Surface tension

Surface tension is the specific free energy of the surface layer at the interface of two phases, as measured by the work that must be spent on education 1 cm2 interfaces with a constant temperature. Surface tension is measured in ergs·cm-2. Surface tension can be directly measured only in case the interface of liquid - gas (saturated steam) or liquid - liquid. P. N., measured under the conditions when the surface layer has not yet reached equilibrium state, referred to as dynamic, as measured in the equilibrium conditions - static P. N. Time to reach the equilibrium state of the surface layer of pure liquids and solutions of low molecular substances is usually not more than a few thousandths of a second.
For colloidal solutions and solutions of macromolecular substances that time much more and is measured sometimes many hours.
In the biochemical, physiological, and pharmacological studies of the most common are the following methods of measurement of surface tension.

stalagmometer the Traub
Fig. 1. Stalagmometer The Traub.

1. Stalagmometric method. Consists of determining the number of droplets formed by leaking this volume of fluids from capillary holes special pipettes - part of the stalagmometer (Fig. 1; 1 - top tag, 2 - bottom label). Having determined using the same part of the stalagmometer the number of drops (n) of the investigated liquid, and the number of drops (n0) standard liquid (usually water) with the famous P. N. σ0, calculate P. N. investigated liquid and according to the formula:
           
where d and d0 - beats. weight investigated and the standard liquid, respectively.
2. A method of punching the air bubbles (proposed P. A. Rebinder) based on the pressure p is required to penetrate the air bubble through capillary of being in contact with the surface of the investigated liquid, and pressure P0 required to penetrate the air bubble through the same capillary, which is in contact with the surface of the standard liquid, surface tension σ0 known. P. N. investigated liquid σ found by the equation:
           

the device Rehbinder
Fig. 2. The device Rehbinder: 1 - vessel for liquid; 2 - capillary; 3 - tap to the respirator; 4 - gauge.

Method Rehbinder more accurate than stalagmometry.
These methods are suitable not only for measuring the surface tension at the interface of liquid - gas (air), but at the interface of two liquid phases. In the latter case, the drops of one liquid push through a capillary in the device Rehbinder (Fig. 2) or cause to flow from part of the stalagmometer other liquid.
When dissolved in the liquid of any substance observe the following cases. 1. The dissolved substance increases, usually slightly, P. N. solvent (such substances called poverkhnostnymi). Poverkhnostnymi substances to water are many inorganic salts, acids and organic substances, as glycerin, alpha-amino acids, sugar and other 2. The dissolved substance lowers P. N. solvent (such substances are called surface active). In relation to the water surface-active substances are many organic compounds (e.g., alcohols, fatty acids, esters, glycosides, proteins and other), the molecules of which are dipolnym and consist of non-polar hydrophobic hydrocarbon residue and polar hydrophilic groups. To polar groups include groups-OH, -COOH, -NH2, -SH-SNO and other Measure surface activity of substances is the attitude changes P. N. Δσ to the corresponding change in the concentration Δс of this substance in the solution (i.e. Δσ/Δс). Pursuant to rule Duclos - Traub surface active substances of the same homological series is increased approximately 3 times with increasing hydrocarbon prices on CH2-group. This rule is true for dilute solutions. In some cases, biological activity (for example, drug effect, bactericidal and others) substances of the same homological series increases with their surface activity in accordance with rule Duclos - Traub.
Surface active substances adsorbed positively on the interface, surface-inactive - negative (see Adsorption).
The surface tension of cytoplasm of living cells on the border with the surrounding liquid medium has a very low value on the average 0,2-2,0 erg·-2. The less P. I. on the border of contact between the cells, the easier cells are isolated from one another. The change of the surface tension on the surface of cells and organelles plays a significant role in such phenomena as cell division, phagocytosis and pinocytes, amoeboid motion, change of permeability of cell membranes and other
Static P. N. human blood is far less than dynamic P. N.; this value is close to P. N. water (73,04 erg·-2 at 18 degrees), whereas static P. N. oxalate the blood of men is on average 55-57, women - 59-62 erg·-2 at 18 degrees. Dynamic P. N. blood plasma is subject to considerable fluctuations in various diseases; the most dramatic change was observed in anaphylactic shock. P. N. blood plasma of the patient tumors is lower than normal. With the age the surface tension of the blood serum reduced.