Hydrolysis and Condensation of Tetraethyl Orthosilicate at the Air–Aqueous Interface: Implications for Silica Nanoparticle Formation

Abstract

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.

Investigation of Water Evaporation Process at Air/Water Interface using Hofmeister Ions

“…The schematic layout of the SFG experiments is presented in Figure a. SFG vibrational spectroscopy is an ideal spectroscopic tool to selectively probe the conformation and structure of the moieties lie in the interfacial region and is based on second-order nonlinear optical process. ,,− We consider the SFG spectrum at the pristine air/water interface as the reference spectrum to observe the comparative impact of various salts (Figure b). The spectra (Figure b–d) consist of a broad region from 3000 to 3650 cm –1 assigned with three peak positions at around 3230, 3445, and 3550 cm –1 .…”

“…The resultant sum frequency signal reflected at an angle of 61° is then allowed to pass through the detection optics. The intensity of the SFG signal is given as ,,− Here, I 1 and I 2 are the intensities of visible and IR beams. χ (2),eff is a second-order nonlinear susceptibility tensor, and can be expressed as Here, χ NR (2) is the nonresonant component of the susceptibility, and the second term is the resonant part represented by a Lorentzian function.…”

Elucidating the Molecular Structure of Hydrophobically Modified Polyethylenimine Nanoparticles and Its Potential Implications for DNA Binding

“…The spectral assignments of pristine water have been well documented and elaborated on in detail in the literature. 23,[27][28][29]41,47 The lower-wavenumber region centered at ∼3225 cm −1 represents the characteristic of strongly hydrogenbonded (H-bonded) water molecules, whereas the higherwavenumber region depicts weakly H-bonded water molecules with peak contributions centered at ∼3450 and ∼3560 cm −1 , respectively. 27,29,41,47 The prominent peak located at 3705 cm −1 represents the free (dangling) OH groups protruding toward the air.…”

“…Hence, we have preliminarily acquired the SFG spectra of the air−pristine water interface before analyzing the spectral signatures of samples (Figure3, black line). The spectral assignments of pristine water have been well documented and elaborated on in detail in the literature 23,[27][28][29]41,47. The lower-wavenumber region centered at ∼3225 cm −1 represents the characteristic of strongly hydrogenbonded (H-bonded) water molecules, whereas the higherwavenumber region depicts weakly H-bonded water molecules with peak contributions centered at ∼3450 and ∼3560 cm −1 , respectively 27,29,41,47.…”

“…Hence, water plays an imperative role in defining its fundamental properties for wide application usage, especially in the biomedical field. − In the past, some reports demonstrated the existence of a hydration layer and its impact on the restructuring of polymeric side chains at the interface. ,,, In view of the significance of the confined nature of the surface and interface and their relevance in defining the molecular behavior in an aqueous medium, sum-frequency generation vibrational spectroscopy (SFG-VS) has come out as a novel spectroscopic tool that targets the extraction of the interfacial molecular signal from the noncentrosymmetric media. In contrast to the conventional spectroscopic techniques, SFG-VS is a label-free spectroscopic tool that is interface-specific and readily characterizes the in situ vibrational features of the molecular groups present in the interfacial region. ,,− A lot of research has been done in interfacial polymer science using SFG-VS to characterize the conformation of the polymer at discrete interfaces, determine polymer–surfactant interactions, and carry out polymer blend studies. ,− …”