at a high pressure and the resultingd 32 ¼0.7mm. (a) Why is it beneficial to add NaCl
to the aqueous phase? (b) What type of emulsion is formed and why? (c) What is the
concentration of lauric acid in the aqueous phase after emulsification? (d) Does the
surface excess at the droplets reach the plateau value? (e) What would have been the
result if not 15 but 10 mg of lauric acid had been added to the oil and why? Tip:
Consult Figure 10.6.
Answers
(b) O–W. (c) 2:04 g?L^1. (d) Yes. (e) Larger droplet size.
Question 2
A food company makes an O–W emulsion by using a high-speed stirrer. The aqueous
phase consists of a solution of skim milk powder, protein content 4%, ionic strength
0.1 molar. The oil volume fraction is 0.2. A problem is that the emulsion shows
significant creaming. Microscopic examination shows drops that are on average
about 2mm in diameter, which is considered to be too large. Since the stirrer is
already used at its highest intensity, some surfactant is added prior to emulsification,
and 0.03%SDS does indeed reduce creaming. To reduce the creaming further, 0.12%
SDS is applied, but now creaming is enhanced rather than reduced. Can you explain
these observations?
Answer
For a diameter of 2mm andj¼0.2, the specific droplet surface area would be about
660 : 2 = 2 ¼ 0 :6m^2 ?ml^1. The solution contains about 40 mg of protein per ml, far
more than sufficient to cover the droplets (see, e.g., Figure 11.15). Covering the
droplets with SDS only would giveG&1mg?m^2 , (see, e.g., Figure 10.8), which
would need 0.6 mg SDS per ml (0.06%)þ0.8 times the CMC (about 0.025%), i.e.,
almost three times the amount added, hence the surface layers of the drops would
still consist predominantly of protein. However, we have seen that small
concentrations of SDS may be sufficient to reduce markedly the recoalescence
during emulsification. Hence a decrease in droplet size, hence reduced creaming. If
0.12%SDS is added, it would almost completely replace the protein at the interface
(see Figure 10.15). Although the drops obtained will be smaller, they may now
readily coalesce afterwards. This is because SDS stabilizes the emulsion by means of
electrostatic repulsion, but the ionic strength of the aqueous phase would be high
enough to reduce greatly this repulsion, as mentioned above. Coalescence leads to
large drops and hence to enhanced creaming.