Relative Vapor Pressure Measurements Using a Frequency-Modulated Tunable Diode Laser, a Tool for Water Activity Determination in Solutions

Abstract

Two-tone frequency modulation spectroscopy (TTFMS) is applied to the determination of water activity using an 820-nm GaAlAs diode laser. By measuring the relative light absorption of water vapor over a solution and over a pure water reference, the water activity in the solution is determined. Fast, nonintrusive, and selective measurements are demonstrated. The detection limit for measuring a relative water vapor pressure change was 0.3%. The application of laser modulation spectroscopy to the determination of water activity is discussed, and suggestions on increasing the sensitivity and accuracy are presented. IntroductionThe water vapor concentration in a gas in chemical equilibrium with a solution depends on the water activity in the solution. The aim of this work is to investigate the possibility of determining the water activity in a solution by measuring the water vapor pressure over the solution relative to the vapor pressure over a pure water reference, using a diode laser, wavelength-tuned to a water molecular transition. With laser techniques the water vapor concentration can be measured independently, of other components present in the gas. This opens new possibilities in chemical analysis.The water activity depends on the composition and the concentrations of the dissolved molecules in the solution and is coupled to the interactions between the molecules.' For example, in colloid chemistry, dealing with solutions that contain aggregates in the range of 1-1000 nm, water activity measurements have given important information on the interaction between colloidal aggregates like proteins and liquid crystals.233 The chemical potential p, an important quantity in describing thermodynamic properties of water solutions, can be determined from the activity a of the water bywhere po is the standard chemical potential of pure liquid water, R the universal gas constant, and T the solution temperature. Today, several techniques for water activity determination exist.In most of them a test solution of unknown water activity is compared with a reference solution, where the water activity can be altered so that the system reaches equilibrium. Among others, three different types of techniques can be distinguished:1. Osmometer with Semipermeable M e m b r~n e .~ A test solution is set in contact with pure water through a semipermeable membrane that only permits water to pass. A small inflow of water establishes a pressure difference between the solutions and equalizes the water activity on both sides of the membrane. This pressure difference is the osmotic pressure of the test solution. Small activity differences can be measured, but the dissolved molecules must be large, e.g., macromolecules, so just water passes through the membrane. evaporates from the solution of high water activity and condenses into the solution with lower activity. At the isopiestic point, the chemical potential of water and the water activity are the same in the two solutions. By measuring the composition of the two solutions, one can determine...

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