Comparison of Vibrational Spectroscopic Techniques for Quantification of Water in Natural Deep Eutectic Solvents

Abstract

Vibrational spectroscopic techniques, i.e., attenuated total reflectance infrared (ATR-IR), near infrared spectroscopy (NIRS) and Raman spectroscopy (RS), coupled with Partial Least Squares Regression (PLSR), were evaluated as cost-effective label-free and reagent-free tools to monitor water content in Levulinic Acid/L-Proline (LALP) (2:1, mol/mol) Natural Deep Eutectic Solvent (NADES). ATR-IR delivered the best outcome of Root Mean Squared Error (RMSE) of Cross-Validation (CV) = 0.27% added water concentration, RMSE of Prediction (P) = 0.27% added water concentration and mean % relative error = 2.59%. Two NIRS instruments (benchtop and handheld) were also compared during the study, respectively yielding RMSECV = 0.35% added water concentration, RMSEP = 0.56% added water concentration and mean % relative error = 5.13% added water concentration, and RMECV = 0.36% added water concentration, RMSEP = 0.68% added water concentration and mean % relative error = 6.23%. RS analysis performed in quartz cuvettes enabled accurate water quantification with RMECV = 0.43% added water concentration, RMSEP = 0.67% added water concentration and mean % relative error = 6.75%. While the vibrational spectroscopic techniques studied have shown high performance in relation to reliable determination of water concentration, their accuracy is most likely related to their sensitivity to detect the LALP compounds in the NADES. For instance, whereas ATR-IR spectra display strong features from water, Levulinic Acid and L-Proline that contribute to the PLSR predictive models constructed, NIRS and RS spectra are respectively dominated by either water or LALP compounds, representing partial molecular information and moderate accuracy compared to ATR-IR. However, while ATR-IR instruments are common in chemistry and physics laboratories, making the technique readily transferable to water quantification in NADES, Raman spectroscopy offers promising potential for future development for in situ, sample withdrawal-free analysis for high throughput and online monitoring.

Controlling Water Delivery to an Electrochemical Interface with Surfactants

“…For these spectra, the concentration of water was 10 M and that of CTAB was 1 mM in dDMSO as the dominant solvent. The water spectrum corresponds to the OH stretch and is blueshifted (3550 cm –1 ) with respect to the expected peak for pure bulk water (3480 cm –1 ). , This behavior is expected for water diluted in organic solvents . The signature of CTAB is the symmetric (2850 cm –1 ) and antisymmetric (2929 cm –1 ) CH stretches of the aliphatic chain .…”

“…66,67 This behavior is expected for water diluted in organic solvents. 68 The signature of CTAB is the symmetric (2850 cm −1 ) and antisymmetric (2929 cm −1 ) CH stretches of the aliphatic chain. 69 The dDMSO signature peaks in the 1050 cm −1 region are previously assigned to the SO stretch and its combinations with other modes in the molecule.…”