Effect of buffers on aqueous solute-exclusion zones around ion-exchange resins
Graphical abstract
Charged latex particles in buffer solutions are uniformly excluded from several-hundred-micron-thick shells surrounding ion-exchange beads regardless of charge sign polarity.
Introduction
Previous work from this laboratory has revealed a feature of aqueous solutions that is unexpected. Adjacent to hydrophilic surfaces, an aqueous zone several hundred micrometers wide excludes colloidal particles and various solutes [1], [2]. Because this zone exhibits broad exclusionary features, it has been labeled the “exclusion zone” or EZ.
One feature of the EZ is its sensitivity to pH. Studies have shown that the size of the exclusion zone is very much dependent on pH [1]. In pure water the EZ is smallest near neutral pH, while a five-fold increase in size is observed as the pH is adjusted downward to 2 or upward to 12. In aqueous systems a shift in pH is analogous to an oxidative state change, which impacts the bulk charge distribution [3], [4]. Change of pH also brings about change of structural and thermodynamic properties [5], [6] that can be augmented by the addition of a polyelectrolyte surface [7].
From these previous studies, it appeared as though a critical variable in establishing EZ size might be charge. Hence it seemed worthwhile to check this expectation by using buffers to attenuate charge [8], [9]. We found indeed that buffers exerted a stabilizing influence on the EZ, and also that in the presence of buffers, EZs could be seen adjacent to both negatively charged and positively charged surfaces.
Section snippets
Methods
Colloidal particle exclusion from both cation (H+ form) and anion (OH− form) exchange-bead surfaces was observed under low magnification (50×) on a Zeiss Axiovert 35 microscope. Ion exchange resin beads (Bio-rad, Hercules, CA, Cat. # 142-6425 & Cat. #142-7425), composed of cross-linked polystyrene divinylbenzene backbones functionalized with either sulfonic groups (H+ form) or quaternary ammonium groups (OH− form), were first rinsed with spectroscopy-grade methanol and then flushed with
Results
Representative results obtained using an imidazole buffer are presented in Fig. 1. Panel (a) shows the results obtained with a negatively charged cation exchange bead, while panel (b) was obtained using a positively charged anion exchange bead. In both situations, negatively charged sulfate microspheres were used.
Substantial EZs developed over several minutes in both cases (Fig. 1(c)). The EZs formed in the shape of an enveloping shell. Their thickness was approximately 200 μm. Microspheres
Discussion
The results show that exclusion zones similar to those found in pure water or in water with salt are also found in buffered solutions. Previous work had indicated that pH was an important determinant of EZ size [1], and the present work was carried out with a series of buffers designed to stabilize pH. We found that the buffers did stabilize the EZ, making it more robust. In cases where pure water showed little or no EZ, addition of buffer in even modest concentrations elicited large EZs, which
References (29)
- et al.
Adv. Colloid Interface Sci.
(2006) - et al.
Phys. Rev. E
(2003) - et al.
Limnol. Oceanogr.
(1972)- et al.
Phys. Chem. Chem. Phys.
(2001) - et al.
J. Am. Chem. Soc.
(2000) Fundamentals of Acids, Bases, Buffers and Their Application to Biochemical Systems
(2000)- et al.
Electroanalysis
(1998) - et al.