Lesson on the topic "surface tension". How to measure surface tension What is surface tension

ARTICLE UPDATED: 12/29/2019

The surface tension of water is one of the most interesting properties of water.

Examples of the surface tension of water

For a better understanding of the surface tension of water, here are some of its manifestations in real life:

• When we see water dripping from the tip of a faucet instead of pouring, this is the surface tension of water;
• When a raindrop in flight takes a rounded, slightly elongated shape, this is the surface tension of water;
• When water on a waterproof surface takes on a spherical shape, this is the surface tension of water;
• The ripples that occur when the wind blows on the surface of water bodies is also a manifestation of the surface tension of water;
• Water in space takes on a spherical shape due to surface tension;
• The water strider insect stays on the surface of the water thanks to this very property of water;
• If a needle is carefully placed on the surface of the water, it will float;
• If liquids of different density and color are alternately poured into a glass, we will see that they do not mix;
• Iridescent soap bubbles are also a wonderful manifestation of surface tension.

Surface Tension - A Few Precise Definitions

Big Medical Encyclopedia

Surface tension (P. n.) is the force of attraction with which each section of the surface film (the free surface of a liquid or any interface between two phases) acts on adjacent parts of the surface. Internal pressure and P. n. The surface layer of the liquid behaves like an elastic stretched membrane. According to the idea developed by Chap. arr. Laplace (Laplace), this property of liquid surfaces depends on "molecular forces of attraction, rapidly decreasing with distance. Inside a homogeneous liquid, the forces acting on each molecule from the molecules surrounding it are mutually balanced. But near the surface, the resultant of the forces of molecular attraction is directed inward; it tends to draw surface molecules into the bulk of the liquid. As a result, the entire surface layer, like an elastic stretched film, exerts a very significant pressure on the internal mass of the liquid in the direction normal to the surface. According to calculations, this "internal pressure", under which the entire mass of the liquid is located, reaches several thousand atmospheres. It increases on a convex surface and decreases on a concave one. By virtue of the tendency of free energy to a minimum, any liquid tends to take on a form at which its surface - the site of action of surface forces - has the smallest possible value. The larger the surface of the liquid, the larger the area occupied by its surface film, the greater the amount of free surface energy released during its contraction. The tension with which each section of the contracting surface film acts on adjacent parts (in the direction parallel to the free surface) is called the tension tension. In contrast to the elastic tension of an elastic stretched body, P. n. does not weaken as the surface film is compressed. … Surface tension is equal to the work that must be done to increase the free surface of the liquid by one. P. n. observed at the boundary of a liquid with a gas (also with its own vapor), with another immiscible liquid, or with a solid body. In the same way, a solid body has a P. n. at the interface with gases and liquids. Unlike P. n., a cut liquid (or solid body) has on its free surface, bordering on a gaseous medium, tension on the inner boundary of two liquid (or liquid and solid) phases, it is convenient to designate a special term adopted in German literature , the term "boundary tension" (Grenzflachenspannung). If a substance is dissolved in the liquid that lowers its P. n., then the free energy decreases not only by reducing the size of the boundary surface, but also through adsorption: a surface-active (or capillary-active) substance is collected in an increased concentration in the surface layer ...
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Big medical encyclopedia. 1970

All of the above can be summarized in this way - the molecules that are on the surface of any liquid, including water, are attracted by the rest of the molecules inside the liquid, as a result of which surface tension arises. We emphasize that this is a simplified understanding of this property.

Surface Tension Coefficient - Definition

Polytechnic terminological explanatory dictionary

Surface tension coefficient - the linear density of the surface tension force on the surface of a liquid or at the interface between two immiscible liquids.

Polytechnic terminological explanatory dictionary. Compiled by: V. Butakov, I. Fagradyants. 2014

Below we give the values ​​​​of the coefficient of surface tension (C.T.S.) for various liquids at a temperature of 20 ° C:

• K. p. n. acetone - 0.0233 Newton / Meter;
• K. p. n. benzene - 0.0289 Newton / Meter;
• K. p. n. distilled water - 0.0727 Newton / Meter;
• K. p. n. glycerin - 0.0657 Newton / Meter;
• K. p. n. kerosene - 0.0289 Newton / Meter;
• K. p. n. mercury - 0.4650 Newton / Meter;
• K. p. n. ethyl alcohol - 0.0223 Newton / Meter;
• K. p. n. ether - 0.0171 Newton / Meter.

Surface tension coefficients of water

The surface tension coefficient depends on the temperature of the liquid. We present its values ​​at various water temperatures.

The concept of surface tension

surface tension is called the thermodynamic characteristic of the interface, defined as the work of the reversible isothermal formation of a unit area of ​​this surface. For a liquid, surface tension is considered as a force acting per unit length of the surface contour and tending to reduce the surface to a minimum for given phase volumes.

Oil is an oil dispersed system consisting of a dispersed phase and a dispersion medium.

The surface of a particle of a dispersed phase (for example, an associate of asphaltenes, a water globule, etc.) has some excess free surface energy F s, proportional to the area of ​​the interface S:

Value σ can be considered not only as the specific surface energy, but also as a force applied to a unit length of the contour limiting the surface, directed along this surface perpendicular to the contour and tending to shrink or reduce this surface. This force is called surface tension.

The action of surface tension can be visualized as a set of forces that pull the edges of the surface to the center.

The length of each arrow of the vector reflects the magnitude of the surface tension, and the distance between them corresponds to the accepted unit of length of the surface contour. As a dimension of quantity σ both [J/m 2 ] = 10 3 [erg/cm 2 ] and [N/m] = 10 3 [dyne/cm] are equally used.

As a result of the action of surface tension forces, the liquid tends to reduce its surface, and if the influence of the earth's gravity is insignificant, the liquid takes the form of a ball with a minimum surface per unit volume.

Surface tension is different for different groups of hydrocarbons - maximum for aromatic and minimum for paraffinic. With an increase in the molecular weight of hydrocarbons, it increases.

Most heteroatomic compounds, having polar properties, have a surface tension lower than hydrocarbons. This is very important, since their presence plays a significant role in the formation of water-oil and gas-oil emulsions and in the subsequent processes of destruction of these emulsions.

Parameters affecting surface tension

Surface tension essentially depends on temperature and pressure, as well as on the chemical composition of the liquid and the phase in contact with it (gas or water).

As the temperature increases, the surface tension decreases and at the critical temperature it is equal to zero. With increasing pressure, the surface tension in the gas-liquid system also decreases.

The surface tension of petroleum products can be found by calculation using the equation:

Recalculation σ from one temperature T0 to another T can be carried out according to the ratio:

Surface tension values ​​for some substances.

Substances whose addition to a liquid reduces its surface tension are called surfactants(surfactant).

The surface tension of oil and oil products depends on the amount of surface-active components present in them (resinous substances, naphthenic and other organic acids, etc.).

Petroleum products with a low content of surface-active components have the highest value of surface tension at the border with water, with a high content - the smallest.

Well-refined petroleum products have a high surface tension at the interface with water.

The decrease in surface tension is explained by the adsorption of surfactants at the interface. With an increase in the concentration of the added surfactant, the surface tension of the liquid first decreases rapidly and then stabilizes, which indicates the complete saturation of the surface layer with surfactant molecules. Natural surfactants that dramatically change the surface tension of oils and petroleum products are alcohols, phenols, resins, asphaltenes, and various organic acids.

Wetting and capillary phenomena are associated with surface forces at the interface between the solid and liquid phases, on which the processes of oil migration in reservoirs, the rise of kerosene and oil along the wicks of lamps and oilers, etc. are based.

Experimental determination of surface tension

Various methods are used to experimentally determine the surface tension of oils and petroleum products.

The first method (a) is based on the measurement of the force required to separate the ring from the interface between two phases. This force is proportional to twice the force of the circumference of the ring. With the capillary method (b), the height of the rise of the liquid in the capillary tube is measured. Its disadvantage is the dependence of the height of the rise of the liquid not only on the magnitude of the surface tension, but also on the nature of the wetting of the walls of the capillary by the liquid under study. A more accurate variation of the capillary method is the hanging drop method (c), which is based on measuring the mass of a liquid drop detached from a capillary. The measurement results are affected by the density of the liquid and the size of the drop and are not affected by the wetting angle of the liquid on the solid surface. This method makes it possible to determine the surface tension in pressure vessels.

The most common and convenient method for measuring surface tension is the method of the highest pressure of bubbles or drops (r), which is explained by the simplicity of design, high accuracy, and independence of the determination from wetting.

This method is based on the fact that when an air bubble or liquid drop is squeezed out of a narrow capillary into another liquid, the surface tension σ at the boundary with the liquid into which the drop is released, in proportion to the greatest pressure necessary to extrude the drop.

A liquid is an aggregate state of matter, intermediate between gaseous and solid, therefore it has the properties of both gaseous and solid substances. Liquids, like solids, have a certain volume, and like gases, they take the shape of the vessel in which they are located. Gas molecules are practically not interconnected by the forces of intermolecular interaction. In this case, the average energy of the thermal motion of gas molecules is much greater than the average potential energy due to the forces of attraction between them, so the gas molecules scatter in different directions, and the gas occupies the entire volume provided to it.

In solid and liquid bodies, the forces of attraction between molecules are already significant and keep the molecules at a certain distance from each other. In this case, the average energy of the chaotic thermal motion of molecules is less than the average potential energy due to the forces of intermolecular interaction, and it is not enough to overcome the forces of attraction between molecules, so solids and liquids have a certain volume.

X-ray diffraction analysis of liquids showed that the nature of the arrangement of liquid particles is intermediate between a gas and a solid. In gases, molecules move randomly, so there is no pattern in their mutual arrangement. For solids, the so-called long range order in the arrangement of particles, i.e. their orderly arrangement, repeating over long distances. In liquids, the so-called short range order in the arrangement of particles, i.e. their ordered arrangement, repeating at distances comparable to interatomic ones.

The theory of fluid has not been fully developed to date. Thermal motion in a liquid is explained by the fact that each molecule oscillates for some time around a certain equilibrium position, after which it jumps to a new position, which is at a distance of the order of the interatomic distance from the initial one. Thus, the molecules of a liquid move quite slowly throughout the mass of the liquid, and diffusion occurs much more slowly than in gases. With an increase in the temperature of the liquid, the frequency of oscillatory motion increases sharply, the mobility of molecules increases, which is the reason for the decrease in the viscosity of the liquid.

Attractive forces act on each molecule of the liquid from the side of the surrounding molecules, rapidly decreasing with distance, therefore, starting from a certain minimum distance, the forces of attraction between molecules can be neglected. This distance (approximately 10 -9 m) is called molecular action radius r , and a sphere of radius r-sphere of molecular action.

Select a molecule inside the liquid BUT and draw a sphere of radius around it r(fig.10.1). It is sufficient, according to the definition, to take into account the action on a given molecule of only those molecules that are inside the sphere

Fig.10.1. molecular action. The forces with which these molecules act on the molecule BUT, are directed in different directions and, on average, are compensated, so the resulting force acting on a molecule inside the liquid from other molecules is equal to zero. The situation is different if the molecule, for example the molecule AT, located at a distance from the surface r. In this case, the sphere of molecular action is only partially located inside the liquid. Since the concentration of molecules in the gas located above the liquid is small compared to their concentration in the liquid, the resultant force F, applied to each molecule of the surface layer, is not equal to zero and is directed inside the liquid. Thus, the resulting forces of all the molecules of the surface layer exert pressure on the liquid, called molecular(or internal). Molecular pressure does not act on a body placed in a liquid, since it is due to forces acting only between the molecules of the liquid itself.

The total energy of liquid particles is the sum of the energy of their chaotic thermal motion and the potential energy due to the forces of intermolecular interaction. To move a molecule from the depth of the liquid to the surface layer, work must be expended. This work is done at the expense of the kinetic energy of the molecules and goes to increase their potential energy. Therefore, the molecules of the surface layer of the liquid have a greater potential energy than the molecules inside the liquid. This extra energy possessed by molecules in the surface layer of a liquid is called surface energy, is proportional to the layer area Δ S:

Δ W=σ Δ S,(10.1)

where σ – coefficient of surface tension, defined as the surface energy density.

Since the equilibrium state is characterized by a minimum of potential energy, the liquid, in the absence of external forces, will take such a shape that, for a given volume, it has a minimum surface, i.e. ball shape. Observing the smallest droplets suspended in the air, we can see that they really have the shape of balls, but somewhat distorted due to the action of the forces of gravity. Under conditions of weightlessness, a drop of any liquid (regardless of its size) has a spherical shape, which has been proven experimentally on spacecraft.

So, the condition for stable equilibrium of a liquid is a minimum of surface energy. This means that the liquid for a given volume should have the smallest surface area, i.e. liquid tends to reduce the free surface area. In this case, the surface layer of the liquid can be likened to a stretched elastic film in which tension forces act.

Consider the surface of a liquid bounded by a closed contour. Under the action of surface tension forces (they are directed tangentially to the surface of the liquid and perpendicular to the section of the contour on which they act), the surface of the liquid contracted and the considered contour moved. The forces acting from the selected area to the adjacent areas do the work:

Δ A=fΔ lΔ x,

where f=F/Δ l -surface tension force, acting per unit length of the liquid surface contour. It can be seen that Δ lΔ x= Δ S, those.

Δ A=f∆S.

This work is done by reducing the surface energy, i.e.

Δ Α =Δ W.

From the comparison of the expressions, it can be seen that

i.e., the surface tension coefficient σ is equal to the surface tension force per unit length of the contour that bounds the surface. The unit of surface tension is newton per meter (N/m) or joule per square meter (J/m2). Most liquids at a temperature of 300K have a surface tension of the order of 10 -2 -10 -1 N/m. Surface tension decreases with increasing temperature, as the average distances between liquid molecules increase.

Surface tension essentially depends on the impurities present in liquids. Substances , liquids that reduce surface tension are called surface-active substances (surfactants). Soap is the best known surfactant for water. It greatly reduces its surface tension (from about 7.5 10 -2 up to 4.5 10 -2 N/m). Surfactants that lower the surface tension of water are also alcohols, ethers, oil, etc.

There are substances (sugar, salt) that increase the surface tension of a liquid due to the fact that their molecules interact with the molecules of the liquid more strongly than the molecules of the liquid interact with each other.

In construction, surfactants are used to prepare solutions used in the processing of parts and structures operating in adverse atmospheric conditions (high humidity, elevated temperatures, exposure to solar radiation, etc.).

Wetting phenomenon

It is known from practice that a drop of water spreads on glass and takes the form shown in Fig. 10.2, while mercury on the same surface turns into a somewhat flattened drop. In the first case, it is said that the liquid wets hard surface, in the second - does not wet her. Wetting depends on the nature of the forces acting between the molecules of the surface layers of the media in contact. For a wetting liquid, the attractive forces between the molecules of the liquid and the solid are greater than between the molecules of the liquid itself, and the liquid tends to increase

surface of contact with a solid body. For a nonwetting liquid, the forces of attraction between the molecules of the liquid and the solid are less than those between the molecules of the liquid, and the liquid tends to reduce the surface of its contact with the solid.

Three surface tension forces are applied to the line of contact of three media (point 0 is its intersection with the plane of the drawing), which are directed tangentially into the contact surface of the corresponding two media. These forces, per unit length of the line of contact, are equal to the corresponding surface tensions σ 12 , σ 13 , σ 23 . Corner θ between the tangents to the surface of a liquid and a solid is called edge angle. The condition for the equilibrium of a drop is the equality to zero of the sum of the projections of the surface tension forces on the direction of the tangent to the surface of the solid, i.e.

–σ 13 + σ 12 + σ 23 cos θ =0 (10.2)

cos θ =(σ 13 - σ 12)/σ 23 . (10.3)

It follows from the condition that the contact angle can be acute or obtuse depending on the values σ 13 and σ 12 . If a σ 13 >σ 12 , then cos θ >0 and angle θ sharp, i.e. liquid wets a solid surface. If a σ 13 <σ 12 , then cos θ <0 и угол θ – blunt, i.e., the liquid does not wet the hard surface.

The contact angle satisfies condition (10.3) if

(σ 13 - σ 12)/σ 23 ≤1.

If the condition is not met, then the drop of liquid for any values θ cannot be in balance. If a σ 13 >σ 12 +σ 23 , then the liquid spreads over the surface of the solid, covering it with a thin film (for example, kerosene on the surface of glass), - we have complete wetting(in this case θ =0).

If a σ 12 >σ 13 +σ 23 , then the liquid shrinks into a spherical drop, in the limit having only one point of contact with it (for example, a drop of water on the surface of paraffin), - we have complete non-wetting(in this case θ =π).

Wetting and non-wetting are relative concepts, i.e. A liquid that wets one solid surface does not wet another. For example, water wets glass but does not wet paraffin; Mercury does not wet glass, but it does wet clean metal surfaces.

The phenomena of wetting and non-wetting are of great importance in technology. For example, in the method of flotation enrichment of ore (separation of ore from waste rock), finely crushed ore is shaken in a liquid that wets the waste rock and does not wet the ore. Air is blown through this mixture, and then it settles. At the same time, rock particles wetted with liquid sink to the bottom, and grains of minerals “stick” to air bubbles and float to the surface of the liquid. When machining metals, they are wetted with special liquids, which facilitates and accelerates surface treatment.

In construction, the phenomenon of wetting is important for the preparation of liquid mixtures (putties, putties, mortars for laying bricks and preparing concrete). It is necessary that these liquid mixtures wet well the surfaces of the building structures to which they are applied. When selecting mixture components, not only the contact angles for mixture-surface pairs are taken into account, but also the surface-active properties of liquid components.

You see its manifestation whenever you watch water slowly dripping from a faucet. A film of water emerges from the faucet and begins to stretch, like a thin rubber shell, under the weight of the liquid contained in it. This film, attached to the faucet opening, gradually lengthens until its weight suddenly becomes too great. The film, however, does not break, as a cutter would break if overloaded. Instead, it "slides" off the coccyx of the faucet and, as if embracing a small amount of water, forms a freely falling droplet. Undoubtedly, you have observed more than once that falling droplets take on an almost spherical shape. If there were no external forces, they would be strictly spherical. What you are observing is one of the manifestations of the unusual ability of water to "contract", "self-compact", or, in other words, its ability to cohesion (cohesion). A drop of water dripping from a faucet contracts into a tiny ball, and of all possible geometric bodies, the ball has the smallest surface area for a given volume.

Due to adhesion, tension is formed on the surface of the water, and in order to break the surface of the water, physical force is required, and, oddly enough, quite a lot. An undisturbed water surface can hold objects that are much "heavier" than water, such as a steel needle or a razor blade, or some insects that glide through the water as if it were not a liquid, but a solid body.

Of all liquids except mercury, water has the highest surface tension.

Inside the liquid, the attraction of molecules to each other is balanced. But not on the surface. Water molecules that lie deeper pull down the topmost molecules. Therefore, a drop of water, as it were, tends to shrink as much as possible. It is pulled together by surface tension forces.

Physicists calculated exactly which weight should be suspended from a column of water three centimeters thick in order to break it. The weight will need a huge one - more than a hundred tons! But this is when the water is exceptionally clean. There is no such water in nature. There is always something in it. Let at least a little, but foreign substances break the links in the strong chain of water molecules, and the cohesive forces between them decrease.

If drops of mercury are applied to a glass plate, and drops of water to a paraffin one, then very small droplets will have the shape of a ball, while larger ones will be slightly flattened by gravity.

This phenomenon is explained by the fact that between mercury and glass, as well as between paraffin and water, attractive forces (adhesion) arise that are smaller than between the molecules themselves (cohesion). When water comes into contact with clean glass, and mercury with a metal plate, we observe an almost uniform distribution of both substances on the plates, since the forces of attraction between glass and water molecules, metal and mercury molecules are greater than the attraction between individual molecules of water and mercury. This phenomenon, when a liquid is evenly distributed on the surface of a solid, is called wetting. This means that water wets clean glass, but does not wet paraffin. Wettability in a particular case can indicate the degree of contamination of the surface. For example, on a cleanly washed plate (porcelain, faience), water spreads in an even layer, in a cleanly washed flask the walls are evenly covered with water, but if the water on the surface takes the form of drops, this indicates that the surface of the dish is covered with a thin layer of a substance that does not wet water , most often fat.