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The silica (SiO2) nanoparticles of a well-known silica precursor tetraethyl orthosilicate (TEOS) are generally synthesized via a promising solution-gelation inorganic polymerization process. The monodisperse silica nanoparticles have potential applications ranging from the formation of dental nanocomposites to antireflective coatings. In the present study, we have systematically investigated the in situ interfacial molecular structure of TEOS and its impact on the interfacial water structure during the processes of hydrolysis and condensation at the air–aqueous interface using sum-frequency generation (SFG) vibrational spectroscopy. With the presence of water, a gradual decrease in the SFG intensity in the CH-stretch region for each concentration of TEOS with time reflects the elimination of ethoxy groups, which is a signature of the hydrolysis process. Further, the impact of the hydrolysis process is revealed from the significant enhancement in the SFG signal in the OH-stretch region. The hydrolysis of TEOS is then followed by condensation in which the SiO– charged species are replaced by forming the SiOSi bridging network. The signature of the condensation process is reflected with the gradual decrease in the observed enhanced SFG signal in the OH-stretch region. The formation of monodispersed silica nanoparticles as an end product of size variation from 1.75 to 4.67 nm with the increase in TEOS concentration is confirmed with the DLS measurements. We have also probed the pH-dependent SFG studies at three different pH values (2.0, 5.8, and 9.0). The dominant pH-dependent hydrolysis process is revealed from the observed molecular structure of TEOS at the air–aqueous interface. [ABSTRACT FROM AUTHOR]

Vibrationalsum-frequency generation spectroscopy is valued forits ability to selectively probe molecules at a variety of interfaceswithout the use of extrinsic chromophores. The spectra contain valuableinformation regarding the molecular structure and the interfacialenvironment through the observation of vibrational resonances associatedwith specific moieties. Chemical information is obtained by closeinspection of the frequencies of these bands and the amplitude ofthe response under conditions of different beam polarizations. Suchsensitivity motivates the development of techniques that can providestructural details. We illustrate several approaches by which varioustypes of calculations and molecular simulations may be used to enhancethe sought structural interpretation of experimental data. By applyingthese techniques to the adsorbate molecules, interfacial water, andthe substrate surfaces themselves, we are able to achieve a holisticpicture of the adsorption environment. [ABSTRACT FROM AUTHOR]

A three-dimensional-phospholipid monolayer with tunable molecular structure was created on the surface of oil nanodroplets from a mixture of phospholipids, oil, and water. This simple nanoemulsion preparation technique generates an in situ prepared membrane model system with controllable molecular surface properties that resembles a lipid droplet. The molecular interfacial structure of such a nanoscopic system composed of hexadecane, 1,2-dihexadecanoyl-sn-glycero-3-phosphocholine (DPPC), and water was determined using vibrational sum frequency scattering and second harmonic scattering techniques. The droplet surface structure of DPPC can be tuned from a tightly packed liquid condensed phase like monolayer to a more dilute one that resembles the liquid condensed/liquid expanded coexistence phase by varying the DPPC/oil/water ratio. The tunability of the chemical structure, the high surface-to-volume ratio, and the small sample volume make this system an ideal model membrane for biochemical research.

Interfacial water commonly distinguishes itself from the bulk phase by adopting a polar, ordered structure. It is well-established that electrolytes can act to perturb this structure; however, the nature of this perturbation remains a topic of interest. In this study, surface- and structure-sensitive nonlinear vibrational spectroscopy is used to monitor electrolyte-induced changes in interfacial water structure. Solution ionic strength was varied over 5 orders of magnitude, and spectra were collected from two mineral surfaces (fused silica and calcium fluoride) and two polymer surfaces (polystyrene and poly(methyl methacrylate)). Analysis of the spectra reveals striking similarities and differences between these four aqueous interfaces; trends in overall intensity do not always follow changes in the spectral shape. Our results reveal the complex interplay between surface charge, ion adsorption, and hydrophobicity in determining interfacial water structure in the presence of dissolved ions.