Neutron reflection bilayer

Neutron reflectivity is ideally suited to this problem, since concentration profiles can be resolved on the nanometer level and since, for an infinitely sharp interface, Rkjf will approach asymptotically a constant value. In addition, neutron reflectivity is nondestructive and multiple experiments can be performed on the same specimen. Figure 4 shows a plot of Rk Q as a function of bilayer of protonated... 

Langmuir mono/bilayers of mono/multidendrons based on 3, 5 - dihydroxyben-zyl alcohol at the air - water interface were studied using pressure - area isotherms and neutron reflectivity by Frechet et al. [108],... 

The more recent neutron reflectivity studies have established that flattened surface micelle or fragmented bilayer structure in more detail and with more certainty, using contrast variation in the surfactant and the solvent [24, 31]. However, the extent of the lateral dimension (in the plane of the surface) and the detailed structure in that direction is less certain. From those neutron reflectivity measurements [24, 31] and related SANS data on the adsorption of surfactants onto colloidal particles [5], it is known that the lateral dimension is small compared with the neutron coherence length, such that averaging in the plane is adequate to describe the data. The advent of the AFM technique and its application to surfactant adsorption [15] has provided data that suggest that there is more structure and ordering in the lateral direction than implied from other measurements. This will be discussed in more detail in a later section of the chapter. At the hydrophobic interface, although the thickness of the adsorbed layer is now consistent with a monolayer, the same uncertainties about lateral structure exist. 

A characteristic of the early neutron reflectivity studies of nonionic surfactant adsorption was some variability in the pattern of adsorption. This was investigated in more detail and more systematically by McDermott et al. [55], who compared the adsorption of Ci2E6 onto a range of different substrates, amorphous silica, crystalline quartz, and the oxide layer on a silicon single crystal. The adsorbed surfactant was found to form a bilayer with an overall thickness 49 4 A, with a structure similar to that determined in the previous studies (see Fig. 4). 

In contrast to the measurements by McDermott et al [58], neutron reflectivity measurements for the Ci2E6/Ci6TAB mixture in 0.1 M NaBr at the air-water interface and SANS measurements of the mixed micelles show close to ideal mixing. Penfold et al. [60] has used neutron reflectivity to investigate this mixture at the solid-solution interface. For the hydrophilic silicon surface, the surface composition of the mixed surfactant bilayer adsorbed at the interface depended strongly upon the solution pH. At pH 2.4, the surface composition... 

Penfold et al. [62] have also used neutron reflectivity to study the adsorption (structure and composition) of the mixed anionic/nonionic surfactants of SDS and C12E6 at the hydrophilic silica-solution interface. This is rather different case to the cationic/nonionic mixtures, as the anionic SDS has no affinity for the anionic silica surface in the absence of the Ci2E6. The neutron reflectivity measurements, made by changing the isotopic labelling of the two surfactants and the solvent, show that SDS is coadsorbed at the interface in the presence of the Ci2E6 nonionic surfactant. The variations in the adsorbed amount, composition, and the structure of the adsorbed bilayer reflect the very different affinities of the two surfactants for the surface. This is shown in Fig. 7, where the adsorbed amount and composition is plotted as a function of the solution composition. 

Koenig BW, Krueger S, Orts WJ, Majkrzal CF, Berk NF, Silverton JV, Gawrisch K. Neutron reflectivity and atomic force microscopy studies of a lipid bilayer in water absorbed to the surface of a silicon single crystal. Langmuir 1996 12 1343-1350. 

Table 3.2 Best-fit results describing a mixed DMPC-cholesterol bilayer deposited at the electrode surface as a function ofthe applied potential. The results are obtained from the best-fit results ofthe measured neutron reflectivity curves in Figures 3.16-3.18.

Next, the use of polyelectrolyte multilayers, prepared by the layer-by-layer deposition protocol, as well as the use of polymer cushions prepared by plasma-polymerization is introduced. Evidence for the proper structural and functional characteristics of the corresponding tethered bilayers is derived from neutron reflectivity and from IR data, and by the observation of the functional incorporation of proteins. 

Abstract Neutron reflection has proved to be a very successful technique for investigating the structure of layers of surfactants adsorbed at the air/water interface, especially when isotopic substitution may be used to resolve the structure within the layers. Here the extension of the technique to surfactant layers adsorbed at the solid/liquid interface is discussed. The experimental conditions necessary for probing surface structure accurately are mainly determined by the possibilities available for varying the contrast through isotopic substitution. The simplest variation is of the H/D ratio in water, but definitive evidence for bilayer structures at the surface can usually only be obtained by varying... 

Experimentally the most accessible hydrophilic surface is the silica surface, although it is by no means simple. Except at very low concentrations, where there may be coulombic interactions between the charged surfactant and charged surface, adsorption of surfactants is generally expected to be dominated by the hydrophobic effect, which will cause the surfactant to adsorb in a bilayer or related structure. In principle, neutron reflection can identify a bilayer structure either from its overall thickness or from the surface coverage. In practice, for the two surfactants that have so far been most systematically studied by neutron reflection, the non-ionic surfactant Ci2Eg [17, 18] and the cationic surfactant hexadecyl trimethyl ammonium bromide. 

Whereas all the neutron reflection studies of bilayer surfactant structures have relied on using the overall thickness of the layer to establish that it is a bilayer. Fragneto et al. were able to show this directly by using partially labelled CieTAB molecules [8]. The series of isotopic species 0 Ci6 dC ,dTAB with m = 4, 8 and 12, where 0 indicates... 

Fig. 8 Neutron reflectivity and calculated profiles at the Si/Si02/Ci6TAB water interface for different isotopes of C.gTAB, ( ) 0 Ci2dC4TAB, ( ) 0 CgdCsdTAB, and (O) 0 QdCiidTAB in (a) D2O and (b) water (2.07 x 10" A" ) matched to silicon. The continuous lines are the best fits of a model of a surfactant bilayer to the data [8]...

Mr,w M = 1.12), was spin-coated from a 1-butanol solution directly onto the lower layer. The PHOSt layer is loaded widi a 5 % mass fraction of the photoacid generator, di(tert-butylphenyl) iodonium perfluorooctanesulfonate. The bilayer is subjected to another PAB for 90 s at 130 C. The model bilayer stack was exposed with a broadband UV dose of 1000 mJ/cm to generate acid within the top PHOSt layer followed by PEB at 110 C for varying times of 15 s, 20 s, 30 s, and 90 s. The original PHOSt layer and the soluble deprotected d-PBOCSt reaction products were removed (developed) by immersion in a 0.26 N tetramethylammonium hydroxide (TMAH) solution for 30 s followed by a rinse with deionized water. The use of deuterated PBOCSt facilitates the measurement of the deprotection profile using neutron reflectometry, described below. The approximate deprotection reaction of d-PBOCSt into PHOSt is shown schematically in figure 1. Neutron Reflectivity... 

Kim, J., Kim, G., and Cremer, P. S. 2001. Investigations of water structure at the solid/liquid interface in the presence of supported lipid bUayers by vibrational sum frequency spectroscopy. Langmuir 17 7255. Koenig, B. W., Krueger, S., Orts, W. J., Majkrzak, C. F, Berk, N. F., Silverton, J. V., and Gawrisch, K. 1996. Neutron reflectivity and atomic force microscopy studies of a lipid bilayer in water adsorbed to the surface of a silicon single crystal. Langmuir 12 1343. 

Figure 8.14 Neutron reflectivity profiles for a deuterated-protonated polystyrene bilayer annealed at 120°C for 0 min (top curve) 2 min (middle curve) 3900 min (bottom curve). Curves are displaced for clarity. Broken lines are fits to the data. Reproduced with permission from ref. 67.

Table 3.6 The Results Obtained after Fitting the Experimental Neutron Reflectivity Curves of Bilayer Samples PA-61/d-PS-g-MA, after Different Annealing Conditions...

Johnson SJ, Bayerl TM, McDermott DC, Adam GW, Rennie AR, Thomas RK, Sackmann E. Structure of an adsorbed dimyris-toylphosphatidylcholine bilayer measured with specular reflection of neutrons. Biophys. J. 1991 59 289-294. 

The inner structure of polyelectrolyte multilayer films has been studied by neutron and X-ray reflectivity experiments by intercalating deuterated PSS into a nondeut-erated PSS/PAH assembly [94, 99]. An important lesson from these experiments is that polyelectrolytes in PEMs do not present well-defined layers but are rather interpenetrated or fussy systems. As a consequence, polyelectrolyte chains deposited in an adsorption step are intertwined with those deposited in the three or four previous adsorption cycles. When polyelectrolyte mobility is increased by immersion in NaCl 0.8 M, the interpenetration increases with time as the system evolves towards a fully mixed state in order to maximize its entropy [100], From the point of view of redox PEMs, polyelectrolyte interpenetration is advantageous in the sense that two layers of a redox polyelectrolyte can be in electrochemical contact even if they are separated by one or more layers of an electroinactive poly ion. For example, electrical connectivity between a layer of a redox polymer and the electrode is maintained even when separated by up to 2.5 insulating bilayers [67, 101-103]. 

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