Desirable Properties of Emulsifying Agents.
·
Must be surface active, to reduce surface
tension to below 10 dynes/cm
·
Is adsorbed on droplets as non-adhering
condensed film
·
Impart enough electrical potential for repulsion
of droplets
·
Increase viscosity
·
Effective in low concentration
Interfacial tension:
When lowered,
the surface energy is lowered and therefore stability is increased. For
spherical particles,
Δ F = 6γV Where V = volume of dispersed phase
d d
= mean diameter of particles.
Thus energy input can be calculated.
When γ is
decreased, Δ F is decreased.
Because the
system tends to lose this energy, coalescence ensues.
Surface-active agents lower
surface tension according to Gibbs’s equation:
Γ
= _ c . δγ
RT δc
Where Γ =
surface excess (moles/ml2
Δγ = Change in surface tension with
δc concentration of amphiphile
c = bulk phase concentration.
2. Film
formation.
A film around the dispersed
droplets is a major requirement. The film can be a monolayer, multiplayer or a
collection of particles. This forms a barrier to prevent coalescence of
particles that can come in to contact with each other. The film should not rupture;
squeeze out when sandwiched between droplets. It should have elasticity to
preserve integrity
.
3.
Electrical
potential
We have already discussed the
origin of a potential. The charge causes repulsion of approaching drops. This
is very important for ionic surfactants.
4.
Emulsion
rheology
Emulsifying agents affect
rheology according to chemical, concentration, emulsion type, thickness of the
film, electroviscous effect. The study of the effect is difficult because
droplets are deformable, which affects the viscosity.
5.
Concentration
of the emulsifier
The concentration must be just
sufficient to provide the condensed film around the droplet. An increase
in the concentration achieves less apart from increasing viscosity and
can be undesirable because of foaming.
Types of emulsifiers
a. surface
active agents: these are adsorbed at oil/water interface and form a
monomolecular layer, and reduce surface tension.
b. Hydrophilic
colloids – form a multimolecular film around the globule in o/w emulsions
c. Fine
solid particles – these are adsorbed between immiscible liquid phases, form
a “film” of particles around the globule.
** the common factor is the formation of the film.
Monomolecular
Absorption (Adsorption theory)
Surface active agents (SAA) reduce interfacial tension
when adsorbed at O/W interface, forming a monomolecular layer. They reduce the
surface free energy and hence the tendency to coalesce.
Recall: w = γowΔA
And ΔF = 6γV
d
More important the dispersed
droplets are surrounded by a monolayer which prevents coalescence as the
droplets approach each other. The film should be easily when disturbed, i.e.
should be elastic e.g on shaking. Additionally there is surface charge which
cause repulsion between adjustment
particles.
In practice emulsifiers are combined. A hydrophilic
emulsifier (in aqueous phase) and a hydrophobic emulsifier (in oil phase) form
a complex film at the interface. For example sodium cetylsulphate and
cholesterol. Used alone they form a poor emulsion, but as a combination, they
provide an excellent emulsion. Some combination can produce poor emulsions,
especially when complexation is poor, e.g cetylalcohol and sodium oleate.
HLB sytem
Developed by Griffin ,
this is an arbitrary numerical scale of values extending from 1-50 based on a
balance between hydrophilic and lipophilic tendencies of surface active agents.
It helps determine the efficiency of a SAA.
Originally, Griffin
arrived at the scale by experiments, but later he developed equations by which
values for many non-ionic compounds can be estimated.
For polyhydric alcohols fatty acid esters e.g. glyceryl
monostearate,
HLB = 20 (1 – S )
A
Where
S is saponification value ( The
number of mg of potassium hydroxide
(KOH) required to neutralize the
free acids in 1g and to hydrolyze the esters in 1 g of the substance).
A is Acid value (the
number of mg of potassium hydroxide required to neutralize the free acids in 1
g of the substance).
For beeswax and lanolin derivatives, (where saponification
number is difficult to obtain),
HLB = E+P
5
Where E = % w/w of oxyethylene
chain
P = % of
polyhydric alcohol groups (e.g in
glycerol)
A rough approximation of HLB may
be obtained by water solubility of the agent.
e.g. No dispersibility in water HLB
= 1-4
Poor
dispersion HLB = 3-6
Milky
dispersion HLB = 6-8
Clear solution
HLB = 13
and above
HLB values
Thus the higher the HLB the more
hydrophilic it is. By the knowledge of HLB we can tell or correlate various
properties:
HLB 3-6 Emulsifying agents (w/o)
7-9
Wetting agents
8-18 Emulsifying
agents (o/w)
13-15 detergents
15-18
solubilizing agent
Values of HLB for emulgents are
also listed e.g :
Acacia HLB = 8; Polysorbate HLB = 15; Tragacanth HLB = 13.2
Glyceryl
monostearate HLB = 5.5
HLB’s are algebraically additive. Therefore can prepare agent with intermediate
value if you mix high HLB agent with that of low HLB.
Problem 1. Calculate
the amount of Span and Tween in the formula below, if the required HLB is 10.5
Liq.
Paraffin 50g
Span
80 HLB
4.3
Tween
80 5g HLB
15
Water,
qs 100g
The required = HLB 10.5
Let proportion of span 80 be x
Then proportion of Tween 80 = 1 – x
Therefore 4.3 x + 15 (1-x) =
10.5
4.3
x – 15 x + 15 = 10.5
10.7 x = 4.5
x = 4.5/10.7 = 0.42
1-x = 1-0.42 = 0.58
If the required weight = 5g then the wt of span 80 = 5 x
0.42 = 2.1g, and that of Tween 80 = 5 x 0.58 = 2.9 g
Problem 2:
Calculate the weights of the emulsifiers required in the following formula:
Cetyl
alcohol 1.25g HLB 15
Wool
fat 11.25g HLB 10
Arachis
oil 14
g HLB
7.5
Emulgents 5
g
Arlacel 60 HLB 4.7
Tween 40 HLB 15.6
Water
qs 100g
% of oil phase = 1.25 + 11.25 + 14 = 26.5
Proportion of oils: cetyl
Alcohol 1.25 = 4.72%
26.5
Wool fat 11.25 = 42.45%
26.5
Arachis
oil 14 = 52.83%
26.5
Total 100.00
Required HLB:
Cetyl
Alcohol: 15
x 0.0472 = 0.71
Wool
fat: 10 x 0.4245 = 4.25
Arachis
oil 7.5 x 0.5283 = 3.96
Total 8.92
Arlacel 60 HLB 4.7
Tween 40 HLB
15.6
Proportion
Arlacel = x , = Tween 40 = x – 1
Therefore:
Contribution of Arlacel + contribution of Tween on HLB = 8.92
4.7
x + 15.6 ( 1 – x) = 8.9
4.7
x + 15.6 x – 15.6 = 8.9
10.9 x = 6.7
x = 6.7 = 0.61
10.9
1 – x = 0.39
Emulsifying mixture = Arlacel 60 = 61%
Tween 40 = 39%
But % emulgent = 5%
Therefore
Arlacel = 5 x 0.61 = 3.05
Tween = 5 x 0.39 = 1.95
