Liquid interfaces are omnipresent in our daily life and essential in many modern technologies. The fundamental and applied research is highly interdisciplinary and aims at producing knowledge required to understand existing technologies and improving then. This requires a large amount of data produced by reliable experimental methods and analyzed by experts using corresponding mechanisms. Often, data are published but they are either not directly available or their quality cannot be verified. This is in general the origin for the creation of a platform called FAIR – Findable, Accessible, Interoperable, Reusable. Unfortunately, such a data base does not exist yet for the science field of interfacial research and technology.
The present publication aims at defining datasets such that they are most reliable and reusable by experts now and by later generations. A team of European experts proposes a framework for
1. discusses the relevant liquid phases to be studied;
2. defines surface-active species of interest;
3. describes the interfacial properties required for all kinds of applications;
4. the selection of respective methods and their correct use.
The expert team proposes the structure and requirements of an “Interface Science Data Repository” as a common platform on which data are provided for anybody’s use. The repository is proposed in such a way that it can also be used in future by Artificial Intelligence as machine-operable database.
1. The liquid-fluid interface
Both liquid-gas (water-air) and liquid-liquid (water-oil) interfaces are discussed. It is emphasized that the purity of all involved liquids is of enormous importance. It can be proved via measurements of the surface or interfacial tension. For a deposition of experimental data, the measured tension values have to be provided and the method has to be mentioned. The measured values should be identical with (very close to) literature data at the given temperature. When purification processes have been applied, also these have to be mentioned as metadata.
2. The adsorbing species
The repository is organized for all kinds of adsorbing species, such as classical surfactants and polymers, but also biosurfactants, proteins, lipids and nanoparticles. Here again the accurate definition of the surface-active compound and its purity are essential. Also the protocols for their chemical synthesis or separation from natural resources, and the way of analysis of their purity are important facts to be provided for the data base.
3. The interfacial properties
Essential properties of liquid interfacial layers are considered in this repository, in the first place the dynamic surface/interfacial tensions at different concentrations, temperatures and salts added to the aqueous solutions. Moreover, the dilational visco-elasticity of the interfacial layers is in the focus of this data collection. In addition to the interfacial elasticity and viscosity as a function of concentration, also the frequency of oscillations has to be considered. To complete the set of data, all solution conditions and the details of the applied experimental method have to be reported. For synthetic polymers, biopolymers and in particular proteins, also the shear interfacial visco-elasticity data are of great importance.
Besides equations state and adsorption isotherms of adsorbing species, also the thickness of the interfacial layers and their electric properties are significant properties. Moreover, the behavior os systems like foams and emulsions can be the target of provided data sets.
In addition to all experimental details, including the peculiarities of the applied method, also the details of the applied theoretical basis have to be provided in order to clearly describe how the individual interfacial parameters have been obtained.
4. The experimental methods
The professional use of experimental tools is another extremely important ingredient for the production of a basis of reliable interfacial data for the use by authors who produced them and for all those experts who will use them in future. Existing sets of data, supposed they have been measured with well defined substances at well defined interfaces, are a great treasure for our scientific community. Reliably measured data can only be obtained when the selected methods are practiced in a correct way. The protocols must be monitored and with increasing experiences even adapted to new standards when required.
The drop/bubble profile analysis tensiometry is obviously the working horse in interfacial science. It allows to measure dynamic surface/interfacial tensions, to determine equilibrium parameters as a function of concentration (isotherms) as well as the dilational interfacial visco-elasticity as frequency dependence as well as isotherm. Moreover, contact angles can be measured in the most accurate way. Our PAT-1M is one of the most efficient instruments of this type, however, only its correct use guarantees a high quality of the measured data. When used with an exactly determined magnification, and a control of all other required settings, including the use of droplets/bubble in the optimum range of the Bond number, the reliability of the measured values is very high.
The use of the bubble and drop pressure tensiometry, instruments like our BPA-2S or the DPA-1M, respectively, provides dynamic tension data in a maximum available time range. It is, however, extremely important for these measurements to know how the measured pressure values are transferred into dynamic tension data. Moreover, the effective surface age, available only when the bubble deadtimes are determined correctly, is the parameter needed to construct a useful dynamic surface tension curve. It is important here also to note that this effective surface age is often not provided but instead the surface tension as a function of bubble frequency or bubble time, which is a rather complex dependency that does not allow to compare the data of one instrument with those of another. For reconsideration, all instrumental parameters are required, such as capillary diameter, length and material, and its immersion depth into the liquid, inner gas volume of the instrument, pressure sensor accuracy.
The shear rheology of interfacial layers, formed essentially by polymers or proteins, is a possibility to appreciate the formation of structures at liquid interfaces. For this purpose, our interfacial torsion wire shear rheometer ISR-1. It is a unique experimental tool of very high sensitivity, and it was shown that it can be successfully complemented by measurements with the Anton Paar rheometer MCR301 equipped with the ISR module.
The publication is based on an immense pool of well selected publications (377 references) of pioneering work or serving as suitable examples for producing, presenting and analyzing reliable experimental data for reuse by the scientific community.
Reinhard Miller, TU Darmstadt, Germany