Hydrophobically modified gelatin and its interaction in aqueous solution with sodium dodecyl sulfate

The interaction between a series of hydrophobically modified gelatins and the anionic surfactant sodium dodecyl sulfate SDS in a pH = 6.6/34 mM buffer has been investigated by surface tension, pulsed-gradient spin-echo NMR, viscosity, and small-angle neutron scattering. In the absence of the surfactant, the bulk viscosity of all the gelatins examined here is dependent only on the gelatin concentration and is independent of the number and size of any grafted hydrophobe. Similarly, the small-angle neutron scattering for all samples could be well-described by a two length-scale model. In this model, the smaller dimension represents the local structure of the gelatin molecule and is unaffected by the presence of the hydrophobe(s), while a larger dimension characterizing the network structure increases significantly with the presence of the hydrophobe but not their size. On addition of the anionic surfactant sodium dodecyl sulfate (SDS), the interaction between surfactant and the hydrophobic gelatin with low grafting densities starts at a slightly lower surfactant concentration (CMC(1)) compared with a "blank" material that has no grafted hydrophobe. At higher grafting densities, CMC(1) is greater than the CMC(1) values for the lower grafting density series. For both grafting densities, neither the length nor the number of hydrophobes present has an appreciable effect on CMC(1). There is no change in the amount of SDS bound to the gelatin at saturation. The viscosities of the hydrophobically modified gelatin/SDS solutions do depend on the length of the hydrophobe, albeit rather weakly, but only at the higher grafting density. In summary, the interaction between these gelatin samples and SDS is dominated by the electrostatics of the system; the effects of the hydrophobe are proposed to be largely manifestations of the change in the electrostatic character of the gelatin induced by grafting process, rather than the hydrophobe per se.