative number of those groups. These methods are described below. 

96% humidity environment for 40 days. The hydrophilic aerogels exhibited mass increases of 10-60% within the first five days. Rao et al. [43] and Rao and Haranath [44] performed water uptake studies by floating aerogels directly on water for 3-6 months and considered an increase of less than 2% in mass as an indication that the aerogel was hydrophobic. 

Rao and Kulkami [45] show that the direct measurement and height/width technique yield similar results. 

Fourier Transform Infrared (FTIR) and solid-state Nuclear Magnetic Resonance (NMR) spectroscopy have been used to complement water uptake or contact angle studies. Both methods give direct structural information about the chemical bonds present in an aerogel sample. The differences in stmcture of hydrophobic and hydrophilic aerogels result in observed differences in their FTIR and NMR spectra. 

When a sample containing nuclei that have spin 0 is placed in a magnetic field, formerly degenerate spin states for those atoms are split into slightly different energy levels. 

---