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Unformatted text preview: the hydrocarbon (hydrophobic) portion with a sphere representing the polar (hydrophilic) group attached at one end.
The hydrocarbon chains are straight because rotation around carbon
carbon bonds bends, coils and twists them. Sodium Lauryl
12 Molecules and ions that are adsorbed at interfaces are termed surface active agents, surfactants or amphiphile The molecule or ion has a certain affinity for both polar and nonpolar solvents. Depending on the number and nature of the polar and nonpolar groups present, the amphiphile may be hydrophilic, lipophilic or be reasonably wellbalanced between these two extremes. It is the amphiphilic nature of surface active agents which causes them to be adsorbed at interfaces, whether these be liquid/gas or liquid/liquid. 13 A scale showing classification of surfactant function on the basis of HLB values of surfactants. The higher the HLB of a surfactant the more hydrophilic it is. Example: Spans with low HLB are lipophilic. Tweens with high HLB are hydrophilic.
14 Determination of HLB
Determination of HLB Polyhydric Alcohol Fatty Acid Esters (Ex. Glyceryl monostearate) (HLB = 20 ( 1 – S / A
S = Saponification number of the ester
A = Acid number of the fatty acid Surfactants with no Saponification no (Ex. Bees wax and lanolin) HLB =E + P / 5
E = The percent by weight of ethylene oxide P=The percent by weight of polyhydric alcohol group in the molecules Surfactants with hydrophilic portion have only oxyethylene groups HLB =E / 5
15 When a liquid is placed on the surface of other liquid, it will spread as a film if the adhesion force is greater than the cohesive forces. 16 As surface or interfacial work is equal to surface tension multiplied by the area increment. The work of cohesion, which is the energy required to separate the molecules of the spreading liquid so as it can flow over the sublayer= Wc = 2 γ L Where 2 surfaces each with a surface tension = γ L
The work of adhesion, which is the energy required to break the attraction between the unlike molecules= Wa = γ L + γ S γ LS Where: γ L =the surface tension of the spreading liquid γ S =the surface tension of the sublayer liquid γ LS =the interfacial tension between the two liquids. Spreading occurs if the work of adhesion is greater than the work of cohesion, i.e. Wa > Wc or Wa Wc > 0 17 Spreading Coefficient is The difference between the work of adhesion and the work of cohesion S = Wa Wc = (γ L + γ S γ LS ) 2 γ L S = γ S γ L γ LS S = γ S – (γ L + γ LS ) Spreading occurs (S is positive) when the surface tension of the sublayer liquid is greater than the sum of the surface tension of the spreading liquid and the interfacial tension between the sub
layer and the spreading liquid. If (γ L + γ LS ) is larger than YS , (S is negative) the substance forms globules or a floating lens and fails to spread over the surface. Liquid Substrate 18 Factor affecting Spreading Coefficient Molecular Structural:
o The greater the polarity o...
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This document was uploaded on 02/05/2014.
- Spring '14