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Scattering approaches to probing surface layers under confinement

Zengyi Wei, Stuart W. Prescott

PII: S1359-0294(15)00062-XDOI: doi: 10.1016/j.cocis.2015.09.001Reference: COCIS 975

To appear in: Current Opinion in Colloid & Interface Science

Received date: 1 September 2015Revised date: 8 September 2015Accepted date: 8 September 2015

Please cite this article as: Wei Zengyi, Prescott Stuart W., Scattering approaches toprobing surface layers under confinement, Current Opinion in Colloid & Interface Science(2015), doi: 10.1016/j.cocis.2015.09.001

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Scattering approaches to probing surface layers under confinement

Zengyi Weia, Stuart W. Prescotta,∗

aSchool of Chemical Engineering, UNSW Australia, UNSW Sydney NSW 2052, Australia

Abstract

The confinement of soft materials between surfaces is central to the interactions that lead to adhesion, lubrication andcolloidal stability. Force measurements have long been able to describe the potential that exists between various coatedsurfaces, but it is only more recently that scattering techniques have permitted the detailed study of surface layers underconfinement. By focusing on the characterization of interfacial layer structures, there is a growing understanding ofthe structural basis for the previously reported interaction potentials. Work to date has illustrated that the nanoscaleconfinement of adsorbed polymers, polymer brushes, polyelectrolyte multilayers, liquid crystals and lipid bilayers fre-quently leads to unexpected structural features. Approaches to probing interfacial structures with X-ray reflection andneutron reflection are described along with some of the advances in sample environment design that have facilitatedthese explorations.

Keywords: confinement, neutron reflection, X-ray reflection, surface forces, polymers

1. Introduction

Many applications of soft matter utilize molecules atsolid or fluid interfaces; commonly studied examples in-clude the polymer layers that are found at the solid/liquidinterface in coatings, paints, and biomedical implants.Other practical surface layer systems include grafted poly-mer brushes, polyelectrolyte multi-layers and liquid crys-tals. Understanding the forces between surfaces and howthese forces are mediated by the structures of the macro-molecules is essential to on-going efforts to predict andtune surface properties to make bespoke interfaces for in-dividual applications.

The forces between two surfaces must clearly be dif-ferent for the contrasting goals of adhesion and lubrica-tion [1–3], yet these two attributes can both be obtainedby coating the surfaces with polymer brushes [4, 5]. Thesurface force apparatus and atomic force microscope havebeen extensively used to measure interactive forces of in-terfacial systems but they cannot provide information onmolecular structure. Going beyond measurements of forcesand additionally measuring the near-surface structures hasbeen an experimental challenge for a number of decades,with the answers to structure–force questions starting toemerge in recently reported work. The key experimentaldifficulty is that the surface layer structures are at theirmost interesting under confinement, while also being prob-lematic to probe owing to the macroscopic solids creatingthe confined geometry. Obtaining sufficiently reproducibleand accurate confinement is challenging, particularly when

∗Corresponding authorEmail address: s.prescott@unsw.edu.au (Stuart W. Prescott)

the restrictions of any given analytical technique are im-posed.

Scattering techniques are very sensitive to surfacestructures and properties such as layer thickness, layerdensity and surface roughness can be explored [6]. Inparticular, neutron and X-ray reflectometry combine thissensitivity to supramolecular structures with highly pene-trating radiation that can be used to probe what is hap-pening between the two solid surfaces that are providingthe confinement. The demands on a sample environmentthat can impose nanometric confined geometries over asufficiently large sample area for measurement by neutronreflection are strict; however, recent work has shown thata surface-forces-type apparatus for neutron reflection isindeed able to probe interfacial structure under confinedgeometries [7].

Here, we will explore the ability of scattering ap-proaches to probe confined soft matter and examine someof the interesting interfacial structures that have beenfound. We will see that current theory is unable to ra-tionalize some of the effects that have been reported andthat molecules in nanoscale confined geometries can ex-hibit effects that differentiate these environments from thebulk.

1.1. Forces and confinement

The study of confined soft matter has its origins in thesurface forces apparatus (SFA) and the atomic force micro-scope (AFM). These two instruments permit fine controlover the forces and distances at which soft matter can beconfined.

The first simple and direct measurements of attractiveforces between surfaces and molecules were conducted by

Preprint submitted to CURRENT OPINION IN COLLOID AND INTERFACE SCIENCE September 6, 2015

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Derjaguin in 1956 [8]. The method involved bringing twosurfaces close together and measuring the attractive forceas a function of the separation. Most of these measure-ments were performed with glass surfaces and the forceswere measured by deflection of a beam balance. This typeof experiment showed the changes in attractive van derWaals forces with respect to surface separation. The sepa-ration distances were typically between 80 and 2000 nm [9]and provided an experimental verification of van der Waalsinteractions.

In what will become a common theme in this reviewof confinement experiments, the roughness of the surfaceswas the main constraint that prevented the separationdistance going below 100 nm. Freshly cleaved mica, whichcan be molecularly smooth over a large area [10], laterprovided sufficiently smooth surfaces for Tabor and Win-terton to reduce the surface separation to 5 nm using thenow familiar crossed cylinder geometry. These pioneeringdesigns were later refined into the surface forces apparatus[9] that has been widely used to confine systems and mea-sure the forces involved in attraction, repulsion, frictionand lubrication across various systems such as adsorbedpolymers and polymer brushes [11].

Another instrument that directly measures molecularforces is the AFM [12, 13]. AFM force measurements fre-quently make use of a colloidal particle that is attachedto a cantilever tip with known spring constant and thedeflection of the cantilever is measured optically [13]. Bycompressing the tip into a surface, the forces required toconfine and then separate can be measured. The colloidprobe is readily functionalized and thus the forces of con-finement of a range of materials have been studied [2, 13].

1.2. Neutron and X-ray reflection

Though SFA and AFM techniques enable the directmeasurement of forces between molecules or interfaces,they do not give information on the structure of the stud-ied systems. When it comes to structural characteriza-tion, neutron and X-ray reflection have been used withliquid crystals, adsorbed polymers, lipid bilayers and poly-electrolyte multilayers [14–16]. By analyzing reflectiondata, structural properties such as layer thickness, inter-face roughness, and layer density can be obtained [6, 17].

For the study of confinement, specular reflection is thedominant technique, where elastic scattering means thatthe magnitude of the wavevector of the beam does notchange after scattering, as illustrated in Figure 1. Forboth X-ray and neutron reflection, the reflectivity is usu-ally written in terms of the change in wavevector betweenthe incident and reflected beams. The scattering vector isgiven by Q = k1−k0 where k0 and k1 are the incident andreflected wavevectors respectively. For specular reflection,it is only the magnitude of this vector that is importantand this is normal to the surface (by convention, the zdirection):

Q = Qz =4π sin θ

λ(1)

Figure 1: In a reflectometry experiment, the incident beam ap-proaches the interface at a shallow angle (exaggerated in this figure)and may reflect off each interface in a layered structure. For specu-lar reflection, the incident and reflected wavevectors have the samemagnitude k and it is only the magnitude of the scattering vector,Q, that is considered.

where λ is the wavelength of the incident radiation andθ is the angle of incidence. Data are normally presentedas either a reflectivity profile (R vs Q), or a reflectivityprofile in which the Fresnel component of the reflection isremoved (RQ4 vs Q).

As shown in Figure 1, the reflection of the incidentbeam from different interfaces leads to a variation in theoverall path length from source to detector. Thus, therefection profile describes the thickness of each of the layersin the structure in terms of an interference pattern, withBragg peaks arising from structures with an internal peri-odicity and Kiessig fringes describing the overall thicknessof the sample [6].

In the layered structure shown in Figure 1, each layer isalso constructed from different materials and so interactsdifferently with the beam. The scattering length density(SLD) of a material is a component of the refractive indexfor the beam in the material and spatial changes in SLDlead to the observed scattering patterns. The beam isrefracted as it passes through each interface and steps inρ(z) can lead to total internal reflection at sufficiently lowQ, giving a sharp feature in R(Q) known as a critical edge.Overall, the R(Q) profile depends on the SLD profile,ρ(z), including both the SLD and the thickness of eachlayer. While this description has been developed in termsof discrete layers, it is readily generalized to smooth SLDprofiles.

Neutron and X-ray reflection are similar techniques,however, the basic physics of the interaction is different:X-ray scattering is sensitive to electron density while neu-tron scattering is sensitive to the nuclei present. For eachmaterial, the SLD for X-ray and neutron beams is con-sequently different and so quite complementary data areobtained from the two techniques. In both cases, it is ρ(z)that is used in the data analysis; iteratively fitting plausi-ble structural models to the experimental data providesthe desired information, with a ρ(z) being a necessaryintermediate result.

A significant difference between X-ray and neutronscattering is that isotopic substitution provides a simpleway to alter the SLD in neutron scattering, while the SLDis invariant for the equivalent X-ray experiment. Conse-quently, in neutron reflection, interfaces can be made invis-ible in an approach analogous to optical refractive indexmatching: matching the SLD of two layers and making

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ρ(z) a constant across that interface hides the interfacefrom view. The most common substitution is D (2H) for H,with the prevalence of hydrogen in the solutes, adsorbatesand solvents used in soft matter systems making this an ex-tremely versatile approach to selectively highlighting partsof the sample. Contrast matching or contrast variationgenerates richer data sets that allow robust conclusions tobe drawn from the data analysis.

2. Confining adsorbed polymers

The wide range of applications for adsorbed polymershas made these systems highly desirable subjects for con-finement investigations and a wealth of SFA and AFMforce measurements underpin understanding in this field.Partnered with the experimental characterization of ad-sorbed polymers, the theoretical framework of Scheutjens–Fleer theory [18] provides a model to describe the polymervolume fraction profile in a solvent; this lattice modelmakes experimentally verified predictions for the volumefraction profiles [19–21] with NMR, small-angle neutronscattering (SANS) and neutron reflection playing impor-tant roles.

In probing the adsorbed polymers with neutron re-flection, a significant technical limitation must be consid-ered: a large neutron beam footprint (500 to 2500mm2)is required in order to finish one experiment in a rea-sonable time frame [7]. The confined interface needs tobe uniformly distributed over a large area with a verylow amount of roughness and excellent parallelism of thetwo surfaces. It was not until 1994 that Cosgrove andZarbakhsh developed a thin film confinement apparatus inan attempt to measure the structures of confined adsorbedpolymers [22, 23]. The apparatus was equipped with twoquartz plates between which the sample was placed and theupper plate was controlled by a hydraulic ram; a pressureas high as 130 bar could be applied to the upper plate.

The parallel-plate confinement cell of Cosgrove et al.

enabled the study of the overlap of 20 × 106 g/mol poly-styrene adsorbed from a poor solvent [23]. A pressure–separation relationship was derived that indicated thereis a monotonically increasing repulsion as separation dis-tance decreases. The onset of the interaction betweenthe polymer layers started at around 330Rg, where theradius of gyration, Rg, of the polymer used was 590Å.Volume fraction profiles were obtained from the reflectivityprofiles; they showed increasing polymer fractions at thesurface with increasing compression, which means polymerlayers start to overlap under compression.

The structure of block copolymers under compres-sion was investigated by Cosgrove et al. using the sameneutron reflection apparatus [22]. In this experiment, apoly(styrene-b-ethylene oxide) copolymer was chosen andadsorbed on the quartz blocks from solvents includingtoluene and octane. Fitting the reflectivity data with thesurface Guinier equation shows that under pressure, thesecond moment of the thickness of the layer decreases,

Figure 2: The parallel-plate confinement cell aims to achieve con-finement between two silicon or quartz plates as in (a) but tends tosuffer from (b) long range waviness in the blocks, (c) block misalign-ment, (d) entrained dust. The self-aligning design of Kuhl et al. [24]addressed the alignment problem while the flexible-film confinementcell of de Vos et al. [7] shown in (e) also addressed the remainingissues.

indicating a strong repulsive interaction. This finding isconsistent with the observation that there will be an in-crease in required pressure to move two plates closer. Uponswitching from toluene (good solvent) to octane (poor sol-vent), a 35% decrease in the pressure required to maintaina separation of 700Å was observed. In poor solvents, thepolymer collapses and the steric barrier to confinement isreduced.

The parallel-plate design also suffered from some limi-tations in its ability to confine materials for the reflectionexperiments. However, the original pressure system ofthe apparatus was unstable; the hydraulic system couldnot maintain constant pressure, leading to a variation inthe plate separation distance [22]. Cox et al. introduceda pneumatic system to the apparatus which reduced thepressure drift to 0.1 bar [16]. The various experimentalchallenges faced by this style of parallel-plate confinementdesign are illustrated in Figure 2. Maintaining the parallelalignment of the blocks over such a large area was found tobe a problem, with the flatness of the blocks (long-rangeflatness not short-range roughness) significantly limitingthe achievable confining separations. Cleanliness over thefull area of the blocks was also difficult to maintain andentrained dust prevented close approach of the confiningblocks.

3. Confining polymer brushes

Polymer brushes, dense arrays of end-attached poly-mers, have been widely studied as ways of robustly tai-loring interfaces. While adsorbed polymers show an equi-librium between the free and bound states [25], polymerbrushes are more stable. Recent synthetic work indicatesthat it is entirely feasible to produce polymer brushes thatare responsive to stimuli such as pH, ionic strength and

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temperature [26]; understanding the structures of stimuliresponsive brushes is an area of on-going research.

The conformation and equilibrium properties of poly-mer brushes have been extensively studied with a combi-nation of experimental and theoretical approaches. Thestructures of brushes, both confined and unconfined, aredependent on the balance between the steric crowdingwithin the brush and the interactions with the solvent.Brushes have a parabolic volume fraction profile [27] andthe thickness of the brush layer is linearly dependent onmolecular weight [5, 28].

Self-consistent field theory has also been used to lookat the energy required to confine brushes, predicting thatthe free energy required to compress the brush increasesas a markedly with decreasing compressed distance [27].

Considerable experimental effort has been expendedprobing the structures of polymer brushes under confine-ment although, until recently, progress was limited for thesame reasons as described earlier: ultra-flat surfaces overa relatively large beam area are difficult to prepare. Avariation on the confinement cell of Cosgrove et al. was de-veloped by Kuhl et al., adding the significant features of aself-alignment gasket to maintain parallelism of the blocksand the ability to apply lateral shear stress to the confinedsample [24, 29], as shown in Figure 3a. This neutronreflection apparatus confined fluids between quartz andsapphire surfaces and was used to investigate the structureof a range of materials including a poly(styrene-b-ethyleneoxide) brush under shear [29]. It was concluded the brushstructure is robust and shear rate does not change thebrush volume fraction profile.

The effects of confinement on brushes of poly(styrene-b-2-vinylpyridine) (PS-P2VP) in toluene were also stud-ied [3]. In this system, the P2VP chains wet the sapphiresubstrate surface effectively making a PS brush extendinginto the toluene. The experiments revealed that uponconfinement, the density profile of the overlap region flat-tens out and there is an increase in PS density at theP2VP-PS interface (Figure 4). In agreement with mean-field theory predictions, the PS brushes compress underconfinement, with interpenetration also occurring in theoverlap region [3]. The effect of shear on the structureof the PS brush was also studied in this confinement cellusing a ball slide attached to a motor to translate one ofthe confining surfaces [30]. It was found that a new disen-tangled structure was formed as a result of the applicationof shear to the brushes and that the relaxation of thisstructure appeared to take weeks.

Better stabilizing the confinement pressure permittedhigher quality data to be obtained at a separation of1000Å (at 400 psi) [31]. It was found that the PS brushescompressed to 25% of their original extension in tolueneand that the polymer volume fraction profile became in-creasingly flat with increasing pressure [32]. Mulder et

al. [33] studied the interpenetration of brushes by selec-tively deuterating the brush on one of the confining sur-faces (one side d-PS, the other side h-PS). By also varying

Figure 3: (a) Cross-section of the parallel-plate confinement cell ofKuhl and coworkers. Loads were applied with a hydraulic ram andthe mounts for the quartz blocks allowed the blocks to self-alignto parallel and also permitted shear experiments on the confinedmaterial. From [3], Journal of Polymer Science, Part B: Polymer

Physics, 42(17), 3290. Copyright c© 2004 Wiley Periodicals, Inc.(b) Schematic diagram of the flexible-film neutron confinement cellof de Vos and coworkers. From [7], reprinted with permission fromReview of Scientific Instruments, 2012, 83, 113903. Copyright c©2012, AIP Publishing LLC.

the deuteration of the toluene to match the scatteringdensity of h-PS, the extension of the high contrast d-PSside was observed. The overall approach to the analysisis shown in Figure 5, including the raw reflection data,the fitted SLD profile and the consequent volume frac-tion profiles. By increasing the confining pressure, theinterpenetration amount increases and the brush extensionheight decreases.

In a rather different approach to the problems ofachieving parallelism and good contact over a large area, aneutron confinement cell design that has one solid surfaceand one flexible surface was developed by de Vos et al. [7]as shown in Figure 3b. The flexible surface is a 50µm thickpoly(ethylene terephthalate) film that is able to conformto long range waviness or bend around any entrained dust(illustrated in Figure 2e), thus solving the problems of flat-ness and cleanliness highlighted above. The confinement

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Figure 4: A PS brush under confinement in air (in the absence ofsolvent). (a) The reflectivity profile showing the oscillations due tothe polymeric layer. (b) The corresponding SLD profile showing thephase-separated P2VP and PS layers with an air gap. These dataillustrate the difficulty in making molecular contact between the twosurfaces in the parallel-plate confinement cells. From [3], Journal of

Polymer Science, Part B: Polymer Physics, 42(17). Copyright c©2004 Wiley Periodicals, Inc.

limit was pushed well below 100 nm by this design andmolecular contact was achieved between the two interfacesover areas upwards of 10 cm2 [7].

The flexible-film apparatus was used to study the hy-dration behavior of poly(ethylene oxide) (PEO) and poly-(acrylic acid) (PAA) under confinement as functions ofgrafting density and pH [34]. Consistent with previousstudies and with numerical self-consistent field theory cal-culations (nSCF), the volume fraction of both PEO andPAA brushes changes from parabolic to uniform (also de-scribed as ‘block-like’) under confinement. The experi-mental data also showed that the pressure required todehydrate the polymer brushes is not dependent on the

Figure 5: Interpenetration of h-PS and d-PS brushes as studied in aparallel-plate confinement cell at three different confinement separa-tions (1440, 1125, and 980Å) (a) Reflectometry data (b) SLD profilesthat are fitted to the reflection data (c) volume fraction profilesshowing the change in the degree of interpenetration. Reproducedfrom Ref. [33] with permission from The Royal Society of Chemistry.

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grafting density but for weakly charged brushes (PAA),higher pressure is required at higher pH, which means fullycharged polymer brushes can resist moderate mechanicalconfinement.

Further comparison with nSCF calculations shows thatwhile the theory successfully predicts the overall trendsin volume fraction profile under confinement, there is asignificant difference in the magnitude of the pressure re-quired to confine brushes. Theory seems to indicate thatapproximately 100 bar is required to collapse the brushes,while experimentally, Abbott et al. [34] found that brushcollapse could be effected with only 1 bar. SFA and AFMforce measurements similarly agree that a polymer brushcollapses at a much lower pressure than would be expectedby the straightforward application of osmotic pressure ornSCF theories. It is not clear at this stage what form oftheoretical framework will be required to rationalize theobserved difference between theory and experiment. Itappears that the confinement of brushes is an exemplarcase of the inapplicability of bulk intuition to the nanoscaleand where confinement can produce some truly unusualresults. Naturally, further investigation of these effects ison-going.

The interpenetration of brushes has also been probedin the flexible-film confinement cell [35]. By coating bothof the confining interfaces with different polymer brushes,it was possible to probe the degree of interpenetrationunder different conditions of charge and polymer identity;both uncharged (h-PEO and d-PEO) polymers and weakpolyelectrolytes (poly(dimethylaminoethyl methacrylate),PDMAEMA) were studied. While uncharged polymers donot interpenetrate (as predicted by nSCF theory), when acharged polymer is confined against an uncharged poly-mer, the Coulombic interactions are sufficient to driveinterpenetration by reducing the density of charges withinthe charged brush. It was found that the degree of inter-penetration is switchable by varying the pH across thepKa of the brush, thereby tuning the charge on the weakpolyelectrolyte.

4. Confining polyelectrolyte multilayers

Polyelectrolyte multilayers (PEMs) are formed fromthe alternating deposition of cationic and anionic polyelec-trolytes on a suitable substrate. Layer-by-layer depositioncan be undertaken by spin coating or dip coating and thePEMs produced have properties that are attractive fora range of applications including photonic crystals [36],tunable permeability membranes for ultrafiltration, andin drug delivery [37]. The water content in the PEM is animportant parameter since the degree of hydration altersthe mechanical properties [38]; the hydration and overallthickness of the PEM have been found to be dependenton the last (terminal) layer added to the PEM, followinga saw-tooth pattern known as the odd–even effect [39].The influence of temperature, salt concentration and pHon the hydration of PEMs have been studied [39] although

comparatively little work has been undertaken on under-standing the changes in PEM structure. That the PEM isalready a layered structure makes reflectometry an idealtechnique.

In neutron reflection experiments using the flexible-filmconfinement cell described above, de Vos et al. investigatedthe structure and hydration behavior of PEMs under con-finement [40, 41]. The poly(styrene sulfonic acid)/poly-(allylamine hydrochloric acid) (PSS/PAH) PEMs havebeen widely studied with a variety of techniques includ-ing neutron reflection. The availability of deuterated PSSoffers a means of altering the contrast within the layer-by-layer structure to highlight changes in internal structurevia changes in Bragg peaks from the repeating deuteratedlayers. As with other samples, changes in the overallthickness of the hydrated layer are visible in changes inthe Kiessig fringes. The water content within the layerstructure was found to vary non-uniformly as the PEMswere confined; however, for PAH-terminated PEMs, innerlayers (near the silicon substrate) are dehydrated morereadily than outer layers. The degree of hydration ofPSS-terminated PEMs was always higher than that ofPAH-terminated PEMs even under confinement. Theseobserved changes in hydration and compressibility are thestructural manifestation of the odd–even effect and, impor-tantly, illustrate that the effect is not only a surface effectbut is instead a structural change throughout the PEM.It appears that the difference in the charge distributionwithin the PEM structures as induced by the charge ofthe final absorbed layer is responsible [40, 41].

5. Confinement in polymer films

Molecules can also be confined within solid films and,in the case of macromolecules, the confinement is achievedas the thickness of the film approaches the radius of gyra-tion of the polymer. Thin polymer layers, formed by spincoating on a substrate, confine the chains even withoutan external applied force [42, 43]. In a series of stud-ies, grazing incidence small-angle neutron scattering andgrazing incidence small-angle X-ray scattering were usedto investigate the behavior of confined polystyrene filmsand polystyrene/poly(p-methyl styrene) blends [43, 44].Confined within the thin films, the in-plane length scale ofpolymer chains was seen to increase, indicating a distortionin the structure. The polymer blend films also started toundergo a confinement-induced phase separation as thestrong repulsion forces increased. Dewetting behavior af-ter annealing was also observed, with the confinement ofthe chains in the film rendering the films less stable andprone to dewetting. A critical film thickness was calculatedfor the film annealing process.

Another confinement method for thin polymer filmswas investigated by Lambooy et al. [45], who used a sil-icon oxide cap layer (deposited by a vapor depositiontechnique) to confined thin organic films. The struc-tural behavior of diblock copolymers such as poly(styrene-

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Figure 6: Neutron reflectivity data and fits for PSS-terminated PEMs with (a) 10.5 bilayers, (b) 22.5 bilayers, and (c) 46.5 bilayers. Theimpact of confinement is seen in the movement of both the fringes and the Bragg peaks to higher Q (smaller lengths) and was analyzed byAbbott et al. in terms of the non-uniform dehydration of the PEM [40]. Reprinted with permission from Macromolecules, 2014, 47(10), 3263,Copyright c© 2014 American Chemical Society.

b-methyl methacrylate) was studied by neutron reflection,with the perturbation of the lamellar structure being ofprincipal interest. The studies showed that under confine-ment, the layers could either contract or expand with themain requirement being that the multilayer structure hadan integer number of layers [46–49]. The energy of thesystem is a fine balance of wetting and demixing/phaseseparation such that an unexpected outcome, the expan-sion of the layers upon confinement, could be observed.

6. Confining liquid crystals

Within liquid crystals (LCs), molecules are orientedto form different phase domains within the bulk liquid.Orientation is typically driven by anisotropic molecularshape and changes to the ordering can be induced by var-ious factors such as temperature, concentration and elec-trical fields. These ordered structures are widely used inelectronic displays in which molecular orientation leads totunable optical properties. LCs also offer lubricity underconfinement, with the structural aspects of the LC beingkey to this property [50, 51].

Studies of LCs under confinement were enabled by thedevelopment of the X-ray surface forces apparatus (X-SFA) by Idziak et al. [15], in which complex fluids canbe confined between two crossed-cylinders and exposedto shear stresses. Using this equipment combined withsynchrotron X-ray scattering, confinement-induced effectson the layer orientation of the LC 4-cyano-4′-octylbiphenyl(8CB) were studied [52]. With an inter-surface separationof 3900Å, shearing did not change the 8CB domain ori-entation, indicating the dominant effect of confinement onlayer orientation [15]. Later improvements to the X-SFAallowed changes of stiffness in the confining surfaces [53].8CB shows a critical gap for the orientation transition onlyfor soft confining surfaces, which permit the LC domainsto rearrange and break into smaller and less orientateddomains [54].

The effects of temperature and confinement on thestructure of thermotropic LCs 8CB and 10CB were studiedby neutron reflection using the flexible-film confinementcell of de Vos et al. [55]. The results of these measure-ments indicated that, in very thin films, neither confiningpressure nor temperature have effects on the phase of 8CBunder confinement. Lyotropic LCs of CTAB, hexanol andwater were also studied under confinement but no conclu-sive evidence of confinement influencing phase orientationwas found [55].

Lipids bilayer stacks are another LC system that hasbeen studied in confined geometries. In a lipid bilayer,the hydrophobic tails of the lipid are separated from thewater in a structure that has a close resemblance to a cellmembrane. Understanding the structural behavior of lipidbilayers under different conditions, including confinement,offers insights into the relationships between biomolecularstructure and function within the cell membrane. Workin this area has the potential to help explain the behaviorof drug delivery vectors and, in the long term, offer newinsights into their design [56].

Figure 7 shows preliminary neutron reflection ex-periments conducted with the flexible-film confinementcell on lipid bilayer stacks of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) swollen with D2O [7]. Fromthe Bragg peaks in Figure 7, it can be deduced that theinter-layer separation reduces as the confining pressure isincreased. Further experiments investigated the structureand hydration behavior of stacks of various chain lengthphosphatidylcholine lipids (DLPC, DMPC, DPPC, DSPC,DOPC) [55]. A confinement-induced phase transition wasidentified, whereby confinement changes the water contentaround the hydrophilic heads of the lipid molecules. Thischange permits the lipid tails to straighten within eachbilayer and causes the inter-layer spacing of the bilayerstack to increase. These data would appear to have someinteresting consequences for the further development of arobust understanding of the biophysics of cell membranes.That a modest confining pressure can cause the head of

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Figure 7: Confinement of a stack of approximately eight lipid bilayersof 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) swollen withD

2O. The shift in Bragg peak position indicates an overall reduc-

tion in the inter-layer spacing, while further data processing showsa reduction in head hydration with confinement. From [7], reprintedwith permission from Review of Scientific Instruments, 2012, 83,113903. Copyright c© 2012, AIP Publishing LLC.

a lipid to dehydrate and result in a phase change withinthe tails of the lipid bilayer implies that the collisionsand confinement of lipid bilayers will include a rich phasebehavior and that the dynamic, confined systems mustbe considered in addition to equilibrium, unperturbedsystems.

7. Conclusions

Confinement has been seen to result in changes to ma-terial properties by inducing changes to mixing, wetting orcrystal forms. The confinement-induced phase transitionsof lipid bilayer stacks (section 6) are a particularly goodillustration of the unexpected effects that can be observedin confinement and why confinement is important. Con-fining soft matter to nanoscale dimensions can profoundlychange the internal structure and the resultant behaviorof the material.

While much progress has been made in understandingthe structural basis of interfacial properties such as lubric-ity and adhesion, there is considerable room for further de-velopment of experimental techniques for conducting con-finement experiments. It is clear from the survey presentedhere that none of the fields of research outlined above couldbe considered complete; along with further experimentsto generalize the effects that have been reported to date,there is also a clear need to extend the current theoreti-cal understanding of soft matter at interfaces so that theeffects of confinement can be adequately rationalized andpredicted.

Experimentally, it is reasonable to suppose that a fur-ther generation of confinement cell will in time be de-veloped to build upon the innovative design ideas of therecent confinement cells. The ability to impart a lateral

shear stress on the confined materials is a feature thatoffered suggestive and intriguing data; combining this abil-ity with the experimentally forgiving flexible-film designswould enable a range of further confinement experiments.Continued advances in beam-line technology, particularlyincreases in the available flux or the ability to capture largeQ ranges in a short period of time, will be beneficial. Ahigher flux will allow designs to use smaller footprints andthat relaxes many of the difficult design constraints thathave limited current sample environments. The feasibilityof kinetic experiments would also improve, permitting asophisticated understanding not only of the initial andfinal states of a confinement experiment, but also the in-termediate structures involved.

Overall, we see that confinement is about much morethan just a dimension; it is also about restricting the shapeand motion of molecules in such a way that new propertiesmay be observed.

Acknowledgements

Many thanks to Prof. Robert Richardson (Universityof Bristol) and A/Prof. Patrick Spicer (UNSW) for theirhelpful comments on this review. The financial supportof a Faculty Research Grant from UNSW Engineering isgratefully acknowledged.

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Graphical abstract

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• Nano-scale confinement of soft matter leads to unexpected interfacial

structures

• Confined geometries can cause molecular rearrangement and structural

changes

• Non-uniform dehydration, phase separation and phase changes have been

observed

• Confined interfacial structures can be probed by neutron and X-ray

reflectometry

• Development of new sample environments has enabled progress in the

field

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