Transition at the deliquesce point in single salts

Background: Deliquesce points for single salts are in general considered to occur at a specific relative humidity and are also shown as such in phase diagrams. For this reason, salts are used for calibration purpose. According to Gibbs phase rule, the crystalline solid and the saturated solution coexist indefinitely, thus, the reaction rate vanishes at equilibrium and the reaction rate is thereby strongly influenced by the relative humidity.

In Linnow & Steiger [2006] several measurements with varying distance to the deliquesce point was made making it possible to extrapolate the infinite reaction time. With these dynamic X-ray diffraction measurements they found the deliquescence point to 75.4 +/- 0.5 % RH at 25˚C. They found that in case of NaCl and a RH 1.2 % RH above the traditional used reference deliquesce point by e.g.

Greenspan equilibrium was reached after 44.7 hours. In [Dai et al., 1997] a stepwise change in salt state is explained by the diffusion of hydrated Na+ and Cl- ions as hydration of surface ions weakens ionic bond strength to the point where they become mobile. According to [Peters & Ewing, 1997] is NaCl added thin film water at RH above 40 % and they measured the increasing numbers of thin film water till 20 mbar at 25˚C whereas the deliquescence point is at 24 mbar.

These results suggest a stepwise change in the state of the salt. During preparation to salt calibration tests (in a Dynamic Vapour Sorption equipment (DVS)) the author noticed that some single salts have a very sudden and accurate change in salt state whereas another salt changed inaccurate as was noticed with NaCl (seen in more than 10 salt preparations).

In the present work, the inaccurate transition between the solid NaCl to NaCl in solution was investigated with a cooling stage (CS) in an ESEM FEI electron microscope at salt crystal level being able to visually observe changes in crystal structure. CS in an ESEM is a tool to create a specific temperature, pressure or humidity.

Being able to control the temperature and pressure a well-defined relative humidity is present. The main function of the CS is the thermoelectric module which is a small wafer composed of PN semiconductor layers. When current passes through these elements, one side of the wafer heats and the opposite one cools.

Reversing the polarity switches cold and hot sides. This is refered to as the Peltier effect. With the CS it was possible visually to observe changes in the crystal structure at changing RH. It was seen that in case of LiCl the transition deliquesce point occurred within a narrow % RH and a short time period in contrast to in case of NaCl where the transition seemed to occur over a wider % RH range or as a consequence of a reaction time of many hours.

Putting into perspective. The velocity of dissolution varies for the different salts can have a major practical influence. Unlike the common comprehension the state of the salt will not necessarily change instantaneously when a specific RH comes into existence and might even not come into existence if the relative humidity is changed into the original stage before the reaction had been completed.

Salts with a slow transition at the deliquesce point are less useful for calibration purposes as equilibrium might not come into existence within the calibration duration (11 hours is the suggested salt calibration duration in a DVS) unless the equilibrium state existed prior to the calibration.