I doubt that it is possible to find one single book, which covers all this phenomenon. Hydroscopicity has a simple thermodynamic explanation but encompass too broad range of specific cases. It corresponds to a process of water transfer from its pure liquid through vapor phase to

(1) surface of hydrophilic adsorbent (eg. celulose), or the crystalline phase of hydrate (eg. that of cyclodextrin or inorganic salts), or the product of chemical hydration, like with CaO.

(2) liquid solution formed as a result of dissolution of a target solid substance in water vapor (eg. CaCl2, sugar, etc., )

(3) solution in liquid substance (eg. TEG).

Respectively, in the case of hydroscopicity, Gibbs energy of this transfer should be large negative. For this,

(1) Adsorbent should have metal cations, or hydroxylic, or peptide groups on its surface, or any combination of these. In other words, there should be both proton-acceptor and proton-donor centers in its pores. A crystalline receptor should be able to include or coordinate water;

(2) Solubility of a solid in water should be high; the data can be found in any handbook with properties of organic and inorganic substances;

(3) The liquid should have a negative or small positive parameter of hydrophobicity LogP, where P is a 1-octanol/water partition coefficient, which can be found in various databases or calculated by software available online.

I doubt that it is possible to find one single book, which covers all this phenomenon. Hydroscopicity has a simple thermodynamic explanation but encompass too broad range of specific cases. It corresponds to a process of water transfer from its pure liquid through vapor phase to

(1) surface of hydrophilic adsorbent (eg. celulose), or the crystalline phase of hydrate (eg. that of cyclodextrin or inorganic salts), or the product of chemical hydration, like with CaO.

(2) liquid solution formed as a result of dissolution of a target solid substance in water vapor (eg. CaCl2, sugar, etc., )

(3) solution in liquid substance (eg. TEG).

Respectively, in the case of hydroscopicity, Gibbs energy of this transfer should be large negative. For this,

(1) Adsorbent should have metal cations, or hydroxylic, or peptide groups on its surface, or any combination of these. In other words, there should be both proton-acceptor and proton-donor centers in its pores. A crystalline receptor should be able to include or coordinate water;

(2) Solubility of a solid in water should be high; the data can be found in any handbook with properties of organic and inorganic substances;

(3) The liquid should have a negative or small positive parameter of hydrophobicity LogP, where P is a 1-octanol/water partition coefficient, which can be found in various databases or calculated by software available online.

For detail

http://pubs.acs.org/doi/abs/10.1021/ja01424a014

Thanks a lot, Valery, for your detailed answer, and Alwin, for the paper.

Both have been very helpful to me.

Do you think it would be correct to summarize it as follows?

1) In ALL cases, hygroscopocity is based on the vapor pressure exerted by the substance. Air will be dehumidified as long as its vapor partial pressure 'pv ' exceeds the vapor pressure of the saturated aqueous solution 'pvsat' (and viceversa).

pv > pvsat

--> Ergo: the substance must have affinity with water, but the mechanism in which water molecules are attached to the substance may vary:

2) These mechanisms of attachment may be hydratation, chemical reaction,...and depend on polarity, porosity, solubility...etc. of the substance. We may observe absorption or adsorption.

And how high this affinity is, depends on the Gibbs energy ( from Valery part II points 1,2,3). This would be the base of hygroscopicity potential.

3) The rate or hygroscopic capacity depends on:

* driving force dp = pv-pvsat

Where:

pv = f(humidity ratio)

pvsat = f(concentration, temperature, substance)

--> ergo: characteristics of the substance (again part II points 1,2,3 from Valery). We could add maybe diffusion, viscosity...?

* of course mass transfer parameters such as exchange surface, velocity of flow,...(but not relevant to my question).

Thanks again!

Christina, I agree with almost all you wrote, except for the driving force of hydroscopicity (difference in water chemical potentials) should be defined rather by ratio of pv at equilibrium and pvsat than by their difference. This is a thermodynamical factor, which determines dehydration extent at equilibrium. In practical aspect, kinetic factors may be more important, which have very complex mathematical description where a concentration gradient in dynamic equilibrium depends also on the type of hygroscopic material, system geometry, etc, as you described in your "ergo" and (*) notes.

A hygroscopic substance is one that readily attracts water from its surroundings, through either absorption or adsorption. Examples include honey, glycerin, ethanol, methanol, concentrated sulfuric acid, and concentrated sodium hydroxide (lye). Calcium chloride is so hygroscopic that it eventually dissolves in the water it absorbs: this property is called deliquescence.Materials and compounds exhibit different hygroscopic properties, and this difference can lead to detrimental effects, such as stress concentration in composite materials.

WATER

Following

Hygroscopic substances have a 'magnetic' affiliation to water

vapor pressure is affected by hygroscopy not the other way around.

Your contributions are correct

I completely agree with that, A hygroscopic substance is one that readily attracts water from its surroundings, through either absorption or adsorption. Examples include honey, glycerin, ethanol, methanol, concentrated sulfuric acid, and concentrated sodium hydroxide (lye). Calcium chloride is so hygroscopic that it eventually dissolves in the water it absorbs: this property is called deliquescence.Materials and compounds exhibit different hygroscopic properties, and this difference can lead to detrimental effects, such as stress concentration in composite materials.

Very interesting discussion, thanks to all

Good afternoon!

Do you want to increase the hygroscopicity of inorganic salts?

Moisture in the atmosphere, where the sample was placed.

Hi Irina,

I just wanted to understand the mechanism of hygroscopicity. But it would be also very interesting to know about how to increase the hygroscopicity of inorganic salts :)

Hygroscopicity of inorganic salts may be increased by their dispersion. However, a hygroscopic salt powder normally forms an impermeable crust on its surface in contact with water vapors. So, for better result, this powder should be immobilized on some less hygroscopic material with a rough texture, for example, cotton tissue or wool.

Hygroscopicity of solid organic compounds may be changed by modification of their polymorphic state as described in https://www.researchgate.net/publication/316467078_Smart_Polymorphism_of_Thiacalix4arene_with_Long-Chain_Amide_Containing_Substituents (DOI: 10.1021/acs.cgd.7b00463)

The initial mechanisms of "hygroscopicity" correlate to

1) adsorption(s) and then correlate to other

2) different surface absorption(s) mechanisms. Later on, other complex

3) insertion (often intercalation) mechanisms might follow,

depending on the

a) resulting (liquid or solid) phases and

b) on the boundary conditions

for water based molecular and different ionic species' diffusion.

The dispersion, obviously, plays a crucial (but different) role(s) in all the (basic three) mechanisms' classes, as well as the (liquid or) solid phases.

So, try to prepare some films, the thin, the better[1], in order to study most of the, above, aspects. Thin films offer many preparation parameters on this issue: how to increase the impact of the "hygroscopicity" (in inorganic salts) during your studies.

1. This technology, among other positive conditions, help into optimization of the (increase of the) surface over volume, ratio.

What could be the possible effect of freezing temperature -80 degree over hygroscopicity of substance

Hii Cristina!

Hygroscopy is ability of compound to uptake water.

In case of CaCl2, the size of the ion Ca2+ is too small. It allows water molecules to bond to it, and the total size of the sphere (ion with the bonded water molecules) becomes suitable for crystal packing. Similar behavior occurs in the Cu2+ compound.

Therefore consider the following generalization:

When the cation has a small size, it bonds some water molecules to increase the sphere size in order to have reasonable size to fit inside the crystal.

Dear Anand,

All there you mentioned LiCl, TAG alcohol and cellulose can be hygroscopic in nature.

Zinc chloride and calcium chloride, as well as potassium hydroxide and sodium hydroxide (and many different salts), are so hygroscopic that they readily dissolve in the water they absorb: this property is called deliquescence. Not all salts are hygroscopic in nature. Pure Sodium chloride does not attract water from just moist air. If relatively, humidity is low, the mineral Halite (NaCl) will not change in decades.

Justifications of three compounds

1. Lithium chloride is one of the most hygroscopic salts known. In a closed environment, a saturated solution of lithium chloride will form equilibrium at a relative humidity of about 12.4% (20C). Moisture absorption behaviour of natural fibres.

2.Cellulose, which forms the major part of the natural fibre, is hydrophilic in nature and it can absorb water molecules.

3. Alcohal TAG is hygroscopic in nature, having the ability to absorb moisture from the air. When exposed to water, isopropyl alcohol absorbs moisture until reaching a state of equilibrium. This mixture state is contains 65% isopropyl alcohol and 35% water.

A hygroscopic substance is one that readily attracts water from its surroundings, through either absorption or adsorption. Examples include honey, glycerin, ethanol, methanol, concentrated sulfuric acid, and concentrated sodium hydroxide. Hygroscopic materials will generally be supplied in sealed bags to reduce moisture absorption but even sealed bags will pick up moisture if stored in a moist cold area. Good storage is simple common sense. Keep the material dry and keep it as warm as possible.

But it’s also worth noting that the rate of water absorption and its quantity is changing depending on season or time of the day – because this is depending on moisture in the air.

Sample preparation

Hygroscopic substances start very quickly absorption of water contained in air that can be observed during weighing, when in short time there is increasing of measured value on the balance, even though there is no more substance added. Controlled conditions can be achieved by placing a balance with sealed and weighted substance (previously subjected to drying process in the appropriate conditions) in so called glove-box also know as dry box or its equivalent insulating the contained objects from the surrounding atmosphere. In some cases absorption of water contained in air by substances can effect on their weight or viscosity, but also on other physical properties.

For example, anhydrous sulfates absorb moisture until their natural, hydrated form is reached. We can include many organic compounds to hygroscopic substances. Among them is e.g. biphenyl, which is used as pesticide, and its content in food samples is controlled.

Preparation methods of LiCl, TAG alcohol and Cellulose

TAG alcohol- contains 65% isopropyl alcohol and 35% water.

LiCl- It is made by the action of hydrochloric acid on lithium hydroxide. The resulting solution is evaporated to get a mixture of saturated solution and lithium chloride crystals.

Cellulose- Natural fibres are characteristically hydrophilic in nature because of the existence of a large number of hydroxyl groups (-OH) in cellulose and hemicellulose. However, not all constituents contribute to the absorption of moisture. Cellulose, which forms the major part of the natural fibre, is hydrophilic in nature and it can absorb water molecules. Even though cellulose has a large –OH to carbon (C) ratio, a small amount –OH groups are exposed oraccessible as cellulose is semicrystalline. The highly crystalline region of the cellulose is virtually inaccessible to water molecules but the water molecules are able to penetrate and gain access into the amorphous region of the cellulose. On the other hand, hemicellulose is predominantly amorphous with high –OH to C ratio making it highly accessible to water molecules.Sol-gel process could be used for further study.

Hope, this is helpful for you.

Ashish

Dear Cristina,

All there you mentioned LiCl, TAG alcohol and cellulose can be hygroscopic in nature.

Zinc chloride and calcium chloride, as well as potassium hydroxide and sodium hydroxide (and many different salts), are so hygroscopic that they readily dissolve in the water they absorb: this property is called deliquescence. Not all salts are hygroscopic in nature. Pure Sodium chloride does not attract water from just moist air. If relatively, humidity is low, the mineral Halite (NaCl) will not change in decades.

Justifications of three compounds

1. Lithium chloride is one of the most hygroscopic salts known. In a closed environment, a saturated solution of lithium chloride will form equilibrium at a relative humidity of about 12.4% (20C). Moisture absorption behaviour of natural fibres.

2. Cellulose, which forms the major part of the natural fibre, is hydrophilic in nature and it can absorb water molecules.

3. Alcohal TAG is hygroscopic in nature, having the ability to absorb moisture from the air. When exposed to water, isopropyl alcohol absorbs moisture until reaching a state of equilibrium. This mixture state is contains 65% isopropyl alcohol and 35% water.

A hygroscopic substance is one that readily attracts water from its surroundings, through either absorption or adsorption. Examples include honey, glycerin, ethanol, methanol, concentrated sulfuric acid, and concentrated sodium hydroxide. Hygroscopic materials will generally be supplied in sealed bags to reduce moisture absorption but even sealed bags will pick up moisture if stored in a moist cold area. Good storage is simple common sense. Keep the material dry and keep it as warm as possible.

But it’s also worth noting that the rate of water absorption and its quantity is changing depending on season or time of the day – because this is depending on moisture in the air.

Sample preparation

Hygroscopic substances start very quickly absorption of water contained in air that can be observed during weighing, when in short time there is increasing of measured value on the balance, even though there is no more substance added. Controlled conditions can be achieved by placing a balance with sealed and weighted substance (previously subjected to drying process in the appropriate conditions) in so called glove-box also know as dry box or its equivalent insulating the contained objects from the surrounding atmosphere. In some cases absorption of water contained in air by substances can effect on their weight or viscosity, but also on other physical properties.

For example, anhydrous sulfates absorb moisture until their natural, hydrated form is reached. We can include many organic compounds to hygroscopic substances. Among them is e.g. biphenyl, which is used as pesticide, and its content in food samples is controlled.

Preparation methods of LiCl, TAG alcohol and Cellulose

TAG alcohol- contains 65% isopropyl alcohol and 35% water.

LiCl- It is made by the action of hydrochloric acid on lithium hydroxide. The resulting solution is evaporated to get a mixture of saturated solution and lithium chloride crystals.

Cellulose- Natural fibres are characteristically hydrophilic in nature because of the existence of a large 295 number of hydroxyl groups (-OH) in cellulose and hemicellulose. However, not all constituents contribute to the absorption of moisture. Cellulose, which forms the major part of the natural fibre, is hydrophilic in nature and it can absorb water molecules. Even though cellulose has a large –OH to carbon (C) ratio, a small amount –OH groups are exposed oraccessible as cellulose is semicrystalline. The highly crystalline region of the cellulose is virtually inaccessible to water molecules but the water molecules are able to penetrate and gain access into the amorphous region of the cellulose. On the other hand, hemicellulose is predominantly amorphous with high –OH to C ratio making it highly accessible to water molecules.Sol-gel process, Liquid flame spray (LFS) nanocoatings and Dip coating and single-step deposition methods could be used for further study.

Hope, this is helpful for you.

Ashish

Ionic nature makes a substance hygroscopic. Adsorption property makes substance hygroscopic.