Investigating the pH dependence of thermal signatures in monohydric and polyhydric alcohols using time-resolved thermal lens spectroscopy

Abstract

We study the influence of pH on the thermophysical properties of monohydric and polyhydric alcohols through a femtosecond laser-induced thermal lens (TL) spectroscopic technique. Relative changes in the thermal signatures of monohydric and polyhydric alcohols are identified at different pH levels using both dual-beam Z-scan and time-resolved TL measurements. We found the TL signatures of the alcohols not to be affected in the acidic region, but a significant change is noticed in the alkaline environment. It is observed that in an alkaline environment (pH > 7), the TL signal significantly increases for short-chain (MeOH and EtOH) monohydric alcohols. In contrast, the TL signal for longer chain length monohydric alcohols (HxOH and phenol) and polyhydroxy alcohols (Ethylene glycol & Glycerol) fall enormously. These results depend strongly on the molecular characteristics of solvents and their interaction strength with the solute as a function of changing pH strength. Our results show the influence of pH on the natural drifting and the heat dissipation characteristics of solvent molecules, which prominently participate in changing the TL signatures of samples. This event could be attributed to changes in molecular arrangements by developing solvation shells (through ions-solvents interactions) in the systems. We investigate the influence of various factors, namely, chain length, structure, polarity, and inter or the intra-H-bonding ability of solvent molecules and ion concentration, which effectively alter the solute-solvent interaction strengths at different pH values (especially in an alkaline environment). The findings through our TL measurements explore this event in great detail for the first time. We try to correlate the molecular characteristics and its ions-solvent interaction effects with the thermophysical properties of alcohols.