Glossary

Dynamic surface tension with overlapping time intervals

The adsorption dynamics is best studied by measuring the dynamic surface tension, or in case of interfaces between two immiscible liquids the dynamic interfacial tension. There are various methods available for such measurements having different time windows. Methods with complementary time windows are needed, even for the study of one surfactant, as the rate of adsorption depends on its surface activity and the concentration in the solution bulk. This is best seen from the following graph which shows the dynamic surface tension measured with a drop shape technique (see PAT-1M and PAT-2S, products of SINTERFACE).
Scientific Setup

Figure 1: Experimental setup for profile analysis tensiometry.

This method yields values for very long adsorption times (here up to about 10.000 s) however, for short times no data are provided by this technique. For a concentration up to 7×10-6 mol/l this time interval seems to be sufficient, for higher concentrations, however, the surface tensions at 5 seconds (the lowest time measured by the PAT-1M here, actually it can start from about 1 s), are significantly lowered due to adsorption. For such surfactant concentrations measurements at much shorter adsorption times are required. The only technique that provides data at adsorption times down to 1 ms and even less is the bubble pressure tensiometry (see BPA-1S and BPA-2S from SINTERFACE). For dynamic interfacial tensions short adsorption times like 1 ms are not accessible by any existing method. The following schematic gives an overview of existing methods for dynamic surface and interfacial tensions and the corresponding time windows. The drop pressure module (DPA-1 and ODBA-1) is the first experimental set-up which allows to measure at short adsorption times. Depending on experimental details, times even below 10 ms could be accessible.  
Scientific Setup

Figure 1: Experimental setup for profile analysis tensiometry.

The choice of a suitable method is sometimes not so easy, as for certain problems many methods exist, and all of them have advantages against others, in respect to easy handling, available temperature control, applicability to liquid-gas and liquid-liquid interfaces etc. What however, becomes evident as well is, that there is no method that covers the whole range of time and needs. Note, for highly concentrated solutions, i.e. for most surfactant solutions above the CMC, even 100 μs appear to be a long adsorption time. An actual summary of experimental possibilities using drop and bubble methods and an excellent selection of experimental examples has been published recently (R. Miller and L. Liggieri (Eds.), Bubble and Drop Interfaces, in “Progress in Colloid and Interface Science”, Vol. 2, Brill Publ., Leiden, 2011, ISBN 978 90 04 17495 5).
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