1. Intermolecular forces week 3 HYDROGEN BONDING, HYDROPHOBIC INTERACTION, SURFACE TENSION

2. A hydrogen bond is the electrostatic attractive interaction between polar molecules, in which hydrogen (H) is bound to a highly electronegative atom, such as nitrogen (N), oxygen (O) or fluorine (F). The name hydrogen bond is something of a misnomer, as it is not a true bond but a particularly strong dipole-dipole attraction, and should not be confused with a covalent bond.

3. A hydrogen bond is the electrostatic attraction between two polar groups that occurs when a hydrogen (H) atom covalently bound to a highly electronegative atom such as nitrogen (N), oxygen (O), or fluorine (F) experiences the electrostatic field of another highly electronegative atom nearby.

5. Intermolecular hydrogen bonding is responsible for the high boiling point of water (100 °C) compared to the other group 16 hydrides that have much weaker hydrogen bonds. Intermolecular hydrogen bonding is partly responsible for the secondary and tertiary structures of proteins and nucleic acids. It also plays an important role in the structure of polymers, both synthetic and natural.

11. 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. However, this is not the case. The behavior of a fat droplet in water has more to do with the enthalpy and entropy of the reaction than its intermolecular forces.
The tendency of nonpolar molecules in a polar solvent (usually water) to interact with one another is called the hydrophobic effect. The interactions between the nonpolar molecules are called hydrophobic interactions.

12. Hydrophobic interactions are more correctly called hydrophobic exclusions.

13. SUPER HYDROPHOBIC COATING
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.
Super hydrophobic technology makes water bounce, it stops it, rolls it off the surface.

14. 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) .

18. Surface tension Surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force.

19. Effects of surface tension
Water beading on a lea
Water dripping from a tap
Water striders stay atop the liquid because of surface tension

20. Surface tension Surface tension is measured as the energy required to increase the surface area of a liquid by a unit of area. The surface tension of a liquid results from an imbalance of intermolecular attractive forces, the cohesive forces between molecules: A molecule in the bulk liquid experiences cohesive forces with other molecules in all directions. A molecule at the surface of a liquid experiences only net inward cohesive forces. A microscopic view of water illustrates the difference between molecules at the surface of a liquid and water molecules within a liquid.

21. Physical units
Surface tension, usually represented by
the symbol γ, is measured in force per unit length.
Its SI unit is newton per meter.
But the cgs unit of dyne per centimeter is also used.

22. Data table Liquid Temperature (°C) Surface tension, γ Acetic acid 20
Surface tension of various liquids in dyn/cm against air[33]Mixture compositions denoted "%" are by massdyn/cm is equivalent to the SI units of mN/m (millinewton per meter)
Liquid
Temperature (°C)
Surface tension, γ
Acetic acid 20
27.60
Acetic acid (40.1%) + Water 30
40.68
Acetic acid (10.0%) + Water
54.56
Acetone
23.70
Diethyl ether
17.00
Ethanol
22.27
Ethanol (40%) + Water 25
29.63
Ethanol (11.1%) + Water
46.03
Glycerol
63.00
n-Hexane
18.40
Hydrochloric acid 17.7 M aqueous solution
65.95
Isopropanol
21.70
Liquid helium II
−273
[0.37
Liquid nitrogen
−196
8.85
Mercury 15
487.00
Methanol
22.60
n-Octane
21.80

23. Interface is the boundary between two or more phases
exist together
The properties of the molecules forming the interface are
different from those in the bulk that these molecules are
forming an interfacial phase.
Several types of interface can exist depending on whether
the two adjacent phases are in solid, liquid or gaseous state.
Important of Interfacial phenomena in pharmacy:
Adsorption of drugs onto solid adjuncts in dosage forms
Penetration of molecules through biological membranes
Emulsion formation and stability
The dispersion of insoluble particles in liquid media to form
suspensions.

24. LIQUID INTERFACES Surface and Interfacial Tensions
In the liquid state, the cohesive forces between
adjacent molecules are well developed.
For the molecules in the bulk of a liquid
They are surrounded in all directions by other molecules for which they have an equal attraction.
For the molecules at the surface (at the liquid/air interface)
Only attractive cohesive forces with other liquid molecules
which are situated below and adjacent to them.
They can develop adhesive forces of attraction with the
molecules of the other phase in the interface
The net effect is that the molecules at the surface of the
liquid experience an inward force towards the bulk of the
liquid and pull the molecules and contract the surface with
a force F .

25. To keep the equilibrium, an equal force must be applied to
oppose the inward tension in the surface.
Thus SURFACE TENSION [γ ] is the force per unit length that must be applied parallel to the surface so as to counterbalance the net inward pull and has the units of N/m or dyne/cm.
INTERFACIAL TENSION is the force per unit length existing at the interface between two immiscible liquid phases and has the units of dyne/cm.
Invariably, interfacial tensions are less than surface tensions because an adhesive forces, between the two liquid phases forming the interface are greater than when a liquid and a gas phase exist together.
If two liquids are completely miscible, no interfacial tension exists between them.
Greater surface tension reflects higher intermolecular force of attraction, thus, increase in hydrogen bonds or molecular weight cause increase in ST

26. The work W required to create a unit area of surface is known as SURFACE FREE ENERGY/UNIT AREA (ergs/cm2)
erg = dyne . cm
Its equivalent to the surface tension γ
Thus the greater the area A of interfacial
contact between the phases, the greater the free energy.
W = γ ∆ A
For equilibrium, the surface free energy
of a system must be at a minimum.
Thus Liquid droplets tend to assume a spherical shape since a sphere has the smallest surface area per unit volume.

28. Methods for measuring surface and interfacial tension
Measurement of Surface and Inter­facial Tensions
Methods for measuring surface and interfacial tension
1- Capillary rise method
2- Ring (Du Nouy) tensiometer
3- Drop weight method (Stalagmometer)
The choice of the method for measuring surface
and interfacial tension depend on:
Whether surface or interfacial tension is to be determined.
The accuracy desired
The size of sample.