Re: Who first discovered that soap effects the surface tension of water?

My apologies for the delay in answering. I found information on this 
question in a very good book, "The World of Physical Chemistry" by Keith
Laidler (a noted physical chemist, himself).

It would appear that the more general question of various organic molecules
affecting the surface tension of water was something known to the ancient
Greeks and Romans. Pliny the Elder (23-79 AD) wrote of divers adding oil
to water to make it easier to see to the bottom. However, the first person
to try to put things on an experimental footing was none other than 
Benjamin Franklin. He studied the effects of oil in smoothing wind ripples
on pond and began the investigations of the effect of organic substances
on surface tension.

However, with regard to soap, it would appear that the first person to
study this scientifically was Agnes Pockels (1862-1935). She was an 
"amateur" german scientist (a lot of scientists were - particularly woman
as they were denied access to higher education) of whom her sister-in-law
once wrote:

... what millions of women see every day without pleasure and are anxious
to clean away i.e., the greasy washing up water, encouraged this girl to
make observations and eventually [to enter into] scientific investigations

She was supported in her efforts by Lord Rayleigh who saw to it that her
initial work on soap films was published in the March 12,1891 issue of 
Nature (still one of the most influential science journals!) Basically, she 
studied the relationship between the area occupied by a film and the 
surface tension. Her results indicated a distinct break between two regimes 
which was later determined to be the point were surface coverage was a 
mono-layer. Above and below this point, the surface behaves quite 
differently.

For more information, please try Laidler's book.