Thermal lensing in a supercritical water medium

Area of Science:

• Analytical Chemistry
• Physical Chemistry
• Chemical Thermodynamics

Background:

• Determining solute concentrations in supercritical fluids is crucial for chemical processes.
• Traditional methods face challenges due to the unique properties of supercritical water.
• Thermal lensing offers a sensitive optical technique for probing fluid properties.

Purpose of the Study:

• To investigate the feasibility of using thermal lensing spectroscopy to quantify aqueous solutes in supercritical water.
• To establish the sensitivity and linearity of the thermal lensing signal with solute concentration.
• To explore the influence of supercritical conditions on thermal lensing measurements.

Main Methods:

• A two-beam thermal lensing experiment was employed using a pulsed YAG laser (266 nm) as the pump and a continuous Ar ion laser as the probe.
• Measurements were conducted in a supercritical water cell, focusing both beams to induce and detect thermal lensing.
• The overlap of the pump and probe beams was optimized to maximize signal intensity.

Main Results:

• The thermal lensing signal strength for aqueous benzoic acid in supercritical water demonstrated a linear relationship with concentration in the sub-millimolar range.
• Despite significant mirage effects and water absorption, a measurable signal was achieved.
• The sharp density gradient near the critical point of water enhanced the thermal lensing signal intensity.

Conclusions:

• Thermal lensing is a viable technique for determining aqueous solute concentrations in supercritical water, even at low concentrations.
• Supercritical conditions, particularly the density gradient near the critical point, significantly improve the sensitivity of thermal lensing.
• Further optimization of beam overlap can maximize signal strength for quantitative analysis.