Probing molecular structure at interfaces for comparison with bulk solution behavior: water/2-propanol mixtures...

Area of Science:

• Physical Chemistry
• Surface Science
• Spectroscopy

Background:

• Understanding liquid/vapor interfaces is crucial for chemical processes.
• The behavior of binary mixtures, like 2-propanol/water, at interfaces is complex.
• Molecular orientation influences macroscopic properties.

Purpose of the Study:

• To investigate the orientation of the isopropyl group in 2-propanol/water mixtures at the liquid/vapor interface.
• To correlate molecular orientation with bulk properties.
• To utilize vibrational sum frequency spectroscopy for interfacial analysis.

Main Methods:

• Vibrational sum frequency spectroscopy (VSFS) was employed.
• CH(3) stretch modes of the isopropyl group were analyzed.
• A novel united atom approach modeled the (CH(3))(2)X moiety.
• Molecular susceptibility changes were calculated as a function of tilt and twist angles.

Main Results:

• Isopropyl groups orient differently based on 2-propanol concentration.
• At low concentrations, methyl groups lie parallel to the surface.
• As concentration increases, methyl groups twist, with one aligning towards the surface normal at the azeotrope (x(iso) = 0.68).
• Orientation stabilizes above the azeotropic composition.

Conclusions:

• The orientation of 2-propanol at the interface is concentration-dependent.
• Changes in molecular orientation correlate with bulk activity coefficients.
• Surface structure and bulk properties of 2-propanol/water mixtures are interconnected.