An anomalous maximum in the ionic strength dependent electrophoretic mobility curves has been observed in previous reports from particles dispersed in colloids. This maximum has been considered anomalous because it is contradictory with the Gouy-Chapman model. The existence of such a maximum has been attributed to specific ionic adsorption, a hairy layer at the surface, or the effect of the anomalous change of surface conductivity in different studies. It was also pointed out that the O'Brien-White approach based on the Gouy-Chapman model could be used to understand this maximum in electrophoretic mobility curves and lead to understandable zeta potential curves. This implied that the observed maximum was actually not "anomalous". In this work we report our simulation of ionic strength dependent zeta potential curves based on the O'Brien-White approach and experimental studies of the ionic strength dependent electrophoretic mobility of the hexadecane droplets in the hexadecane-water emulsions at different pH or in the presence of sodium dodecyl sulphate at varied concentrations. In some cases, the simulation shows that the calculation with the O'Brien-White approach does change the trend in the concerned ionic strength dependent curves. However, the simulation in some other cases also leads to similar trends in the ionic strength dependent electrophoretic mobility curves and zeta potential curves. In the experiments, both the existence and non-existence of such a maximum were observed and demonstrated to be system dependent. The corresponding molecular structure of the oil-water interface was then discussed with the analyses of the zeta potential curves and second harmonic generation signals recorded at the hexadecane-water interface.
It is important to investigate the influence of surfactants on structures and physical/chemical properties of oil/water interfaces. This work reports a second harmonic generation study of the adsorption of malachite green (MG) on the surfaces of oil droplets in a hexadecane/water emulsion in the presence of surfactants including sodium dodecyl sulfate, polyoxyethylene-sorbitan monooleate (Tween80), and cetyltrimethyl ammonium bromide. It is revealed that surfactants with micromolar concentrations notably influence the adsorption of MG at the oil/water interface. Both competition adsorption and charge-charge interactions played very important roles in affecting the adsorption free energy and the surface density of MG at the oil/water interface. The sensitive detection of the changing oil/water interface with the adsorption of surfactants at such low concentrations provides more information for understanding the behavior of these surfactants at the oil/water interface.
Dimethyldodecylamine N-oxide (DDAO) shows high surface activity with two distinct energy states at the hydrophilic silica/aqueous solution interface studied by total internal reflection (TIR) Raman spectroscopy combined with ratiometric and kinetic analysis.
Probing the behavior of surfactants at oil–water interfaces is crucial to understand their functionality. In this work, we present detection of the adsorption of several common surfactants at the hexadecane–water interface with second harmonic generation (SHG) and zeta potential measurements. Water molecules were used as reliable indicators of the adsorption of ionic surfactants in SHG analysis. With the change of the interfacial potential monitored by both SHG and zeta potential measurements, unique information about the multiple steps involved in the adsorption of typical surfactants at the oil–water interface is provided. It was revealed that the adsorption of sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) at the hexadecane–water interface is initialled by a step dominated by the adsorption of the hydrophobic part of the surfactant, and a latter step involves comparable contributions from both the hydrophobic part and the counterion. The adsorption free energies involved in the initial step can be quantitatively analyzed. In addition, the adsorption of two oil-soluble amphiphiles at the hexadecane–water interface was also studied. Analysis of the ionic strength dependent SHG signal at the hexadecane–water interface also reveals that the origin of the SHG emission is mainly the water molecules at the interfacial layer. The preferential orientation of water molecules is with the hydrogen atoms pointing to the oil phase.
By probing the electric potential at the hexadecane-water interface with second harmonic generation and the zeta potential at the surface of a hexadecane droplet in its emulsion, we show that hydronium ions don't have a specific affinity to the oil-water interface although hydroxide ions do. The observed apparent affinity of the hydronium ions to the hexadecane-water interface is more likely a result of the electro-static attraction effect. The adsorption free energy of the hydroxide ions at the oil-water interface was estimated to be -8.3 kcal mol. This study provides more experimental evidence for understanding the behavior of hydronium and hydoxide ions at the oil-water interface.
To understand and control the interfacial properties of polydiacetylenes (PDAs) vesicles with π-conjugated backbone is very important for their colorimetric sensing of chemical and biological targets. In this work, we adopted 10, 12-pentacosadiynoic acid (PCDA) as the model molecule to prepare PDAs vesicles in aqueous solution with different forms (from monomer to blue-to-purple-to-red phase) by controlling the UV irradiation dose. The variations of the interfacial conformation of PDAs vesicles during chromatic transitions were inspected by the adsorption behaviors of probe molecules (4-(4-diethylaminostyry)-1-methylpyridinium iodide, D289) on vesicle surface with surface-specific second harmonic generation (SHG) and zeta potential measurements. Resonant SHG signal from D289 adsorbed on vesicle surface attenuated sharply, and the adsorption free energy as well as the corresponding two-photon fluorescence signal decreased slightly in chromatic transitions. While, the change in the surface density of the adsorbed D289 molecules for PDAs vesicles with different forms was relatively small as estimated from zeta potential measurements. The attenuation of the SHG intensity was thus attributed to the overall order-disorder transition and the changed orientation of D289 molecules caused by the gradual distortion of carboxyl head group driven by backbone perturbation.
The population genetic structure of invasive species can be strongly affected by environmental and landscape barriers to dispersal. Disentangling the relative contributions of these factors to genetic divergence among invading populations is a fundamental goal of landscape genetics with important implications for invasion management. Here, we relate patterns of genetic divergence in a global invasive agricultural pest, Colorado potato beetle (CPB; Leptinotarsa decemlineata), to environmental and landscape factors along an invasion front in Northwestern China. We first used microsatellite markers and spatial‐temporal samples to assess broad patterns of genetic diversity as well as fine‐scale changes in patterns of genetic divergence. We then distinguished the relative contributions of five factors to genetic divergence among front populations: geographic distance (isolation by distance), climate dissimilarity (isolation by environment), and least‐cost distances (isolation by resistance) modeled with three factors: climate suitability, cropland cover, and road networks. Genetic diversity broadly decreased from West to East, with the exception being Eastern China. Low levels of genetic diversity and varying degrees of divergence were observed in Northwestern China, reflecting the potential effect of landscape heterogeneity. Least‐cost distance across cropland cover was most positively correlated with genetic divergence, suggesting a role of croplands in facilitating gene flow. The contribution of climate to genetic divergence was secondary, whether modeled in terms of local adaptability or connectivity of the climatic landscape, suggesting that constraints to CPB gene flow imposed by a harsh climate may be ameliorated in agricultural landscapes. No evidence was found for an obvious effect of road networks on genetic divergence and population structuring. Our study provides an example of how agricultural landscape connectivity can facilitate the spread of invasive pests, even across a broad climatic gradient. More broadly, our findings can guide decisions about future land management for mitigating further spread.
The snow leopard (Panthera uncia) is a cryptic and rare big cat inhabiting Asia’s remote and harsh elevated areas. Its population has decreased across the globe for various reasons, including human–snow leopard conflicts (HSCs). Understanding the snow leopard’s distribution range and habitat interactions with human/livestock is essential for understanding the ecological context in which HSCs occur and thus gives insights into how to mitigate HSCs. In this study, a MaxEnt model predicted the snow leopard’s potential distribution and analyzed the land use/cover to determine the habitat interactions of snow leopards with human/livestock in Karakoram–Pamir, northern Pakistan. The results indicated an excellent model performance for predicting the species’ potential distribution. The variables with higher contributions to the model were the mean diurnal temperature range (51.7%), annual temperature range (18.5%), aspect (14.2%), and land cover (6.9%). The model predicted approximately 10% of the study area as a highly suitable habitat for snow leopards. Appropriate areas included those at an altitude ranging from 2721 to 4825 m, with a mean elevation of 3796.9 ± 432 m, overlapping between suitable snow leopard habitats and human presence. The human encroachment (human settlements and agriculture) in suitable snow leopard habitat increased by 115% between 2008 and 2018. Increasing encroachment and a clear overlap between snow leopard suitable habitat and human activities, signs of growing competition between wildlife and human/livestock for limited rangeland resources, may have contributed to increasing HSCs. A sound land use plan is needed to minimize overlaps between suitable snow leopard habitat and human presence to mitigate HSCs in the long run.
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