57 Equation 1: Calculation of the Solid Surface Zeta Potential
While certain applications benefi t from the desirable adsorption of substances onto surfaces, others necessitate its prevention. Consequently, the evaluation of the dynamic streaming potential proves valuable across numerous industries for examining the adsorption kinetics of substances onto various solid surfaces.
In Equation 1, εo is the vacuum permittivity, and εr, η and KB are the dielectric constant, the electrolyte viscosity and conductivity
of the electrolyte solution.
The zeta potential of the sample’s surface depends on several factors.
It is strongly pH dependent. Measurements at different pH values give valuable information on the composition of the sample’s surface, i.e. the presence of acidic or basic functional groups. The pH dependence of the zeta potential is therefore frequently recorded to understand the nature of surface functional groups and to determine the isoelectric point (pH of the aqueous solution where the zeta potential reverses its sign).
The concentration of ions (ionic strength) in the aqueous phase also has an infl uence on the zeta potential. That means different electrolyte solutions return different zeta potential values. If more ions are available, the initial surface charge can be compensated more effectively, which results in a minor zeta potential.
When exchanging acid or base for pH adjustment by a different chemical compound (e.g. surfactant, polyelectrolyte, polypeptide, protein), the interaction of such a compound with the material’s surface may be investigated.
Dynamic streaming potential
The measurements of the dynamic streaming potential is an extension of the classic surface zeta potential analysis, monitoring the interaction between solutes and solid surfaces in real time. When assessing the dynamic streaming potential, the focus lies on temporal changes rather than static processes. This involves examining variations, such as the accumulation of dissolved substances, as well as the effects of adsorption and desorption on solid surfaces, over time. The principle is quite easy to understand - the adsorbate is added to the electrolyte solution and the changes of the streaming potential are recorded over time.
If the adsorption of substances on a surface is reversible, the process of desorption can be achieved by exchanging liquids, for example. Adsorbed substances desorb from the surface and the initial state of the solid surface is successively approached.
In the assessment of the dynamic streaming potential, the primary emphasis lies on monitoring changes in charge throughout the whole measurement. It is necessary to understand that this emphasis does not imply a change in the sign of the zeta potential, but rather the change of the net charge of the solid surface. Dynamic processes at the solid- water interface can be accomplished by adjusting different factors such as the liquid composition of the electrolyte solution, allowing for fl exibility in achieving the desired changes in charge.
Applications
The phenomena of adsorption and desorption play essential roles in various aspects of daily life, spanning activities such as textile laundry, hair washing, and applications within the medical and life sciences.
Figure 2: Dynamic streaming potential results for hair treated with shampoo followed by conditioner.
At fi rst, the untreated hair exhibits a negative streaming potential (corresponding to a negative zeta potential). The application of shampoo lowers the magnitude of the zeta potential. Subsequent rinsing of the shampoo returns the streaming potential to its initial values, suggesting the complete desorption of shampoo components. Upon the adsorption of the conditioner, the streaming potential reverses its sign, indicating the conditioner’s adsorption. After rinsing the conditioner, the streaming potential reduces in magnitude but remains positive. This suggests that a part of the conditioner becomes irreversibly bound to the hair surface, and some excess amount is removed during the rinsing process.
Adsorption processes occur in numerous applications, which are impossible to count in this article. We therefore focus on three examples and discuss selected results in detail.
Cosmetics
Hair care is a versatile area as there are different hair types, different trends and other factors that infl uence hair health, all of which require specifi c hair care formulations. Developing such versatile formulations necessitates the understanding of the effect of haircare products on the hair texture directly. Investigating surface zeta potential at the interface between the hair fi bre and an aqueous emulsion of the hair care product helps us understand how formulation components of the product interact with the hair surface and allows for precise adjustments of the formulation composition.
By measuring the dynamic streaming potential, when applying a haircare formulation on human hair, the cosmetic chemist gets information about the interaction with the hair structure during application. Figure 2 shows a typical hair washing cycle. Here, part of a tress of Caucasian hair was fi rst rinsed with an aqueous buffer solution and then shampooed. The shampoo was washed out before a conditioner was applied.
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