SURFACE ENERGY AND SURFACE TENSIONSURFACE ENERGY AND SURFACE TENSION

Subject: Kimia Koloid dan Permukaan

by

Dr. Fitria Rahmawati

Surface energy

Surface energy quantifies the disruption of

intermolecular bonds that occur when a surface intermolecular bonds that occur when a surface

is created.

May defined as the excess energy at the

surface of a material compared to the bulk.

• It means that the molecules at surface position have binding energy 75 % of binding energy of molecules at bulk

position.

Surface molecules have 25% higher energy

than bulk molecules

if l is the length of moveable wire, the increase in

area is 2 (l dx), therefore:

f dx = γ (γ (γ (γ (2 l ) dx

Surface tension of liquid :

f/ 2l = γγγγ

Surface tension is force that opposes the

increase in surface area of the liquid. increase in surface area of the liquid.

SI unit is Newton/ meter (N/m) which is

equal to the increase of Gibbs free energy

with area, Joule/m2

Magnitude of surface tension

Public domain from wikipedia

Examples of surface tension:

MEASUREMENT OF SURFACE TENSION

Capillary rise method

∆ρ : different of density between air

and liquid, g : gravity acceleration

(m/s2)

Wilhemy plate method

Du Nuoy Ring method

http://en.wikipedia.org/wiki/Pierre_Lecomte_du_No%C3%BCy

Maximum Bubble Pressure Method

xx

waterwater

x

water

nd

nd=

γγ

Maximum bubble pressure before a bubble is produced = surface pressure

Force to depress surface = 2πrγForce to pull out the bubble = 2πhrdg2πrγ + 2πhrdg = Pmaks π r2

r

)hdg(r

P

rP)hdg(r

max

max

+γ=

π=+γπ2

2 2

)gdhP(r

xxmax −=γ2

)gdhgdh(r

xxwaterwaterwater −+γ=γ2

Sessile drop

Measuring contact angle

Discussion will be continued to interface phenomena......