First of all, Tween (the numbers show the degree of polymerization) belongs
to the polyethylene ester group of surfactants (detergents to the
uninitiated). It is also known as Polysorbate, but Tween is specifically
purified for medicinal purposes, and is used as an emulsifier in
medications for human internal use. Chemically, it is an oleate ester of
sorbitan, which makes it a bit more complex than the other surfactants in
the group. Use-wise, it should behave just like any other non-ionic wetting
agent. (OK, there are three types: negative-ionic, positive-ionic, and
non-ionic; for photographic purposes we normally use only the non-ionic as
they do not react with alkalinic or acidic materials. Most household
detergents are ionic, which is why they don't do well in photography; ionic
is cheaper.)
So Tween, Polysorbate - they're identical and you can use what you can get.
For our purposes, the molecular polymerization number is almost meaningless
(the most common is 80, but I note 20 is marketed; these are not
concentrations, but merely how many molecules are in each polymer). And,
remember, this is stuff that is used in medicinal products, so ingestion is
harmless (cheaper surfactants are used as emulsifiers in most food
products). Good to know in these days of chemical scare tactics!
Any questions?
Sil Horwitz, FPSA
Technical Editor, PSA Journal
silh@iag.net