Surfactants as Emulsifiers¶
As per the previous notes, these are only the components of the lectures that has not already been covered in Surface and Colloids.
General Requirements for an Emulsifier¶
- Reduce the interfacial tension between the oil and the water
- Adsorb quickly at the oil-water interface
- Resist thinning when droplets collide
- Should be slightly more soluble in the continuous phase
- Aid stability to flocculation
- Aid stability to coagulation
- Increase bulk viscosity (helps to stabilise the emulsion if the droplets can’t move as quickly)
- Perform well at low concentration
- Be inexpensive
- Be Generally Recognised as Safe (GRAS)
- Be chemically compatible with the oil
- Emulsify!
HLB¶
The formulation for HLB is simple:
You start with 7 and you add the HLB of the specific constituent groups within the molecule. The values come from this table:
| Hydrophobic Group | HLB Value | Hydrophilic Group | HLB Value |
|---|---|---|---|
| \(\ce{-CH=}\) | -0.475 | \(\ce{-SO4Na}\) | +38.7 |
| \(\ce{-CH2-}\) | -0.475 | \(\ce{-COOK}\) | +21.1 |
| \(\ce{H3C-}\) | -0.475 | \(\ce{-COONa}\) | +19.1 |
| \(\ce{NR4+}\) | +9.4 | ||
| Derived Group | HLB Value | \(\ce{-COOR-}\) (sorbitan ring) | +6.8 |
| \(\ce{-(CH2-CH2-O)-}\) | +0.33 | \(\ce{-COOR-}\) (free) | +2.4 |
| \(\ce{-(CH2-CH(CH3)O)-}\) | -0.15 | \(\ce{-COOH}\) | +2.1 |
| \(\ce{-OH}\) (free) | +1.9 | ||
| \(\ce{-O-}\) | +1.3 | ||
| \(\ce{-OH}\) (sorbitan ring) | +0.5 |
Example 1
Calculate the HLB for \(\ce{CH3(CH2)12COOH}\) at low and high pH (asusme sodium salt) Low:
High:
The function of a surfactant will vary based on its HLB value:
| HLB | Function | Appearance |
|---|---|---|
| 1-3 | not dispersible | |
| 3-6 | w/o emulsifiers | poor dispersion |
| 6-8 | wetting agents | milky dispersion after vigorous shaking |
| 8-10 | o/w emulsifiers | stable milky dispersion |
| 10-13 | o/w emulsifiers | translucent to clear dispersion |
| >13 | detergents and solubilisers | clear solution |
HLB of Oils¶
The only new thing here is that if the o/w and w/o HLB of an oil are too close together, they can be particularly difficult to form either emulsion, as it will constantly try to invert.
A combination of surfactants or oils has a linear combination of their HLB values.
Example 2
Three oils with required HLB values of 4, 9 and 12 are mixed in a 1:1:2 ratio. Calculate the required HLB of this oil mixture.
To find the ratio that would give the desired HLB, we can use the equation from Surface and Colloids.
Example 3
If an emulsion containing an oil with an HLB requirement of 12.2 is prepared using myristic acid, at what pH will it have optimal stability? Myristic acid = CH3(CH2)12COOH, pKa = 4.90, NaCl Electrolyte
Low pH:
High pH:
Ratio:
39% protonated/61% deprotonated $$ \begin{align} pH&=pK_a+log\frac{[S-]}{[\ce{SH}]}\ &=4.90+log\frac{[0.61]}{[0.39]}\ &=5.35 \end{align} $$