ASTM D1331-20

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Designation: D1331 − 14 D1331 − 20
Standard Test Methods for
Surface and Interfacial Tension of Solutions of Paints,
Solvents, Solutions of Surface-Active Agents, and Related
Materials
This standard is issued under the fixed designation D1331; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 These test methods cover the determination of surface tension and interfacial tension of a variety of liquid materials, including
but not restricted to paints, solvents, and solutions of surface-active agents, as defined in Terminology D459. Four methods are
covered as follows:
Method A—Surface Tension by du Noüy ring.ring;
Method B—Interfacial Tension by du Noüy ring.ring;
Method C—Surface Tension by Wilhelmy plate.plate; and
Method D—Interfacial Tension by Wilhelmy plate.
1.2 Method A originally was written primarily to cover aqueous solutions of surface-active agents, but is also applicable to
aqueous paints, nonaqueous solutions (including paints) and mixed solvent solutions.
1.3 Method B is applicable to two-phase solutions. More than one solute component may be present, including solute components
that are not in themselves surface-active.
1.4 Method C is applicable to surface active liquids and, unlike du Noüy ring, no buoyancy corrections are needed and results are
not affected by moderate viscosities (1-10 Pa-sec) of the liquid. It is the recommended method for use with paints and resin
solutions.
1.5 Method D is applicable to two-phase solutions and mixtures.
1.6 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use. Material Safety Data Sheets are available for reagents and materials. Review
them for hazards prior to usage.
1.8 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
These test methods are under the jurisdiction of ASTM Committee D01 on Paint and Related Coatings, Materials, and Applications and are the direct responsibility of
Subcommittee D01.24 on Physical Properties of Liquid Paints & Paint Materials.
Current edition approved Dec. 1, 2014Dec. 1, 2020. Published January 2015January 2021. Originally approved in 1954. Last previous edition approved in 20112014 as
D1331D1331 – 14. – 11. DOI: 10.1520/D1331-14.10.1520/D1331-20.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D1331 − 20
2. Referenced Documents
2.1 ASTM Standards:
D459 Terminology Relating to Soaps and Other Detergents
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
3. Terminology
3.1 Definitions of Terms Specific to This Standard:
3.1.1 du Noüy ring—ring, n—platinum wire ring.
3.1.2 Wilhelmy plate—plate, n—flat, thin plate made of glass or platinum.
4. Summary of Test Methods
4.1 du Noüy Ring Methods—The ring is suspended from a tensiometer or balance. The test liquid is brought into contact with the
ring until the latter is covered. For interfacial tension measurements, the ring must be at the interface between the two liquids. The
ring is then slowly withdrawn until a maximum force is attained, from which the surface or interfacial tension may be calculated.
4.2 Wilhelmy Plate Methods—The plate is suspended from a tensiometer or balance. The test liquid or interface between two
liquids is brought into contact with the bottom of the plate, causing the plate to be pulled down into the liquid by the surface tension
force. The force applied to the plate from above is then increased to bring the bottom edge of the plate level with the flat surface
of the liquid. The force acting on the plate is measured and used to calculate the surface tension of the liquid or the interfacial
tension between two liquid phases.
5. Significance and Use
5.1 These methods provide data that are useful in evaluating the effectiveness of surface active agents in reducing surface tension.
In addition, surface tension data can predict interactions between liquids and solid surfaces or other liquids and can be used to
establish wetting properties of paints, solvents, and other liquids.
5.2 A number of laboratories have found the Wilhelmy plate to be easier to use, easier to clean and generally better for use with
pigmented paints.
6. Interferences
6.1 The du Noüy ring is difficult to clean when used with pigmented paints. Paints high in titanium dioxide have a tendency to
coat the ring with a layer that is almost impossible to remove. If left on the ring, this layer of material affects the surface tension
values that are measured.
6.2 Paint measurements with the du Noüy ring are affected by viscosity above a level of 10 Poise (1 Pa-sec). Many coatings have
a viscosity at rest greater than that level. Above 10 P, the resistance of the ring to being pulled is as much due to viscosity as to
surface tension.
METHOD A—SURFACE TENSION BY DU NOÜY RING
7. Apparatus
7.1 du Noüy Tensiometer—Either the du Noüy precision tensiometer or the du Noüy interfacial tensiometer, equipped with either
the 4 or the 6-cm circumference platinum ring, as furnished by the manufacturer, may be used. The tensiometer shall be placed
on a sturdy support that is free from vibrations and other disturbances such as wind, sunlight, and heat. The wire of the ring shall
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be in one plane, free of bends or irregularities, and circular. When set in the instrument, the plane of the ring shall be horizontal,
that is, parallel to the surface plane of the liquid being tested.
7.2 Sample Container—The vessel for holding the liquid shall be not less than 6 cm in diameter, and sufficiently large to ensure
that the contact angle between the ring and the interface is zero.
8. Preparation of Apparatus
8.1 Clean all glassware thoroughly. The use of fresh chromic-sulfuric acid cleaning mixture, followed by a thorough rinsing in
distilled water, is recommended.
8.2 Clean the platinum ring by rinsing thoroughly in a suitable solvent and in distilled water, before taking a set of measurements.
Allow the ring to dry, and then heat to white heat in the oxidizing portion of a gas flame.
9. Calibration of Apparatus
9.1 Calibrate the tensiometer or balance according to the manufacturer’s directions.
10. Procedure
10.1 After the tensiometer has been calibrated, check the level and insert the cleaned platinum ring (Note 1) that will be used in
the measurement. Check the plane of the ring, and set the dial and vernier at zero. Adjust the rear adjusting screw so that the index
level of the arm is opposite the reference mark on the mirror, that is, the ring system is at the zero position.
NOTE 1—Extreme care must be taken to have the sample vessel and platinum ring clean. Contamination of the liquid surface by dust or other atmospheric
impurities during measurement should be avoided.
10.2 Place the solution to be tested (Note 2), contained in the thoroughly cleaned vessel (Note 1), on the sample platform. Raise
the sample platform by means of its adjusting screw until the ring is just submerged.
NOTE 2—For surface active agents: since the surface tension of a solution is a function of the concentration, care must be taken that the concentration
is adjusted and recorded within known limits. The presence of solutes other than the surface-active agent should be ascertained and reported qualitatively
and quantitatively, insofar as possible. This includes hardness components in the water. Care should be taken that the solution is physically homogeneous.
Measurements made near or above the cloud point or other critical solubility points can be in serious error. This is particularly true when the solute is
a surface-active material.
10.3 Lower the platform slowly, at the same time applying torsion to the wire by means of the dial-adjusting screw. These
simultaneous adjustments must be carefully proportioned so that the ring system remains constantly in its zero position. As the
breaking point is approached, the adjustments must be made more carefully and more slowly. Record the force reading from the
dial or the digital display when the ring detaches from the surface.
10.4 Make at least two measurements. Additional measurements shall be made if indicated by the over-all variation obtained, the
total number of readings to be determined by the magnitude of that variation.
10.5 Record the temperature of the solution and the age of the surface at the time of testing. Since the submerging of the ring (10.2)
may constitute a significant disturbance of the surface, take the age as the elapsed time between submersion and breakaway of the
ring. The accuracy of this time observation may be indicated in the usual manner. In most cases an accuracy of 65 s is reasonable,
and sufficient for this test method.
11. Calculation and Report
11.1 The dial reading, obtained from a measurement carried out in the foregoing manner with a calibrated instrument, is actually
the pull per linear centimetre on the ring (both inner and outer circumference being considered) at the break-point, expressed in
dynes. This value, called the uncorrected surface tension, must be multiplied by a correcting factor, F, to give the corrected surface
tension. F is a function of the contours of the liquid surface in the neighborhood of the ring at the instant of breakaway. It can be
numerically specified in terms of R, the mean radius, in centimetres, of the ring; r, the radius, in centimetres, of the wire from which
the ring is made; and V, the maximum volume of liquid elevated above the free surface of the liquid. For liquids of low surface
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tension, such as surface-active agents, F is, in general, appreciably less than unity. It must, therefore, be ascertained and applied.
3 3
Values of F in terms of two compounded parameters, R /V and R/r have been compiled and tabulated by Harkins and Jordan. In
order to look up F in the tables, the values of these two parameters must be calculated. Values for R and r are furnished by the
manufacturer with each ring. The value of V is calculated from the following equation:
V 5 M/ D 2 d (1)
~ !
where:
M = weight of liquid raised above the free surface of the liquid,
D = density of liquid, and
d = density of air saturated with vapor of the liquid.
To calculate M, multiply the tensiometer dial reading by the factor which converts this reading into grams pull on the ring. The
factor D can be measured by the usual procedures, and the value d can be obtained from published data. The corrected surface
tension in dynes per centimetre is obtained by multiplying the uncorrected surface tension value by F.
11.2 Unless specified, the surface tension values reported shall be corrected values. Report also the temperature at which the
measurement was made. If it is desired to report the surface tension value of an aqueous solution at some standard temperature,
for example, 25°C, and the measurement was actually made at a temperature within about 3°C of this value (that is, 22 to 28°C),
a correction factor of 0.14 dynes/cm·°C may be used. Subtract this correction factor from the surface tension when the temperature
of the test is lower than the reported temperature, and add it to the surface tension when the temperature of the test is higher than
the reported temperature. This value for the correction factor is not valid for nonaqueous liquids, and should be used only where
the solvent is preponderantly water.
METHOD B—INTERFACIAL TENSION BY DU NOÜY RING
12. Procedure
12.1 Determine interfacial tension as described in Section 10 for surface tension, with the following modifications:
12.1.1 Always move the ring from the aqueous side of the interface through to the nonaqueous side. With liquids lighter than
water, it is accordingly possible to use the precision tensiometer as well as the interfacial tensiometer. With liquids heavier than
water, where the ring must be pushed downward, the interfacial tensiometer should be used.
12.1.2 Use fresh
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