Phase-sensitive vibrational sum frequency generation is employed to investigate the water structure at phospholipid/water interfaces. Interfacial water molecules are oriented preferentially by the electrostatic potential imposed by the phospholipids and have, on average, their dipole pointing toward the phospholipid tails for all phospholipids studied, dipalmitoyl phosphocholine (DPPC), dipalmitoyl phosphoethanolamine (DPPE), dipalmitoyl phosphate (DPPA), dipalmitoyl phosphoglycerol (DPPG), and dipalmitoyl phospho-l-serine (DPPS). Zwitterionic DPPC and DPPE reveal weaker water orienting capability relative to net negative DPPA, DPPG, and DPPS. Binding of calcium cations to the lipid phosphate group reduces ordering of the water molecules.

An account is given of the current state of understanding of aqueous salt, acid, and lipid/water surfaces, interfacial depth, and molecular organization within the air-solution interfacial region. Water structure, hydration, surface propensity of solutes, and surface organization are discussed. In this perspective, vibrational sum frequency generation spectroscopic studies of aqueous surfaces are interpreted. Comment on future directions within the field of aqueous surface structure is provided.

Hydration and orientation of the phosphate group of dipalmitoylphosphatidylcholine (DPPC) monolayers in the liquid-expanded (LE) phase and the liquid-condensed (LC) phase in the presence of sodium ions and calcium ions was investigated with vibrational sum frequency generation (SFG) spectroscopy at the airaqueous interface in conjunction with surface pressure measurements. In the LE phase, both sodium and calcium affect the phosphate group hydration. In the LC phase, however, sodium ions affect the phosphate hydration subtly, while calcium ions cause a marked dehydration. Silica-supported DPPC monolayers prepared by the Langmuir-Blodgett method reveal similar hydration behavior relative to that observed in the corresponding aqueous subphase for the case of water and in the presence of sodium ions. However, in the presence of calcium ions the phosphate group dehydration is greater than that from the corresponding purely aqueous CaCl 2 subphase. The average tilt angles from the surface normal of the PO 2 -group of DPPC monolayers on the water surface and on the silica substrate calculated from SFG data are found to be 59°( 3°and 72°( 5°, respectively. Orientation of the phosphate group is additionally affected by the presence of ions. These findings show that extrapolation of results obtained from model membranes from liquid surfaces to solid supports may not be warranted since there are differences in headgroup organization on the two subphases.

Inorganic salts in marine aerosols play an active role in atmospheric chemistry, particularly in coastal urban regions. The study of the interactions of these ions with water molecules at the aqueous surface helps to elucidate the role of inorganic cations and anions in atmospheric processes. We present surface vibrational sum frequency generation (SFG) spectroscopic and molecular dynamics (MD) studies of aqueous MgCl 2 surfaces as models of marine aerosol. Spectroscopy results reveal that the disturbance of the hydrogen bonding environment of the air/aqueous interface is dependent on the MgCl 2 concentration. At low concentrations (<1 M) minor changes are observed. At concentrations above 1 M the hydrogen bonding environment is highly perturbed. The 2.1 M intermediate concentration solution shows the largest SFG response relative to the other solutions including concentrations as high as 4.7 M. The enhancement of SFG signal observed for the 2.1 M solution is attributed to a larger SFG-active interfacial region and more strongly oriented water molecules relative to other concentrations. MD simulations reveal concentration dependent compression of stratified layers of ions and water orientation differences at higher concentrations. SFG and MD studies of the dangling OH of the surface water reveal that the topmost water layer is affected structurally at high concentrations (>3.1 M). Finally, the MgCl 2 concentration effect on a fatty acid coated aqueous surface was investigated and SFG spectra reveal that deprotonation of the carboxylic acid of atmospherically relevant palmitic acid (PA) is accompanied by binding of the Mg 2þ to the PA headgroup. magnesium chloride | fatty acid | air/aqueous interface | sum frequency spectroscopy | molecular dynamics I norganic salts present in marine boundary layer (MBL) aerosol originate from turbulent wave action at the surface of the ocean (1). These aerosols, typically of the micron size range and smaller, travel over continental regions by being entrained in the air mass in which they were created, and have been detected more than 900 km inland (2). Aerosols play a key role in the modification of global climate through their effect on cloud condensation nuclei prevalence, radiative balance, and level of precipitation (3). Aerosol composition and size have also been correlated to thunderstorm severity (4). Alkali metals (Na, K, Li, and Rb), alkaline earth metals (Mg, Ca), and ammonium (NH þ 4 ) make up the majority of cationic species, and halides (F, Cl, Br, and I) and oxidized sulfur and nitrogen ions make up the majority of inorganic anionic species found in MBL aerosol (5, 6). While calcium and magnesium are the most prevalent divalent cations in seawater and MBL aerosols, chloride anion is the dominant halide species (5). Although recent cloud drop measurements in the MBL show that calcium concentrations are about four times higher than magnesium concentrations, (6) the small size and high charge density of Mg 2þ gives rise to a strongly hydrated complex in aqueous solut...

Interfacial water structure plays a key role in many chemical, biological, and environmental processes. Here, in addition to conventional VSFG, we employ phase-sensitive sum frequency generation (PS-SFG) to investigate the average direction of the transition dipole of interfacial water molecules that is intrinsically contained in the sign of the second-order nonlinear susceptibility, χ((2)). The orientation of water at air/aqueous inorganic salt interfaces of Na(2)CO(3) and NaHCO(3) was inferred from the direct measurement of the transition dipole moment of the interfacial water molecules. It is found that bicarbonate and its counterion sodium do not significantly perturb the interfacial water structure, whereas carbonate strongly orients water so that the water hydrogens point down toward the bulk solution. This is consistent with the picture of carbonate anions residing many layers below the water surface with a preference for the sodium cations to be above the anions and thereby closer to the topmost layer of the water surface.