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Understanding Surface Tension & Surfactants and HLB Values

Surface TensionTo understand surface tension requires an in-depth understanding of the concept of “phase”, which in physical chemistry simply means that a phase consists of a homogeneous substance with homogeneous physical and chemical properties. Phase boundaries are used between different phases. A surface is a type of phase boundary, which is simply the line that separates two “regions”. We can easily understand that the nature and meaning of the boundary line is certainly different from the boundary inside, especially in the field of science, because this field is especially about the conditions, the definition of the conditions is never ambiguous, which makes the boundary part of the special, its properties may be consistent with the left side of the boundary, may also be consistent with the right side of the boundary, or may be not consistent with both. This is why the science of surfaces is called out, and it is usually discussed separately in the field of physical chemistry.

Surface tension is caused by different intermolecular forces on both sides of the surface (phase boundary), why are they different? Because they are different phases, such as gas and liquid (water) phases, one side of the molecular density, the other side of the molecular density is small, so the molecules at the interface, in the water side may be subjected to more forces, such as hydrogen bonding, and the overall effect is that this “more forces” makes the liquid has a spontaneous tendency to shrink up, to shrink until it can balance out this part of the liquid, and to shrink to the point where it can be balanced out. The overall effect is that this “extra force” gives the liquid a spontaneous tendency to contract itself up to the point where it can balance out the extra force.

I. What is surface tension

Surface tension is a force that causes the surface of a liquid to shrink and is generated by the mutual attraction between molecules in the surface layer. The intermolecular force is called van der Waals force and contains both attractive and repulsive forces. When the liquid is not external force, the internal molecules are also in a balanced state of unstressed, that is, the attraction and repulsion are equal, the molecules are close to a little bit of repulsion, away from a little bit of attraction. And on the surface of the liquid, often some molecules with high energy break free from the attraction of other molecules and escape, which is the vaporization of the liquid. In this role, the liquid surface layer of the distance between the molecules than the distance between the molecules within the larger, between the molecules embodied in the tension, which is the surface tension. The presence of surface tension tends to minimize the area of the surface of the liquid, so that the droplets tend to be spherical. Now understand why the water droplets are spherical it, the reason is because of surface tension, if there is no surface tension, the droplet shape is strange, may be square, triangular, oval and so on!

II. Understanding of surface tension

The magnitude of surface tension depends on the nature of the liquid and the environmental conditions. Generally speaking, surface tension increases as the intermolecular attraction of the liquid increases, and decreases as the temperature rises. Simple and rough understanding, the smaller the surface tension of the liquid, the more likely to be “violated”, the greater the surface tension of the liquid “the more tenacious, not easy to be violated”, the surface tension of this property, there are many applications in life.

Detergents and soaps: During the washing process, detergents and soaps can reduce the surface tension of water, making it easier to penetrate into dirt and grease, thus cleaning the surface of objects more effectively.

Droplet shape: Surface tension makes water droplets spherical. This is because surface tension causes water molecules to be attracted to each other on the surface, making the droplets as small as possible and thus reducing the surface area.

Principle of action of absorbent paper: The capillary phenomenon is one of the important applications of surface tension. When a tiny tube (such as a glass capillary) is inserted into a liquid, the liquid rises or falls to a position higher or lower than the level of the liquid in the container. This is because surface tension creates a force for the liquid to rise or fall in the tiny tube.

A drop of mercury on a glass plate is essentially spherical because all the molecules within the thin layer on the outer surface of the drop are in a state of high potential energy. Calculations show that if the total potential energy of the molecules is kept very small, the surface must be spherical. If the effect of gravity is eliminated, for example, by placing the drop in another liquid of the same specific gravity that does not chemically react with the drop, or by free-falling in a vacuum, or in the weightless environment of artificial satellites and rockets, the drop will take on the ideal spherical shape. Spherical soap bubbles and spherical dewdrops on lotus leaves can also be explained by the same reasoning.

Third, the application of surface tension in coatings

Leveling of coatings: The leveling of a coating refers to the ability of the coating to be evenly distributed on the surface of the coating. If the surface tension of a coating is too high, it will form a raised surface, resulting in an uneven coating. By adjusting the surfactant in the coating, the surface tension can be lowered so that the coating can be more easily and evenly distributed on the surface, thus improving the leveling of the coating.

Preventing coating dispersion: Adding surfactants to coatings reduces the surface tension between liquid particles, thus preventing coating dispersion. This ensures even distribution of pigments and fillers in the coating and provides better coverage and color consistency.

Improve coating adhesion: Proper adjustment of surface tension can help coatings adhere better to the surface of the substrate. By reducing the surface tension, the coating can better penetrate into the tiny pores of the substrate and form a closer bond with the substrate surface, improving the adhesion and durability of the coating.

Reduce blistering of coatings: Air bubbles and blistering problems in coatings may lead to uneven coating surfaces. Proper adjustment of surface tension can reduce the formation of bubbles in coatings, thus improving the appearance and quality of coatings.

Coating Dispersants: Pigments and fillers in coatings are often in the form of particles that are prone to agglomeration. By adding surfactants as dispersants, the surface tension between the particles can be reduced so that the particles can be better dispersed in the coatings and the stability and uniformity of the coatings can be improved, etc.

Fourth, why read surface tension

So, through the above examples to understand why now the paint engineers mouth has been talking about reducing surface tension it. Because, with the continuous development of water-based paint, many companies are in the oil to water, from oil paint to water-based paint, and the original oil-based paint in a variety of solvents in the surface tension are concentrated in between 20-40, only the surface tension of the water system 72.8mN/m (20 ℃), so understand why the difficulty of the water-based paint is relatively large, and now the paint engineers why the mouth of the mouth to talk about reducing the surface tension of the bar, because that is very stubborn and stubborn. Because that is very stubborn and stubborn.

Hydrophile-lipophile balance HLB value of surfactant molecules in the hydrophilic and lipophilic groups on the oil or water of the combined affinity known as hydrophile-lipophile balance (HLB) will be surfactant HLB value range is limited to 0 ~ 40, of which the nonionic surfactant HLB value range of 0 ~ 20, i.e., completely by the hydrophobic hydrocarbon groups composed of paraffin molecules, the HLB value range of 0 ~ 20, i.e., completely by the hydrophobic hydrocarbon groups composed of the paraffin molecules of the hydrophobic hydrocarbons. The HLB value of paraffin molecule composed entirely of hydrophobic hydrocarbon groups is 0, the HLB value of polyoxyethylene composed entirely of hydrophilic oxyethylene is 20, and the HLB value of surfactants with both hydrocarbon and oxyethylene chains is in between. the higher the HLB value, the more hydrophilic, and vice versa, the more lipophilic.
Surfactants with HLB values of 3-6 are suitable as W/O emulsifiers; surfactants with HLB values of 8-18 are suitable as O/W emulsifiers. The HLB value as solubilizer is 13 ~ 18 ,the HLB value as wetting agent is 7 ~ 9, and the HLB value as detergent is 13 ~ 16.
Critical micelle concentration CMC

The lowest concentration at which surfactant molecules associate to form micelles is the critical micell concentration (CMC).At CMC, the surface tension of the solution is almost minimum.

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