Knowledge Bank

Sum frequency generation (SFG) is a second order optical spectroscopy that probes regions of non centrosymmetry, interfaces, and allows for the understanding of molecular organization at air/aqueous interfaces. An overview of our work in this area is presented with emphasis on phase-sensitive SFG (PS-SFG) spectroscopy. PS-SFG is a variant of SFG and is used in our laboratory to investigate the average direction of the transition dipole of interfacial water molecules. The orientation of water at air/aqueous inorganic salts interfaces of CaCl$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$, NaCl, Na$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$SO$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$, (NH$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$)$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$SO$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$, and Na$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$CO$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$ is inferred from the direct measurement of the transition dipole moment. We find that charge separation at the air/water interface is most obvious for the aqueous ammonium sulfate solution where the local electric field has a greater magnitude at this interface relative to the other salt solutions. The magnitude of the electric field in the surface extending to the subsurface regions decreases in the order: (NH$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$)$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$SO$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$ $>$ Na$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$SO$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$ $>$ Na$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$CO$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$ $\ge$ CaCl$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ $>$ NaCl; the electric field is opposite in direction for the sulfates and carbonate relative to the chloride salts.