While taking Surface Tension readings using Pendant Drop method, the reading continuously decreases with time for surfactant solutions. For some surfactant concentrations drop breaks from needle after some time.
Which reading should be taken as correct for these time varying data?
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You have difficulty with the definition of the surface tension of this method. Use simple plate method, method of counting drops ..
Thanks Yuri for his suggestion.
My result is similar to the graph show in Fig 5 of the attached paper. What value should be taken as Surface Tension of Surfactant solution?
True surface tension see Fig.7 when the surface tension does not depend on the Worthington number W0.
The surface tension is the value at total equilibrium. That is no overall changes in temperature, (pressure) and composition.
When the drop falls before a plateau is reaches please make the drop smaller or take a thicker flat needle.
It Is even more complicated when you measure interfacial tensions via the pendent drop as here the surrounding oil phase must be saturated with water and surface active components (usually the heavy phase).
I normally use a 1.65 mm needle and use a 3 -5 minutes monitoring time and checking for the plateau.
Loosing the drop is normal part of the experiment and requires some practice but it's worth it.
Alos make plots of the equilibrium surface tension against the log of the concentration that should become constant in slope.
In case of interfacial tensions I use saturated oil phase and not too often replacement of the oil phase (when increasing the concentration of the surface active component).
Hope this helps.
Harms comments are very helpful practical hints, however, surface tension itself is not defined as equilibrium property. Before equilibrium is reached, it is constantly changing as referred to dynamic surface tension, which can be also measured by pendant drop method (http://www.sciencedirect.com/science/article/pii/0927775796035510?np=y). Equilibrium values maybe calculated from this.
Titus Sobisch has put forward a very remarkable point about the surface tension vis-a-vis equilibrium. Pendant drop formation around the solid glass wall in open environment is troublesome. Residual forces that operate as continuous thin film at surface make sample molecules to adhere to solid surface of glass wall to get accumulated till its mass becomes equal to the cohesive forces or the surface tension. So no equilibrium occurs but adherence of sample mass on the surface may remain constant for each pendant drop formation at specific temperature and pressure. Dropping of sample from top and formation of pendant numbers could be a steady state and not the equilibrated state. But there are difficulties for developing the pendant drop with the stalagmometer and other which remains in the open environment. The liquid sample get evaporates due to a difference in temperature of the sample and of the environment along with pressure fluctuations. The gases present in environment also interact with the samples. Considering all these difficulties now the survismeter is being used to measure not only surface tension but interfacial tension, viscosity, wettability, activation energy, friccohesity also of the sample together.
https://www.youtube.com/watch?v=X0oJ1sQ6pcg