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Polyelectrolyte-mediated interaction between similarly charged surfaces: Role of divalent counter ions in tuning surface forces
YKI – Ytkemiska institutet.
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2001 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 17, p. 8321-8327Article in journal (Refereed)
Abstract [en]

The effects of divalent salts (CaCl2, MgCl2 and BaCl2) in promoting the adsorption of weakly charged polyelectrolyte (polyacrylic acid), PAA, Mw ~ 250000 g/mol) on mica surfaces and their role in tuning the nature of interactions between such adsorbed polyelectrolyte layers were studied using the interferometric surface forces apparatus. With mica surfaces in 3 mM MgCl2 solutions at pH ~8.0-9.0, the addition of 10 ppm PAA resulted in a long-range attractive bridging force and a short-range repulsive steric force. This force profile indicates a low surface coverage and weak adsorption. The range of the force can be related to the characteristic length scale RG of polyelectrolyte chains using a scaling description. An increase of the PAA concentration to 50 ppm changed the attractive force profile to a monotonic, long-range repulsive interaction extending up to 600 Å due to the increased surface coverage of polyelectrolyte chains on the mica surfaces. Comparison of the measured forces with a scaling mean field model suggests that the adsorbed polyelectrolyte chains are stretched, which eventually give rise to the polyelectrolyte brush like structure. When the mica surfaces were preincubated in 3 mM CaCl2 at pH ~8.0-9.0, in contrast to the case of 3 mM MgCl2, the addition of 10 ppm PAA resulted in a more complex force profile: long-range repulsive forces extending up to 800 Å followed by an attractive force regime and a second repulsive force regime at shorter separations. The long-range electrosteric forces can be attributed to strong adsorption of polyelectrolyte chains on mica surfaces (high surface coverage) which is facilitated by the presence of Ca2+ ions, while the intermediate range attractive forces can be ascribed to Ca2+ assisted bridging between adsorbed polyelectrolyte chains. Also interesting is to note various relaxation processes present in this system. In contrast to both MgCl2 and CaCl2 systems, with mica surfaces in 3 mM BaCl2 solution at pH ~8.0-9.0, the addition of 10 ppm PAA resulted in precipitation of polyelectrolyte chains on mica surfaces, resulting in an extremely long-range monotonic repulsive force profile. In summary, our study showed that divalent counterions (Mg2+, Ca2+, and Ba2+) exhibit significantly different behavior in promoting PAA adsorption on mica surfaces, modifying and controlling various surface interactions.

Place, publisher, year, edition, pages
2001. Vol. 17, p. 8321-8327
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Natural Sciences
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URN: urn:nbn:se:ri:diva-27217OAI: oai:DiVA.org:ri-27217DiVA, id: diva2:1054221
Note
A1579Available from: 2016-12-08 Created: 2016-12-08 Last updated: 2017-11-29Bibliographically approved

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