2-Substituted ethyl methacrylate monomers were characterized using sum frequency generation vibrational spectroscopy (SFGVS) to study the effect of substituent groups in the organization of the monomers at the monomer-hydrophilic quartz interface. The SFG spectra of the methacrylate-based functional monomers collected at the hydrophilic quartz interface were found to be different from those collected at the air-monomer interface. The various spectral profiles indicated different conformations of the molecular groups at the solid-liquid interface. Moreover, a peak shift of the methyl symmetric stretch from ∼2900 cm to ∼2880 cm was observed from the air-liquid to solid-liquid interface, respectively. This observation of peak shifting is due to two factors: (1) a change in the chemical environment of the methacrylate-based monomer from air-to-liquid surfaces and (2) interaction between the ester carbonyl group of the monomer and the surface hydroxyl silanol group of the amorphous quartz surface. Also, the monomers were characterized in the carbonyl region, which showed the presence of the C[double bond, length as m-dash]O stretch in the SFG spectrum. This result is indicative of the hydrogen bonding interactions existing between the carbonyl group of the monomers and the Si-OH groups of the hydrophilic quartz interface. In addition, the changes in the SFG intensity of the C[double bond, length as m-dash]O peak at ∼1730 cm revealed that the conformation of the C[double bond, length as m-dash]O groups is affected by bulky substituents. Furthermore, the conformational changes of these functionalized monomers at the hydrophilic solid-neat liquid interface was investigated using SFGVS.

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Functionalization of monomers is important for various applications of polymers from electronics to surface coatings. Polymer chemists utilize methacrylate-based monomers to design polymers with wider application features. In this work, ethyl methacrylate-based monomers are functionalized with varying bulky substituents, methoxy (−OCH3), ethoxy (−OEt), isopropoxy (−O i Pr), tert-butoxy (−O t Bu), and phenoxy (−OPh), at the ethyl end position. These substituents are selected to understand the steric consequences of modification with a bulkier group. This study allows the determination of how the substitution affects the overall conformation of the monomer at the air–liquid interface using the sum frequency generation spectroscopic (SFGS) technique. The SFG spectral results were different for all monomers. To emphasize, the SFG intensity profile at ∼2910 cm–1 (assigned as the methyl symmetric stretch, −CH3 SS) is more noticeable in the SSP spectra compared to other vibrational modes in the SSP spectra. On the basis of the spectral and global fitting, the change in the intensity of the ∼2910 cm–1 peak was affected by an increase in the number of methyl groups in the chemical structure of the monomers. This observation has become more visible for −O i Pr- and −O t Bu-substituted monomers. Orientation distribution analysis was performed for the −CH3 SS of substituted −OCH3 and −OPh monomers using the calculated amplitude ratios of SSP and PPP polarizations. This analysis shows a narrow distribution angle of <30° for average tilt angles near the surface plane for the −OCH3-substituted monomer. Meanwhile, narrow distribution angles were obtained for the average tilt angles closer to the surface normal for the −OPh-substituted monomer. On the basis of the results, the change in the intensity is affected by the number density of the vibrational modes present at the interface and/or the orientation distribution of the interfacial molecules. The SFG spectral results were compared to our measured surface tension (ST) results. The ST values followed a trend with increasing number of −CH3 groups of the substituted monomers according to −OCH3 > −OEt > −O i Pr > −O t Bu. The −OPh-substituted monomer, 2-phenoxyethyl methacrylate (PhEMA), had a high ST value, which is the result of the intermolecular forces, including π-stacking interactions. The ST value of PhEMA favored the SFG spectral data because the aromatic CH stretch was observed and can only be detected if the −OPh group is oriented perpendicular to the surface plane. These results facilitate understanding the effects of the substituents on the conformations of the monomers. Moreover, these results will help correlate these effects with the physicochemical properties of the respective polymers.

The self-assembly of surfactant monolayers at interfaces plays a sweeping role in tasks ranging from household cleaning to the regulation of the respiratory system. The synergy between different nanoscale species at an interface can yield assemblies with exceptional properties, which enhance or modulate their function. However, understanding the mechanisms underlying coassembly, as well as the effects of intermolecular interactions at an interface, remains an emerging and challenging field of study. Herein, we study the interactions of gold nanoparticles striped with hydrophobic and hydrophilic ligands with phospholipids at a liquid–liquid interface and the resulting surface-bound complexes. We show that these nanoparticles, which are themselves minimally surface active, have a direct concentration-dependent effect on the rapid reduction of tension for assembling phospholipids at the interface, implying molecular coassembly. Through the use of sum frequency generation vibrational spectroscopy, we reveal that nanoparticles impart structural disorder to the lipid molecular layers, which is related to the increased volumes that amphiphiles can sample at the curved surface of a particle. The results strongly suggest that hydrophobic and electrostatic attractions imparted by nanoparticle functionalization drive lipid–nanoparticle complex assembly at the interface, which synergistically aids lipid adsorption even when lipids and nanoparticles approach the interface from opposite phases. The use of tensiometric and spectroscopic analyses reveals a physical picture of the system at the nanoscale, allowing for a quantitative analysis of the intermolecular behavior that can be extended to other systems.

The geometry, arrangement, and orientation of a quaternary ammonium surfactant flanked by two methyl groups, a benzyl head, and an octyl tail were assessed at the air–water and air-deuterium oxide (D2O) interfaces using sum frequency generation vibrational spectroscopy. Remarkably, symmetric and asymmetric N–CH3 stretches (at ∼2979 and ∼3045 cm–1, respectively, in the SSP polarization combination) were visible in water but negligible in deuterium oxide. We concluded that D2O addition triggers the average reorientation of the dimethyl amino units parallel to the interface and possibly changes the overall conformation of the surfactant. A reduced number of gauche defects in the surfactant octyl chain is also observed in D2O. Tilt angles for the octyl chain (1.0–10.8°) are consistent with an ordered monolayer at the air–liquid interface.