Obtaining extended insight into molecular systems by probing multiple pathways in second-order nonlinear spectroscopy.

Second-order nonlinear spectroscopy is becoming an increasingly important technique in the study of interfacial systems owing to its marked ability to study molecular structures and interactions. The properties of such a system under investigation are contained within their intrinsic second-order susceptibilities which are mapped onto the measured nonlinear signals (e.g. sum-frequency generation) through the applied experimental settings. Despite this yielding a plethora of information, many crucial aspects of molecular systems typically remain elusive, for example the depth distributions, molecular orientation and local dielectric properties of its constituent chromophores. Here, it is shown that this information is contained within the phase of the measured signal and, critically, can be extracted through measurement of multiple nonlinear pathways (both the sum-frequency and difference-frequency output signals). Furthermore, it is shown that this novel information can directly be correlated to the characteristic vibrational spectra, enabling a new type of advanced sample characterization and a profound analysis of interfacial molecular structures. The theory underlying the different contributions to the measured phase of distinct nonlinear pathways is derived, after which the presented phase disentanglement methodology is experimentally demonstrated for model systems of self-assembled monolayers on several metallic substrates. The obtained phases of the local fields are compared to the corresponding phases of the nonlinear Fresnel factors calculated through the commonly used theoretical model, the three-layer model. It is found that, despite its rather crude assumptions, the model yields remarkable similarity to the experimentally obtained values, thus providing validation of the model for many sample classes.

Utilization of fruit pomace, overripe fruit, and bush pruning residues from Andes berry (Rubus glaucus Benth) as antioxidants in an oil in water emulsion.

Crude extracts were prepared from residues of Rubus glaucus Benth by using food-grade solvents. Their efficacy protecting lipid oxidation of an oil in water (O/W) emulsion as well as of a bulk oil was tested. Stability of lipids during storage of an O/W emulsion was tested by the hydroperoxides and thiobarbituric acid reactive species (TBARS) measurements. Bulk oil stability was measured by the Rancimat method. Fruit pomace crude extracts were the best controlling lipid oxidation of an O/W emulsion, with crude extracts from overripe fruit and bush pruning residues acting as pro-oxidants as measured by the hydroperoxides levels. Neither of the crude extracts was able to inhibit lipid oxidation of the bulk oil. Mathematical modelling revealed that despite total phenolic content and partition coefficient of the crude extracts are important parameters to control lipid oxidation of an O/W emulsion, they do not totally explain their behavior in food-like systems.