I recently measured the surface tension using the pendant drop method. What I found was that the surface tension decreases with time for pure water and salt water. I also noticed that this kind of thing reported by other publications. But I haven't found any paper discussed the reason for this phenomenon. Does any one know why? Thanks.
Yes , it does reduce with time due to presence of air as surfacetant...
Journal of Colloid and Interface Science
Volume 141, Issue 1, January 1991, Pages 1-9
Abstract:An improved method has been developed for the determination of surface and interfacial tensions from primary drop shape data. In addition, wetting angles of sessile drops may be determined. The method has been built around a commercial pendant drop instrument and an IBM-compatible PC with a frame grabber card. In order to differentiate the drop profile, a filter routine using a local threshold and interpolation technique has been developed that is combined with an edge-tracing algorithm. The program for calculation of surface tension is divided into two parts. The first part is based on the traditional optical method and uses inflection of the drop profile. By means of several polynomial interpolations and curve fitting of theoretical profiles, the form factor β and surface tension γ are determined. The second part of the calculation utilizes the above values as a first estimate and then performs a further optimalization of γ by comparison between experimental and theoretical Young—Laplace profiles. With a PC AT with a 80287 mathematical coprocessor the measurements take about 5 s and the reproducibility is typically 0.01–0.03 mN/m for a wide range of known liquids.
Thank you Anoop~ but I didn't find anything related to the reduction of surface tension in the paper you recommended. Would you please specify a little bit what did you mean "air as a surfactant"?
Hello Yumeng,
There is a book called “Mechanics of Immiscible Fluids in Porous Media” (Corey, 1994) that gives a description of surfactants in section 1.5.2 (p. 19). This section is available on Google Books (https://books.google.ca/books?id=2K8iFk9PsCoC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q=surface%20tnesion&f=false). According to it, surfactants are agents that tend to accumulate preferentially in the boundary layer, although the concentration in the phase as a whole may be very small, and can reduce the surface tension of water.
Hope this helps.
Marcos
REFERENCE
Corey, A.T., 1994. Mechanics of Immiscible Fluids in Porous Media. Water Resources Publications.
Thank you Marcos.
I am also thinking of whether there are some impurity presented in the air functioned as surfactant and adhered to the surface of pure water droplet. One interesting thing I noticed is that if I encapsulated the drop with a small transparent container that inhibited the evaporation process, the surface tension of pure water will be constant. This means that the evaporation was a key factor that reduced the surface tension. I am wondering whether it was caused by the impurity of water itself and the evaporation brought surfactant to the droplet surface. In this case, it means the D.I. water I used was not pure enough (but actually I think the water should be Ok); Or another explanation is that the evaporation process itself other than surfactant caused the decrease of surface tension, this may related to adsorption, diffusion, humidity etc. I guess. I am not an expert in surface science or thermodynamics so I don't know what exact caused this phenomenon. But this is really an interesting topic, isn't it?
Hi Yumeng Zhao
I am interested to know why surface tension changes within 1hr of the same solutions under the same conditions.
I saw the reasons you postulated: the evaporation and impurity
I do not go with evaporation reason, because it is a short period, and for impurities, I am using DI water also.
I guess it depends on the surfactant molecules distribution in the solution, surfactants tend to attracted toward the air-water interface, not sure.
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