Introduction to the Classification and Selection of Wetting Agents

Nov 16, 2021

A substance that can make solid materials more easily wetted by water. By reducing its surface tension or interfacial tension, the water can spread on the surface of the solid material or penetrate the surface, thereby wetting the solid material.
Wetting agent is a surface active agent that can make solid materials more easily wetted by water by reducing its surface energy.

Such as sulfonated oils, soaps, pull open powder BX, etc., soy lecithin, mercaptans, hydrazides and mercaptan acetals, etc. can also be used. Wetting agents are increasingly being used in the ceramic industry. Generally, a kind of poly-oxyethylene alkylate ether with high water-resistant hardness (Poly-oxyethylene alkylate ether) is commonly used. Sulfonated oils and soaps all have moderate wetting properties, excellent detergency and solubilization tendency.
The water droplets spread out into a thin layer on clean glass, and become spherical when dropped on wax-coated glass. The former is called wetting, and the latter is called non-wetting.
Solids that can be wetted by water are called hydrophilic solids, such as silicates, nitrates, sulfates, quartz, and metal oxides. Solids that cannot be wetted by water are called hydrophobic solids, such as wax, graphite, sulfide, and various solid pesticides.

Water can soak glass because the silicates of glass are polar substances, and their interaction is strong and can be close to each other to reduce the surface energy of water. Water cannot wet because wax is a non-polar substance, and the force between them is weak and cannot be close to each other to reduce the surface energy of water. When the surface energy is high, the water automatically shrinks into a spherical shape.
Wetting agents are surfactants, which are composed of hydrophilic and lipophilic groups. When in contact with a solid surface, the lipophilic group adheres to the solid surface, and the hydrophilic group extends outward into the liquid, so that the liquid forms a continuous phase on the solid surface. This is the basic principle of wetting.

Classification
According to the effect
(1) Solvents with low surface tension and water miscibility, including ethanol, propylene glycol, glycerin, dimethyl sulfoxide, etc.
(2) Surfactants, such as anionic surfactants, certain polyol type surfactants (Span type, Tween type), polyoxyethylene type surfactants.
According to the route of administration
(1) Wetting agents used in external preparations include surfactants and alcohols that have low surface tension and are miscible with water.
(2) Wetting agents used in oral preparations include ethanol, glycerin, Tween, etc., which have low surface tension and are miscible with water.
(3) Wetting agents for injection administration include ethanol, propylene glycol, glycerin, polyethylene glycol 200-400, etc., and Tween-80, which have low surface tension and are miscible with water. [1] 
According to nature
Wetting agents include anionic and nonionic surfactants.
Anionic surfactants include alkyl sulfates, sulfonates, fatty acid or fatty acid ester sulfates, carboxylic acid soaps, phosphate esters, and the like.
Nonionic surfactants include polyoxyethylene alkylphenol ethers, polyoxyethylene fatty alcohol ethers, polyoxyethylene polyoxypropylene block copolymers and the like.
There is also a new class of silanol nonionic surfactants on the market, also known as wetting agents. Features: low molecular weight and multi-hydrophobic groups with umbrella-shaped symmetrical structure. Compared with traditional active agents, it has performance of wetting and permeability. Extremely excellent and efficient, it is a revolutionary new-generation surfactant. The dynamic and static surface tension is extremely low, containing dihydroxy, reactive activator, chemically inert, generally does not participate in the chemical reaction of the system, good acid and alkali resistance, stable chemical properties. The typical model is: GSK-588/582/585 and other series.

application
When the pesticide is a hydrophobic solid powder, its surface is not easy to be wetted by water, and it is difficult to dissolve in water and sink and agglomerate. If a wetting agent is added, since the wetting molecules are aligned on the surface of the solid, the surface energy of the water is reduced, so that the water can wet these solids, and the solid powder is dispersed in the water to become a stable suspension. For another example, there is wax on the surface of plant leaves and insects. When the liquid medicine is sprayed on, it will become spherical and roll off due to the inability to wet or the water will evaporate. In order to make the medicinal liquid have good wetting properties and spread on the surface of plant leaves and insects to fully exert its effectiveness, it is often necessary to add a wetting agent to the pesticide. The wetting effect is more practical in chemical experiments. After the experiment, the glass apparatus must be cleaned. There should be no water droplets on the wall of the cleaned glass instrument, but a thin and even water film should be laid. If there are water drops on the glass, it means that the oil is not clean and must be washed again.

Choose
In general, for drugs that are not very hydrophobic, ethanol, propylene glycol, glycerin, etc. can be ground with them to achieve the purpose of wetting; and for drugs or medicinal materials with strong hydrophobicity, the surface with an HLB value of 6~15 is generally selected. Active agent. You can also choose from any of the following indicators:
Contact angle
Contact angles: It is the angle between the droplet and the spreading surface. The selected wetting agent should be able to reduce the contact angle of the hydrophobic drug to less than 90°. When the contact angle is 0, it can be completely wetted; when the contact angle is 0～90°, it can be wetted, and it is a hydrophilic drug; when the contact angle is greater than 90° and less than 180°, it cannot be wetted; when the contact angle is 180° It is completely non-wetting and is a hydrophobic drug.
Interfacial tension
Contact angle (interfacial tensions): Some wetting agents change the interfacial tension between the hydrophobic drug and the solvent.
Wetting amount
Wetting point: Wetting point refers to the amount of solvent needed to wet 100g of hydrophobic drugs. Adding wetting agent can reduce the wetting point, that is, reduce the amount of solvent.