9 junio, 2024

Menisci (chemistry): what it is and types

What is the meniscus (chemistry)?

He meniscus is the curvature of the surface of a liquid. It is also the free surface of a liquid at the liquid-air interface. Liquids are characterized by having a fixed volume, being not very compressible.

However, the shape of liquids varies by adopting the shape of the container that contains them. This characteristic is due to the random movement of the molecules that form them.

Liquids are flowable, have high density, and diffuse rapidly into other liquids with which they are miscible.

They occupy by gravity the lowest area of ​​the container, leaving a free surface that is not completely flat at the top. In some circumstances they can take special forms such as drops, bubbles and bubbles.

Liquid properties such as melting point, vapor pressure, viscosity, and heat of vaporization depend on the strength of the intermolecular forces that hold liquids together.

However, liquids also interact with the container through adhesion forces.

The meniscus then arises from these physical phenomena: the difference between the forces of cohesion between the particles of the liquid, and those of adhesion, which allows them to wet the walls.

What does the meniscus consist of?

As just explained, the meniscus is the result of several physical phenomena, among which can also be mentioned the surface tension of the liquid.

cohesive forces

Cohesive forces is the physical term that explains the intermolecular interactions within the liquid. In the case of water, the cohesive forces are due to dipole-dipole interaction and hydrogen bonding.

The water molecule is bipolar in nature. This is because the oxygen in the molecule is electronegative because it has a greater avidity for electrons than the hydrogens, which determines that the oxygen is left with a negative charge and the hydrogens are positively charged.

There is an electrostatic attraction between the negative charge of a water molecule, located on the oxygen, and the positive charge of another water molecule, located on the hydrogens.

This interaction is what is known as dipole-dipole interaction or force, which contributes to the cohesion of the liquid.

adhesion forces

On the other hand, water molecules can interact with the glass walls, by partially charging the hydrogen atoms of the water molecules that bind tightly to the oxygen atoms on the glass surface.

This constitutes the adhesion force between the liquid and the rigid wall, which colloquially means that the liquid wets the wall.

When a silicone solution is placed on the surface of the glass, the water does not completely permeate the glass, but rather droplets form on it that are easily removed.

Thus, it is indicated that with this treatment the adhesion force between water and glass is reduced.

A very similar case occurs when the hands are oily, and when washing them with water, very defined drops can be seen on the skin instead of the moistened skin.

types of menisci

There are two types of menisci: concave and convex. In the image above, the concave is A and the convex is B. The dotted lines indicate the correct flush when reading a volume measurement.

concave meniscus

The concave meniscus is characterized in that the contact angle θ formed by the glass wall with a line tangent to the meniscus, which is introduced into the liquid, has a value of less than 90º.

If a quantity of the liquid is placed on the glass, it tends to spread over the surface of the glass.

The presence of a concave meniscus shows that the cohesive forces within the liquid are less than the liquid-glass wall adhesion force.

Therefore, the liquid bathes or wets the glass wall, retaining a quantity of liquid and giving the meniscus a concave shape. Water is an example of a liquid that forms concave menisci.

convex meniscus

In the case of the convex meniscus, the contact angle θ has a value greater than 90º. Mercury is an example of a liquid that forms convex menisci. When a drop of mercury is placed on a glass surface, the contact angle θ has a value of 140º.

The observation of a convex meniscus indicates that the cohesive forces of the liquid are of greater magnitude than the adhesion force between the liquid and the glass wall. It is said that the liquid does not wet the glass.

The surface forces of cohesion (liquid-liquid) and adhesion (liquid-solid) are responsible for many phenomena of biological interest, such as surface tension and capillarity.

Surface tension

Surface tension is a net force of attraction exerted on the molecules of the liquid that are on the surface and that tends to introduce them into the liquid.

Therefore, the surface tension tends to cohere the liquid and give more concave menisci. In other words, this force tends to remove the surface of the liquid from the glass wall.

The surface tension tends to decrease as the temperature increases, for example: the surface tension of water is equal to 0.076 N/m at 0 °C and 0.059 N/m at 100 °C.

While the surface tension of mercury at 20 °C is 0.465 N/m. This would explain why mercury forms convex menisci.

capillarity

If the contact angle θ is less than 90°, and the liquid wets the glass wall, the liquid inside the glass capillaries can rise until it reaches an equilibrium condition.

The weight of the liquid column is balanced by the vertical component of the cohesive force due to surface tension. The adhesion force does not intervene because it is perpendicular to the surface of the tube.

This law does not explain how water can ascend from the roots to the leaves through the xylem vessels.

Actually, there are other factors that intervene in this regard, for example: evaporating the water in the leaves allows the water molecules in the upper part of the capillaries to be sucked.

This allows other molecules from the bottom of the capillaries to rise to take the place of the evaporated water molecules.

References

Whitten, Davis, Peck & Stanley. Chemistry. CENGAGE Learning.
Helmenstine, Anne Marie, Ph.D. How to Read a Meniscus in Chemistry. Retrieved from thoughtco.com

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