Microalgae as Soft Permeable Particles

Abstract

The colloidal stability of non-motile algal cells in water drives their distribution in space. An accurate description of the interfacial properties of microalgae is therefore critical to understand how microalgae concentrations can change in their biotope or during harvesting processes. Here, we probe the surface charges of three unicellular algae�Chlorella vulgaris, Nannochloropsis oculata, and Tetraselmis suecica�through their electrophoretic mobility. Ohshima's soft particle theory describes the electrokinetic properties of particles covered by a permeable polyelectrolyte layer, a usual case for biological particles. The results appear to fit the predictions of Ohshima's theory, proving that all three microalgae behave electrokinetically as soft particles. This allowed us to estimate two characteristic parameters of the polyelectrolyte external layer of microalgae: the volume charge density and the hydrodynamic penetration length. Results were compared with transmission electron microscopy observations of the algal cells' surfaces, and in particular of their extracellular polymeric layer, which was identified with the permeable shell evidenced by electrophoretic measurements. Noticeably, the algal surface potentials estimated from electrophoretic mobility using the soft particle theory are less negative than the apparent zeta potentials. This finding indicates that electrostatics are expected to play a minor role in phenomena of environmental and industrial importance, such as microalgae aggregation or adhesion.

Investigation of the Electrokinetic Properties of HIV-Based Virus-Like Particles

“…Accordingly, differences between bVLP and eVLP could not be detected, and similar adsorption behavior can be expected during chromatographic purification. In addition, the values for both parameters are comparable with values reported for other biological particles in the literature. − Sonohara et al and de Kerchove and Elimelech reported a hydrodynamic penetration length of approximately 0.7 nm and a number density of charges ZN of about −145 mmol l –1 for E. coli cells.…”

“…A well-known and commonly used model to describe the electrokinetic phenomena for monodispersed, nonconducting particles is based on the Helmholtz–Smoluchowski equation − μ = ζ ε 0 ε r η …”

“…where μ is the electrophoretic mobility, ζ is the ζ-potential, η is the dynamic viscosity of the dispersion medium, ε 0 is the vacuum permittivity, and ε r is the dimensionless relative permittivity of the medium. This model refers to a rigid, ion-impenetrable particle with all charges located on the surface (hard particle). ,− Additionally, the electrical double layer has to be much thinner than the particle radius b in order to consider a locally planar particle surface. , The electrical double-layer thickness can be described by the Debye length κ – , 1 , and the validity for eq can be written as κ b ≫ 1. , κ is the Debye–Hückel parameter and is defined for a symmetrical z – z electrolyte as 16 κ = ( 2 z 2 n e 2 ε 0 ε normalr normalk normalB T ) 1 / 2 with temperature T , Boltzman’s constant k B , ionic valence of electrolyte z , and elementary electric charge e . Number density of electrolyte n can be described as the product of the Avogadro constant N A and the electrolyte concentration for symmetrical electrolyte n = N A · c …”

“…This special particle setup has an impact on the electrokinetic behavior. Typically, the electrophoretic mobility μ of a hard particle tends to zero at high ionic strength, whereas a soft particle approaches a nonzero limiting value μ ∞ . − …”

“…Here, ZN means the number density of fixed charges in the polyelectrolyte layer, λ –1 the softness parameter or hydrodynamic penetration length of the polyelectrolyte layer, f ( d a ) a function of core radius a and thickness of the polyelectrolyte layer d , Ψ 0 the surface potential, and Ψ D the Donnan potential. − ,, Ψ D represents the potential deep inside the polyelectrolyte layer and Ψ 0 at the boundary between polyelectrolyte layer and dispersion medium. , These parameters are sketched in Figure b, in which a characteristic change of the potential across the polyelectrolyte layer with respect to the distance to the particle core is shown. Additionally, Ohshima introduced a Debye–Hückel parameter of the polyelectrolyte layer κ m , as shown in eq .…”