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Surface tension
SURFACE TENSION by Norbert Sajdera Kocour Co., Chicago Surface tension is defined as a cohesive molecular force acting on the surface of all liquids, which tends to force the solution to its smallest volume. Due to this force, the surface of the liquid becomes an elasticlike film that exerts its force perpendicular to the surface of the solution (if viewed from inside the liquid, a force pulling inward at the surface). A liquid with a high surface tension can, therefore, form a larger volume than a liquid with a low surface tension. The dyne/cm is the unit used for measurement of surface tension. This represents a force of 1 dyne acting perpendicularly on a segment of the surface 1-cm long. The value for water has been measured accurately and is accepted to be 72.75 _+ 0.05 dyne/cm at 20°C. When a sheet of metal is immersed in a liquid and then removed, a layer of liquid clings to the surface of the sheet. This layer will hold more liquid when the surface tension is high than when the surface tension is low; therefore, a solution that has a low surface tension becomes easier to rinse because it has less solution on it. This property is valuable when implementing control of alkaline cleaners and acid dips. The proper control of the surface tension will reduce drag-out and facilitate rinsing between process solutions. When surfactants (surface-active agents) are used to reduce surface tension, occasionally more than one benefit can be achieved. For instance, in some nickel and zinc plating solution formulations, the surfactant's primary function is as an antipitting agent; however, the reduction of surface tension on the cathode surface also tends to enhance the effect of other additives on the character of the electrodeposit. In many cases, the analysis of the surfactant is complex and the simplest control is the measurement of the surface tension. In the case of chromium plating solutions, the sole purpose of the surfactant is to suppress the emission of the chromium mist produced during operation of the solution. Close control is required because the surfactant is destroyed by the chromic acid and removed by drag-out. Many methods have been used to measure the surface tension of liquids. The capillary method seems to be the most convenient. The procedure requires immersing one end of a small diameter ( < 1 mm) capillary into the liquid. The liquid will then rise up the capillary due to the surface tension of the liquid. The height reached in the capillary can be used to measure the surface tension from the formula: Surface tension = h r d g / 2 where: h = height in centimeters; r = capillary radius in centimeters; d = density of liquid, g/cm3; and g = gravitational constant, cm/sec2. This method has been used to measure the surface tension of pure liquids very accurately; however, the addition of surfactants introduces problems such as bubbles in the column, difficulty in measuring the lower height rise caused by the decreasing surface tension, and problems with cleaning fine capillaries. Because of these problems, this method has not been widely used by electroplaters. The surface tension of a liquid influences the size of the drop that will be formed when the liquid is suspended from a glass tip. This fact has been used to devise various different drop-weight methods. Surface tension can be calculated very accurately from the relationship: Surface tension = mg/rXF where m is the mass of a slowly formed drop, which falls from the horizontal tip of a polished capillary tube due to acceleration force g and whose outside diameter is 2r. F is a function of V/r 3,
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The mass and volume (V) are measured from the accumulation of 30 or more drops. F is obtained from Volume 4 of the International Critical Tables (p. 435). With careful measurement, the method is capable of -+0.1% accuracy. The drop-weight method is also the basis for the stalagmometer. The stalagmometer is a modification of a pipette that permits a more controlled formation of the drops. There are two points etched on the stem of the stalagmometer that contain a definite volume of liquid. Below this calibrated volume, a capillary is inserted to control the speed of drop formation and the bottom tip is formed into a larger diameter that is polished flat. The test is performed by filling the stalagmometer with the test liquid and allowing the liquid to slowly form drops. The operator counts the number of drops that form and fall as the top of the solution moves from the upper to the lower etched point. This method does not directly measure the surface tension, but compares the number of drops of the test liquid to the number of drops of pure water formed with this stalagmometer tube. The surface tension is then calculated with the following formula: Surface tension =
72.75 × Number of water drops × Solution specific gravity Number of solution drops
This method is widely used by electroplaters because the equipment is inexpensive and precise measurements are easily obtained. The surface tensiometer is a force gauge that uses a torsion spring to apply a pulling force to a platinum-iridium ring. The tensiometer consists of a container to hold the test solution mounted on a table that can be raised and lowered smoothly. The torsion spring has a dial to indicate the force applied to the ring. The test is performed by lowering the platinum-iridium ring into the test solution, then lowering the solution and adjusting the torsion spring until the rings breaks free from the surface of the solution. Currently, manual and semiautomatic models are available. The semiautomatic model provides more uniform motion for raising and lowering both the test ring and the sample table. This test is sensitive to vibration and requires care in cleaning the sample container and the platinum-iridium ring. The maximum bubble pressure method uses the pressure differential between two probes with unequal orifice diameters. A constant volumetric gas flow is delivered to each probe, and the pressure differential between the probes is measured while bubbles are formed at the orifices, which are submerged in the solution. This pressure is proportional to the surface tension of the solution. Modern instruments use computers to collect and analyze the pressure data and compute the surface tension. Although this method is used largely in the manufacture of inks, paints, dyes, adhesives, and so on, it is mentioned here because it is the only method available that can be used for automated surface tension control. The stalagmometer has been used to measure surface tension directly. The greatest use for this method, however, has been for comparing the surface tension of the test liquid to the surface tension of water. Another common use is to estimate the quantity of surfactant in the process solution. This is accomplished by adding known amounts of surfactant to a process solution and measuring the surface tension at each concentration. In the case of the stalagmometer, only the number of drops needs to be plotted. Surface tension can be measured by several methods. The method chosen depends on the accuracy needed, cost of equipment, time available for testing, and skill of the operator.
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The mass and volume (V) are measured from the accumulation of 30 or more drops. F is obtained from Volume 4 of the International Critical Tables (p. 435). With careful measurement, the method is capable of -+0.1% accuracy. The drop-weight method is also the basis for the stalagmometer. The stalagmometer is a modification of a pipette that permits a more controlled formation of the drops. There are two points etched on the stem of the stalagmometer that contain a definite volume of liquid. Below this calibrated volume, a capillary is inserted to control the speed of drop formation and the bottom tip is formed into a larger diameter that is polished flat. The test is performed by filling the stalagmometer with the test liquid and allowing the liquid to slowly form drops. The operator counts the number of drops that form and fall as the top of the solution moves from the upper to the lower etched point. This method does not directly measure the surface tension, but compares the number of drops of the test liquid to the number of drops of pure water formed with this stalagmometer tube. The surface tension is then calculated with the following formula: Surface tension =
72.75 × Number of water drops × Solution specific gravity Number of solution drops
This method is widely used by electroplaters because the equipment is inexpensive and precise measurements are easily obtained. The surface tensiometer is a force gauge that uses a torsion spring to apply a pulling force to a platinum-iridium ring. The tensiometer consists of a container to hold the test solution mounted on a table that can be raised and lowered smoothly. The torsion spring has a dial to indicate the force applied to the ring. The test is performed by lowering the platinum-iridium ring into the test solution, then lowering the solution and adjusting the torsion spring until the rings breaks free from the surface of the solution. Currently, manual and semiautomatic models are available. The semiautomatic model provides more uniform motion for raising and lowering both the test ring and the sample table. This test is sensitive to vibration and requires care in cleaning the sample container and the platinum-iridium ring. The maximum bubble pressure method uses the pressure differential between two probes with unequal orifice diameters. A constant volumetric gas flow is delivered to each probe, and the pressure differential between the probes is measured while bubbles are formed at the orifices, which are submerged in the solution. This pressure is proportional to the surface tension of the solution. Modern instruments use computers to collect and analyze the pressure data and compute the surface tension. Although this method is used largely in the manufacture of inks, paints, dyes, adhesives, and so on, it is mentioned here because it is the only method available that can be used for automated surface tension control. The stalagmometer has been used to measure surface tension directly. The greatest use for this method, however, has been for comparing the surface tension of the test liquid to the surface tension of water. Another common use is to estimate the quantity of surfactant in the process solution. This is accomplished by adding known amounts of surfactant to a process solution and measuring the surface tension at each concentration. In the case of the stalagmometer, only the number of drops needs to be plotted. Surface tension can be measured by several methods. The method chosen depends on the accuracy needed, cost of equipment, time available for testing, and skill of the operator.
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