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On the phase behaviour of hydrogels: A theory of macroion-induced core/shell equilibrium
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Pharmacy, Department of Pharmacy.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Colloidal macroions are known to interact very strongly with oppositely charged polyionic hydrogels. Sometimes this results in a non-uniform distribution of the macroions within the gel, a phenomenon that is not fully understood. This thesis is a summary of four papers on the development of a theory of the thermodynamics of macroions interacting with hydrogels, aimed at explaining the phenomenon of core/shell separation in spherical gels. It is the first theory of such interactions to use a rigorous approach to whole-gel mechanics, in which the elastic interplay between different parts of the gel is treated explicitly.

The thesis shows that conventional theories of elasticity, earlier used on gels in pure solvent, can be generalised to apply also to gels in complex fluids, and that the general features of the phase behaviour are the same if mapped to corresponding system variables. It is found that the emergence of shells is due to attractions between macroions in the gel, mediated by polyions. Since the shell state is unfavourable from the perspective of the shell itself, being deformed from its preferred state, there will be a hysteresis between the uptake and the release of the macroion, like already known to occur with the uptake and release of pure solvent.

Due to the elastic interplay, growth of the shell makes further growth progressively more favourable. Thus, unless there is a limited amount of macroions available the system will not reach equilibrium until complete phase transition has taken place. If the amount is limited the core/shell separation can be in equilibrium, so the volume of the solution that the gel is in contact with plays a very important part in determining the thermodynamic resting point of the system. The ability of a macroion/hydrogel to phase separate thus depends on the molecular properties whereas the ultimate fate of such a separation depends on the proportions in number between the ingoing components.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. , 70 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Pharmacy, ISSN 1651-6192 ; 169
Keyword [en]
polymer, polyelectrolyte, surfactant, thermodynamics, elasticity
National Category
Pharmaceutical Sciences Physical Chemistry
Research subject
Pharmaceutical Science
Identifiers
URN: urn:nbn:se:uu:diva-188151ISBN: 978-91-554-8565-8 (print)OAI: oai:DiVA.org:uu-188151DiVA: diva2:576845
Public defence
2013-02-08, B42, BMC, Husargatan 3, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2013-01-18 Created: 2012-12-12 Last updated: 2013-04-02Bibliographically approved
List of papers
1. A model describing the internal structure of core/shell hydrogels
Open this publication in new window or tab >>A model describing the internal structure of core/shell hydrogels
2011 (English)In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 7, no 21, 10327-10338 p.Article in journal (Refereed) Published
Abstract [en]

We apply a theory to the constrained swelling of gel particles, explicitly accounting for the propagation of elastic forces through the particle. This approach, together with conventional thermodynamics of gel swelling, allows modelling of the equilibrium state of gels with properties that are spatially inhomogeneous. In our case we consider both a discrete inhomogeneity in the form of assigning different water solubilities to the core and shell domains of the particle, and a continuous inhomogeneity in allowing the density of chemical cross-links to vary gradually through the network. The model is used to understand the behaviour of temperature-sensitive poly(N-isopropyl acrylamide) core/poly(N-isopropyl methacrylamide) shell microgels investigated in an earlier experimental study. How the swelling of the core and shell is affected by the presence of each other at different temperatures is investigated and explained from a mechanical and thermodynamic perspective.

National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:uu:diva-161771 (URN)10.1039/c1sm05694h (DOI)000296026700065 ()
Available from: 2011-11-21 Created: 2011-11-17 Last updated: 2017-12-08Bibliographically approved
2. Core-shell separation of a hydrogel in a large solution of proteins
Open this publication in new window or tab >>Core-shell separation of a hydrogel in a large solution of proteins
2012 (English)In: Soft Matter, ISSN 1744-683X, E-ISSN 1744-6848, Vol. 8, no 42, 10905-10913 p.Article in journal (Refereed) Published
Abstract [en]

Upon absorption of large oppositely charged electrolytes such as proteins, polyionic hydrogels are frequently observed to separate into dense shell–swollen core states. We have developed a theory that in a detailed way takes into account the inhomogeneous swelling and distribution of the protein within such a core–shell separated gel. With this we investigated whether the core–shell separation can be an equilibrium state or if it must be understood as a dynamical phenomenon. Restricting ourselves to spherical gels with an unlimited supply of protein, we found that as an intermediate between a swollen and a collapsed gel the core–shell state can indeed be the one of lowest free energy but this state is not stable. In such cases where formation of a shell could occur spontaneously there was no further thermodynamic barrier to complete collapse of the gel (but possibly dynamical ones). The core–shell separation was favourable in systems of high charge and low ionic strength and was explained, within our theory, by the fact that the energy gain in packing proteins and polyions closely together outweighs the entropy loss of the uneven distribution.

Place, publisher, year, edition, pages
Royal Society of Chemistry, 2012
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-184518 (URN)10.1039/C2SM26227D (DOI)000310829400014 ()
Available from: 2012-11-08 Created: 2012-11-08 Last updated: 2017-12-07Bibliographically approved
3. Hysteresis in the surfactant-induced volume transition of hydrogels
Open this publication in new window or tab >>Hysteresis in the surfactant-induced volume transition of hydrogels
2015 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 119, no 4, 1717-1725 p.Article in journal (Refereed) Published
Abstract [en]

The discontinuous uptake and release of surfactants by hydrogels and the accompanying discontinuous volume transition is known to occur with a hysteresis. We have performed a theoretical analysis in order to find the mechanistic origin of this phenomenon. Using a mean-field model, we have quantitatively reproduced the experimental behavior by considering the cost of elastically deforming the gel material to allow phase coexistence. The major part of the hysteresis is due to the high phase coexistence cost of the swelling transition, since in this direction the coexistence cost depends not only on the elasticity of the network (being a weak force in comparison) but also on the entropy of the monovalent nonsurfactant electrolyte present in the system.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-188149 (URN)10.1021/jp5087416 (DOI)000348753600047 ()25567724 (PubMedID)
Available from: 2012-12-12 Created: 2012-12-12 Last updated: 2017-12-07Bibliographically approved
4. Stable core/shell separation in hydrogels induced by surfactants
Open this publication in new window or tab >>Stable core/shell separation in hydrogels induced by surfactants
(English)Manuscript (preprint) (Other academic)
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-188150 (URN)
Available from: 2012-12-12 Created: 2012-12-12 Last updated: 2013-02-11

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