Effect of (De-)protonation on the UV Spectra of Sodium Oleate: Experimental and Electronic Structure Investigation

Abstract

Sodium oleate (NaOL), an anionic surfactant used in mineral processing, is challenging to quantify in aqueous solutions using UV spectroscopy. This study examined ultraviolet spectral shifts of sodium oleate with and without common pH regulators. Experimental results were supported by quantum chemical calculations at the CAM-B3LYP//6–31++G(d,p) level of theory. Two main absorbance peaks appeared in sequence, at 190 and 232 nm, corresponding to the π → π* and the n → π* electronic transitions of the CC double bond and COO– functional group, respectively. Under the typical concentration range used in adsorption and flotation studies, the first peak redshifted to reach λmax = 203 nm at 2 mM NaOL, while both solution deprotonation and protonation resulted in hypo/hyperchromicity. By influencing the electronic environment, acidic and alkaline treatments triggered a broadening and narrowing of the NaOL UV spectra. This difference in behavior was associated with the dissociative equilibrium of oleic acid, where protonation was favorable to molecular species, whereas deprotonation favored the ionic oleate species. The HOMO-LUMO excitation energies were higher in the protonated than in the deprotonated environment, explaining the broader spectra obtained experimentally in the former and higher reactivity in the latter. The peak disruptions give rise to significant discrepancies during quantitative measurements of NaOL in aqueous solutions, which may be managed by appropriate precautions and corrective techniques. These findings can be extended to systems similar to the oleate system and may guide experimental and data-processing procedures in UV spectrometry.

CO2 Switchable Pickering Emulsion Stabilized by Responsive Janus SiO2 Nanoparticles for Enhanced Oil Recovery

“…When sodium oleate and SiO 2 JNPs absorb at the oil–water interface, the hydrophilic groups of oleic acid and the amino moieties on SiO 2 JNPs are present in the aqueous phase, while the hydrophobic tails are both located in the oil phase. Nevertheless, upon the introduction of CO 2 into the W/O emulsion, the ionized carboxyl group (−COO – ) of sodium oleate underwent protonation, converting it into a carboxylic acid group (−COOH). ,,, After CO 2 was introduced into the response system for 1.5 min, the pH decreased. The pH tests also confirmed the protonation of −COO– (see Figure S5).…”