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What is the smallest surfactant molecule?
I was just writing about surfactants and emulsifiers and it occurred to me that there probably has to be some minimum size for a molecule to be a surfactant. Anyone know what would be the smallest sized surfactant (from a molecular standpoint)?
I did a little searching but didn’t find a satisfying answer. Could Ethyl Alcohol be considered a surfactant?
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7 Replies
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I suspect the smallest commonly encountered one in cosmetics is sodium laurate
sodium caprate (C10), sodium caprylate (C8) and sodium hexanoate (C6) have been shown to have some surfactancy, but below C6 the carbon chain is apparently too short to form micelles
my view is that if it can form micelles, it’s a surfactant; if it can’t, it’s not
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Limiting the definition of surfactant to something that can form micelles seems like a pretty good parameter
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Surfactant: Surfactants are compounds that lower the surface tension (or interfacial tension) between two liquids or between a liquid and a solid.
(Wikipedia) - alcohol, eg methanol, does that, so alcohol by definition must be a surfactant (as well as a solvent). -
I guess that any amphiphilic molecule can be
considered as surfactant. Of course you cannot stabilize emulsion with short
chain alcohol, but they can be considered as co-surfactants as they have
affinity for the interface.I don ‘t know if there is a definition
linked to a specific value of surface tension that have to be reached to
consider a molecule as surfactant.In my opinion, there is no point to describe a surfactant by
its size (unless you want to demonstrate the kinetic of interface adsorption or
for large molecules steric stabilization), HLB or other scales that describe
the amphiphilicity of the molecule are more convenient to describe surfactant
molecules. -
@jeremien I’d disagree - there are a few situations in which the shape, and hence the size, of the surfactant/emulsifier plays an important part in determining the overall properties of the system
one example is the difference between using stearyl alcohol ethoxylates and oleyl alcohol ethoxylates as emulsifiers
the two are almost identical in terms of chemistry and HLB, but you get very different emulsions because the latter is wedge-shaped rather than linear, and takes up more space at the interface than the former
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@Bill_Toge I agree with you. Of course the chemical structure of your surfactant is important to understand
the difference in properties (maybe in that example, the PIT (phase inversion temperature) could be the best system to explain the differences).I guess my comment was confusing, i mean there is no sense to add a minimum
size (the same as a maximum) to the definition of a surfactant. -
you can run and plot surface tension as a function of SAA concentration (we used DN tensiometer with ring method) and determine critical micelle concentration (CMC). Area/molecule can then be calculated.
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