Nonlinear vibrational spectroscopies such as visible-infrared sum-frequency spectroscopy may serve as powerful probes of interfacial structure. Obtaining quantitative orientation information, however, has been limited by the required knowledge of the corresponding molecular-level nonlinear optical properties. We provide a general scheme for calculating the vibrational hyperpolarizability of any infrared- and Raman-active mode, regardless of the molecular symmetry or complexity of the structure. Our method involves all atoms and therefore does not rely on making any local mode approximations. We show how this information is used together with experimental data to arrive at the tilt and twist angles of a surfactant headgroup at the air/water interface. Since our approach is completely general, it may be used for the analysis of any adsorbate at an isotropic interface.

Whole-molecule approach for determining orientation at isotropic surfaces by nonlinear vibrational spectroscopy.

This study investigated the effect of cationic, anionic (saturated and unsaturated), and nonionic surfactants on the formation, morphology, and surface properties of silica nanoparticles synthesized by the ammonium‐catalyzed hydrolysis of tetraethoxysilane in alcoholic media. Results indicate that at a relatively low surfactant concentration (1 × 10−3–1 × 10−6 M), cationic surfactants significantly affected the growth of silica particles as measured by dynamic light scattering and transmission electron microscopic analyses. In contrast, the anionic and nonionic surfactants showed relatively minor effects in the low concentration range. The magnitude of negative zeta potential was reduced for silica colloids that were synthesized in the presence of cationic surfactant because of charge neutralization. The presence of anionic surfactants only slightly increased the negative zeta potential while the nonionic surfactant showed no obvious effects. At high surfactant concentrations (>1 × 10−3 M), catio...

Effect of surfactants on the formation, morphology, and surface property of synthesized SiO2 nanoparticles

The crystallization mechanisms and kinetics of micellar sodium dodecyl sulfate (SDS) solutions in water, under isothermal conditions, were investigated experimentally by a combination of reflection optical microscopy (OM), differential scanning calorimetry (DSC), and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). The rates of nucleation and growth were estimated from OM and DSC across temperatures ranging from 20 to −6 °C for 20% SDS-H2O, as well as for 10 and 30% SDS-H2O at representative temperatures of 6, 2, and −2 °C. A decrease in temperature increased both nucleation and growth rates, and the combined effect of the two processes on the morphology was quantified via both OM and ATR-FTIR. Needles, corresponding to the hemihydrate polymorph, become the dominant crystal form at ≤ −2 °C, while platelets, the monohydrate, predominate at higher temperatures. Above 8 °C, crystallization was only observed if seeded from crystals generated at lower temperatures. Our results pr...

Isothermal Crystallization Kinetics of Sodium Dodecyl Sulfate–Water Micellar Solutions

A novel synthetic method is described for the preparation of bulk titanium oxide mesophase (TOM) material with wormhole texture based on templating by inexpensive dodecyl sulfate surfactant. To contol hydrolysis and condensation reactions, a mixture of the Ti(III) salt of dodecyl sulfate and Ti(IV) isopropoxide are reacted in an alcohol solvent. Humidified air is passed over or through this solution eliminating alcohol by evaporation and introducing water. Under these controlled conditions, gelation takes place over a period of several days. The sulfated TOM compound produced using this method has limited thermal stability which precludes elimination of surfactant by calcination. However, anion exchange of the sulfate surfactant from the TOM can be easily accomplished using dilute solutions of oxo-anions such as molybdate, tungstate, and vanadate, and this makes pore space available under mild conditions. Progressive filling of the pores produces micro- to mesoporous multicomponent oxides that are stable ...

Anionic Surfactant Templated Titanium Oxide Mesophase: Synthesis, Characterization, and Mechanism of Formation

The crystallization kinetics of sodium dodecyl sulfate (SDS)-water micellar solutions, under linear cooling conditions, were experimentally investigated using optical microscopy, differential scanning calorimetry, and infrared spectroscopy. Cooling rates were systematically varied, from 0.1 to 50 °C min–1, encompassing environmental to near-“isothermal” temperature changes, between 22 and −5 °C, for a reference concentration of 20% SDS-H2O. The cooling rate was shown to determine the dominant crystal morphologies, with platelets and needles predominating at the lowest and highest rates, respectively. The results were rationalized in terms of isothermal crystallization data and the time–temperature cooling profile. Rates 0.1, 5.0, and 10 °C min–1 yield morphologies and kinetics analogous to those of isothermal quenches at the corresponding crystallization temperature window. Nontrivial deviations were observed for intermediate rates (0.5, 1.0 °C min–1), due to commensurate changes in temperature and crysta...

W.K. Thompson

Papers

The vibrational spectra of sodium dodecylsulphate 18 hydrate (α-phase)