Montserrat Hernandez-Viñas

Saline lakes exhibit complex spectral behaviour due to the coexistence of shallow brine, wet sediments and evaporitic crusts. Instead of a clearly defined shoreline, these environments exhibit hydro-evaporitic transition zones in which surface states vary gradually, producing continuous spectral variations within individual pixels. As a consequence, conventional water and salinity indices often show limited transferability across saline systems with contrasting surface conditions. This study proposes a Sentinel-2 Salinity Water Index (S2SWI) derived from optimised spectral band combinations. Index coefficients were derived using three different approaches: Particle Swarm Optimisation (PSO), Ridge Regression (RR), and a hybrid U-Net + Ridge Regression (UNET+RR) framework. Reference surface classes (water, wet saline margin and crust) were generated through Random Forest classification. The index was developed using three saline lakes within a Leave-One-Lake-Out (LOLO) validation framework to promote spectral formulations that generalize across different environments. Its transferability was subsequently evaluated in three independent lakes by generating S2SWI maps and applying an unsupervised K-means clustering approach; the results were compared with Random Forest (RF) reference classifications. Results indicate that the S2SWI provides a consistent ordering of surface conditions across the six lakes, ranging from liquid hypersaline water to dry evaporitic crusts. Among the three formulations evaluated, the hybrid UNET+RR approach showed the strongest overall transferability, particularly in spectrally challenging systems characterized by extensive transition zones.The proposed S2SWI offers a computationally simple and transferable tool for Sentinel-2 monitoring of saline lakes. Moreover, the comparison between optimization, regression, and hybrid spatial-spectral approaches helps clarify the environmental conditions under which spectral indices remain robust, as well as those in which spatial information can improve the interpretation of surface conditions.

Photoinduced proton transfer reactions of harmane (1-methyl-9H-pyrido[3,4-b]indole) (HAR) in the presence of a proton donor/acceptor such as dihydrogen phosphate anions in aqueous solution have been studied by stationary and time-resolved fluorescence spectroscopy. The presence of high amounts of dihydrogen phosphate ions modifies the acid/base properties of this alkaloid. Thus, by keeping the pH constant at pH 8.8 and by increasing the amount of NaH₂PO₄ in the solution, it is possible to reproduce the same spectral profiles as those obtained in high alkaline solutions (pH >12) in the absence of NaH₂PO₄. Under these conditions, a new fluorescence profile appears at around 520 nm. This result could be related to the results of a recent investigation which suggests that a high intake of phosphates may promote skin tumorigenesis. The presence of β-cyclodextrin (β-CD) avoids the proton transfer reactions in this alkaloid by means the formation of an inclusion complex between β-CD and HAR. The formation of this complex originates a remarkable enhancement of the emission intensity from the neutral form in contrast to the cationic and zwitterionic forms. A new lifetime was obtained at 360 nm (2.5 ns), which was associated with the emission of this inclusion complex. At this wavelength, the fluorescence intensity decay of HAR can be described by a linear combination of two exponentials. From the ratio between the pre-exponential factors, we have obtained a value of K = 501 M for the equilibrium of formation of this complex.

Density and sound velocity data for aqueous solutions containing nonionic surfactants of the homologue series of polyoxyethylene(n) nonyl phenyl ethers (NPE_n, n = 5 and 40) were analyzed in the absence and presence of β-cyclodextrin (β-CD) at 298 K. Thus, the critical micelle concentration of the surfactants and their apparent and partial molar volumes and compressibilities were measured. From a pseudophase separation model, the partial molar volumes and compressibilities of both pure surfactants in the micelle state and those of NPE₄₀ in the monomer phase have been determined directly. For the ternary systems, increases of the molar volumes and compressibilities of NPE₅ and NPE₄₀ at infinite dilution and shifts of the cmc were observed compared to the binary systems. Luminescent measurements of the complexation process between NPE₄₀ and β-CD showed 1:1 + 1:2 (NPE_n/2β-CD) stoichiometries for the complexes, with thermodynamic equilibrium constants that were in good agreement with previous results for NPE₅ in the presence of β-CD. This resemblance allowed us to use these results to indirectly determine the molar partial properties of NPE₅ in the monomer state and understand the changes in the thermodynamic properties of NPE₅ due to aggregation. From the aggregation and complexation data, a folding of the surfactants at the monomer state, in which the hydrophobic moieties of NPE_n are surrounded by the EO chain, has been found. The oxyethylene group contributions at the monomer and micelle state of the NPE_n homologue series have been estimated. The values of the transfer thermodynamic properties of both surfactants and β-CD at infinite dilution conditions have been discussed in terms of a new extended model, in which the balance between the released water from the cavities of two β-CDs and the different hydrophobic moieties of the surfactant that enter the macrocycle was considered.

Absorption and fluorescence measurements for aqueous solutions at 298 K containing pentaoxyethylene nonyl phenyl ether (NPE5), in the absence and presence of beta-cyclodextrin (beta-CD), were analyzed to determine the effect of the complexation on the aggregation of the surfactant. For the binary system, the appearance of a new emission band and the presence of an isoemissive point in the emission spectra at the time and frequency domains indicate the formation of an excimer within the micellar core. The addition of beta-CD induces the formation of an inclusion complex strong enough to break the aggregates and avoid the excimer formation. For the ternary system, the increase in fluorescence has been used to assess the binding constants of 1:1 + 2:1 stoichiometries. Static light scattering, 1H NMR diffusion-ordered spectroscopy (DOSY), and two-dimensional rotating-frame Overhauser enhancement spectroscopy (ROESY) experiments were used to characterize the cloud point of NPE5 at 298 K, and to ascertain the effects of complexation on the clouding process. In the presence of beta-CD, the analysis of the 1H NMR spectra and the self-diffusion coefficients reveal the existence of interactions between the beta-CD and the aggregates that increase the cloud-point concentration more than expected. Under conditions of excess of beta-CD, ROE enhancements point to a complex of dominant 2:1 stoichiometry (beta-CD:NPE5) in which the hydrophobic moiety of the surfactant threads two beta-CDs.

¹H NMR spectra, diffusion-ordered NMR (DOSY), and 2D rotating-frame Overhauser enhancement spectroscopy (ROESY) experiments for aqueous solutions at 298 K containing the gemini surfactant, bis (dodecyl dimethylammonium)diethyl ether dibromide (12-EO₁-12), in the absence and presence of β-cyclodextrin (β-CD) were used to characterize the surfactant and to determine the effects of the complexation in the micellization. For the binary system, the critical micelle concentration (cmc), the aggregation number, the stepwise micellization constant, and the size of the monomer have been obtained by studying the dependence of the chemical shifts and the self-diffusion coefficients with the concentration of surfactant. For the ternary system, the analysis of the ¹H NMR spectra and the self-diffusion coefficients reveal the formation of complexes of 1:1 and 2:1 stoichiometry (β-CD:gemini), with a calculated stability constant for the second binding step higher than that of the first. The values of the hydrodynamic radii of the complexes were obtained from the calculated diffusion coefficients. The presence of β-CD modifies the cmc in an extension that indicates mainly the formation of a 2:1 complex. The analysis of the chemical shifts of the surfactant indicates the nonparticipation of the complexes into the micelles. ROE enhancements depend substantially on the amount of the macrocycle added and therefore on the stoichiometry; at low concentrations of β-CD, one of the hydrocarbon chains binds favorably with the cavity whereas the other interacts with the outer face. By contrast, at higher concentrations of β-CD, the two hydrocarbon tails are included in two different macrocycles.