MANOJ KUMARIII SemesterME

Seminar on

HYDROPHOBIC AND HYDROPHILIC

• Some of the common natural Hydrophobic materials are waxes, oil and fats.

Hydrophobicity comes also from the greek word Hydro(water) and Phobicity (fear) it refers to the physical property of a material that repels a mass of water.

WHAT IS MEANT HYDROPHOBICITY ?

The evaluation of hydrophobicity is made through water contact angle measurements.

A water droplet would be spherical so the water contact angle will be significantly high.

• At the molecular level, the hydrophobic effect is important in driving protein folding formation of lipid bilayers and micelles, insertion of membrane proteins into the nonpolar lipid environment and protein-small molecule interactions. Substances for which this effect is observed are known as hydrophobes.

HYDROPHOBIC EFFECT

The hydrophobic effect represents the tendency of water to exclude non-polar molecules. The effect originates from the disruption of highly dynamic hydrogen bonds between molecules of liquid water.

Continued….

SUPER HYDROPHOBIC COATING

Super hydrophobic technology makes water bounce, it stops it, rolls it off the surface.

The process of coating the surface of a material with hydrophobic property material in order to avoid sticking of liquids on that surface.

This is absolutely unique way of coating unlike conventional which shrink continuously during drying to produce low porosity films.

Young's equation is used to describe the interactions between the forces of cohesion and adhesion and measure what is

referred to as surface energy.

www.ramehart.com/contactangle.htm

• A drop with a contact angle over 90° is hydrophobic. 

• This condition is exemplified by poor wetting, poor adhesiveness and the solid surface free energy is low.

Continued..

Hydrophobic interactions

• Hydrophobic interactions describe the relations between water and hydrophobes (low water-soluble molecules).

• Hydrophobes are nonpolar molecules and usually have a long chain of carbons that do not interact with water molecules.

• The mixing of fat and water is a good example of this particular interaction.

• The common misconception is that water and fat doesn’t mix because the Van der Waals

• forces that are acting upon both water and fat molecules are too weak.

Causes of Hydrophobic Interactions

• American chemist Walter Kauzmann discovered that nonpolar substances like fat molecules tend to clump up together rather that distributing itself in a water medium, because this allow the fat molecules to have minimal contact with water.

Hydrophobic interactions - ChemWiki

Formation of Hydrophobic Interactions

• The mixing hydrophobes and water molecules is not spontaneous; however, hydrophobic interactions between hydrophobes are spontaneous.

• When hydropobes come together and interact with each other, enthalpy increases ( is positive) because some of hydrogen bonds that form the clathrate cage will be broken.

• Tearing down a portion of the clathrate cage will cause the entropy to increase ( is positive), since forming it decreases the entropy.

According to the formula: ΔG= ΔH-TΔS

ΔH=Small positive value ΔS=Large positive value

Result :ΔG= Negative

A negative ΔG indicates that hydrophobic interactions are spontaneous

What does the strength of Hydrophobic Interactions depend on?

• Temperature

• Number of carbons on the hydrophobes

• The shape of the hydrophobes

In order of Effectiveness:

APPLICATIONS

• A primary purpose of hydrophobic coatings such as polytetrafluoroethylene(PTFE) or polyxylylene is to act as a barrier against water commonly seen in automobiles

• Used in fabrication on metallic nano rod to prevent icing.

• Its is widely used in aerospace industry for providing anti-icing coating on the surface of the aeroplane .

• Hydrophobic self cleaning glasses are installed in traffic sensor control unit.

• We induce hydrophobic recovery after plasma treatment, a physical contact treatment (PCT) .

Hydrophilicity, also comes from the Greek word Hydro (water) and Philicity (friendship) it refers to a physical property of a material that can transiently bond with water (H2O) through hydrogen bonding. Furthermore it allows the liquid to enter the pores of a material and totally wet it.

WHAT IS MEANT HYDROPHILICITY ?

• Almost all natural materials are hydrophilic in nature.

The evaluation of hydrophilicity is made through water contact angle measurements. A water droplet would occupy as long surface of the hydrophilic material as possible. So the water contact angle will be significantly low.

• This is thermodynamically favorable, and makes these molecules soluble not only in water but also in other polar solvents. There are hydrophilic and hydrophobic parts of the cell membrane.

• A hydrophilic molecule or portion of a molecule is one that is typically charge-polarized and capable of hydrogen bonding, enabling it to dissolve more readily in water than in oil or other hydrophobic solvents.

Hydrophilic coatings wet the surface very easily, and maintain the wetness for longer time. Thus, using hydrophilic coatings eliminates the need for additional lubricants.

HYDROPHILIC COATING

Continued…

Here ,also we use same figure of young’s equation to explain hydrophilicity

A drop with a small contact angle is hydrophilic. 

This condition reflects better wetting, better adhesiveness, and higher surface energy.

Hydrophilic Interactions

Interactions between water and other molecules such that the other molecules are attracted to water are called hydrophilic interactions.

Molecules that have charged parts to them are attracted to the charges within the water molecule.

This is an important reason why water is such a good solvent

Continued….

Glucose molecules have polar hydroxyl(OH) groups in them and these attract the water to them. When sugar is in a crystal the molecules are attracted to the water and go into solution.

Once in solution the molecules stay in solution at least in part because they become surrounded by water molecules. This layer of water molecules surrounding another molecule is called a hydration shell.

What does the strength of Hydrophilic Interactions depend on?

• Inhibitory effect mainly depended on their concentration and to a lesser extent on the ion charge and hydrated ion radii. 

• Stepwise regression analysis proved that the strength of interaction depends on the polarity parameters of amino acids and is independent of their chemical structure.

APPLICATIONS

• Spray Painting

• Film coating

• Bio-Medical

Wenzel and Cassie States of Wetting

Here are two natural parameter:

A first natural and simple effect arising from the presence of microstructures is an increase of the surface area of the material.

r =1+2πbh/p2r =1+2πbh/p2

we can define two levels; namely, the top of the posts and the ground on which they sit.

A second natural parameter is the ratio ϕS between the top and bottom surface areas, which is also the post density.

ϕS = πb2/p2ϕS = πb2/p2

When depositing a water drop on a such solid .It can adopt at least two type of configuration-

Quéré D , Reyssat M Phil. Trans. R. Soc. A 2008;366:1539-1556

conforming to the solid surface (Wenzel situation)

Sitting on the top of the posts((Cassie or fakir state)

www.me.umn.edu/~lixxx099/EFRI_CAES/Research/texturing.htm

The Second and simplest picture is the so-called Wenzel state (figure the drop just follows the structures on the material (Wenzel 1936).

This will affect both the wetting (the value of the equilibrium angle, that is the one which minimizes the surface energy of the drop) and the adhesion (characterized here by the value of the contact angle hysteresis).

cosθ*=r cosθcosθ*=r cosθ

Wenzel state

Cassie state-Fakir state

Third fig. implies the creation of a liquid–vapour interface below the drop, on a fraction of surface of the order of 1−ϕS, considering these interfaces as flat (the drop is large compared with the pillars, implying a negligible curvature of the liquid–vapour interface at the scale of pillars).

The cosine of the apparent contact angle is then an average between the cosine of the Young angle and the cosine of 180°, the angle of a water drop on air, where the weights for the average are, respectively, ϕS and 1−ϕS.

cosθ*= -1 + Φs(1+cosθ)cosθ*= -1 + Φs(1+cosθ)

(Cassie & Baxter 1944)

THANKS