Pure versus Commercial Surfactants

Pure compounds are beautiful but mixtures are wonderful

To begin, I must mention that I started my career in organic chemistry, and it influences my approach to understanding physico-chemical phenomena and complex formulations, as I always strive for a molecular comprehension of these processes.

Your statement claiming that a surfactant blend in C8-C18 is more effective than a well-defined C12 surfactant is both true and false for several reasons:

1. Surfactants are used for very different applications, such as solubilizing oil in water, forming stable and creamy foam, wetting a solid surface, or preparing a W/O or O/W emulsions. Therefore, a good surfactant for one application may be highly ineffective for another.
2. I will assume that you are interested in the "simple" case (governed by thermodynamics) of solubilizing a complex oil in water with the minimum surfactant. I will exemplify most of my responses by considering the case of ethoxylated fatty alcohols CiEj as they are well-known and because CiEj still represents 70% of industrial non-ionic surfactants. However, my arguments would be similar for other surfactant families
3. First academic example: Which surfactant is most effective for dissolving 30% heptane in water at 25°C? Just 5% of C12E5 is sufficient to achieve a microemulsion indefinitely stable but not robust to changes in oil, temperature, or surfactant purity. In this case, the pure surfactant is at least twice as effective as an equivalent industrial-grade surfactant. [doi.org/10.1016/j.cis.2019.102099]
4. Second industrial example: Which surfactant to choose for dissolving 30 wt.% perfume (20 to 200 individual molecules) in water to obtain a perfume without ethanol that leaves no residue on skin or fabric after evaporation? A blend of a small amount of a "true" surfactant plus 5 to 10% of a volatile hydrotrope works perfectly, whereas using the hydrotrope alone require a much higher quantity. [dx.doi.org/10.1016/j.colsurfa.2013.11.024]
5. In general, why is an industrially impure surfactant more effective than an ultra-pure surfactant in industrial applications? According to my perspective, employing impure surfactants offers several advantages:
• Low cost and ease of synthesis: Natural fatty alcohols and acids are much cheaper than their pure counterparts. Moreover, industrial syntheses (ethylene oxide polycondensation for CiEj and Fisher condensation for APG) yield mixtures of hydrophilic oligomers that are practically impossible to separate. More selective synthesis routes are significantly more expensive and are not industrially developed.
• Reduction of Krafft point: Several C12 surfactants have Krafft points (TK) above room temperature. For instance, the TK of dodecyl-β-D-glucoside is 38 °C. This surfactant would be unusable in its pure form as it would not form micelles at room temperature. However, the addition of "impurities" lowers the melting temperature in accordance with Raoult's law [doi.org/10.1016/j.cis.2019.06.003].
• Increase in cloud point: For ethoxylated surfactants, the usage temperature must be below the cloud point. Pure C12E4 has a Cloud Point of 6°C [doi.org/10.1016/j.cis.2019.102099], making it impractical in its pure state at 25°C. However, when mixed with more hydrophilic surfactants, its effective cloud point increases. For example, Brij 30, sold as industrial C12E4, has a cloud point of 62°C [Chem Sci Rev Lett 2017, 6(24), 2163], making it one of the most popular surfactants in formulation.
• Co-solubilization of residual fatty acids and alcohols and hydrophobic oligomers: Industrial syntheses leave significant proportions of unreacted fatty acids or alcohols, which are insoluble in water. Instead of expensive removal processes, surfactant producers prefer to leave them, as they become water-soluble in the presence of more hydrophilic oligomers, forming micelles.
• Auto-adaptation of "HLB" based on physical and chemical fluctuations: Formulations must remain stable over a large temperature range (typically between 5 and 40°C) despite fluctuations in the composition of natural (crude oil, essential oils, greasy soil) or synthetic oils (perfumes). The ideal surfactant varies for each temperature and oil molecule. If the surfactant is a blend, the actual composition of the interfacial film will evolve and adapt during fluctuation of the environment. For example, as the temperature rises, surfactants most effective at room temperature will migrate partially into the oil phase (partitioning phenomenon) and cease to perform their role. However, they will be replaced in the interfacial film by more hydrophilic surfactants. A formulation designed with an industrial surfactant is therefore more robust against changes than an ultra-pure surfactant.

Comment from Dr David Scheuing

Here is the comment from LinkedIn that seems to me to nicely complement Prof Aubry's insights:

For ethoxylates, there are old papers on the partitioning of stuff like dodecyl alcohol or molecules with only 1 or 2 EO groups into “oils” in ternary surfactant-oil-water systems that show how the shape of the “fish body” is severely distorted. The efficiency of solubilization of a given oil by different versions of the “same” surfactant can often be blamed on the striking changes in the “oil phase” when a small amount of “junk” like C12-OH or C12-EO1 or C12-EO2 sneaks into it. As you’ve mentioned in other works – we still have a language problem in surfactant science – with many terms thrown around that can be vague, confusing, or downright wrong. So – one could say that the junk has partitioned into the known “oil” phase, changing the overall “polarity” of the oil. Another description is to say the “EACN” of the (often) complex “oil” has been modified by the junk. And then there is the approach that describes the junk as “hydrophobic linkers” – which can boost solubilization efficiency – especially when a “hydrophobic” linker is combined with a “hydrophilic linker”. So the junk could be called an uncontrolled “linker” that would require the addition of a partner linker to restore the efficiency of the system!