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At charged aqueous interfaces, the second-order nonlinear optical response originates from water molecules within the diffuse part of the electrical double layer, which are ordered by the surface field and from water that additionally experiences chemical and physical interactions with the surface in the Stern layer. These two environments can either reinforce or diminish the overall signal and can be disentangled by varying the coherence length of their interaction with external laser fields. Here, we demonstrate a method in which the angle of incidence is varied to afford a significant change in the coherence length. When this technique was applied to the silica–water interface, it was observed that water molecules in the Stern and diffuse layers direct their hydrogen atoms toward the mineral surface at a low ionic strength and neutral pH. A decrease in the signal with increasing ionic strength is attributed to hydrated cation adsorption that competes with free water for deprotonated silanol sites.

ABSTRACTThis paper comprehensively reviews driving behaviour in the context of heterogeneous traffic and poor lane discipline condition. Organising such driving behaviour models into two main groups (lateral movement model and longitudinal movement model), the systematic review primarily focusses on chronological development of the studies, intricate components of modelling framework (input and output variables, their selection procedure, methodology, calibration and validation) and strength and limitations of each model. In addition, the models are also scrutinised to determine to what extent they are applicable to capture the complex interaction of various vehicle types in poor or no lane discipline condition. Finally, the state-of-the-art is compared with the universal modelling requirement to pinpoint the research gap, provide appropriate direction to bridge the gap and develop a comprehensive model of driving behaviour in heterogeneous and non-lane-based traffic condition.

Polylactic acid (PLA) biopolymer appears to provide environmental advantages over the petroleum‐derived polymers but often ends up with limited applications owing to their poor mechanical performance and brittleness. Herein, we present a PLA polymer compatible graphene oxide (GO) based crosslinker with the intention of improving the mechanical properties. Lactic acid (LA) functionalized GO (GO‐LA) crosslinker was prepared and had been crosslinked with the PLA chains through a one‐step polycondensation reaction. The mechanical properties of the as‐synthesized GO crosslinked PLA (GO‐C‐PLA) were investigated by compression tests and compared with neat PLA, and GO reinforced PLA (GO‐PLA) with no crosslinking. With 0.3% of GO‐LA crosslinker in GO‐C‐PLA, the compressive modulus increased by nine times compared to that of the neat PLA. The compressive strength also increased to 46 MPa, which was four times higher than the neat PLA. This strategy for improving the mechanical properties by introducing GO‐based crosslinker can be used potentially for many polycondensation polymers and thus be useful for many high‐performance applications.

The molecular origin of overcharging at mineral oxide surfaces remains a cause of contention within the geochemistry, physics, and colloidal chemistry communities owing to competing “chemical” versus “physical” interpretations. Here, we combine vibrational sum frequency spectroscopy and streaming current measurements to obtain molecular and macroscopic insights into the pH-dependent interactions of calcium ions with a fused silica surface. In a 100 mM CaCl2electrolyte, we observe evidence of charge neutralization at pH ∼ 10.5, as deducted from a minimum in the interfacial water signal. Concurrently, adsorption of calcium hydroxide cations is inferred from the appearance of a spectral feature at ∼3610 cm–1. However, the interfacial water signal increases at higher pH, while adsorbed calcium hydroxide appears to remain constant, indicating that overcharging results from hydrated Ca2+ions present within the Stern layer. These findings suggest that both specific adsorption of hydrolyzed ions and ion–ion correlations of hydrated ions govern silica overcharging with increasing pH.

The structure of water adjacent to silica is sensitive to the degree of deprotonation of surface silanol groups. As a result, close inspection of signals originating from these water molecules can be used to reveal the surface charge density. We have used nonlinear vibrational spectroscopy of the water O–H stretching band over a temperature range of 10–75 °C to account for the increase in surface potential from deprotonation. We demonstrate that the behavior at the silica surface is a balance between increasing surface charge and a decreasing contribution of water molecules aligned by the surface charge. Together with a model that accounts for two different types of silanol sites, we use our data to report the changes in enthalpy and entropy for deprotonation at each site. This is the first experimental determination of these thermodynamic parameters for hydrated silanol groups at the silica surface, critical to a wide range of geochemical and technological applications.

The study of interfacial properties of thin films such as polymers is an important area of surface science. The application of visible–infrared sum-frequency generation spectroscopy to such systems requires a careful interpretation of the results, as the electric field magnitude and phase at each interface must be determined in a manner that takes thin-film interference effects into account. Several schemes have been proposed for handling the local field corrections, and these methods all have their origins in linear optics. We first provide an extensive discussion of the cases in which the film is sufficiently thick that multiple beam interference can be ignored or sufficiently thin in which the relevant expressions collapse to simple forms. Then, we illustrate a straightforward method that has a concise analytic solution in the case of a single thin film that exhibits interference effects. We demonstrate a visualization technique that allows the experimental geometry to be tuned to select the interface of interest and rapidly switch between the interfaces when the film thickness is chosen to accommodate this.

Vibrational sum frequency generation (SFG) spectroscopy was utilized to distinguish different populations of water molecules within the electric double layer (EDL) at the silica/water interface. By systematically varying the electrolyte concentration, surface deprotonation, and SFG polarization combinations, we provide evidence of two regions of water molecules that have distinct pH-dependent behavior when the Stern layer is present (with onset between 10 and 100 mM NaCl). For example, water molecules near the surface in the Stern layer can be probed by the pss polarization combination, while other polarization combinations (ssp and ppp) predominantly probe water molecules further from the surface in the diffuse part of the electrical double layer. For the water molecules adjacent to the surface within the Stern layer, upon increasing the pH from the point-of-zero charge of silica (pH ∼2) to higher values (pH ∼12), we observe an increase in alignment consistent with a more negative surface with increasing pH. In contrast, water molecules further from the surface appear to exhibit a net flip in orientation upon increasing the pH over the same range, which we attribute to the presence of the Stern layer and possible overcharging of the EDL at lower pH. The opposing pH-dependent behavior of water in these two regions sheds new light on our understanding of the water structure within the EDL at high salt concentrations when the Stern layer is present.

Using second harmonic generation and sum frequency generation spectroscopy, we monitor the influence of sodium and potassium halides on acid–base processes at the negatively charged silica/aqueous electrolyte interface. We find that the two types of acidic silanols at the surface are very sensitive to the presence of halides in the aqueous phase. As the halide size increases, the pH at which half the more acidic sites are deprotonated (pH0.5) shifts to lower pH. Conversely, the pH0.5of the less acidic sites shifts to higher pH with increasing halide size. We also observe titration curves of increasing sharpness as the halide size increases, indicative of positive cooperativity. Using a simple cooperative model, we find that the cooperative unit for the dissociation of more acidic surface sites is ∼1, 2, and 3 for the chloride, bromide, and iodide electrolytes, respectively, which reveals that the larger anions promote deprotonation among the more acidic silanol groups. We also find that the fraction of more acidic sites, proportional to the relative surface charge density at neutral pH, increases from 20% to 86% as the sodium halide is varied from chloride to iodide. As the percentage of more acidic sites and the surface charge at neutral pH increases, the effective acidity of the less acidic sites decreases, indicating that greater surface charge density renders the remaining silanol groups more difficult to deprotonate. As the relative amount of less acidic sites increases, their deprotonation events exhibit negative, rather than positive, cooperativity revealing charge repulsion between neighboring silanol groups.

A new chelating resin was synthesized by modification of Amberlite XAD-4 resin with 1,8-diaminonaphthalene through an azo linkage (NN). The synthesized resin was characterized by scanning electron microscope, Fourier transform infrared spectroscope, elemental analysis and thermogravimetric analysis. It was applied to preconcentrate and separate Cu(II), Cd(II) and Pb(II) ions, which were determined subsequently by graphite furnace atomic absorption spectrometry. Various physico-chemical parameters that influence the quantitative preconcentration and recovery of the metals were optimized by both static and dynamic techniques. Among them, optimum pH was in the range 6–7. 2.0M HNO3was selected as eluent. Optimum flow rates of sample and the eluent were found to be 2 and 4mLmin−1, respectively. Preconcentration factors were 220 for Cu(II) and Cd(II) and 240 for Pb(II), sorption half times (t1/2) were 5min for Cu(II) and Cd(II) and 6min for Pb(II), respectively. The Langmuir, Freundlich and Temkin adsorption isotherms were also studied in detail. The procedure was validated by analysis of a certified reference material (A-11, powder milk). The new method was employed to estimate the metal ion concentrations quantitatively from dairy cow’s milk collected from dairy farm, Savar, Dhaka, Bangladesh.

Resistance to algae contamination is an important characteristic of insulators used in overhead power distribution in coastal environments. It is therefore important to understand the parameters governing algae adhesion onto polymer insulator materials such as silicone. Flow cell-based shear experiments were conducted in order to characterize the adhesion strength of algae onto polydimethylsiloxane surfaces, comparing fresh polymer substrates with those that have been soaked in water and saline solutions for 1 month. Both freshwater algae and seawater species could withstand considerably less drag force and were therefore more easily removed when the polymer was soaked in salt water. The polymer surface was found to be unaltered in terms of its roughness, contact angle, and lack of water uptake; no macroscopic surface characterization was therefore able to account for the differences in cell adhesion strength resulting from the soaking treatment. Surface-specific nonlinear vibrational spectroscopy, however, revealed subtle differences in the orientation of surface methyl groups that resulted from the water and saline exposure.

Second-order nonlinear optical techniques have recently been established as sensitive probes of charged interfaces through the nonlinear susceptibility of water and provide an attractive route to elucidate the surface potential. We discuss methods that have been proposed using electronic second-harmonic generation and vibrational sum-frequency generation. A detailed comparison is provided, using a unified notation and including a discussion of the assumptions that are either convenient or necessary in order to arrive at the surface potential. We then illustrate that, when using the full solution of the Poisson–Boltzmann equation that is applicable to a wide range of surface potentials, several benefits may be realized. The first is that, when using off-resonance phase measurements, the surface potential can be determined from the phase alone, without any additional information or calibrated intensity schemes. Next, we illustrate a scheme for surface potential measurement using intensity-only data off-resonance by changing the angle of incidence. Finally, we discuss the possibility of measurements that target the vibrational resonance with water and illustrate how the surface potential can be isolated from such data.

The structure of silicone polymer surfaces at elevated temperatures is of interest to industrial applications such as sealants and electrical insulators, and for scientific studies including the thermal control of droplets in microfluidic devices. Nonlinear vibrational spectroscopy has been used to study the temperature-dependent surface structure of poly(dimethylsiloxane) when exposed to water and a perfluorinated hydrophobic liquid. Quantitative analysis of the methyl plane orientation was performed using a combination of peak ratios and peak amplitudes that enable the proposed structures to be identified. For both environments, the tilt and twist of the methyl plane was found to increase with temperature in a reversible manner. This has been attributed to be a consequence of the backbone reorganization due to temperature-dependent density changes.

Knowledge of the thickness of films, combined with the ability to control and ideally tune the angle of incidence, provides the possibility for vibrational sum-frequency generation experiments to selectively probe either interface of the film. This is of interest to studies targeting the adhesive properties of polymer films on substrates, or cases where the structure of the exposed side of the film is sought in relation to its environmental conditions, for example, solvent exposure. A straightforward theory is presented that is readily implemented for samples according to their thickness and refractive index. This results in a procedure for planning experiments that reveals whether there is a combination of film thickness and angle of incidence in different beam polarizations that enables multiple beam interference effects to be used to their advantage. The most complicated case where all beams are p-polarized is discussed in detail, as this is often a necessary experimental configuration for dielectric or metal samples.