Neutron reflectivity , interfacially

In the last 10-15 years, neutron reflectometry has been developed into a powerful technique for the study of surface and interfacial structure, and has been extensively applied to the study of surfactant and polymer adsorption and to determine the structure of a variety of thin films [14, 16]. Neutron reflectivity is particularly powerful in the study of organic systems, in that hydrogen/deu-terium isotopic substitution can be used to manipulate the refractive index distribution without substantially altering the chemistry. Hence, specific components can be made visible or invisible by refractive index matching. This has, for example, been extensively exploited in studying surfactant adsorption at the air-solution interface [17]. In this chapter, we focus on the application of neutron reflectometry to probe surfactant adsorption at the solid-solution interface. 

Interface between two liquid solvents — Two liquid solvents can be miscible (e.g., water and ethanol) partially miscible (e.g., water and propylene carbonate), or immiscible (e.g., water and nitrobenzene). Mutual miscibility of the two solvents is connected with the energy of interaction between the solvent molecules, which also determines the width of the phase boundary where the composition varies (Figure) [i]. Molecular dynamic simulation [ii], neutron reflection [iii], vibrational sum frequency spectroscopy [iv], and synchrotron X-ray reflectivity [v] studies have demonstrated that the width of the boundary between two immiscible solvents comprises a contribution from thermally excited capillary waves and intrinsic interfacial structure. Computer calculations and experimental data support the view that the interface between two solvents of very low miscibility is molecularly sharp but with rough protrusions of one solvent into the other (capillary waves), while increasing solvent miscibility leads to the formation of a mixed solvent layer (Figure). In the presence of an electrolyte in both solvent phases, an electrical potential difference can be established at the interface. In the case of two electrolytes with different but constant composition and dissolved in the same solvent, a liquid junction potential is temporarily formed. Equilibrium partition of ions at the - interface between two immiscible electrolyte solutions gives rise to the ion transfer potential, or to the distribution potential, which can be described by the equivalent two-phase Nernst relationship. See also - ion transfer at liquid-liquid interfaces. 

Polymer-polymer interfaces are an important area of study since the interfacial behaviour is fundamental to the bulk properties of the system. This is particularly true when two or more polymers are mixed to form a blend, but the interface also plays a dominant role in areas such as adhesion, welding, surface wetting and mechanical strength. To understand fully polymer behaviour in such applications, the interface must be characterised at a microscopic level. Through deuterium labelling the interface between otherwise indistinguishable polymers can be studied, and neutron reflectivity provides unprecedented detail on interfacial width and shape. In addition to the inherent interdiffusion between polymers at a polymer-polymer interface, the interface is further broadened by thermally driven capillary waves. Capillary waves... 

Extensive neutron reflectivity studies on surfactant adsorption at the air-water interface show that a surfactant monolayer is formed at the interface. Even for concentration cmc, where complex sub-surface ordering of micelles may exist,the interfacial layer remains a monolayer. This is in marked contrast to the situation for amphiphilic block copolymers, where recent measurements by Richards et al. on polystyrene polyethylene oxide block copolymers (PS-b-PEO) and by Thomas et al. on poly(2-(dimethyl-amino)ethylmethacrylamide-b-methyl methacrylate) (DMAEMA-b-MMA) show the formation of surface micelles at a concentration block copolymer, where an abrupt change in thickness is observed at a finite concentration, and signals the onset of surface micellisation. 

For all these specialty polymers, deuterium can be used as a label on one or the other monomer. Deuterium labeling allows the use of techniques based on ion detection such as forward recoil spectrometry (FRES), nuclear reaction analysis (NRA) or secondary ion mass spectrometry (SIMS). If a high-resolution depth profile of the interfacial region is needed, neutron reflectivity can also be used. The main drawback of that approach is the cost of the deuterated polymers while deuterated styrene and methyl methacrylate are expensive but commercially available, other monomers need to be synthesized and the cost can be quite prohibitive. 

Measurements of the interfacial width of the interfaces by neutron reflectivity [73] show that this composition corresponds to the situation where the interface is symmetrically broadened. However, the simple approximation used to obtain Xrcp-PS and Xrcp-PMMA n0 longer holds since Xps-pmma is composition-dependent in this case. 

Interfacial agents, such as block copolymers, are known to reduce the Interfaclal tension and hence are expected to Increase the degree of dispersion in blends. The measurement of Interfacial tension for polymer systems is not easy. Most measurements have been made by the pendant drop technique. Measurements of Interfacial thickness are also difficult. They have been made using electron microscopy and, mostly in the case of block copolymers, by x-ray and neutron scattering. Recent results using neutron reflection suggest that this will be a useful technique in the future. 

In this review we focus on polyelectrolyte-surfactant interactions at solid-liquid interfaces as studied with surface force measuring techniques. The last years have seen much progress in this area, and it is timely to recapitulate some main findings. It is, however, clear that in order to understand interfacial properties of polyelectrolyte-surfactant systems one needs to understand bulk association. Further, a multitude of experimental techniques needs to be applied. Recent advances have been made using ellipsometry [34,35], reflectometry [36,37], neutron reflectivity [38], and surface sensitive spectroscopic techniques [39,40], It is also our belief that the... 

Polymerization of coniferyl alcohol is attempted at the air/water interface. The polymerization process was monitored by surface tension, ellipsometry and neutron reflectivity. The formation of the interfacial layer was found to proceed according to two steps formation of a dilute layer and then densification. 

Therefore, complete unfolding of the polypeptidic chain at the interface leading to a train-loop-tail model, similar to the conformation of copolymers at interfaces, is to be considered as a limiting case only. Even for a disordered and uncross-linked protein such as /3-casein, this type of conformation at air-water or oil-water interface may not in fact be totally realistic [200]. It provides however a convenient basis to derive a thermodynamical analysis of protein interfacial layers from polymer theories [201-203]. Recent specular neutron reflectance studies on protein adsorption layers at the air-water interface [204] show that the protein density profile normal to the interface is qualitatively similar for /3-casein and BLG and consists of a protein-rich layer, about 15 A thick, close to the interface, and a... 

As mentioned above, the study of the dynamics of adsorption layers at liquid interfaces is mainly restricted to surface and interfacial tension measurements. Only for slow adsorption processes, methods such as radiotracer technique [163, 164], the significantly improved surface ellipsometry [165, 166], or the very recently developed technique of neutron reflectivity [167, 168, 169, 170] can be used to directly follow the change of surface concentration with time. Neutron reflectivity allows even distinguishing between different species adsorbed at a fluid interface [171, 172, 173]. These techniques are reviewed in more detail in the preceding chapter 3 as they yield data most of all for the equilibrium state of adsorption layers. 

Figure 1 shows the interfacial thickness X, measured by ellipsometry for the blend systems PS/PMMA, PMMA/SAN-5.7 and PMMA/SAN-38.7. The systems containing random copolymers show a relatively thick interface. This is caused by their small polymer-polymer interaction parameter Xab and will be discussed below. The interfacial thickness in the system PS/PMMA increases slightly with temperature. The value at 120 °C was obtained by neutron reflectivity and was taken from ref. 4. 

Figure 1. Temperature dependence of the interfacial thickness measured by ellipsometry. The value at 120 °C was obtained by neutron reflectivity and was taken from the literature... 

Nevertheless, much is known about the structure of adsorbed 6-casein, certainly more flian is known for any other food protein, and various techniques have been used to study the adsorbed protein. The first evidence from DLS showed that 6-casein adsorbed to a polystyrene latex caused an increase in the radius of the particle by 10 to 15 nm (84). Later studies using small-angle X-ray scattering confirmed this and showed, in addition, that the bulk of the mass of the protein was close to the interface, so the interfacial layer was not of uniform density throughout (85). Neutron-reflectance studies also showed that most of the mass of protein was close to the interface (86). Only a relatively small portion of the mass of the adsorbed protein extends from the tightly packed interface into the solution, but it is this part which determines the hydrodynamics of the particle and which is almost certainly the soiuce of the steric stabilization which the 6-casein affords to emulsion droplets (84). It is to be noted that all of the studies just described were performed on latex particles or on planar interfaces however, it has also been demonstrated that the inter-facial structiues of 6-casein adsorbed to emulsion dro plets resemble those of the model particles (39, 85). Although detailed control of emulsion droplets dining their... 

If we take typical values of x and substitute into equations (4.3.11) and (4.3.12) we will typically find that we would expect the interfacial width to be a few tens of angstrom imits wide and the interfacial tension a few mJm . Experimental results are relatively sparse these rather low values of interfacial tension are very difficult to measure in polymer melts with their very high viscosities even for relatively modest molecular weights. Interfacial widths of these thicknesses are only practicably measurable by neutron reflectivity. 

Experiments in which interfacial widths were directly measured have been summarised by Stamm and Schubert (1995). The most studied pair has been polystyrene and poly(methyl methacrylate), whose interfacial width has been measured directly using neutron reflectivity. The initial experiments were done by two groups, Anastasiadis, Russell and co-workers (Anastasiadis et al. 1990) and Fernandez, Higgins and co-workers (Fernandez et al. 1988) both obtained the virtually identical result that the interface was described by a tanh profile with a width of 25 A This result is consistent with the interfacial tension measurement, but the prediction of equation (4.3.11) using an independently measured value of % (Russell et al. 1990) is for a width of 14.9 A. 

Interpretations of the experiments that have been performed in this area have concentrated on the second aspect. Karim and co-workers (Karim et al. 1990) and Stamm and co-workers (Stamm et al. 1991) both used neutron reflectivity to look at the very early stages of the interdiflfusion of polystyrene and deuterated polystyrene. Figure 4.26 shows the results of Karim et al. The graph depicts the extent of interfacial broadening as a fimction of time and temperature, whereby a WLF shift factor (equation (4.4.9)) has been used to reduce the data for a number of different temperatures on to one curve. At very early times... 

Figure 4.26. Interfacial broadening as a function of time for polyst5n-ene and deuterated polystyrene, both of relative molecular mass 200000, as measured by neutron reflectivity. Data taken at various temperatures have been reduced to a reference temperature of 120 C by using a WLF shift factor. After Karim et al. (1990).

The cases in which one or both of the copolymer blocks form a wet brush are somewhat more complicated and are discussed by Dai et al. (1992). Our discussions in section 6.2 should lead us to suspect that this scaling approach is not likely to be completely accurate - we know from neutron reflectivity measurements (for example those shown in figures 6.14 and 6.20) that, even in the dry-brush regime, there is substantial penetration of the brush by the homopolymer. Nonetheless, the theory does succeed in capturing much of the physics and allows at least semi-quantitative predictions of the interfacial excess and interfacial tension. 

Figure 7.3. Fracture energies of interfaces between polystyrene and poly(/)-methyl styrene) of various relative molecular masses (A, PS 1 250 000 and PpMS 570 000 o, PS 310 000 and PpMS 570 000 and O, PS 862 000 and PpMS 157 000) as functions of their interfacial widths, measured by neutron reflectivity. After Schnell et al. (1998).

Another interfacial rate process that has been invoked to explain dynamic surface tension data for some pure nonionic surfactants is reorientation of adsorbed surfactant molecules between a state in which they he nearly flat along the surface, which is favored at low values of T, and a state in which their orientation is nearly vertical, which is favored at high values of E (Fainerman et al., 1996 Miller et al., 1999). Neutron reflection data are eonsistent with sueh an interpretation (Lu et al., 1993). Further discussion and references dealing with... 

For a given model of the structure normal to the interface, no matter how complex, it is possible to calculate the neutron reflectivity exactly using the same formulae, apart from the difference in the refractive index, as for light polarized at rightangles to the plane of reflection. For a multilayer structure the optical matrix method [4] can then be used, in which the interface is divided into as many layers as are required to describe it with adequate resolution. This method lends itself especially well to machine calculations and is therefore the most widely used method of analysing neutron reflectivity. However, it does not reveal the relatively simple relation between reflectivity and interfacial structure, which can be done more clearly using the kinematic approximation. In the kinematic approximation the reflectivity profile is given by [5,6]... 

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