SU1753368A1 - Method of determining the liquid surface tension - Google Patents

Abstract

Uses: The invention relates to the field of research of surface properties, in particular to the study of the surface tension of liquids, and can be used to study the laws of adsorption and prediction of the properties of surface-active substances and their compositions, with the aim of improving the accuracy, reliability and rapidity of determining surface tension liquids characterized by a non-zero wetting angle and a slow establishment of an equilibrium surface tension value, from ep dissolved height lifting the capillary, the distance between the line phase and the top of the wetting perimeter of the meniscus is determined by capillary constant on the basis of the numerical solution of the basic equation capillarity and surface tension is calculated. 1 or 1 tab. &

Description

The invention relates to the field of research of surface properties, in particular to the study of the surface tension of liquids, and can be used to study the laws of adsorption and predict the properties of surface-active substances and their compositions.

A known method for determining the surface tension of liquids is based on measuring the elevation of liquid in a capillary,

The essence of the method lies in the fact that for liquids that completely wet the walls of the capillary, on the basis of measurements of its elevation (Hi) in

a calibrated capillary of radius R0 is determined. VRo / Hi value. unambiguously related to a2, determined from pre-compiled tables, which, in turn, is related to the magnitude of the surface tension of the fluid (o).

The disadvantage of this method is its unsuitability for determining the surface tension of liquids characterized by a wetting angle, the value of which is significantly different from zero.

The closest in technical essence to the present invention is a method for determining the surface tension of liquids, based on the measurement of high temperature |

SP

Oj

oe with

00

You are capillary raising and determining the surface tension, taking into account the magnitude of the wetting angle.

The method consists in determining the magnitude of the capillary lift (Hi) in a calibrated capillary with a radius of RO. the value of vPЈ / Hi is calculated, and then at a previously known value of the wetting angle according to previously compiled tables, the value of a2 is determined, which is uniquely related to the surface tension by the ratio

dq R2,

„Ag up a -f -j.

where d d - dr dx,

yj is the density of the liquid;

dr is the gas density; --...,.

g - free fall acceleration.

In order to implement the method, it is necessary to know the magnitude of the wetting angle of the capillary walls of the test oil. This value cannot be measured directly on the capillary wall due to optical effects due to the curvature of its surface and the relaxation of the wetting angle. , ...; ,,.

The purpose of the invention is to improve the accuracy, reliability and speed of the determination of the surface tension of liquids characterized by a non-zero wetting angle and a slow establishment of the equilibrium value of the surface tension,;, -:

The drawing shows the removal of the process of capillary lifting of a liquid in a cylindrical capillary tube of radius Ro dipped into a vessel filled with the liquid under study with a radius significantly larger than R0.

Designations adopted in the drawing;

Hi is the height of a capillary lift (elevation of the top of the meniscus above the flat surface of the liquid in the vessel); ;: On-distance between the line of three-phase wetting perimeter and the top of the meniscus;

Z and r-cylindrical coordinates;

A, B, C-turntables corresponding to marks of the line of the three-phase base perimeter, vertex of the meniscus and the level of the flat-surface of the liquid vessel, respectively

0- "wetting angle; ,

The method for determining the surface tension of liquids consists in measuring the height of the capillary lift Hi and the distance between the three-phase wetting perimeter line and the top of the H2 meniscus, determining the capillary constant

and on the basis of the numerical solution of the basic capillary equation, v- (

, 2 v, Yxx, Yxx

+

(f + YSy 5 (CP + U2)

with boundary conditions Y (0) Yi: Yx (0) 0; Y (1) -Yi + Y2;

(one)

(2).

(3)

(four)

Where

Y - ...

l -; To

Yl-iii.

 "about :

Ohno

five

five

0

five

0

five

  dY: -

 Yx .dx - ..;. ; ....;

v d2Y dg o2 Vxx,:

g and Z cylindrical coordinates of the interfacial surface; 0Ro is the capillary radius;

d dx - dr d (d is the density of the liquid, dr is the density of the gas),

g is the acceleration of free fall inheriting the calculation of the surface weight of 5 tons of rib rib formula

C7 R2o. ,...(five)

d a c

The essence of the method, in contrast to the well-known technical solution, in which taking into account 0 the influence of the wetting angle on the height of the capillary lift, is based on its measurement in the Independent experiment, the measurement of the additional parameter H2 The value of Hi enables the direct numerical calculation of the value of the capillary constant a, which is uniquely related to the surface tension of the liquid a. ;

This calculation is based on the physical interrelation between the parameters characterizing the process of capillary elevation shown in the drawing.

It is known that the height of the capillary lift Hi is determined by the difference of hydrostatic pressures above the flat surface of the liquid in vessel 2 and above the curved surface of the liquid in capillary 1. At the point corresponding to the top of the Yenisk, the indicated pressure difference AP dgHi according to the Laplace equation is related to the surface tension of the fluid a and the radius of curvature of the surface liquid at the apex of the meniscus R by the ratio

2g

DR

TT. R-value (not indicated), not available for direct experimental determination.

ki, in turn, depends on the magnitude of the surface tension, which tends to give the meniscus the shape of a spherical surface, and on the magnitude of the hydrostatic pressure of the liquid column with height H, which leads to flattening of the meniscus and depending on the magnitude of wetting angle 0 and the density of the liquid

Thus, the value of H2, which can be determined directly during the observation of capillary elevation, reflects the effect on the elevation of the capillary elevation wetting angle.

The calculated value of the surface tension according to the experimentally determined parameters Hi and H2 is calculated on the basis of a numerical solution of the basic capillary equation (1) with the boundary conditions (2) - (4) and formula (5) /

Example 50 ml of bidistilled water is poured into vessel 2 A glass cylindrical capillary tube with a radius of RO (0.1090 + 0 0005) cm is fixed in a special holder and lowered into the test solution. The vessel with the capillary is thermostated to 25 ° C for 40 minutes. After an equilibrium capillary lift has been established, which is controlled with the cathetometer by periodically monitoring the position of the meniscus tip with a discreteness of 10 min, the measurement process is started. The cathetometer is read three levels x - corresponding to the NI three-phase wetting perimeter line (mark A in the drawing), at the top of the meniscus N2 (mark B) and on the flat border of the liquid with the vessel N3 (mark C)

Based on the measured values of NI, N2 and N3, the values of capillary lift Hi N2 - N3 are calculated. the distance between the line of three-phase wetting perimeter and the tip of the meniscus; On NI - N2 and dimensionless parameters Yi Hi / R0; Y2H2 / Ro, after which the capillary constant a2 is determined by numerical solution of differential equation (1) with boundary conditions (2) - (4) by the method of successive approximations and by the relation

a d | R2o

3

the surface tension is calculated.

The data of seven consecutive series of measurements and calculations are presented in the table.

As can be seen from the table a- (71 61 ± 0 02 dyn / cm: -0 03%

The magnitude of the surface tension of plain water, determined with the same / c fish according to a known method, is (71.85 ± 0.5) dyne / cm. Thus, in this case, both methods determine (within the limits of experimental error) consistent with each other and with water (71.96 dyn / cm),

A special advantage of the proposed method is the accuracy of determining the surface tension of a specified class of liquids. This is due to the fact that all measured values are determined in the same experiment in identical experimental conditions. At the same time, directly in the process of measuring by observing the position of the top of the meniscus in the field of vision of the cathetometer monitors all the relaxation processes in the system including the changes in time of the wetting angle of the cap walls ll ra test liquid that, in principle, not possible with the known method

Claims (1)

Invention Formula The method of determining the surface tension of liquids, including measuring the height of the capillary lift Hi of a liquid in a calibrated capillary radius RO and determining the capillary constant characterized in that in order to improve the accuracy, reliability and expressiveness of determining the surface tension of liquids characterized by non-zero wetting angle and slow establishment of the equilibrium surface tension value additionally measure the distance H2 between the line of the three-phase perimeter cmz h Vani and the top of the meniscus, capillary constant and determined based on a numerical solution of the basic equation capillarity , 2 / YXX x  AA | LA (1 + Yx)% (XO + Y) 1 by the relevant conditions Y (0) Yi, Yx (0) - 0; Y (1) Yi + Y2; X Ј-; Y z Ro Y, - {Jl. Co Ro . P Yxx Y .- “LY. Yx  . -dx2 gig - cylindrical coordinates of the interfacial surface: Ro is the radius of the capillary, calculate the surface tension using the formula p - d 9 R2 where d yj - dr dm. dж - fluid density dr - gas density: g - free fall acceleration.