Trying to measure IFT of emulsions.....high IFT values are obtained when measured by pendant drop (surrounding phase n-heptane)...whereas a homogeneous solution is formed in spinning drop method giving no results.... suggesting very low IFT which is theoretically correct (low IFT of emulsion than surfactant solutions).
why is it so....anomaly in interracial behavior (IFT) in both the methods?
Also, is it possible to measure IFT of emulsion by spinning drop method...if how?
Spinning drop method is generally more appropriate for measurement of low and ultra low IFT, whereas pendant drop method is more reliable for high IFT.
To get accurate results for high IFT in spinning drop, you need to work with high rotational speeds only.
You should use saturated phases in the spinning drop apparatus. My guess is that the heptane you are injecting into the spinning drop apparatus is simply being solubilized in the water phase. So saturate the water phase with heptane before filling the apparatus.
Actually doing the same for the pendant drop measurement will get you you to equilibrium sooner.
there is no interfacial tension 'of an emulsion' except the interfacial tension at the drop - continuous phase interface because it is defined at the boundary of 2 phases only.
Hello Dr. Kumar, can i ask which are the components of the system you are measuring? I understand you are measuring an heptane-water system. Does it have a surfactant added? The spinning drop and pendant drop methods should give similar measurement values at high IFT (nowadays spinning drop apparatus can reach high spinning drop velocities to measure IFTs as high as 50 mN/m). At low IFTs the pendant drop is not a suitable methods because the drop detaches from the syringe tip. If the drop dissapear in the spinning drop capillary, maybe is a matter of the experimental: are you placing both phases in contact to reach equilibrium previous to injection of the drop? For more information i suggest you the following paper where IFT and interfacial rheology properties are measured for both methods. Kind regards https://www.sciencedirect.com/science/article/pii/S0021979718301553
A challenging question!
Based on experiences, the droplet and bulk equilibrium in the pendant drop method is difficult to achieve if IFT reaches values less than 2 or 3 mN/m and the output values are unreliable for this range. It seems to me that the two methods can not be compared this way because they have different measurement limits for the liquid-liquid system.
I agree with Iman. It is very difficult to achieve similar conditions and principles of two measurements are different. Literature shows that microemulsion is having ultra-low IFT compared to surfactant solution and oil system. Spinning drop method cannot be used for IFT for microemulsion. Again pendant drop method is unable to get ultra-low IFT.
The difference is about the factors which will be assumed by each method. For example the pendant drop method considers the gravity of the drop and viscouse forces. The value of IFT assigns to the time when viscouse force and gravity forces are equal. While in spinnig drop, the gravity will not be assumed.
For emulsions due to low range of IFT my suggestion is spinning drop method.
Recently, I did IFT of water-decane (without surfactant) using pendant drop method and the value obtained was in agreement with what has been reported in the literature. Then, I tried determining IFT of water-decane/surfactant using same method (pendant drop). It became very difficult for the decane/surfacant drop to form. I mean extremely difficult. Because of this, I switched to the spinning drop method to determine the IFT of water-decane/surfactant I am investigating and the IFT value obtained was very low. My observation: pendant drop is very good to determining samples having high IFT values especially in the absence of surfactant while spinning drop technique is good for samples with low IFT in the presence of surfactant. The use of spinning drop for samples with high IFT will require increasing the rotation speed but caution is needed not to damage the system due to high speed.
there is a reason for 'It became very difficult for the decane/surfacant drop to form' - likely formation of viscous water-in-oil emulsion
You are very correct Titus Sobisch ... The emulsion formed was extremely viscous and stable.
The cause of this phenomenon goes back to the different theory of these two methods. In the pendant drop method, the dominant mechanism for mass transfer is diffusion; while in the spinning drop method, mass transfer by diffusion is accompanied by convective mass transfer due to the rotational type of measurements which enhances the mass transfer rate.
Moreover, the volume of the bulk phase in the pendant drop procedure is much more than that of the spinning drop method.
While yielding a very precise measurement, the spinning drop (with the Vonnegut eq.) reduces the complications inherent to methods relying on the detection of the whole drop shape, e.g., the Young−Laplace fit of the drop surface, which are more sensitive to refraction effects due to the cylindrical capillary and are affected by any change in the illumination conditions. Article Spinning Drop Dynamics in Miscible and Immiscible Environments
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