Investigation of colloidal dispersions of photographic couplers and surfactants by small-angle neutron scattering

Colloidal dispersions of photographic couplers prepared by condensation methods were studied by small-angle neutron scattering, initially in dilute aqueous solutions, where inter-particle effects can be neglected. These nanoparticle dispersions are composed mainly of coupler and surfactant hydrocarbon chains, which were shown to be intimately mixed as in a liquid. The particle sizes were determined to be in the range 50–250 Å in radius for ten couplers investigated with the surfactant sodium dodecyl sulfate. Selected couplers were studied with two other surfactants. The amount of surfactant associated with coupler in the particles varied with coupler structure and ranged from 50 to 80%. Studies were also performed at higher concentrations where interparticle interactions were observed as manifested by a peak, as opposed to a monotonic fall off in the scattering, as in dilute solutions. For these data, the interparticle scattering function, S(Q), was calculated using the mean spherical approximation, MSA which was used to extract the aggregation number, surface charge, and inner and outer radii of the particle, which define the particle ‘shell’, consisting of surfactant headgroups, counterions and water of hydration.