Ostwald ripening occurs when larger crystals or droplets form while smaller ones disappear due to differences in molecular energy. It was first defined by W. Ostwald and can be seen in consumer products like ice cream and emulsions. Surfactants can be used to reduce this effect. It can also affect image quality in photographic processing.
Ostwald ripening is an effect that occurs in crystals and some liquid mixtures; this happens when larger crystals or droplets of liquid form while smaller ones disappear. The effect is caused by differences in the molecular energy of larger versus smaller crystals or droplets. Larger structures have lower energy because the center of the droplet or crystal has a stable molecular arrangement, which is unlikely to separate, while only the outer surface is available to form a larger crystal or droplet. Smaller sizes are less stable because the outer surface, while small, can connect or fuse with other molecules.
The changes that occur with Ostwald maturation were first defined by W. Ostwald at the end of the 19th century. Initial research found that some materials formed crystals that got rougher or coarser over time. Ostwald developed the theory of crystal size change over time, which was later confirmed with laboratory experimentation. Maturation is rarely seen with the eye, but can be determined by laboratory equipment capable of measuring crystal or droplet size.
Examples of Ostwald ripening can occur in common consumer products. Ice cream is one example; Ostwald ripening occurs when ice crystals form in ice cream after it has been stored in a freezer. This makes the ice cream textured, known as poor mouthfeel in the food industry. Crystals form because finely mixed water molecules have unstable molecular energy and want to connect with other water molecules. Over time, more and more ice crystals will form which will make the product less desirable to eat.
Some emulsions, which are combinations of two or more liquids that do not react or mix chemically, can change properties over time due to Ostwald ripening. Examples of emulsions include mayonnaise, skin creams and many liquid cosmetics. On a larger scale, this effect can be seen if oil is poured into a pan of water. Initially, the oil droplets may be quite small but over time larger droplets will form as the smaller ones disappear.
Surfactants can be used in any process where it is necessary to avoid or reduce Ostwald ripening. These chemicals do not react with any of the ingredients in the blend, but rather form molecules that surround one of the components in the blend. This surfactant coating interrupts or stops the process of small droplets or crystals merging into larger ones and results in ice cream that stays creamy or cosmetics that don’t develop a grainy texture.
Photographic processing uses chemical reactions to form silver molecules which react to light, producing a visible picture or image. Photo printing has become automated since the late 20th century for consumer photographs, but most of the processing is based on the silver chemical process. Ostwald maturation can affect image quality by changing the size of silver crystals after they are deposited on photographic paper. Steps to stop these effects are included in the development of the print, which results in a very small crystal size and improved print quality.
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