Soft matter under confinement

‘Soft matter’ encompasses myriad systems both in synthetic and in biological contexts – and often in combination. An inclusive definition of soft matter might stipulate that the important length scales are at least of the order of nanometers; that the relevant energy scales are thermal (so that the modulus ∼ k_BT/(nm's)³ is indeed ‘soft’), and that entropy plays a major role in determining its properties. A simple rubber band provides a classic example: the enthalpic interactions of the entangled and cross-linked polymer molecules of which it is composed depend hardly at all on their configurations over a large range of extensions, and its elastic properties in this range are due almost entirely to the configurational entropy change of the elongated chains. Indeed, it is instructive that if a given (ideal) polymer chain is stretched to a certain elongated shape, the work involved is essentially the same as if it was instead to be confined (by squeezing, say) to the identical elongated shape.¹ Entropic and confinement effects on soft matter thus come to the fore hand-in-hand: the imposition of an interface or increasing geometrical confinement can decrease substantially the degrees of freedom, as well as introducing new molecular interactions. This can have many ramifications, for example tipping the balance in favour of a more ordered structure. Charged systems too, in aqueous media, derive many of their properties from such confinement and entropy effects. Thus the repulsion between two like-charged surfaces across water or salt solutions may be well understood in terms of the osmotic pressure arising from confinement of the counterions trapped between them (see e.g.ref. 2). Likewise, adsorption of polyelectrolytes from solution onto an oppositely-charged surface arises, ultimately, not from electrostatic attraction between the charges, but rather from the entropy gain as counterions are released into the bulk while the chain is confined to the adsorbing surface.³ A detailed understanding of many of these effects was achieved already by the early masters, including Kuhn, Debye and Flory (all of whom have different length-scales, in the ca. 1–100 nm range characteristic of soft matter, named after them: the Kuhn step length and Flory radius in the case of polymers, and the Debye screening length for electrostatic double-layer interactions ubiquitous in aqueous media). It is fair to say, however, that the powerful simplifying approaches pioneered by Pierre Gilles de Gennes,¹ Sam Edwards⁴ and their colleagues from the 1970's onwards (for example ref. 5), revitalized the field, both theory and experiment, while the advent of ever-faster computation gave rise to far more realistic simulation studies.⁶ The effects of confinement were extended also to dynamic scenarios, such as the ‘tube’ representing the effect of topological constraints on entangled polymers,⁴ and the related seminal paradigm of reptation.¹ These ideas were initially applied – and proved very fruitful – for the case of polymers, but concepts of scaling, hydrodynamic screening and topological confinement were later used more widely in the realm of interfaces and ‘complex fluids’, and now under the banner of ‘soft matter’. Many of these early developments, which were extended enormously with the appearance also of several new journals to accommodate them, are reflected in the papers in this issue.

‘Confinement’ in the context of this themed issue can include situations such as soft (or biological) materials at solid surfaces, at liquid–liquid interfaces, adjacent to bio-membranes, the crowded and fluctuating intra-cellular space, in synthetic nano-pores and natural porous materials, and inside polymer brushes. Encompassing many examples from this list, and with the list of soft materials studied equally long, the papers in this themed issue reflect well the diversity of work in this area.

Another characteristic of this field is the diverse range of experimental and computational tools and techniques applied to study soft confined systems, and their combination often affords the clearest picture of structure, dynamics and mechanism. Within this volume can be seen the application of spectroscopies, scattering methods, force probes, electrochemical sensing, microfluidics and computer simulation. Despite the breadth of work represented here, some key themes emerge as areas of focus, and these are summarized in what follows.

Polymers and macromolecules under confinement

In view of the close relation between their entropy, their properties and their confinement, it is perhaps not surprising that fully half of the papers in this issue relate to polymers or macromolecules under topological constraints. For convenience we divide them into three related groups.

Bio-related systems

Several of the studies are inspired by biological systems. The review by Osmanovic et al. (DOI: 10.1039/c3sm50722j) considers modeling of the nuclear pore complex which separates the nucleus from the cytoplasm in cells and regulates transport between them. In particular, they provide a wide-ranging review of how modeling approaches are used to evaluate the free energy changes when macromolecules – such as different proteins – pass through the pore. The paper by Mellouli et al. (DOI: 10.1039/c3sm51163d) utilizes a microfluidics approach generating aqueous droplets stabilized by lipid monolayers, to probe the effect of confinement and crowding within these microdroplets on the spatial organization of particular fibre-forming proteins (FtsZ) important in cell-division. The issue of biological lubrication, which is not well understood despite decades of study, particularly the very low friction at articular cartilage surfaces in joints, inspires the investigation by Espinoza-Marzal et al. (DOI: 10.1039/c3sm51415c). This paper examines the tribological behavior of polymer brushes, particularly the effect of a viscous solvent (analogous in some sense to synovial fluid permeating the joint space). For macromolecules native to the same cartilage system but with a more structural emphasis, the study by Attili and Richter (DOI: 10.1039/c3sm51213d) examines directly the interaction in surface complexes of hyaluronan (HA) and aggrecan, two major macromolecular components of articular cartilage. Their work, exploring structures reminiscent of those thought to coat the articular cartilage surface in vivo,⁷ reveals the strong swelling of the HA brushes by intercalating aggrecan molecules and considers the implications of this for biological function.

Effects of confinement on polymer size

Other studies examine more closely the relation between the confinement of a polymer and the chain dimensions. The transition between a 2-D and a 1-D configuration of self-avoiding chains is effectively simulated, using Monte Carlo methods, by Hsu and Binder (DOI: 10.1039/c3sm51202a), who confine the polymers to progressively narrower strips on the plane to which they are attached, thereby changing the dimensionality of the confinement. They are thus able to identify the different relative power laws governing their size. True 2-D confinement is not easy to achieve experimentally, as shown in a small-angle neutron scattering study of polymer chains confined by clay particles by Frielinghaus et al. (DOI: 10.1039/c3sm50644d). They recognize the effect of the finite thickness on the chain configuration, finding that they get better agreement with their data by applying the concept of self-avoiding trails – rather than self avoiding exact 2-D walks – to explain the chain dimensions. Nanoslits and nanopores can provide a further experimental realization, respectively, of quasi-2-D and quasi-1-D confinement. Paturej et al. (DOI: 10.1039/c3sm51275d) study star-branched polymers between two parallel confining surfaces (slits) as a function of the surface separation, using both molecular dynamics and self-consistent mean field calculations, and comparing their results critically to different analytical mean field and scaling predictions. Suzuki et al. (DOI: 10.1039/c3sm50907a) use several complementary techniques including X-ray diffraction and dielectric spectroscopy to examine how nanopores in alumina affect the crystallization and local dynamics of a model polymer, varying the pore characteristics including its size and surface properties, as well as cooling rates.

Thin films

The final papers in this group investigate nanometrically-thin polymer films, which provide a natural confinement geometry: both how the thin-film properties are modified relative to the bulk polymers, as well as how they modify the confining surfaces which they contact. Broad-band dielectric spectroscopy is used, together with temperature-modulated calorimetry, by Mapesa et al. (DOI: 10.1039/c3sm51311d), to study the dynamics, including those related to the glass transition, at different length scales of chains confined to layers just a few nm thick. They find variations which depend strongly both on the length scale being probed relative to the layer thickness, and, for entangled chains, also on the entanglement length. Nano-composites of polymers and solid fillers, which can have desirable reinforced structural properties, are strongly influenced by the interactions of the chains at the filler surfaces, and this is treated by Masnada et al. (DOI: 10.1039/c3sm51207j) using a coarse-grained model, in which they are also able to examine the effect of brushes on the filler surfaces. The self-assembly into thin films of electroactive oligomers, as models relevant to organic electronics, was investigated by Dane et al. (DOI: 10.1039/c3sm51407b) using grazing incidence X-ray scattering; the detailed structural information they obtained revealed significant differences to the bulk-assembled materials, with implications for better molecular design of such films.

Ions, or counter-ions, at interfaces and in confinement

Ion specific effects are often intriguing; merely altering the size, charge or valence orbitals of the ions present can have remarkable effects, as has been long known at least since the landmark contribution of Hofmeister.⁸ In this issue dos Santos et al. (DOI: 10.1039/c3sm51057c) investigate, using Monte Carlo simulations, the confinement of poly-cations to water–oil interfaces whilst varying the nature of the simple negatively charged counter-ions present. Ions can be distinguished as having either positive or negative surface activity, depending on whether the energetic cost of partial dehydration offsets the entropic gain for the water. This balance is particularly favoured for large polarizable ions – so called cosmotropes. Dos Santos and Levin show how the adsorption of polycations to the water–non polar interface, an important model for antibacterial action, is particularly enhanced in the presence of cosmotropes such as iodide, which show preference for the surface and so can shepherd the polycation to the interfacial region. The nature of the counterion again comes to the fore in the work of Farina et al. (DOI: 10.1039/c3sm51450a), although here it is the effect of ion valency under inspection. Polyelectrolytes grafted at one end to a solid surface at such a density that they form a brush configuration in water are investigated by simultaneous surface force apparatus and electrochemical methods. Here, trivalent ions are seen to collapse the brush, in contrast to the same ionic strength environment achieved using monovalent ions. Once collapsed, two brush-covered surfaces pressed together lead to an adhesion due to bridging of the multi-valent cations between poly-anionic brushes on opposing surfaces. Counterions between stacks of lipid bilayers provide a further interesting system of confinement, as shown by Dvir et al. (DOI: 10.1039/c3sm51916c): anionic bilayers are separated by a pillow of diffuse counter ions at low electrolyte concentration, but undergo a dramatic condensation at a critical concentration of electrolyte. At this point, free counterions condense onto the lipids and drive a lateral phase separation into domains of charged and neutralised lipids. One further paper concerns properties of ions in confinement, albeit from an entirely new standpoint: Mendonca et al. (DOI: 10.1039/c3sm51689j) study using computer simulation the lubricating properties of ionic liquids between two parallel amorphous carbon sheets. The task involves determination of appropriate potentials and application of MD simulation out of equilibrium to determine the friction force as a function of applied pressure. The success of this method should provides a route to prediction of the lubricating quality, and molecular level mode of action, of different classes of ions and surface types.

Confined particles

Nanoparticles confined in a polymer matrix – polymer nanocomposites (PNCs) – have applications in a multitude of directions from optoelectronics to responsive materials. Two articles in this issue focus on different aspects of PNCs; the first by Srivastava et al. (DOI: 10.1039/c3sm51289d) describe a super-compressible ordered material composed of nanoparticles connected by DNA, with the mechanism underlying the behavior involving a balance between the mechanical stiffness and the entropy of the DNA linkage. Glomann et al. (DOI: 10.1039/c3sm51194d) study how the polymer dynamics and orientation in PNCs varies with the end-termination of the polymer: exchange of hydroxyl to methoxy end-groups leads to a dramatic change from end-grafted chains to adsorbed chains with multiple attachment sites. Another method for controlling the positions or dispersion of particles is through optical trapping, and using this method confinement of particles to certain geometries can be achieved. By imposing 2D pentagonal geometry on a collection of particles as in the work of Skinner et al. (DOI: 10.1039/c3sm51627j) can lead to commensurate or frustrated systems – depending on the number of particles – and thus non-monotonic fluctuations in the phase behavior of the confined particles. In a study by Pandey et al. (DOI: 10.1039/c3sm51879e) solutions of bidisperse particles, attracting one another due to the presence of polymer depletants, are confined between walls: in this case the dynamics of the particles slows monotonically until a gel state is reached.

Wetting & interfacial tension effects in confined systems

Confined systems necessarily involve large surface area to volume ratios, and so wetting and interfacial tension are often crucial in determining the system's properties. Gang et al. (DOI: 10.1039/c3sm51188j) have applied X-ray methods to study the wetting process in highly-defined cylindrical nano-wells, characterizing the stepwise process from thin wetting layer to pore filling to film growth. Setu et al. (DOI: 10.1039/c3sm51571k) study the ‘viscous fingers’, occurring when a liquid of low viscosity displaces a higher viscosity liquid; high resolution microscopy in three dimensions allows detailed analysis of the curvature and flow in the liquid fingers. In the work of Goldian et al. (DOI: 10.1039/c3sm51924d), the amphiphilic protein hydrophobin is used to flip surface polarity depending on the nature of the underlying substrate. This change in protein orientation alters the wettability and hydration properties of the surface, which Goldian et al. are able to correlate with changes in the friction and lubrication behavior between two coated surfaces.

Work in the area of soft materials in confinement is very interdisciplinary in nature, both in terms of the applications and systems under investigation and the methods (experimental and theoretical) used to approach them. This collection provides a snapshot of diverse approaches and highlights, emphasizing the commonalities of confined soft matter from the importance of entropy to ion-specific effects.

Susan Perkin, Oxford University, UK

Jacob Klein, Weizmann Institute of Science, Israel

References

1. P. G. de Gennes, Scaling Concepts in Polymer Physics, Cornell Univ. Press, Ithaca, N.Y., 1979.
2. S. Perkin, R. Goldberg, L. Chai, N. Kampf and J. Klein, Faraday Discuss., 2009, 141, 399.
3. J. F. Joanny, Eur. Phys. J. B, 1999, 9, 117.
4. M. Doi and S. F. Edwards, in The Theory of Polymer Dynamics, Oxford University Press, New York, 1986.
5. P. Pincus, Macromolecules, 1976, 9, 386.
6. K. Binder, Monte Carlo and Molecular Dynamics Simulations in Polymer Science, Oxford University Press, London, 1995.
7. J. Seror, et al., Biomacromolecules, 2011, 12, 3432.
8. F. Hofmeister, Arch. Exp. Pathol. Pharmakol., 1888, 24, 247–260.

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