Have you ever wondered what connects the world? You might think of gravity, that invisible force that pulls everything towards the center of the earth. Or you might think of electromagnetism, the kind of force that allows magnets to stick to your fridge and allows electric current to pass through wires. But did you know what, according to a recent Nature Nanotechnology article,Scientists have recently discovered a new force of nature, which is hidden in a drop of water? This force is called the electrolytic force, and it determines the amazing behavior of tiny particles in water and other liquids. These particles are called colloids, and they are so small that they can only be seen with a microscope. They are everywhere, from milk and paint to blood and clouds. Colloids usually have an electric charge, either positive or negative, and they affect each other and the liquid in which they are located.
You might expect particles of the same charge to repel each other, as if two magnets of the same pole were facing each other. This is the case in classical physics, and what happens in most liquids. But in the water, something strange happens: negatively charged particles attract each other, forming clusters, while positively charged particles remain separated. This is contrary to your expectation and also violates the principle of charge reversal symmetry, which says that the sign of the charge should not affect the interaction.
So what causes the mysterious attraction between particles of the same charge in water? The answer lies in the structure of water molecules and their response to the electric field. The water molecule is shaped like a V with two hydrogen atoms attached to an oxygen atom. The oxygen atom is more electronegative, meaning that it pulls the shared electrons closer to itself, causing the molecule to have a slight negative charge at one end and a slight positive charge at the other end. This makes water a polar molecule, meaning it has a dipole moment, or a separation of charges.
Water molecules are constantly moving and rotating in liquid water, but they tend to align with the electric field, either from an external source or from charged particles. When a water molecule approaches an electrically charged particle, it reorients itself so that its opposite charge is facing the particle, forming a net-charged layer of water molecules surrounding the particle. This layer is called an electrical bilayer, and it affects the interaction between the particles.
The electrical bilayer can be divided into two regions: the inner zone, where the surface of the water molecules and particles are tightly combined; The outer area, where water molecules are loosely connected to the surface of the particles and can be exchanged with the volume of water. The potential difference between the surface of a particle and the volume of water is called Zeta potential, and it is a measure of the effective charge of a particle.
Zeta potential depends on the nature of the particles, water, and ions dissolved in the water. The ions in the water shield the particles' electric field, reducing the repulsive force between the Zeta potential and the particles of the same charge. That's why adding salt to the water makes colloids more stable and prevents them from clumping together. However, this effect alone cannot explain the force of attraction between particles of the same charge in water, as it should apply equally to both charge symbols.
The key to understanding the force of attraction is to look at the inner regions of the electrical bilayer, where the water molecules and the surface of the particles are tightly bonded. Here, instead of spinning freely, the water molecules are constrained by the geometry and chemistry of the surface. Depending on the type of surface, water molecules may take different orientations and alignments, resulting in a local order or disorder of the water structure. This order or disorder affects the free energy of the system, which is a measure of how thermodynamically superior a state is.
The free energy of a system depends on two factors: entropy, which is a measure of how disordered a system is; and enthalpy, which is a measure of how much heat is released or absorbed by a system. Generally, systems tend to minimize free energy, by maximizing entropy and minimizing enthalpy. However, sometimes these two factors compete with each other, creating a trade-off between order and disorder.
This is what happens in the inner area of the electrical bilayer. The water molecules on the surface have a lower entropy than the water molecules in the volume of water because they are more ordered. However, they also have lower enthalpy as they form stronger bonds with the surface and with each other. The equilibrium between these two factors determines the free energy of the system and thus the interaction between the particles.
Researchers who discovered the force of electrosolity have found that the equilibrium between entropy and enthalpy depends on the charge sign of the particle and solvent. In water, the water molecules on the surface of the negatively charged particles have a lower enthalpy than the water molecules on the surface of the positively charged particles because the negative charge strengthens the hydrogen bonds between the water molecules. This means that the free energy of the system is lower for the negatively charged particles than for the positively charged particles, causing a difference in the interactions.
When two negatively charged particles are close to each other in the water, the water molecules in the inner region of the electric bilayer are squeezed out, increasing the entropy of the system and decreasing the enthalpy of the system. The decrease in enthalpy is greater than the decrease in entropy, resulting in a net decrease in free energy and attraction between the particles. This force is called the electrosolic force, and it overcomes electrostatic repulsion and van der Waals attraction, which are the other two components of the total interaction.
The opposite happens when two positively charged particles are close to each other in water. The water molecules in the internal region of the electric bilayer are squeezed out, increasing the entropy of the system and decreasing the enthalpy of the system. The decrease in enthalpy is less than the decrease in entropy, resulting in a net increase in free energy and repulsion between the particles. This force, coupled with electrostatic repulsion and van der Waals attraction, makes the total interaction even more repulsive.
The researchers also found that the sign of the electrolytic force can be reversed by changing the solvent. They tested two types of alcohol, ethanol and isopropyl alcohol, which have different molecular structures than water. Ethanol and isopropanol have the same hydroxyl group (OH) as water, but they also have a hydrocarbon chain (CH3) which is more hydrophobic. This makes them less polar than water, affecting their orientation on the surface of charged particles.
In water, the water molecules on the surface tend to point their oxygen atoms towards the volume, creating a negative interfacial potential. In ethanol and isopropanol, the molecules on the surface tend to point their hydrocarbon chains towards the surface, resulting in a positive interfacial potential. This means that the sign of the electrolytic force is opposite to that in water: positively charged particles attract each other to form clusters, while negatively charged particles repel each other and remain separated.
The researchers demonstrated this phenomenon with colloidal particles with different surface chemistries and measured their interactions in water and alcohol. They use a technique called optical tweezers, which uses a focused laser beam to capture and manipulate individual particles. They measured the force between two particles as a function of their distance and compared it to a theoretical model that took into account the electrosolic force. They found a good agreement between experiment and theory, confirming their findings.
The electrolytic force is a new force of nature that has been neglected for decades, but it has important implications for many fields of science and engineering. It can explain the formation of ordered structures and patterns in colloidal systems, such as crystals, gels, and glasses. It can also affect the behavior of biomolecules, such as proteins and nucleic acids, which often have an electrical charge and interact with water and other solvents. Electrolytic forces can provide a new way to control and manipulate substances in solution by regulating the charge, solvent, and surface chemistry of particles. It can also inspire the design of new materials and devices that take advantage of the unique properties of electrosolic forces.
The force of electrolysis is a fascinating example of how complex and rich the interaction between matter and water can be, and also how much more we still have to learn. The next time you see a drop of water, remember that there is a hidden force at work that shapes the world at the nanoscale.
wang, s., walker-gibbons, r., watkins, b. et al. a charge-dependent long-ranged force drives tailored assembly of matter in solution. nat. nanotechnol. (2024).**10,000 Fans Incentive Plan