Molecular Adsorption and Physicochemical Properties at Liquid/Liquid Nanoemulsion Soft Interfaces: Effect of Charge and Hydrophobicity.

Contrary to the popular adage, "Oil and water do not mix", evidence of mixtures comprising the two "immiscible" liquids is universal. In the presence of an emulsifier, oil and water mix to form a colloidal suspension known as emulsion. Their utility in many areas such as food chemistry, biomedical health sectors, catalysis, and the petroleum industry is well recognized. While their application in our society is pervasive, tantalizing fundamental questions regarding the chemistry that takes place at the oil/water soft interface still linger. For instance, do organic compounds show proclivity for this molecularly thin boundary and, if so, what forces, hydrophobic or pure electrostatic among others, drive the molecular interactions? The focus of this Article is on molecular adsorption at the interface of oil-in-water (O/W) nanoemulsion (NE) droplets. The effect of the interfacial surfactant charge (positive, negative, zwitterionic, and neutral) on the affinity of aromatic organic compounds on the O/W NEs has been studied. Using a second harmonic generation (SHG), a nonlinear light scattering technique, we have explored the adsorption equilibrium of charged and neutral organic dyes. By variation of the surfactant functional group and thereby the interfacial charge properties, the source of the adsorption interaction, if any, has been deduced. The population of surfactants containing a charged functional group at the O/W interface is found to be sparse, yet adsorption at some of these interfaces has been observed. A purely electrostatic Coulomb interaction plays a key role, but the presence of a charged interface does not necessitate molecular adsorption. Hydrophobic interactions are not a major driving force of adsorption for the SHG dyes studied. However, a possible pi-interaction is likely in explaining the accumulation of neutral aromatic compounds at the O/W NE interface. These intricate adsorption features are discussed in the context of NE interfacial charge properties and their stability upon molecular adsorption.

Detection and binding interactions of pharmaceutical contaminants using quartz crystal microbalance - Role of adsorbate structure and surface functional group on adsorption.

HYPOTHESIS: Emerging contaminants (ECs) can interact with soft solid/aqueous interfaces of particulate organic matter and microplastics in the aquatic environment but to what extent? It is hypothesized that EC adsorption can be detected using quartz crystal microbalance (QCM), a sensitive gravimetric tool, and their adsorption energetics and uptake capacity can be measured for various substrates of distinct functional group. This in turn reveals the specific vs. nonspecific interactions. EXPERIMENTS: QCM has been used to detect and measure the adsorption of selected pharmaceuticals, amlodipine (AMP) and carbamazepine (CBZ), onto butyl, carboxyl, amine, and phenyl functionalized self-assembled monolayers (SAMs), mapping out the hydrophobic effect, H-bonding capability, and π- interactions. Adsorption free energy (ΔGads) and maximum interfacial concentration (cmax) for these surfaces are compared. Solvatochromic studies to elucidate the likelihood of H-bonding interactions for CBZ and AMP have been conducted using UV-Vis absorption spectroscopy. FINDINGS: Amlodipine and carbamazepine adsorb onto butyl/aqueous interface with respective ΔGads values of -35.8 ± 1.1 and -37.7 ± 0.1 kJ/mol. Nonspecific interaction allows a greater extent of cmax on the hydrophobic/aqueous interface. CBZ does not bind to the phenyl surface. AMP and CBZ exhibit H-bonding and show proclivity for the amine and carboxyl SAMs. Interfacial chemical environment and adsorbate structural properties play a significant role on EC adsorption.

Synthesis, photophysical and nonlinear optical properties of push-pull tetrazoles.

A 2,5-disubstituted tetrazole with p-nitrophenyl and 3-pyridyl units as acceptors (1a), and three push-pull tetrazoles with p-nitrophenyl as an acceptor and phenyl (1b), 2-(dibenzo[b,d]furan-4-yl) (1c), and 4-(N,N-diphenylamino)phenyl (1d) as donor groups, were synthesized by copper-catalyzed aerobic C-N coupling of p-nitrophenyl tetrazole with appropriately substituted aryl boronic acids. The absorption and emission spectra of 1a-c showed minimal dependence on the polarity of the solvent; however, in the case of 1d a blue shift was noted in the longest absorption band (λ 1) as the polarity increased. The fluorescence intensity of the title compounds was found to be solvent-dependent; however, no apparent correlation to solvent polarity could be established. The absorption and emission characteristics of 1a-d were also influenced by the nature of the substituent as 1d, bearing a strong electron donating 4-(N,N-diphenylamino)phenyl group, displayed a significant red shifted absorption (λ 1) as well as emission (λ em) bands compared to other compounds. Time dependent density functional calculations (CAM-B3LYP/6-311++G**) revealed that the longest wavelength band (λ 1) is associated with an intramolecular charge transfer (ICT) from HOMO/HOMO-1/HOMO-2 → LUMO/LUMO+1 in these molecules. The first hyperpolarizability values, ß HRS, of 1a-d were measured using the solution-based hyper-Rayleigh scattering technique using a femtosecond Ti:Sapphire laser and the highest NLO activity was measured for 1d with the greatest push-pull characteristics. A strong correlation was observed between the calculated hyperpolarizability (ß tot) and experimentally measured values (ß HRS).

Enhanced malachite green photolysis at the colloidal-aqueous interface.

Colloids, such as natural particulate matter and microplastics, can play a significant role in the fate and transport of organic contaminants. Specifically, these small nano-to micron-sized particles provide large surface area; thus, particle-aqueous interfacial chemistry becomes significant. In this work, we present an experimental investigation of interfacial photokinetics of malachite green cation (MG+) adsorbed at the surface of polystyrene carboxyl (PSC) microspheres suspended in aqueous solution. Second harmonic generation (SHG), an interfacial selective laser spectroscopic tool, has been used to probe the buried interface. It is revealed that relative to the bulk, photoinduced degradation of MG+ is accelerated by approximately 10-fold at this noncatalytic particle surface. By measuring the SHG-based surface electronic spectra, we have also demonstrated that N-demethylated intermediates of MG+ remain at the interface until they are further decomposed. MG+ exhibits a bathochromic shift at the interface. Together with strong binding affinity and faster initial rate of photodegradation of MG+ at the interface, this work highlights that adsorption and surface photolysis are important pathways by which organic compounds can be transformed within the aquatic environment. Moreover, this research also stimulates further questions on the enrichment of reactive species at the colloidal-aqueous interface and their influence on facilitating decompositions of organic pollutants.

Interactions of emerging contaminants with model colloidal microplastics, C60 fullerene, and natural organic matter - effect of surface functional group and adsorbate properties.

Surface adsorption of two commonly detected emerging contaminants, amlodipine (AMP) and carbamazepine (CBZ), onto model colloidal microplastics, natural organic matter (NOM), and fullerene nanomaterials have been investigated. It is found that AMP accumulation at these colloidal-aqueous interfaces is markedly higher than that of CBZ. Measurements of surface excess and particle zeta potential, along with pH-dependent adsorption studies, reveal a distinct influence of colloidal functional group on the adsorption properties of these pharmaceuticals. AMP shows a clear preference for a surface containing carboxylic group compared to an amine modified surface. CBZ, in contrast, exhibit a pH-dependent surface proclivity for both of these microparticles. The type of interactions and molecular differences with respect to structural rigidity and charge properties explain these observed behaviors. In this work, we also demonstrate a facile approach in fabricating uniform microspheres coated with NOM and C60 nanoclusters. Subsequent binding studies on these surfaces show considerable adsorption on the NOM surface but a minimal uptake of CBZ by C60. Adsorption induced colloidal aggregation was not observed. These findings map out the extent of contaminant removal by colloids of different surface properties available in the aquatic environment. The methodology developed for the adsorption study also opens up the possibility for further investigations into colloidal-contaminant interactions.

Magnetic vs. non-magnetic colloids - A comparative adsorption study to quantify the effect of dye-induced aggregation on the binding affinity of an organic dye.

HYPOTHESIS: Due to attractive magnetic forces, magnetic particles (MPs) can exhibit colloidal instability upon molecular adsorption. Thus, by comparing the dye adsorption isotherms of MPs and non-magnetic particles of the same size, shape and functional group it should be possible to characterize the influence of magnetic attraction on MP aggregation. EXPERIMENTS: For a range of particle densities, a comparative adsorption study of malachite green (MG(+)) onto magnetic and non-magnetic colloids was carried out using a combination of a separation technique coupled with UV-vis spectroscopy, optical microscopy, and polarization dependent second harmonic generation (SHG) spectroscopy. FINDINGS: Significant MP aggregation occurs in aqueous solution due to MG(+) adsorption. This alters the adsorption isotherm and challenges the determination of the adsorption equilibrium constant, Kads. The dye-induced aggregation is directly related to the MG(+) concentration, [MG(+)]. A modified Langmuir equation, which incorporates loss of surface sites due to this aggregation, accurately describes the resulting adsorption isotherms. The Kads of 1.1 (±0.3)×10(7) and a loss of maximum MP surface capacity of 2.8 (±0.7)×10(3)M(-1) per [MG(+)] has been obtained. Additionally, SHG has been established as an effective tool to detect aggregation in nanoparticles.

Second harmonic studies of ions crossing liposome membranes in real time.

The transport kinetics of the positively charged triphenylmethane dye, malachite green (MG(+)), across liposome bilayers effects the transport of monovalent inorganic cations when ionophores are present in the membrane. Three different types of ionophores characterized by different transport mechanisms have been studied. The ionophores are gramicidin A (gA) (a channel former), valinomycin (VAL) (a lipophilic cyclopeptide that encloses an alkali ion), and carbonyl cyanide-m-chlorophenylhydrazone (CCCP) (a weak acid that functions as a protonophore). The effects of these ionophores on the kinetics and extent of MG(+) crossing into the liposome, investigated using the interface selective second harmonic generation method, were found to be markedly different.