Coil size oscillatory packing in polymer solutions near a surface

The theory developed by Scheutjens and Fleer to describe polymer adsorption and depletion is used to calculate the density profile of nonadsorbing pol ymers near a surface. The theory predicts damped oscillations in the segmen t density profile with a wavelength of about the coil size. As a consequenc e, the interaction energy between two surfaces immersed in a solution of no nadsorbing polymers is an oscillatory function of the separation distance, too. The decay length of the oscillations is proportional to the coil size and independent of the polymer concentration. The oscillations are associat ed with a liquid-like layering of polymer coils near the surface. An increa se in concentration or chain length causes a decrease in the amplitude of t he oscillations, because the stronger interpenetration of the coils suppres ses inhomogeneities. In dilute solutions no oscillations are observed, beca use the decay length of the oscillations is smaller than the depletion corr elation length, in analogy with the Fisher-Widom criterion in simple fluids . For polymer solutions it is found that the Fisher-Widom condition coincid es with the overlap concentration. Our results show that the blob size is n ot the only relevant length scale above the overlap concentration. The size of the individual coils plays a role, too. (C) 2000 American Institute of Physics. [S0021-9606(00)50731-4].