Surface free energy in powder wetting
Contact angle and surface free energy for optimizing dispersibility
Mixtures of powdery substances and liquids are found in a number of technical processes and products. The mechanical effort required for stirring and the tendency to form lumps and sediment depend on the surface free energy of the powder and the interactions with the liquid. Measurements of the wettability and the surface free energy based on the contact angle provide information on the behavior of the powder in a liquid, and contribute towards optimizing the product and process for dispersions.
Lumps of oily cocoa powder, which do not mix well with water, often return to the surface completely un-wetted after immersion. The correlation between wettability and mixability is therefore clear simply from this everyday experience. In technical processes, powders which are to be dispersed are therefore frequently pre-treated to improve wettability by the liquid and therefore also the mixability. One example is the production of compound plastics, in which the mineral powder is often given a hydrophobic coating to facilitate mixing with the likewise hydrophobic polymer melt. On the other hand, hydrophilic coatings, for example of powdered spices and medicines, ensure better wettability with water in an aqueous environment.
Our Force Tensiometer – K100 measures the wettability of a powder based on the Washburn method, and in this way characterizes untreated and treated powders. When using this method, the rate of increase and the increase in mass of different liquids in a powder bulk in an immersed tube are measured. The better the powder wetting, the faster the liquid rises.
Alternatively, our optical drop shape analysis measuring instruments measure the contact angle as a measure of the wetting based on drops on a powder bed. The variation of the contact angle with respect to time provides additional information for powders containing surfactants. It shows how quickly surfactant molecules in the powder reach the liquid/solid interface and improve wetting at this point. This information helps to improve drug recipes, for example, by making active ingredients in powder form bioavailable more quickly.
To improve wettability and dispersibility, surfactants are frequently dissolved in the liquid to be mixed in order to reduce its surface tension and thus improve wetting. As well as surface tension measurements with our tensiometers, contact angle measurements between surfactant solutions and powders are also helpful for selecting and dosing surfactants.
Measurements with several liquids on a powder enable the surface free energy of the powder to be calculated. These together with measurements of the surface tension of the dispersing liquid enable the wetting enthalpy to be determined. The larger this value, the more beneficial the contact between particle surface and liquid, and the better the dispersibility. The results of the measurements enable powder and liquid phase to be optimized for the mixture independently of one another.
Flotation, as a method of separating solids, is based on the different wettability of different dispersed substances. For example, it is used in ore preparation and paper recycling for separating colored pigments ("de-inking"). The contact angle and the surface free energy of the individual substances in the powder mixture have a significant effect on the separating action. Surfactants, as so-called collecting agents, are added in order to enhance the separation process. These make the surface of the substance to be separated hydrophobic and thus improve separation in the aqueous suspension. The contact angle between powder and surfactant solution plays an important role in the investigation of this process.
Our Force Tensiometer – K100 features a method for measuring the sedimentation speed of a dispersion. This enables the results obtained from interfacial chemical measurements to be correlated with the actual stability behavior of a dispersion.
AR224: Dispersability predictions - Some practical examples
The free surface energy of differently coated carbon black particles and different plastics is measured. The degree of mixability in the melt is predicted in each case based on calculated adsorption enthalpies and confirmed by stirring tests.
AR213: Two-Component Surface Energy Characterization of Wettability and Dispersability
Polar and dispersive fractions of the surface free energy or surface tension of the phases are determined on different powder/liquid systems. The article shows how the results can be used for predicting the mixability in each case.
TN302: Wettability studies for porous solids including powders and fibrous materials
The procedure for measuring the contact angle for fibers and powders using the Washburn method is presented. Among other things, the example measurements show results for titanium dioxide pigments and microcrystalline cellulose for the pharmaceutical industry.