Fixed-Path Length Laser-Induced Sound Pinging: A Streamlined Method for Sound Speed Determination in Arbitrary Liquids

摘要

Recently, we introduced laser-induced sound pinging (LISP) as a simple photoacoustic technique for the expedient and fairly rapid (2-3 min) measurement of the speed of sound traversing low-volume samples (25-1000 mu L), built around an inexpensive tattoo-removal laser source (Sens. Actuators B-Chem.2019, 291, 401-410). In this report, we expand on the utility of this technique by improving the facility of measurement and speed of analysis by adopting a fixed-path length (FPL) architecture which allows for much faster analysis (1-5 s) of similar sample volumes. The FPL-LISP setup was employed to determine the speeds of sound at temperatures from 293 to 333 K for four popular hydrophilic deep eutectic solvents (DESs) containing choline chloride (i.e., reline, ethaline, glyceline, and maline), plus several representative decanoic acid-based (i.e., hydrophobic) DESs. The temperature-dependent speeds of sound were used alongside the corresponding experimental densities to calculate the bulk modulus for these illustrative DESs. Interestingly, the bulk modulus was in the 4000-5000 MPa range for choline chloride-derived DESs, essentially twice the value of a hydrophobic DES (typified by 1:2 tetraoctylammonium bromide/decanoic acid), suggesting unique potential for hydraulic applications, for example. The modest volume requirements of FPL-LISP make it suitable for determining the speed of sound in scarce, precious, or hazardous liquids, and the speed of analysis enables integration into a continuous-flow platform for real-time analytics (e.g., beverage quality control).