Temperature-insensitivity with anionic-non-ionic surfactant mixtures

4 Temperature-insensitivity with anionic-non-ionic surfactant mixtures 

It has been known for almost three decades that mixing anionic and ethoxylated non-ionic surfactants allows to produce microemulsions which are insensitive to temperature changes [60]. The expressions of the HLD for the two kind of surfactants (see Eqs. (3.24) and (3.25)) exhibit a different sign before the aj and cj temperature coefficients. The signs express the fact that the affinity of water for an ionic surfactant increases when the temperature increases, whereas the reverse takes place with a polyethoxylated non-ionic surfactant. Coefficient ax is about 0.01 for alkyl benzene sulphonates and 0.02 for alkyl trimethyl quaternary ammoniums, while cT is in the 0.05-0.1 range for ethoxylated alcohols and phenols, with a tendency to increase with the ethoxylation degree and to decrease with increased temperature. The fact is that the effect of the temperature is several times stronger with non-ionics, hence a mixture insensitive to temperature should contain more ionic than non-ionic, so that the effects could cancel out [60-62]. 

The calculation cannot be carried out in an accurate way because, as mentioned before, the mixing rule between anionic and non-ionic surfactants is not actually linear due to a shielding of the ionic group by the polyethylene oxide chain. However, the use of a linear approximation often leads to a fairly good estimate in some cases such as a mixture of alkylbenzene sulphonates and ethoxylated nonylphenols to be considered as an example next [61]. 

Assuming a linear approximation and using an ACN scan technique to find the optimum formulation, then the correlation for a three-phase behaviour (HLD = 0) could be written as 

It is seen that PACNN increases as Cj/k (about 0.24 for ethoxylated nonylphenols) and PACN M decreases as a [/k (0.06 for alkylbenzene sulphonates), i.e. about four times slower. According to HLD expressions (Eqs. (3.24) and (3.25)), the relationship between PACN and T is a linear relation. It holds 

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