R. Aveyard et al., Compression and structure of monolayers of charged latex particles at air/water and octane/water interfaces, LANGMUIR, 16(4), 2000, pp. 1969-1979
We have studied the compression and structure of compressed monolayers of s
ulfate polystyrene latex particles on air/water and octane/water interfaces
. If compressed sufficiently (on a Langmuir trough) the monolayers at air/w
ater surfaces give rafts of hexagonally packed particles, while those at oi
l/water interfaces undergo a transition from the originally hexagonal to a
rhombohedral structure. We have found that beyond collapse the particle mon
olayers on both air/water and octane/water interfaces fold and corrugate, a
nd there is no expulsion of individual particles or particle aggregates fro
m the interface. In the case of air/water interfaces, the structuring of pa
rticle monolayers (below collapse) was found to be very sensitive to the el
ectrolyte concentration in the aqueous phase. At low electrolyte concentrat
ion, a fairly ordered structure resulting from the interparticle repulsion
was observed, while at high electrolyte concentration, the particles form 2
D clusters. In marked contrast, particle monolayers at octane/water interfa
ces remain highly ordered as a result of long-range repulsion, even on conc
entrated electrolyte solution. We attribute the enhanced lateral repulsion
between the latex particles at the octane/water interface to the existence
of residual surface charges at the particle/octane interface. We propose a
simple model, which describes the electrostatic interaction between the ads
orbed particles and includes the effect of image forces. From this we have
derived an analytic formula for the electrostatic surface pressure vs troug
h area, which agrees well with the experimental data over a wide range of s
urface pressure.