Charge change: How electric forces vary in colloids

September 12, 2019

Colloidal suspensions heterogenous mixtures of particles with diameters of about 2-500 nanometers, which are permanently suspended in a second phase, usually a liquid. Owing to the small particle size of the suspended material, a colloid does not separate into its characteristic components even if allowed to remain undisturbed, nor can the suspended material be separated through filtration. Colloids are distinguished from other types of mixtures by several important distinctive properties, one of which is the electrokinetic force in colloidal suspensions, also known as the "zeta potential."

To explore zeta potential, we must first understand what a "slipping surface" is. A slipping surface is an "electrical double layer" that forms on the surface of any object when it is exposed to a fluid. This double layer consists of one layer of charges that adhere to the surface of the object as a result of chemical interactions, and a second layer of opposite charges that are attracted to the first layer. Due to the attraction between these two layers of opposite "ions" or charges, an electric potential is created, and this is the zeta potential. The zeta potential occurs in double layers on the surface of particles suspended in colloids as well.

Prof Hiroyuki Ohshima of Tokyo University of Science has been a lifelong theoretical researcher of electrokinetic phenomena such as the movement of colloidal particles in an electric field and electrostatic interactions between colloidal particles. He has recently summarized some of the major findings in his field in a review published in the journal Advances in Colloid and Interface Science. He asserts the importance of zeta potential in colloidal surface chemistry. According to him, "the dispersion stability of colloidal particles, which is one of the most important issues in colloid surface chemistry, greatly depends on the zeta potential of the particles."

Zeta potential is calculated based on the electrophoretic mobility of the particles. Until now, the no-slip boundary condition of the fluid, which assumes that the fluid will have zero velocity relative to the boundary, has been applied when calculating the zeta potential. However, while this condition is applicable to particles with a hydrophilic ("water-loving") surface, it cannot be applied to particles with a hydrophobic ("water-shy") surface. In this case, the Navier boundary condition, which considers the relative velocity of the fluid, is applied.

In the Navier boundary condition, the effect of the hydrodynamic slip is characterized by the slipping length. When the surface is hydrophilic, the slipping length is considered to be zero, and it progressively increases with the increase in hydrophobicity of the surface, where the molecules of the particle surface weakly interact with the molecules in the surrounding phase so that liquid slip occurs. In accordance, an infinitely large slipping length theoretically corresponds with a completely hydrophobic surface. From this information, theoretical calculations show that electrophoretic mobility and sedimentation potential increase with increasing slipping length.

According to Prof Ohshima, what is more interesting is that if we accept the possibility of the presence of a slipping surface on a spherical solid colloidal particle, we can observe that the electrokinetic properties of this solid particle will be hydrodynamically similar to those of a liquid drop.

These findings highlight the importance of reconsidering how the electrokinetic properties of hydrophilic and hydrophobic surfaces vary and showcase how they affect the dynamics of colloidal suspensions. Prof Ohshima concludes, "We have constructed a general theory describing various electrokinetic phenomena of particles with a sliding surface. By applying this theory, we could expect a more accurate evaluation of zeta potential and colloidal particle dispersion stability in the future."
-end-


Tokyo University of Science

Related Hydrophobic Articles from Brightsurf:

The applications of liquid crystals have been extended to drug encapsulation
Widely used in the manufacture of LCD screens and, more recently, phosphorescent sensors, liquid crystals may also have an important application in biomedicine.

Revealing the identity of the last unknown protein of autophagy
Japanese scientists discovered that Atg9, one of the proteins that function to mediate autophagy, has phospholipid-translocation activity (the lipid scramblase activity) between the two layers of the lipid bilayer?and elucidated that the protein's activity brings about autophagosome membrane expansion.

Probing water for an electrifying cause
An experiment, elegant in its simplicity, helps explain why water becomes electrified when it touches hydrophobic surfaces.

A chemist from RUDN developed a new type of one-molecule thick water-repellent film
A chemist from RUDN University together with colleagues created a new type of two-dimensional nanofilm from an organic material called calixarene.

Research shows potential to improve paints, coatings
New research from Binghamton University, State University of New York could lead to more environmentally friendly paints and coatings.

Novel method of heat conduction could be a game changer for server farms and aircraft
'We are hopeful that the one-way heat transfer of our bridging-droplet diode will enable the smart thermal management of electronics, aircraft, and spacecraft,' said Boreyko.

Pesticides can protect crops from hydrophobic pollutants
Researchers have revealed that commercial pesticides can be applied to crops in the Cucurbitaceae family to decrease their accumulation of hydrophobic pollutants, thereby improving crop safety.

Wrapping up hydrophobic hydration
Studied in detail, the embedding of hydrophobic molecules in water looks quite different than previously assumed.

KIST ensures stability of desalination process with magnesium
A Korean research team found a method to inhibit the fouling of membranes, which are used in the desalination process that removes salt and dissolved substances from seawater to obtain drinking, domestic, and industrial water.

Superhydrophobic magnetic sponge to help purify water from oil products
TPU jointly with the University of Lille developed a new material capable of purifying water effectively from oil products.

Read More: Hydrophobic News and Hydrophobic Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.