Accurate Determination of the Equilibrium Surface Tension Values with Area Perturbation Tests

These protocols can be incorporated into tension measurement protocols. For example, they can be used to measure equilibrium interfacial tension between water and oil phases. Each of the two protocols in this video can be used to obtain robust and reliable equilibrium surface tension values.

These values can be established after testing their stability against area perturbations. To begin, prepare the tensiometer and sample as described in the text protocol. Then, select an inverted stainless steel needle based on the estimated surface tension values and place it at the tip of the dispensing device.

Next, load 40 milliliters of liquid sample into a quartz cell. Place the cell on top of the sample platform. Adjust the height of the inverted needle such that the tip of the needle is at least 20 millimeters below the surface of the liquid sample.

Inject one milliliter of air through the submerged inverted needle to remove impurities that could possibly be present on the tip of the syringe and to improve the surface chemical purity of the air/liquid interface. Then, experimentally determine the initial bubble volume. Dispense the computed initial bubble volume to form a bubble at the tip of the inverted syringe.

Make sure that the bubble is in hydrostatic equilibrium such that the bubble does not move. Measure the dynamic surface tension based on the shape of the produced air bubble at the tip of the needle. Every second, calculate the surface tension based on the axisymmetric drop shape analysis method of the Laplace-Young equation.

Compare the actual shape of the bubble with a calculated shape. If the two shapes overlap, then the equilibrium Laplace-Young equation is valid. This inference is completely valid when the bubble stops moving and the surface tension stops changing.

Measure the surface tension as a function of time until the first steady-state surface tension is achieved. The steady state surface tension is defined as a plane of value beyond which the surface tension changes by less than one millinewton per meter or by less than 5%in several consecutive dynamic surface tension measurements. When steady state surface tension is achieved, record the bubble volume and the surface area.

Then, decrease the bubble volume by removing one microliter of air and record the new bubble volume and area. Continue measuring the dynamic surface tension in the areas until the dynamic surface tension reaches the second steady state surface tension. Next, expand the bubble volume by injecting one microliter of air so that the volume and area are similar to the initial values.

Continue measuring the dynamic surface tension values until a third steady state surface tension is reached. If the three steady state surface tension values differ from each other by less than one millinewton per meter, or by less than 5%then we define their average as the equilibrium surface tension. Place a filled sample holder inside of the spinning chamber of the spinning tensiometer and then spin the tube at 500 RPM.

This should prevent the injected bubble from migrating upward or attaching to the tube wall. Next, load two microliters of air into the syringe. Insert the needle of the syringe through the rubber septum, which seals the spinning tube and inject a two microliter air bubble into the spinning tube.

Increase the rotation frequency of the sample tube so that the bubble moves closer to the axis of rotation and deforms more because of the increased centrifugal forces. Continue to speed it up until the ratio of the horizontal bubble’s length to its radius at the middle of the bubble is eight or greater. Then adjust the tilt angle of the measuring chamber to position the sample tube horizontally.

This will prevent bubble movement and to help achieve gyrostatic equilibrium for an axisymmetric shape assumed in the Laplace-Young equation and algorithm used. Now, measure and record the dynamic surface tension values at one second intervals. Continue at a fixed rotation frequency until the surface tension reaches a steady state value.

Also, record the bubble volume and area. Once recorded, alter the rotation frequency to a second rotation frequency to vary the surface area. Measure the dynamic surface tension at a fixed rotation frequency once it reaches a second steady state value at the new frequency.

At this point, also record the new bubble volume and area. Next, change the rotation frequency so that it is close to the original value. Measure the dynamic surface tension values at this fixed rotation frequency until the third steady state value is reached.

Again, record the new bubble volume and area. Using the emerging bubble method, the steady state surface tension values of a five millimolar solution of Triton X-100 were measured against air. This concentration is above the critical micelle concentration for this surfactant in water.

The steady state surface tension of 31.5 millinewtons per meter was obtained approximately 20 seconds after the bubble was formed. After about 25 seconds, the volume and area of the bubble were reduced and the dynamic surface tension dropped to 31 and within one second and increased back to 31.5, marking the steady state surface tension number two. After about 50 seconds, the volume and the area of the bubble were increased abruptly and the dynamic surface tension value changed little, and hence, the steady state surface tension number three was determined to be 31.5 millinewtons per meter, as well.

The three SST values were all the same, therefore, the equilibrium surface tension was determined to be 31.5 millinewtons per meter. Using the spinning bubble method, steady state surface tension one was found to be 30.9 millinewtons per meter, steady state surface tension two was found to be 30.6, and steady state surface tension three and the equilibrium surface tension was found to be 30.8. The two methods had a 2.2%difference in the equilibrium surface tension values when measuring five millimolar Triton X-100.

This was probably due to certain systematic errors. The most important thing to remember for the emerging bubble method is to maintain conditions close to the hydrostatic equilibriums. For the spinning bubble method, be sure to apply the correct equation.

The methods described in this video can also be applied to determine equilibrium interfacial tension values between water and oil phases. These methods provide a more reliable way of calculating how much a surfactant absorbs when in equilibrium at the air-water interface. It can also be used to determine the extent of surfactant aggregation in solution called micellization.