Inhibitors and Stabilizers Information
Inhibitors and stabilizers alter catalytic reactions or prevent them from occurring. Such reactions include corrosion, oxidation, and electrochemical and biochemical processes.
Inhibitors and stabilizers consist of any agent used for maintenance or stabilization of particular features or properties of products and additives. In terms of chemical mechanisms, a stabilizer's purpose is to hinder reactions between other chemicals whereas a catalyst aims to increase the rate of a chemical reaction.
Chemicals that interfere with the process of separation in suspensions and emulsions are referred to by the same terms. These agents affect the formation of the enzyme-substrate complex or modify an enzyme's active site or some combination of both. Their use in research is focused on mapping out catalyst mechanisms of reaction and metabolic pathways.
Many pharmaceuticals are designed to target enzymes in a similar way. In nature, inhibition occurs when cells regulate metabolic processes by blocking the activity of particular enzymes. Inhibitors and stabilizers are effective even in low concentrations and support a diverse range of uses in modern manufacturing processes and products. These substances target undesirable effects such as corrosion of metals, aging of polymers, and oxidation occurring in fuels and food.
For stabilizers, the most preferred additives in today's marketplace incorporate calcium-based, lead, tin, liquid, and light stabilizers. Typical inhibitors are comprised of chromate, nitrite, arsenic, and bismuth salts, as well as numerous organic compounds. The performance of these substances varies depending on several factors including the amount of product, the point of dispersion, and environmental factors such as temperature.
Types of Inhibitors and Stabilizers
Different types of inhibitors and stabilizers include:
Sequestrants: Deactivate metal ions by forming chelate complexes around them and preventing their oxidation.
Antioxidants: Prevent undesired oxidation in food and materials. Antioxidants include both synthetic (ethyl and propyl gallates, ionol) and natural substances (thyme, sage, raw cottonseed or soybean oil).
Emulsifiers and surfactants: Stabilize emulsions by enhancing solubility of chemical compounds.
Ultraviolet stabilizers: Protect materials such as plastics from UV degradation. These agents consist of the following categories:
- UV absorbers: A type of light stabilizer that absorbs ultraviolet radiation and keeps it from penetrating material. Absorbers change damaging UV radiation to non-threatening infrared radiation or dissipated heat via the polymer matrix. Sunscreen behaves in a similar manner protecting the skin. UV absorbers include carbon black, oxanilides, benzophenones, and rutile titanium oxide. These compounds feature low costs; however, they do not offer long-term protection from UV exposure.
- Quenchers: These chemicals prevent radiation from breaking chemical bonds by releasing it as heat. Many quenchers are reformed phenolates, such as nickel salts. One real world example is nickel quenchers, which are prominent in the production of agricultural film. However, their use is constrained by the contribution of tan or green colors to products and presence of heavy metals.
- Hindered amine light stabilizers (HALS): These long-lasting thermal stabilizers eliminate free radicals at the surface. They operate by scavenging radicals formed by UV absorption via the Denisov cycle. HALS are favored UV stabilizers for many plastic products.
Insecticides: These toxins interfere with and break down essential biological processes.
Therapeutic drugs: Include enzyme inhibitors such as penicillin, an antibiotic that prevents synthesis of bacterial cell walls by inactivating the enzyme necessary to form them. Aspirin is another example of this type of compound.
Corrosion inhibitors: These agents act to reduce the rate of corrosion of materials subject to environmental or other factors prone to enhancing the effect, and include the following:
- Anodic: The consequence of producing an oxide film coating with the ability to stick to the surface of a metal. This generates an anodic shift reducing the object's susceptibility to corrosion by pushing its surface into the passivation region. As a result, they are sometimes called passivators. Anodic inhibitors include chromates, tungstate, nitrates, and molybdates.
- Cathodic: These agents function in two ways: they either slow a cathodic reaction or limit diffusion on cathodic areas of a metal surface.
- Mixed: Inhibitors categorized as "mixed" reduce both cathodic and anodic reactions. They are commonly compounds capable of forming films. They enable precipitates to form on a surface and block anodic and cathodic sites in an indirect fashion. Water rich in calcium and magnesium is not as corrosive as "soft" water due to the propensity of salts present in "hard" water to form a protective film on a metal's surface. Inhibitors of this type are mainly silicates and phosphates. Water softeners utilize sodium silicate to avoid the formation of rust water. While silicates and phosphates are not as effective as chromates and nitrides in retarding corrosion, they are best suited for conditions requiring non-toxic inhibition agents.
- Volatile corrosion inhibitors: These are also known as vapor phase inhibitors (VPI). They are compounds moved in a closed system to the corrosion site through volatilization from a particular source. In a boiler, volatile compounds are moved by steam preventing corrosion in condenser tubes, either by rendering acidic particles, such as carbon dioxide, neutral or by increasing the surface pH to a less corrosive level.
How Inhibitors and Stabilizers Work
Inhibitor agents are classified as either irreversible or reversible. If an agent is irreversible, it acts by destroying or permanently altering the enzyme activity. Reversible inhibitors work by forming a non-permanent complex with an enzyme. Agents constraining enzymes in a reversible manner come in two main types: competitive and non-competitive.
Competitive inhibition is reversed by an increase in substrate concentration. The inhibition percentage present is found by calculating the rati between inhibitor and substrate rather than determining the total inhibitor concentration. In non-competitive inhibition, the agent forms an inactive complex by binding to other sites instead of the substrate site and remains unaffected by any increase in the substrate concentration.
Inhibitors operate by degrading the effectiveness of a catalyst (both of non-biological catalysts and enzymes). They can do so if the agent possesses structural similarity to the reactant. This allows it to bind to the site where the catalyst is active without undergoing a catalytic reaction. As a result, the catalyst is unable to initiate catalysis while the inhibitor is in place. Once the inhibitor is released, the catalyst can once again start the reaction.
Inhibition is not the same as catalyst poisoning. The former serves to hinder a catalyst without changing it, whereas catalyst poisoning refers to the partial deactivation or destruction of the catalyst activity.
Inhibitors and stabilizers serve a vast range of purposes including:
- Corrosion inhibition
- Pest control
- Natural supplements
- UV radiation protection
- Oxidation suppression
- Polymerization inhibition
- Food preservation
Selecting Inhibitors and Stabilizers
Inhibitors and stabilizers feature an assortment of types and perform a multitude of activities. When selecting a product, first consider the nature of the intended use and analyze characteristics of the agent that are important to performing the task. Considerations of price, effective duration, and environmental or other impacts should also be taken into account. Consult manufacturer's guidelines to verify the product's desired function is consistent with the approved applications.
ISO 15688 - Road construction and maintenance equipment - soil stabilizers - terminology and commercial specifications
NAS3400 - Volatile corrosion inhibitor
QPL-24453 - Inhibitor, corrosion, soluble oil
QPL-53021 - Stabilizer additive, diesel fuel