Deliquescence

Deliquescence (Latin deliquescere , to flow) is the process in which a water-soluble substance that is exposed to moist air absorbs moisture from the air and forms a solution as soon as the air humidity is above a limit value that is characteristic of the substance. In particular, all water-soluble salts are subject to deliquescence if the air humidity is only high enough. If enough moisture can be absorbed from the ambient air, the substance dissolves completely into a solution.

The relative humidity, enters deliquescence above which is characteristic of the substance Deliqueszenzfeuchte . It is identical to the relative equilibrium humidity over a saturated solution of the substance in question in water. If the current relative humidity is higher, the water vapor partial pressure of the air is also higher than the equilibrium vapor pressure of the resulting solution, there is supersaturation and air humidity condenses on the existing solution. Conversely, if the current relative humidity is below the deliquescent humidity, water evaporates from the solution and part of the dissolved substance crystallizes out again (“efflorescence”).

Substances with a higher solubility generally have lower deliquescent moisture levels. If the solubility of a substance increases with increasing temperature, then its deliquescent moisture decreases with temperature.

Explanation

Water that contains a dissolved substance has a lower equilibrium vapor pressure than pure water (“vapor pressure reduction”). The more concentrated the solution, the lower its equilibrium vapor pressure. The saturated solution has the highest possible concentration for a given soluble substance and thus the lowest equilibrium vapor pressure. A saturated sodium chloride solution, for example, produces a vapor pressure in equilibrium that is only 75% of the vapor pressure over pure water at the same temperature. The relative humidity corresponding to this vapor pressure is also 75% according to its definition.

If a salt crystal, e.g. a sodium chloride crystal, is in moist air, individual water molecules from the air are adsorbed on the surface . Humidity below the deliquescent humidity is normal adsorptive hygroscopicity , in the case of sodium chloride, depending on the humidity, only a few tenths of a mass percent are deposited, which corresponds to about 2 to 3 layers of water molecules. The thin film of water can loosen atoms from the salt crystal and form a saturated salt solution, but this state is stable and no progressive dissolution processes take place. But as soon as the relative humidity exceeds the relative equilibrium humidity of this saturated solution of 75%, there is supersaturation on the surface of this salt solution and further water vapor from the air condenses on the solution. The solution is diluted by the addition of water, is no longer saturated and more salt goes into the solution until saturation is restored.

This process of continuous condensation and the continued dissolution of salt only ends when either the supply of moisture from the air or the supply of undissolved substance has been used up. If the substance dissolves completely, the solution then continues to absorb air humidity until its increasing equilibrium humidity due to the increasing dilution matches the current air humidity.

Deliquescence as a phase transition

In thermodynamic terms, deliquescence is a first-order phase transition between the solid state and a saturated aqueous solution . The transition takes place at a well-defined relative humidity, which depends on the substance under consideration and the temperature. At relative humidity below the deliquescent humidity, the crystalline state, surrounded by the atmosphere containing water vapor, is the more favorable state; above the deliquescent humidity, the aqueous solution is the more favorable state. In the case of deliquescent moisture itself, the saturated solution (with or without a proportion of undissolved substance) is in equilibrium with the moist air. If the humidity is above this, the unsaturated solution is in equilibrium with the moist air, with the humidity and the concentration of the solution being set such that the water vapor partial pressure of the moist air and the equilibrium vapor pressure above the solution are identical.

Deliquescent moistures

The following table lists the deliquescent moisture levels of some salts at different temperatures as an example:

	0 ° C	10 ° C	20 ° C	30 ° C	40 ° C	50 ° C	60 ° C
Sodium hydroxide , ${\displaystyle \mathrm {NaOH} }$			08.9	07.6	06.3	04.9	03.6%
Lithium chloride , ${\displaystyle \mathrm {LiCl} }$	11.2	11.3	11.3	11.3	11.2	11.1	11.0%
Magnesium chloride , ${\displaystyle \mathrm {MgCl_{2}} }$	33.7	33.5	33.1	32.4	31.6	30.5	29.3%
Sodium chloride , ${\displaystyle \mathrm {NaCl} }$	75.5	75.7	75.5	75.1	74.7	74.4	74.5%
Ammonium sulfate , ${\displaystyle \mathrm {(NH_{4})_{2}SO_{4}} }$	82.3	82.1	81.3	80.6	79.9	79.2

Examples

Humidity regulation

The deliquescence of salts can be used to set a certain relative humidity in closed containers, for example desiccators . If the deliquescence takes place in a closed container, the supply of moisture from the air is limited. If the initial air humidity is above the deliquescent humidity, the relative humidity of the air drops due to the condensation of humidity on the salt solution until it has reached the relative deliquescent humidity and is in equilibrium with the saturated solution.

Conversely, if the air humidity is below the relative deliquescent humidity, the air above the solution is undersaturated and water evaporates from the solution, which increases the air humidity until the deliquescent humidity and thus equilibrium is reached.

This control mechanism can be used to keep the relative humidity in a container at a constant value by introducing a saturated salt solution (with sufficient undissolved sediment), which can be used, for example, for the calibration of hygrometers or for laboratory work that requires a defined relative humidity require. A saturated sodium chloride solution keeps the relative humidity of the container air at a constant 75%. Other salts can be used to set other humidity fix points, for example

Relative humidity above saturated saline solution at 23 ° C
Sodium dichromate , ${\displaystyle \mathrm {Na_{2}Cr_{2}O_{7}\cdot 2\,H_{2}O} }$	52%
Magnesium nitrate , ${\displaystyle \mathrm {Mg(NO_{3})_{2}} }$	53%
Potassium chloride , ${\displaystyle \mathrm {KCl} }$	85%
Ammonium dihydrogen phosphate , ${\displaystyle \mathrm {NH_{4}H_{2}PO_{4}} }$	93%
Potassium nitrate , ${\displaystyle \mathrm {KNO_{3}} }$	94%

To produce the saturated salt solution to be used, the salt in question can be dissolved in distilled water, with sufficient undissolved sediment to be ensured in order to maintain the moisture buffering capacity. If distilled water is not available, a pure salt solution can also be made in small quantities by placing a bowl of dry salt and a bowl of tap water together in a closed container. Water vapor from the water bowl distills over into the salt bowl and forms a saturated salt solution there due to the deliquescence of the salt.

Dehumidification

During the deliquescence process, a considerable amount of water can be removed from the air and bound as a solution. This can be used for air dehumidification . The focus here is not on setting a desired relative humidity, but rather on extracting air humidity. The prerequisite for this is that the deliquescent humidity of the substance used for dehumidification is below the relative humidity of the air to be dehumidified.

Salts with a deliquescent moisture content close to 0% can remove almost all of the moisture from the air and can be used as a drying agent.

Relative humidity via desiccant at 23 ° C
Calcium chloride ,, grain size <3 mm ${\displaystyle \mathrm {CaCl_{2}} }$	≈ 0%
Magnesium perchlorate, ${\displaystyle \mathrm {Mg(ClO_{4})_{2}} }$	≈ 0%
Phosphorus pentoxide, ${\displaystyle \mathrm {P_{2}O_{5}} }$	≈ 0%

Salt damage

Salts contained in the pore space of building materials are often the cause of structural damage. If salt crystallizes out of its solution, the crystallization pressure exerted by the growing crystals can exceed the mechanical strength of the building material and damage the building material. Repeated dissolving and crystallizing is particularly harmful, as the damage then adds up. Salts in the pore space can be dissolved by adding water as a result of rain or condensation and crystallize again in dry periods. Salts whose deliquescent moisture content is in the range covered by the natural fluctuations in relative humidity can, however, also be subjected to a constant cycle of dissolution and crystallization due to the fluctuations in humidity. In some cases, damage can be prevented by keeping the air humidity to which the building material is exposed permanently either below or above the deliquescent humidity of the salt present.

Storage conditions

Deliquescence can impair the chemical and physical stability of commercial goods during their manufacture, transport or storage, especially if they are in powder form. Many chemical reactions take place more easily or quickly in the resulting aqueous solution, which can lead to loss of quality or spoilage. Alternating deliquescence and renewed crystallization (efflorescence) caused by frequent changes in humidity can promote crust formation and lump formation.

Mixtures of deliquescent substances have a deliquescent moisture that is below the deliquescent moisture of the constituents involved. For example, although sodium chloride, the main component of commercial table salt , has a deliquescent moisture content of 75%, deliquescence of table salt can begin at significantly lower humidity levels, as it usually contains various additives in addition to sodium chloride such as magnesium chloride with a deliquescent moisture content of 33%.

The following table shows some examples of deliquescent moisture content of crystalline food ingredients:

Deliquescent humidity at 25 ° C
Ascorbic acid	> 95%
Citric acid anhydrate	74-75%
Citric acid monohydrate	78%
Malic acid	59%
bD- fructose	62-63%
α- glucose anhydrate	89-91%
α-glucose monohydrate	91%
β-glucose	74%
α- lactose monohydrate	95%
β-lactose	95%
Sucrose	85%
Sodium glutamate	87%
Sodium chloride	76%

Demarcation

Deliquescence is to be distinguished from normal hygroscopicity, in which an object attaches water molecules to its surface through physical adsorption (especially also to the surfaces of its inner pore space, if any). The amount of moisture accumulated in this way increases steadily with increasing air humidity, so, in contrast to deliquescence, there is no sudden change. Objects for which deliquescence is possible, such as salt crystals, have this "normal" hygroscopicity below the deliquescence moisture.

Deliquescence must also be distinguished from chemical absorption , in which humidity in the air in the form of crystal water can be bound in the crystal lattice of a substance. If a substance can occur in different hydration stages ( copper sulfate, for example, is available as penta-, tri-, mono- and anhydrate ), then the transition from one stage to the next takes place when a certain relative humidity is exceeded, but with a change in the crystal structure connected, usually not with a solution process.

Web links

Wiktionary: deliquescence  - explanations of meanings, word origins, synonyms, translations

• Deliquescent moisture in SaltWiki

Individual evidence

1. ^ Langenscheidt online dictionary: deliquescere
2. ↑ Entry on deliquescence . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.D01582 Version: 3.0.1.
3. ↑ Encyclopædia Britannica online, keyword Deliquescence
4. ↑ ^{a b c d e f g h i j} LJ Mauer, LS Taylor: Water-Solids Interactions: Deliquescence. In: Annual Review of Food Science and Technology, Vol. 1, pp. 41-63 (April 10, 2010), doi : 10.1146 / annurev.food.080708.100915 ( PDF 0.99 MB ), p. 43
5. ^ MP Bhatt: Studies on impurity association and removal, hygroscopicity, caking and dissolution characteristics of solar salt. PhD Thesis, Gujarat University, URI: http://hdl.handle.net/10603/49053 , S. 148f ( digitized version )
6. ^ LJ Mauer, LS Taylor: Water-Solids Interactions: Deliquescence. In: Annual Review of Food Science and Technology, Vol. 1, pp. 41-63 (April 10, 2010), doi : 10.1146 / annurev.food.080708.100915 ( PDF 0.99 MB ), Table 1, p. 45
7. ^ ^{A b} L. Greenspan: Humidity Fixed Points of Binary Saturated Aqueous Solutions. Journal of Research of the National Bureau of Standards - A. Physics and Chemistry, Vol. 81A, No. January 1-February 1977 ( PDF 8.55 MB )
8. ↑ ^{a b} DIN EN ISO 12572: Heat and moisture behavior of building materials and building products - Determination of water vapor permeability - Method with a test vessel (ISO 12572: 2016). Beuth-Verlag, Berlin 2017.
9. ↑ ^{a b c d} Salt wiki, keyword "Deliquescence moisture"
10. ^ LJ Mauer, LS Taylor: Water-Solids Interactions: Deliquescence. In: Annual Review of Food Science and Technology, Vol. 1, pp. 41-63 (April 10, 2010), doi : 10.1146 / annurev.food.080708.100915 ( PDF 0.99 MB ), Table 1, pp. 48ff
11. ^ LJ Mauer, LS Taylor: Water-Solids Interactions: Deliquescence. In: Annual Review of Food Science and Technology, Vol. 1, pp. 41-63 (April 10, 2010), doi : 10.1146 / annurev.food.080708.100915 ( PDF 0.99 MB ), Table 1, p. 44