# Preface! ✨

It’s your favorite material science & nanotechnology enthusiast! Today, we will cover the mechanisms of hydrophobicity from Young’s Law with a bit of help from surface tension and mathematics!

The article below is fundamental to build the intuition for the equations used in Young’s Law!

## TL;DR → Young’s Law (Part 1)! 🔑

• The main concepts in the article are Young’s Law, Critical Surface Tension of both the solvent and the aerogel, Solvents (Pure & Aqueous), and the overall concentration of the solvents in the solution when it comes into contact with the aerogel!
• In an aerogel, the more of the (concentrated) solvent you add, it can actually decrease the contact angle and reduce hydrophobicity in the aerogel.
• Young’s LawThe contact angle of a liquid or vapor touching the surface of solid material and determines how wet an object can get. The angle is normally measured from the angle between the liquid and the solid.
• Young’s Law (Mechanism)If the contact angle is greater than 90 degrees (90°), the liquid/water in the aerogel will not be able to spread, and the aerogel is impervious/hydrophobic to water. If the contact angle is less than 90 degrees (90°), the liquid/water in the aerogel will spread, and the aerogel is hydrophilic to water, and the water will adsorb until the contact angle is equal to 0 degrees (0°).
• SolventA liquid/dissolving medium where a substance/solid (a.k.a. solute) will dissolve in the liquid to form a solution/homogenous mixture.
• Surface TensionThe strongest intermolecular forces between molecules of a liquid (water) at the surface only. Because the intermolecular forces are the strongest, the water molecules will minimize their surface area.

Authors Note:

This was an overview of the definitions of the fundamental concepts used when calculating the outcome of Young's Law! If this information still leaves you confused, I highly recommend going back to the article above and read it completely before coming back! 🙏🏽

Now that we have our overview and explanation, let’s go over the mathematical concepts in Young’s Law! 😄

# Young’s Equation Explained! 💡

## Review + Definition! 🔑

I will briefly go over what is Young’s Law and how it works before explaining the equation!

Young’s LawThe contact angle of a liquid or vapor touching the surface of solid material and determines how wet an object can get. The angle is normally measured from the angle between the liquid and the solid.

Young’s Law (Mechanism)If the contact angle is greater than 90 degrees (90°), the liquid/water in the aerogel will not be able to spread, and the aerogel is impervious/hydrophobic to water. If the contact angle is less than 90 degrees (90°), the liquid/water in the aerogel will spread, and the aerogel is hydrophilic to water, and the water will adsorb until the contact angle is equal to 0 degrees (0°).

But in some cases, the solvent used in the solution can change the outcome of adsorption in the aerogel, depending on the overall concentration of the solvent.

Concentration (in terms of chemistry) is how much of a substance (the solute, mostly a solid/liquid/chemical) is found in a mixture (solution). Concentration can also mean how much of the solute is in a solution, mostly a huge amount of the solute. This is mostly used for Aqueous Solutions.

Pure Solvents are just the solvent itself (methanol, ethanol, etc.)

Aqueous Solvents are when the solvent is water (The solute can be whatever, methanol, ethanol, etc.).

## Young’s Equation! 🔑

The amount of liquid adsorbed by the surface of the aerogel depends on the surface tension of the liquid that will be adsorbed/wet the surface of the aerogel.

Adsorption Particles/molecules of something will collectively add up on the surface of a material. In some way, you could say the particles are “resting” on the material.

From this, the equation for Young’s Law looks like this:

γ(sv) = Solid-Vapor Interface Energy.

γ(sl) = Solid-Liquid Interface Energy.

γ(lv) = Liquid-Vapor Interface Energy.

cos θ = The angle used to measure the contact angle (to see if the liquid is greater or less than 90 degrees (90°)).

## Young’s Law: Mechanisms! 🔑

There is one thing that can change the outcome of the adsorption/contact angle of the solid vapor interface. It’s called surface energy.

Surface Energy is the measurement of energy at the surface of a material and the interaction between molecules of the solid material/aerogel (it could be called excess energy since there aren’t any molecules above the molecules on the surface of the aerogel).

In a solid, the surface energies are strong because there is strong interaction (intermolecular forces) between the molecules. In a liquid, the surface energies are weaker because of the intermolecular forces between the molecules. Surface Energy is mostly only applied to solids.

If the surface energy of a solid is low, the contact angle will be greater than 90° (γ(sl) ≥ γ(sv)). The water droplet won’t be able to spread and adsorb on the surface of the aerogel.

The reason why the aerogel may have low surface energy is that the bonds at the surface of the aerogel are non-polar (hydrocarbons, alkanes, alkenes, etc.) Non-polar molecules also have weak forces/interaction with each other at the surface, due to Van der Waal Forces occurring in the non-polar bonds (where electronegativity is below 0.5).

If the surface energy of a solid is high, the contact angle will be less than 90° (γ(sv) ≥ γ(sl)). The water droplet will begin to spread and adsorb on the surface of the aerogel completely until the droplet flattens out and the angle is equal to 0°.

High surface energy is possible when the bonds between the surface of the aerogel are polar, covalent, ionic, or even metallic bonds (where electronegativity is equal to or greater than 0.5).

# Closing Thoughts! 💭

This covers up all of the foundational concepts in Young’s Law with the explanation of the mathematical formulas and some of the molecular interactions at the surface of the aerogel (also known as the surface energy)!

Summary:

• If the surface energy of a solid is low, the contact angle will be greater than 90° (γ(sl) ≥ γ(sv)). The water droplet won’t be able to spread and adsorb on the surface of the aerogel. WHY?
• The bonds at the surface of the aerogel are most likely non-polar bonds (electronegativity is below 0.5 with the presence of Van der Waal Forces).
• If the surface energy of a solid is high, the contact angle will be less than 90° (γ(sv) ≥ γ(sl)). The water droplet will begin to spread and adsorb on the surface of the aerogel completely until the droplet flattens out and the angle is equal to 0°. WHY?
• The bonds at the surface of the aerogel are most likely polar, covalent, ionic, coordinate covalent, hydrogen, or metallic bonds (electronegativity is equal to or greater than 0.5.)
• Surface Energy is the measurement of energy at the surface of a material and the interaction between molecules of the solid material/aerogel.

See you tomorrow for the applications of Young’s Law in solvents and further calculations to find the surface energy and surface tension! ✌🏽

# Vocabulary! 📓

Young’s LawThe contact angle of a liquid or vapor touching the surface of solid material and determines how wet an object can get. The angle is normally measured from the angle between the liquid and the solid.

Young’s Law (Mechanism)If the contact angle is greater than 90 degrees (90°), the liquid/water in the aerogel will not be able to spread, and the aerogel is impervious/hydrophobic to water. If the contact angle is less than 90 degrees (90°), the liquid/water in the aerogel will spread, and the aerogel is hydrophilic to water, and the water will adsorb until the contact angle is equal to 0 degrees (0°).

Surface EnergyThe measurement of energy at the surface of a material and the interaction between molecules of the solid material/aerogel.

SolventA liquid/dissolving medium where a substance/solid (a.k.a. solute) will dissolve in the liquid to form a solution/homogenous mixture.

Surface TensionThe strongest intermolecular forces between molecules of a liquid (water) at the surface only. Because the intermolecular forces are the strongest, the water molecules will minimize their surface area.

Pure SolventsIt’s just the solvent itself (methanol, ethanol, etc.)

Aqueous SolventsThe solvent is water (The solute can be whatever like salt, sugar, etc.).

Adsorption Particles/molecules of something will collectively add up on the surface of a material. In some way, you could say the particles are “resting” on the material.

Concentration (#1) (In terms of chemistry) How much of a substance (the solute, mostly a solid/liquid/chemical) is found in a mixture (solution).

Concentration (#2) → How much of the solute is in a solution, mostly a huge amount of the solute.

SoluteThe substance that will get dissolved in the solvent (the main liquid).