Multilayers y-type

X-type multilayer Y-type multilayer Z-type multilayer Alternating X-type... 

X-type films deposited on the downward stroke are hydrophilic because the hydrophobic part of the fatty acid molecules is deposited on the solid surface. Similarly, Z-type films deposited on the upward stroke are hydrophobic because the hydrophilic end of the molecules is deposited on the substrate. Obviously, there is no reason for the resulting contact angles to generate equivalent transfer ratios. Finally, contact angles on multilayered Y-type films depend not only on the type of hydrophilic or hydrophobic molecule ends but also on the number of layers on the composite film (Gaines, 1977). 

On the basis of experimental evidence and taking into account the role of electrical double layers on contact angles and flow patterns, one can certainly argue that the most successful conditions for multilayer Y-type LB deposition take place at a pH equal to the pX of the carboxylic acid, with a subphase where a minimum concentration of a... 

Fig. 4. Multilayer films where ( ) represent a hydrophobic group and (B) a hydrophilic one (a) Y-type, (b) X-type, and (c) Z-type.

Figure 38. Fluorescence spectra for the X- and Y-types of multilayers of L-NaphAla-Ci g and L-PyrAla-Ci g.

As discussed in section 2.2, a mixture of AMP and AA showed two solid condensed phases above and below about 30 mN m- [5,10]. A loosely stacked structure of two porphyrins was proposed for LB films prepared at higher surface pressures than 30 mNmr1, which was caused by squeezing-out of a monomolecular structure formed at lower surface pressure [5,10]. In this section, photoelectric characteristics of LB films containing AMP and AA deposited at two solid condensed phases will be discussed in relation to multilayer structure and the anisotropic intermolecular tunneling rates [87]. Seven monolayers of 1 5 or 1 10 mixture of AMP and AA were deposited at 20 and 50 mN m-1 on an ITO plate at 18 °C to form stable Y-type LB films. Aluminum was vacuum evaporated onto LB films as sandwich-type electrodes at 10-6 Torr. Steady photocurrents were measured in a similar manner as mentioned above. 

Figure 6 compares the electroabsorption spectra of the three type of LB films in the wavelength range that corresponds to the absorption due to the transition moment in the direction of the long molecular axis. The applied field was 3.2 x 10s Vcm1 in each case. In the Y-type deposition film, a small Stark signal is observed nevertheless, the Y-type film is assumed to possess a symmetrical molecular orientation. The reason for this weak signal may be that the fluctuation of molecular orientation across the films induced a small asymmetry in the multilayer structure. 

The quantity and quality of the deposited monolayer on a solid support is measured by a so-called transfer ratio, tr. This is defined as the ratio between the decrease in monolayer area during a deposition stroke, Al, and the area of the substrate, As. For ideal transfer, the magnitude of tr is equal to 1. Depending on the behavior of the molecule, the solid substrate can be dipped through the film until the desired thickness of the film is achieved. Different kinds of LB multilayers can be produced and/or obtained by successive deposition of monolayers on the same substrate (see Figure 4.11). The most common one is the Y-type multilayer, which is produced when the monolayer deposits on the solid substrate in both up and down directions. When the monolayer deposits only in the up or down direction, the multilayer structure is called either Z-type or X-type. Intermediate structures are sometimes observed for some LB multilayers, and they are often referred to be XY-type multilayers. 

Different film architectures can result upon deposition as depicted in Fig. 3.9(e). Y-type multilayers are the most common and can be prepared on either hydrophilic or hydrophobic substrates, and are typically the most stable due to the strength of the head-head and tail-tail interactions. The X-type and Z-type films, with head-tail interactions, are less common. 

Variations on the vertical dipping technique have been utilized to construct films containing divalent metal ions. For example, the quartz crystal microbalance (QCM) has been used to evaluate the horizontal lifting method of CdSt LB Film construction (26). In this method, the QCM quartz plate was touched to monolayers compressed on a subphase and lifted horizontally. Y-type transfer (transfer ratio of 1) was demonstrated with two centrosymmetric monolayers deposited for each cycle. A combination of the vertical and horizontal dipping techniques has been utilized to prepare multilayer films from an amphiphilic porphyrin compound (27). 

Figure 3.53. Schematic representation of monolayer and multilayer transfer (type Y) on a hydrophilic support. Not to scale For details see the text.

If the support is hydrophobic, e.g. silanized glass or silicium, deposition will normally start on the first immersion into the subphase. For Y-type transfer there will be an even number of layers deposited at the end of each completed cycle, in contrast to the odd number of layers transferred to hydrophilic supports. It should be noted that the schemes depicted in figs. 3.53 and 3.54 are highly idealized and Y-, X- or Z-transfers do not always give the corresponding (perfect) multilayer structure. 

Figure 1.5.10 Schematic drawing of a Pockels trough used to form compact molecular monolayers on a water surface. The deposition takes place on the downward stroke if hydrophobic interaction is responsible for deposition (hydrophobic surface) and on the upward stroke if hydrophihc surface-monolayer interactions are more important. If the deposition area is equal to the loss of the monolayer on the air/water interface (deposition ratio equal to 1), it is assumed that perfect deposition has taken place. If the ratio is near unity in both upward and downward strokes, the material is deposited in the Y mode, which is the most stable multilayer structure (b). If the deposition ratio is near unity in the down stroke and zero on the up stroke, the deposition is in the X mode (a). The surface is hydrophobic and am-phiphiles are bound in the A orientation. The converse situation leads to Z-mode multilayers (c B orientation). X- and Z-types often rearrange to Y-type multilayers.

Figure 2. Langmuir-Blodgett film deposition (a) condensed monolayer on water surface (b) transfer of monolayer on upstroke of solid substrate (c) subsequent transfer on downstroke (d) Y-type multilayer film.

Figure 10.3 Schematic representation of the different types of multilayer deposition of LB films, (a) X-type films deposited during immersion only, (b) Z-type films deposited during removal only, (c) Y-type films deposited during immersion and removal...

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