All compounds that attract water as vapor or liquid from their environment are hygroscopic, which is why they are often used as desiccants .
Summary
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- 1 Introduction
- 2 hygroscopic nuclei
- 3 Deliquescence
- 4 Source
Introduction
In the manufacturing process of ceramic floor tiles and in many other cases, related to the study and characterization of clay material, the need arises to carry out prior checks on the most relevant parameters such as:
- Moisture degree1 of the samples; b) cooking temperature;
- Pressing pressure and
- Apparent density (DAP), to name a few. These controls are made on raw samples by different methods, such as the most recent2, with the aim of determining the behavior of electrical properties such as:
- conductivity;
- electrical permittivity;
- grain3 dimensions, and others.
The experimental results of the absorption and thermal emission behavior of more than 100 samples of raw ceramic material (test tubes). The most important electrical property of dielectric materials is the electrical permittivity ε, which is complex for clay compounds, since its calculation and determination is a multi-phase problem (air, water, solid), as reported by 4. A rigorous physical model that describes the behavior of the relative electrical permittivity ε, for this class of atomic systems, is extensive and requires a good calculation machine and enough time available, s
Some of the hygroscopic compounds react chemically with water, such as hydrides or alkali metals . Others trap it as water of hydration in its crystalline structure, such as sodium sulfate . Water can also be physically adsorbed. In these last two cases, the retention is reversible and the water can be desorbed. In the first case, having reacted, it cannot be recovered simply.
Some examples of the best known hygroscopic compounds are:
- Calcium chloride (CaCl 2)
- Sodium chloride (Halite) (NaCl)
- Sodium hydroxide (NaOH)
- Hydroxylamine (NH 2OH)
- Sulfuric acid (H 2SO 4 )
- Copper (II) sulfate (CuSO 4)
- Phosphorous oxide (V) (P 4O 10 )
- Calcium oxide (Quicklime) (CaO)
- Silica gel
- Honey .
For each substance there is a moisture that is called equilibrium , that is, a moisture content such as that of the atmosphere at which the material takes up moisture from the environment at the same rate that it releases it. If the ambient humidity is less than this equilibrium value, the material will dry, if the ambient humidity is higher, it will moisten. Thus, certain minerals such as calcium chloride are capable of capturing water from the atmosphere in almost any condition, because their equilibrium humidity is very high. Substances like these are used as desiccators. Other examples are sulfuric acid, silica gel, etc.
Hygroscopic nuclei
The hygroscopic nuclei refer to the particles of salts or droplets of saline solutions, mainly coming from the seas and oceans, on which the humidity of the air in the atmosphere is condensed. Once in the atmosphere they begin to come into contact with the moisture droplets and dissolve in them. As the vapor pressure of the solution is lower than that of pure water, the steam condenses in it, until it forms larger drops that end up falling in rain. One of the methods to cause rain artificially consists precisely in disseminating hygroscopic nuclei within the clouds.
Automotive brake fluid is also an example of hygroscopic fluid. We must be careful not to touch it and avoid spills. In the vehicle it has a paint paint stripping effect. It has a high boiling point, so it serves to withstand high temperatures (300 ° C). It is usually made up of alcohols or glycerins.
Deliquescence
Deliquescent materials (from the Latin deliquescere , to become liquid) are substances (mostly salts ) that have a strong chemical affinity for moisture and that absorb relatively high amounts of water if exposed to the atmosphere , forming a liquid solution . They are examples of deliquescent substances: calcium chloride , ferric chloride , magnesium chloride , zinc chloride , potassium carbonate , potassium hydroxide and sodium hydroxide.