مجفف

من ويكيبيديا، الموسوعة الحرة
اذهب إلى: تصفح، ‏ ابحث

المُجَفِّف مادة كيميائية استرطابية تجلب - أو تصون - حالة تجفف في محيطها إذا وضعت في وعاء معزول جيّدًا.

أمّا المرطّب فهو أيضًا مادة استرطابية لكنّه يُستخدم ليُعين مُنتَجٍ ما على احتواء الماء، وتُستخدم المواد المرطّبة كثيرًا في مجالات الطّعام والمستحضرات التجميلية - وبشكل خاص الجليسرين.

ومن أوسع المجفّفات انتشارًا مواد صلبة تعمل من خلال الامتصاص أو الامتزاز أو الاثنين معًا، كما تتواجد مجفّفات غير صلبة للتطبيقات المتخصّصة، وقد تعمل تلك الأخيرة على أسس أخرى، كالربط الكيميائي مع جزيئات الماء.

وتُستخدم المجفّفات المُعبّأة مسبقًا (الجاهزة) في الغالب لطرد أي رطوبة زائدة قد تُفاقم أو تُفسد المنتج الحسّاس للماء. ومن أوسع المجفّفات انتشارًا:

التطبيقات[عدل]

يبقي المرطب إذا أضيف إليه على الطاعم رطبا طريا. كما تكون المرطبات أحيانا أحد مكونات الطبقات المقاومة للكهرباء الساكنة التي توضع على اللدائن. وتستخدم المرطبات أيضا في العديد من مستحضرات التجميل لغرض «ترطيب البشرة» - مثلا في منعم الشعر (en)وكريمات البشرة.

كما تستخدم المرطبات في صناعة بعض السجائر ومنتجات التبغ.

Humectants are also used in topical dosage forms to increase the solubility of the active ingredient, to elevate its skin penetration and increase its activity time. Humectants also elevate the hydration of the skin to minimize the dehydrating effect of some active ingredients like corticoids.

A specific usage of a desiccant would be in insulated windows where it is placed inside the spacer between the air space and window edge. This prevents moisture condensation between the panes

Toxicity[عدل]

A desiccant may be chosen for a particular task based not only on its effectiveness at drying, but also perhaps for any natural antibiotic, fungicidal, pesticidal, or virucidal effect, or for a lack of harmful effect on humans. For instance, table salt has a natural antibiotic and anti-fungal effect and is not harmful to humans when consumed in moderate quantities, and is thus commonly used as a drying agent in the preparation of dried foods.

Chemistry[عدل]

A humectant is a hygroscopic substance. It is often a molecule with several hydrophilic groups, most often hydroxyl groups, but amines and carboxyl groups, sometimes esterified, can be encountered as well; the affinity to form hydrogen bonds with molecules of water is crucial here.

Because of their common use in preservation, it is usually desirable for a desiccant to be substantially chemically stable or chemically inert, and a number of useful desiccants with this property have been identified and are in common use, such as silica gel, chalk and clay-based desiccants.

Performance efficiency[عدل]

One measure of desiccant efficiency is the ratio (or percentage) of water storable in the desiccant, relative to the mass of desiccant.

Another measure is the residual relative humidity of the air or other fluid being dried.

The performance of any desiccant varies with temperature and both relative humidity and absolute humidity. To some extent, desiccant performance can be precisely described, but most commonly, the final choice of which desiccant best suits a given situation, how much of it to use, and in what form, is made based on testing and practical experience.

Coloured saturation indicators[عدل]

Indicating Silica Gel

Often some sort of humidity indicator is included in the desiccant to show, by color changes, the degree of water-saturation of the desiccant. One commonly used indicator is cobalt chloride (CoCl2). Anhydrous cobalt chloride is blue. When it bonds with two water molecules, (CoCl2•2H2O), it turns purple. Further hydration results in the pink hexaaquacobalt(II) chloride complex [Co(H2O)6]Cl2.

Drying of solvents[عدل]

Toluene is refluxed with sodium and benzophenone to produce dry, oxygen-free toluene. The toluene is dry and oxygen free when the intense blue coloration from the benzophenone ketyl radical is observed.


Desiccants are also used to remove water from solvents, typically used for moisture free reactions (e.g. the Grignard reaction). The method generally, though not always, involves stirring the solvent with the desiccant.

Often, the desiccant will react with moisture to form an insoluble solid that can be removed by filtration. When a very pure solvent is required, a distillation still is used to remove moisture and other impurities simultaneously.

Types of desiccants[عدل]

propylene glycol (E1520) as well as hexylene glycol and butylene glycol

See also[عدل]

مصادر[عدل]

روابط خارجية[عدل]