Reexamining the Enhanced Solubility of Sodium Laurate/Sodium Oleate Eutectic Mixtures

Mixtures of multiple surfactants that have superior performance to the individual components are highly sought-after commercially. Mixtures with a reduced Krafft point (<i>T</i>_K) are particularly useful as they enable applications at lower temperatures. Such an example is the soap maker's eutectic: the mixture of sodium laurate (NaL) and sodium oleate (NaOl). A true eutectic implies that the two surfactants do not mix in the solid state but mix readily in the micellar solution above <i>T</i>_K, leading to a sharp <i>T</i>_K depression at a specific composition. However, the NaL/NaOl mixture shows a broad <i>T</i>_K depression of >15 °C at a NaOl weight fraction (<i>w</i>_O) of about 0.5. Our tie-line analysis shows that pure NaL and NaOl do not coexist in the solid phase on either side of the <i>T</i>_K minimum. X-ray analysis of the isolated solids with varying <i>w</i>_O reveals that a unique intermediate compound (I.C.) forms in the solid state with a NaL-to-NaOl mole ratio of about 4/3. Below the <i>T</i>_K minimum, NaL and the I.C. coexist in the solids for <i>w</i>_O < 0.5, whereas the I.C. and NaOl coexist in the solids for <i>w</i>_O > 0.5. Each pair of solids exhibits eutectic or monotectic solubility behavior, and the congruent I.C. melting point is so close to that of the eutectic point(s) that a broad <i>T</i>_K minimum ensues. Thermal analysis and modeling via the freezing-point depression approach support the above interpretation. The fact that surfactants with other headgroups but the same blend of chain lengths do not exhibit similar depressed <i>T</i>_K is a topic for further study.