Surface tension of water by capillary rise and effect of detergents

Surface tension is a feature of a liquid surface that manifests itself as a stretched elastic membrane. This effect can be seen in the nearly spherical shape of small liquid drops and soap bubbles. Because of this property, several insects may stand on the water’s surface. The surface tension of water may support a razor blade. When pushed through the water’s surface, the razor blade does not float; it sinks.

The forces of attraction between the particles within a specific liquid, as well as the gas, solid, or liquid in contact with it, determine surface tension. The molecules in a drop of water, for example, have a modest attraction to one another.

Surface tension & surface energy

Surface tension

Surface tension is a feature of liquids that is influenced by intermolecular interactions and is derived from the cohesive forces that exist between molecules in a liquid. According to thermodynamics, systems like paints and inks attempt to achieve a state with the greatest number of beneficial interactions. This means that liquids will take on a structure in which the number of bulk molecules is maximized and the number of surface molecules is minimized.

Surface molecules are partially naked since they are not completely surrounded by their peers at the liquid’s surface. When compared to molecules in the bulk of the liquid, molecules near the liquid-air interface have less favorable interactions and are in a higher energy state. As a result, energy is required to transfer molecules from the liquid’s bulk to the surface. The more energy it takes to expand a liquid’s surface area, the stronger the interactions between the molecules.

The energy (in Joule) required to create 1 m2 of fresh liquid-gas interfacial area is defined as the surface tension of a liquid ( J/m2 ). It has the dimension of the unit N/m (Newton per meter). The interface to which an interfacial tension refers is specified using two subscripts like for liquid-gas interface it is ‘lg’. An interface is a point where two phases meet. As a result, the sign for interfacial tension should incorporate the abbreviation of these phases. The interfacial tension at a liquid-gas interface is known as surface tension. The ‘gas’ phase in paints and inks is usually the air above the system.

Surface energy

The surface energy of solid materials, such as substrates that must be coated, is an important feature. The interfacial tension at a solid-gas interface is known as surface energy. The sign ‘sg’ denotes the attribute, with the subscript ‘s’ denoting solid and ‘g’ denoting gas. Most of the time, the gas above the solid surface is air.

• A solid surface with high surface energy allows for strong interactions.
• Surprisingly, high surface energy is advantageous for substrate wetting and adherence.
• Coating low-surface-energy solids, such as most plastics, is problematic.
• Pre-treatment procedures such as corona treatment, plasma therapy, or flame treatment can increase the surface energy of plastic items, enhancing wetting and adhesion.

The surface tension of water

The International Association for the Properties of Water and Steam (IAPWS) has calculated the surface tension of pure liquid water in contact with its vapor as:

w = 235.8 (1-TTC)1.2561-0.625 (1-TTC)mN/m,

Temperature T and the critical temperature TC = 647.096 K are both given in kelvins. From the triple point (0.01 °C) through the critical point, the complete vapor-liquid saturation curve is valid. When extrapolated to metastable (super-cooled) circumstances, it gives satisfactory findings down to at least 25 °C. IAPWS accepted this formulation in 1976, and it was updated in 1994 to comply with the International Temperature Scale of 1990.

IAPWS provides the uncertainty of this formulation over the entire temperature range. The uncertainty is 0.5% for temperatures below 100 °C.

 What is Capillary?

Capillarity is the rise or descent of a liquid in a narrow passage, such as the gaps between the fibers of a towel or the perforations in a porous material. Capillary action is not restricted to the vertical plane. Water is pulled into the threads of a towel regardless of its orientation.

Liquids rising in small-bore tubes introduced into the liquid are said to wet the tube, whilst liquids depressed beneath the surface of the surrounding liquid are said not to wet the tube. A liquid that wets glass capillary tubes is water; a liquid that does not is mercury. Capillary does not form when there is no wetness.

Surface forces, also known as interfacial forces, cause capillarity. The attraction between the molecules of water and the glass walls, as well as among the molecules of water themselves, causes the water to rise in a thin tube immersed in water. These attractive forces simply balance the gravitational force of the water column that has risen to a particular height. The higher the water rises, the narrower the bore of the capillary tube. Mercury, on the other hand, is depressed to a larger extent as the bore narrows.

Capillary Rise Method

The capillary rise method is a test that is used to assess a liquid’s surface tension or contact angle with soil or pipe material. The increase in a liquid above zero pressure level induced by the net upward force created by the attraction of water molecules to a solid surface is known as capillary rise.

Capillary rise occurs when cohesive and adhesive forces combine to induce liquids to rise up in tubes with very tiny diameters.

A narrow circular capillary is dipped into the liquid to be tested in the capillary rise method. If the adhesion forces (those between the liquid and the capillary wall) are greater than the cohesive forces (those between the liquid molecules), the liquid will wet the wall and rise to a particular level in the capillary tube, forming a concave meniscus.

The liquid level in the capillary tube lowers when the adhesion forces between the liquid and the capillary wall are weaker than the cohesive forces between the liquid molecules, causing the meniscus to become convex.

Effect of Detergents

Among the various toxins, detergent, as a major pollutant, poses a significant threat to natural ecosystems. Detergents can also enter wastewater treatment plants and have a negative impact on their performance. They are an integral element of human life and are consumed for a variety of reasons, including hygiene. As a result, detergent components can reach soil and water bodies through a variety of routes. Detergents have an impact on fauna and flora, as well as ecosystems, both directly and indirectly. More significant are eutrophication, foaming, and changing factors such as temperature, salinity, turbidity, and pH, all of which must be handled and controlled. Researchers discovered that aerobic mechanisms can break down the majority of detergents, however, anaerobic degradation is impossible due to limited metabolic pathways and toxicity. As a result, the creation of environmentally-friendly detergents is a major concern all over the world.

Conclusion 

Surface tensions are common occurrences in our daily lives. There are various instances where surface tension is a factor. Consider the famous example of a spider walking across the water’s surface. If you change the water to ethanol, the spider will perish. Why? Because the surface tension of water is sufficient to support the spider’s weight, ethanol, which has a significantly lower surface tension, is unable to do so. Rain falls in a spherical drop due to the high surface tension of the water. High surface energy causes the water drop to take on the shape of a sphere, which has the least amount of surface area.