title: Adsorbed anthranilic acid molecules cause charge reversal of nonionic micelles.
abstract: The effect of anthranilic acid, an aromatic amino acid, on the structural characteristics of nonionic micelles of Triton X-100 at different pH values was investigated by light scattering and small-angle neutron scattering (SANS) measurements. The scattered light intensity decreases as pH is increased or decreased on either side of the isoelectric point (IEP = 3.4) of the amino acid. Analysis of the SANS data using a sticky hard-sphere model shows that the micelles are oblate ellipsoids with an axial ratio of approximately 2.3. No significant change could be observed in the size of the micelles with a change in the pH, while the stickiness parameter (tau), which is related to the interaction potential (u(0)) increases on either side of the IEP. As tau increases, u(o) becomes less negative, indicating a decrease in the attractive interaction between nonionic micelles. This can be explained in terms of the changes in the surface charge of the micelles resulting from a shift in the acid-base equilibrium of the adsorbed amino acid. The variation of the intermicellar interaction as calculated from the stickiness parameter is consistent with the picture of reversal of charge of amino acids with pH. This is further supported by the observed variation of the cloud point of the solutions at different pH values. The change in the interparticle interaction is also reflected in the diffusion coefficient of the micelles measured by dynamic light scattering.
authors.affiliation: Chemistry Division and Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai-400085, India.
authors.forename: Gunjan
authors.initials: G
authors.lastname: Verma
authors.forename: V K
authors.initials: VK
authors.lastname: Aswal
authors.forename: S K
authors.initials: SK
authors.lastname: Kulshreshtha
authors.forename: P A
authors.initials: PA
authors.lastname: Hassan
authors.forename: Eric W
authors.initials: EW
authors.lastname: Kaler