Intra-particle sorption rate and liquid chromatographic bandbroadening in porous polymer packings III. Diffusion in the polymer matrix as the cause of slow sorption

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In order to discover the physical cause of the slow intra-particle sorption rate of naphthalene in 10-μm spheres of the macroporous poly(styrene-divinylbenzene) (PS-DVB) polymeric HPLC sorbent Hamilton PRP-1, which has been shown to cause excessive bandbroadening of eluted peaks, the sorption-rate curve for naphthalene from methanol-water (85:15) was measured on PRP-1 using the shallow-bed technique. Sorption on PRP-1 follows a two-term theoretical rate equation for sorption on a biporous particle. From the (fast) first term it is found that 91 % of the naphthalene is sorbed on the walls of the large pores and that the diffusion coefficient in these large pores is 3×10−6 cm2/s. This is close to the free-solution diffusion coefficient, which demonstrates that large-pore diffusion is nearly unhindered. From the (slow) second term in the rate equation it is found that 9% of the naphthalene is sorbed into the polymer matrix of PRP-1, in which the effective diffusion coefficient is no larger than 10−12 cm2/s. It is clear from these results that the cause of the slow intra-particle rate, and therefore of excessive Chromatographic bandbroadening, is slow diffusion into the polymer matrix of PRP-1. To provide additional information on the PS-DVB polymer matrix, the sorption rate of naphthalene was also measured on Hamilton PRP-∞ which is a 19-μm diameter, spherical, nominally nonporous PS-DVB Chromatographic packing. The sorption is slow and follows the theoretical rate equation for hindered diffusion into a homogeneous sphere. The effective diffusion coefficient is (4±1)×10−9 cm2/s. Diffusion through the polymer matrices in PRP-1 and PRP-∞ could be either hindered diffusion through micropores in a rigid matrix or diffusion through a flexible polymer ‘gel’.