Adsorption - Adsorption

Japanese: 吸着 - きゅうちゃく（英語表記）adsorption

When two phases are in equilibrium, the concentration of a certain component may differ between near the interface and inside the phase. This phenomenon is called adsorption. When the concentration near the interface is higher than inside the phase, it is called positive adsorption, and when the opposite is true, it is called negative adsorption. Negative adsorption is almost never a problem in practice, except for the case of adsorption on the solution surface in an aqueous solution of inorganic salts, and positive adsorption is generally referred to as adsorption. Also, when an adsorbed substance is released, it is called desorption.

[Toshihisa Yoshida]

Heat of adsorption

Heat of adsorption is the amount of heat generated by adsorption, and since the heat of adsorption is always positive, the lower the temperature, the greater the amount of adsorption. In the case of hydrogen gas adsorption on metals, the amount of adsorption does indeed decrease as the temperature rises in the low temperature range. However, above room temperature, a different type of adsorption occurs again. Adsorption in the low temperature range has a small heat of adsorption (less than about 20 kJ) and can exist even with different types of metals and gases. It is said to be caused by the physical attraction between the gas molecules and the solid surface, and this is called physical adsorption. Adsorption in the high temperature range has a large heat of adsorption (more than about 40 kJ), is extremely slow, and shows a considerable amount of activation heat. It may not occur depending on the combination of metal and gas, so this is called chemical adsorption. Physical adsorption can be understood as being similar to the phenomenon of gas condensing on a solid surface, but chemical adsorption can also be considered to be due to the formation of something similar to normal chemical bonds between the metal and gas molecules.

[Toshihisa Yoshida]

Use of Adsorption

In addition to adsorption between solid and gas phases, adsorption can also be observed at interfaces between solid and liquid phases, liquid and gas phases, and two liquid phases that are insoluble in each other. Familiar examples include activated carbon used to deodorize the inside of refrigerators and filter drinking water. An example of a desiccant is silica gel (silicic acid gel). It is also widely used industrially, and a method of recovering adsorbed substances using adsorbents (carbon-based porous bodies, zeolites, etc.) called the pressure swing adsorption and desorption method (PSA method, in which adsorption is performed at high pressure and desorption is performed at reduced pressure) is used in many fields. Using this method, it is easy to separate adsorbed substances such as nitrogen and oxygen, and to recover organic solvents such as acetone.

[Toshihisa Yoshida]

Separation technology and analysis

In chemical analysis, its application in separation techniques such as gas and liquid chromatography has made a major contribution to modern chemical analysis methods.

[Toshihisa Yoshida]

"Adsorption" by Keii Tominaga (1965, Kyoritsu Shuppan)" ▽ "Adsorption Technology Handbook" edited by Shimizu Hiroshi (1993, NTS)" ▽ "Colloid Science 1: Fundamentals, Dispersion and Adsorption" edited by the Chemical Society of Japan (1995, Tokyo Kagaku Dojin)" ▽ "The Chemistry of Adsorption - Key Technologies for Surface and Interface Control" by Takeuchi Setsu (1995, Sangyo Tosho) ▽ "The Science of Adsorption, 2nd Edition, by Kondo Seiichi, Ishikawa Tatsuo and Abe Ikuo (2001, Maruzen)" ▽ "The Science and Applications of Adsorption" by Ono Yoshio and Suzuki Isao (2003, Kodansha)"

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

2相が平衡にあるとき、ある成分の濃度が界面付近と相内部とで異なることがある。この現象を吸着という。界面付近の濃度が相内部より大きいとき正吸着、逆の場合を負吸着という。負吸着は無機塩類水溶液における溶液表面への吸着の例を除けば、ほとんど実際に問題となることはなく、一般には正吸着を吸着という。また、吸着物が脱離することを脱着という。

［吉田俊久］

吸着熱

吸着に伴う発熱量で、吸着熱はつねに正であるので、温度の低いほうが吸着量は多くなる。金属に対する水素ガスの吸着などでは低温域で確かに温度の上昇とともに吸着量は減少する。しかし常温以上では、ふたたび別種の吸着がおこる。低温域の吸着は吸着熱が小さく（約20キロジュール以下）、金属やガスの種類が異なっても存在する。ガス分子と固体表面の間の物理的引力に起因するといわれ、これを物理吸着という。高温域の吸着は吸着熱が大きく（約40キロジュール以上）、吸着速度もきわめて遅く、かなりの活性化熱を示す。金属とガスの組合せしだいでおこらないこともあるので、これを化学吸着という。物理吸着は固体表面に気体が凝縮する現象に近いものと理解できるが、化学吸着は金属と気体分子間に通常の化学結合に近いものができるためと考えてもよい。

［吉田俊久］

吸着の利用

固相と気相の間の吸着のほかに、固相と液相、液相と気相、互いに溶解しない二つの液相などからなる界面でも吸着は観察できる。身近な例として、冷蔵庫内の脱臭、飲料水の濾過(ろか)などに活性炭が用いられている。乾燥剤としてはシリカゲル（ケイ酸のゲル）などがある。工業的にも大いに使われており、吸着剤（炭素系多孔体、ゼオライトなど）を用いて、圧力変動式吸脱着法（PSA法。高圧で吸着を行い、減圧で脱着を行う方法）という、吸着物質の回収を行う操作法が多くの分野で利用されている。この方法を用いれば、吸着物質としての窒素、酸素の分離などや、またアセトンのような有機溶剤の回収なども容易である。

［吉田俊久］

分離技術・分析

化学分析ではガスおよび液体クロマトグラフィーなどの分離技術に応用され、これは現代の化学分析法に大きく寄与している。

［吉田俊久］

『慶伊富長著『吸着』（1965・共立出版）』▽『清水博監修『吸着技術ハンドブック』（1993・エヌ・ティー・エス）』▽『日本化学会編『コロイド科学1　基礎および分散・吸着』（1995・東京化学同人）』▽『竹内節著『吸着の化学――表面・界面制御のキーテクノロジー』（1995・産業図書）』▽『近藤精一・石川達雄・安部郁夫著『吸着の科学』第2版（2001・丸善）』▽『小野嘉夫・鈴木勲著『吸着の科学と応用』（2003・講談社）』