The formulation triangle: Why good surfactants do not stand still
Harrison Gellini and Ashley Quigley of Surfachem consider how regulation, feedstocks and economics continually reposition established surfactant chemistries
Sustainability, performance and affordability are often discussed as though they can be improved independently. In formulation, that is rarely the case. A useful way of looking at surfactant selection is as a triangle with these three at its three corners( Figure 1).
The ideal material would sit comfortably between all three: high performing, environmentally favourable and inexpensive enough for widespread use. In reality, the position of a surfactant within that triangle is not fixed.
Regulation, feedstock availability, manufacturing technology, consumer expectations and economics can all move it, sometimes without changing the fundamental chemistry or cleaning performance at all. Few surfactants demonstrate this better than sodium laureth sulfate( SLES).
The Goldilocks balance
SLES has been a workhorse in detergents and cleaning formulations for decades and there are good reasons for that. It provides excellent detergency and foam, is readily biodegradable and is available on an enormous industrial scale. More importantly from a formulator ' s perspective, it is remarkably easy to work with.
In many SLES-containing aqueous systems, useful viscosity can be developed through electrolyte addition, with familiar salt-response behaviour giving formulators considerable flexibility. Rawmaterial supply is mature and useful finished-product rheology can often be achieved without necessarily
Figure 1- The Formulation Triangle
requiring high levels of secondary surfactants or polymeric thickeners.
That matters commercially. The true cost of a surfactant system is never simply the purchase price of the primary surfactant. If replacing it requires additional amphoterics, foam boosters, hydrotropes or rheology modifiers to obtain the same viscosity, stability and consumer experience, the economics of the replacement can change very quickly.
This combination of performance, formulation flexibility and cost efficiency is one reason SLES became so difficult to displace. Its sustainability position is also more interesting than the simplistic‘ synthetic sulfate’ label sometimes applied to it.
In a conventional SLES made using a renewable C 12-14 fatty alcohol, most of the carbon in the molecule originates from that fatty chain. For a C 12 alcohol ethoxylated with two moles of ethylene oxide, 12 of the molecule’ s 16 carbon atoms originate from the fatty alcohol, approximately 75 % of its molecular carbon. With
longer chains the proportion can be slightly higher.
The remaining carbon traditionally comes from petrochemical ethylene oxide. Even that is not an unavoidable limitation, however. Industrial routes are established for producing ethylene, and subsequently ethylene oxide, from renewable ethanol. 1 This means that the ethylene-oxidederived carbon in ethoxylated surfactants need not inherently remain fossil-based.
For many years, SLES therefore occupied something close to the‘ Goldilocks’ region of the Formulation Triangle: good cleaning performance, favourable economics and a substantial renewable-carbon contribution when based on vegetablederived fatty alcohol.
When the triangle moves
The complication is 1,4-dioxane, which can occur as a trace, process-related impurity in ethoxylated surfactants, including SLES. Manufacturers have long managed residual levels through process optimisation and
34 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981