Dealkalization is a process of surface modification applicable to glasses containing alkali ions, wherein a thin surface layer is created that has a lower concentration of alkali ions than is present in the underlying, bulk glass. This change in surface composition commonly alters the observed properties of the surface, most notably enhancing corrosion resistance.
Many commercial glass products such as containers are made of soda-lime glass, and therefore have a substantial percentage of sodium ions in their internal structure. Since sodium is an alkali element, its selective removal from the surface results in a dealkalized surface. A classic example of dealkalization is the treatment of glass containers, where a special process is used to create a dealkalized inside surface that is more resistant to interactions with liquid products put inside the container. However, the term dealkalization may also be generally applied to any process where a glass surface forms a thin surface layer that is depleted of alkali ions relative to the bulk. A common example is the initial stages of glass corrosion or weathering, where alkali ions are leached from the surface region by interactions with water, forming a dealkalized surface layer.
A dealkalized surface may have either no alkali remaining or may just have less than the bulk. In silicate glasses, dealkalized surfaces are also often considered "silica-rich" since the selective removal of alkali ions can be thought to leave behind a surface composed primarily of silica (SiO2). To be precise, dealkalization does not generally involve the outright removal of alkali from the glass, but rather its replacement with protons (H+) or hydronium ions (H3O+) in the structure through the process of ion-exchange.
Treatment of glass containers
Motivation
For glass containers, the goal of surface dealkalization is to render the inside surface of the container more resistant to interactions with liquid products later put inside it. Since the treatment is directed primarily at changing the properties of the inside surface in contact with the product, it is also referred to as "internal treatment".
The most common example of its use with containers is on bottles intended to hold alcoholic spirits. The reason for this is that some alcoholic spirits such as vodka and gin have an approximately neutral pH and a high alcohol content, but are not buffered in any way against changes in pH. If alkali is leached from the glass into the product, the pH will begin to rise (i.e. become more alkaline), can eventually reach a pH high enough that the solution begins to attack the glass itself quite effectively. By this mechanism, initially neutral alcohol products can achieve a pH where the glass container itself begins to slowly dissolve, leaving thin, siliceous glass flakes or particles in the fluid. Dealkalization treatment hinders this process by removing alkali from the inside surface. Not only does this mean less extractable alkali in the glass surface directly contacting the product, but it also creates a barrier for the diffusion of alkali from the underlying bulk glass into the product.
The same logic applies in pharmaceutical glass items such as vials that are intended to hold medicinal products. While many of these items are composed of more durable borosilicate glass, they are also at times dealkalized in order to minimize the possibility of alkali leaching from the glass into the product. This action helps to avoid undesired changes in pH or ionic strength of the solution, which not only inhibits eventual attack of the glass as previously described, but can also be important in maintaining the efficacy or stability of sensitive product formulations.
Dealkalization methods
Dealkalizing glass containers is accomplished by exposing the glass surface to reactive sulfur- or fluorine-containing compounds during the manufacturing process. A rapid ion-exchange reaction proceeds that depletes the inside surface of alkali, and is performed when the glass is at high temperature, usually on the order of 500–650 °C or greater.