Temperature of the subphase

The stereochemically directed conformation of the FE isomer affords stronger intermolecular associations. The behavior of the molecules in a monolayer film is relatively insensitive to the temperature of the subphase over the same range of temperatures where the SE isomer is quite sensitive. It is seen from Table 14 that the FE isomer spread less spontaneously from the bulk crystal to a monolayer state. This is an indication that associations in the bulk crystal are stronger for the FE isomer than for the SE. In addition, the entropy of spreading is lower for the FE isomer, indicating a more ordered and conversely a more tightly packed film or a less ordered crystal. 

It is often important to control the temperature of the subphase at a point above or below ambient. This can be achieved either by a water jacket surrounding the trough or by immersing a zig-zag shaped piece of glass tube in the subphase and passing either heated or cooled water through it. 

The clean trough is filled by the liquid on the surface of which the films will be prepared. This liquid is referred to as the subphase. Usually the subphase is either pure water or a dilute aqueous solution of an inorganic salt, typically with a divalent cation, such as cadmium. The nature of the cation and its concentration, as well as the pLI and temperature of the subphase, were found to play an important role in the structure and stability of the films. 

The subphase contained 10 mM Tris-HCl buffer (pH 7.4). CaCl2 (2 mM) or PLL (0.5 mM, concentration of amino acid residues) was added to the subphase. The temperature of the subphase was adjusted to 18°C. Hydrogenase solution of 20 pi (1.1 mg/ml) was spread on the air/water interface of the subphase (surface area of 248 cm2) using the glass-rod method (Hirata et al., 1992). After 10 min, the monolayer was compressed at a rate of 7.5 cm2/min. The layer of hydrogenase was transferred to hydrophilic quartz plates and indium tin oxide (ITO) electrodes (40 x 5 x 1 mm). The vertical deposition onto the substrate was performed a surface pressure of 30 mN/m. The dip-ping and withdrawal rates were 50 and 5 mm/min, respectively. The density of hydrogenase molecules in an LB film was calculated based on the number of molecules spread on the subphase and the transfer ratio (TR). 

Isotherms (interface pressure n [mN/m] area per molecule A [A ]) are usually measured by continuously compressing the film, which has been prepared on top of an electrolytic subphase. In this experiment, the interface pressure k A) = 7o - y A) [i.e. the difference of surface tensions without (y ) and with an interface film] is determined by monitoring the surface tension y A). For the phospholipid L-a-dimyristoyl phosphatidic acid (DMPA) a set of data at pH = 6 and various temperatures of the subphase was measured by Albrecht et al.9 and is reproduced in Fig.l. 

The last phase transition is to the soHd state, where molecules have both positional and orientational order. If further pressure is appHed on the monolayer, it collapses, owiag to mechanical iastabiHty and a sharp decrease ia the pressure is observed. This coUapse-pressure depends on the temperature, the pH of the subphase, and the speed with which the barrier is moved. 

Prior to LB transfer, the surface pressure - molecular area (n-A) isotherms of dialkylsilane under various pH and temperature conditions were investigated. The pH condition of the subphase (water phase under the monolayer) is a crucial factor for the monolayer state. The condensed phase was formed directly without formation... 

The phase behavior of monolayers is determined by the molecular structure of the am-phiphile and the conditions of the subphase. Phospholipids, for example, attract each other because of van der Waals interactions between the alkyl chains. The longer the alkyl chains, the more strongly the phospholipids attract each other. Thus, the LE-LC transition pressure will decrease with increasing chain length (at constant temperature). Double bonds in the alkyl chains increase this phase transition pressure. Charges and oriented dipole moments (see Chapter 6) in the headgroups, lead to a repulsion between the phopholipids and increase the pressure at which the transition occurs. Salts in the subphase, screen this repulsion and decrease the transition pressure. 

It is possible to control the pressures at which the phase transitions occur by fine tuning the strength of intermolecular interactions between the amphiphilic molecules. The interactions between the hydrophobic tails depend on temperature [37], while the interactions between the hydrophilic heads depend on the chemical composition of the subphase, namely its pH and ionic strength [4], For example, the fatty acid molecules in films prepared on subphase with high pH and high concentration of divalent salt, such as CaCl2 or CdCl2, are normal to the surface, i.e. are in solid state, even at low pressures. Pressure-area isotherms of such films are featureless compressed films are stable and easy to transfer [38]. 

For the in-situ FTIR analyses, mixed solutions were spread by the same hion onto a house-made minitrough that fitted into the sample chamber of the FTIR instrument. For every in-situ experiment, the close-packed monolayer was initially formed on DI water that was then replaced with ImM CdCl2 solution by careful circulation of the subphase using a peristaltic pump. The background spectrum was taken for a bare water surface when the sample was Ae monolayer on pure water, and was taken for a close-packed monolayer on pure water when the sample was the one on the cadmium-containing subphase. The latter technique enhanced the carboxylate peak relating to ion adsorption. All the experiments were done at room temperature. 

The temperature of the phase transition in mixed monolayers increases with the increase of n-octadecanol concentration, as in the monolayers on water or glycerol [31,41]. However, the maxima in log Vs vs 1/T plots are not so sharp as for liquid subphases, probably because the silica gel surface is not as homogeneous as that of liquid. At the higher concentration of n- octadecanol in a monolayer (curves f-h) two maxima, appear... 

It is well-known that it is difficult to determine phase boundaries from isotherm measurements. A striking example of this problem is the G-LE transition in pentadecanoic acid (PDA). Harkins and Boyd studied monolayers of PDA at a variety of temperatures and proposed that the G-LE two-phase region ended at a critical point at a temperature in excess of 35°C. The nature of this two-dimensional critical point was examined by Kim and Cannell, who made isotherm measurements at closely spaced temperatures between 14.8 and 34.7°C. (Earlier, Hawkins and Benedek carried out a similar investigation, but their failure to control the pH of the subphase left their results in doubt.)... 

Other properties of fatty acid monolayers such as the phase transition temperature are consistent with the field strength theory stearic acid monolayers formed rigid films on alkaline earth subphases (17, 35). The temperature of the phase transition from rigid to fluid monolayers, estimated by the Devaux talc test, was a function of pH and buffer composition (Figure 17). Thus transition temperature at pH 6 decreased in a weak field sequence (I in Table III) while transition temperatures at pH 8 decreased in an intermediate field sequence (III or IV in Table III). Since variables such as tt are not controlled in these experiments, it is surprising that transition temperature data followed these sequences. 

A number of distinct regions are immediately apparent on examining the isotherm. When the monolayer is compressed, it can pass through several different phases, which are identified as discontinuities in the isotherm, as shown in Figure 5.6. The phase behaviour of the monolayer is mainly determined by the physical and chemical properties of the amphiphile, the subphase temperature and the subphase composition. Various monolayer states exist depending on the length of the hydrocarbon chain and the magnitude of cohe-... 

The LB film prepared in the dark shows temperature dependent UV-vis absorption spectra. At lower temperatures, there is an electronic interaction between the fullerene moieties in the LB film. Upon heating over 47 C, which is the subphase transition temperature of the cast films of the fullerene lipid [28, 36], the electronic interaction of the fullerene moieties is loosened. The electronic interaction between the fullerene moieties can be controlled by the phase change of the film. The fundamental property of the self-assembled bilayer membrane film is maintained in the LB film prepared in the dark, indicating that the molecular orientations of the... 

The trough was mounted on an aluminum base with built-in channels for temperature control of the subphase and enclosed in an environment chamber. The surface pressure-area isotherms were determined at two different temperatures 24.0 0.2 and 36.0 0.2°C. Three compression-expansion isotherms were recorded consecutively. There was no awaiting time between compression and expansion. There was a small deviation between the first and the second isotherms but the third one presented no difference against the second one. 

Figure 3B is a simulation result corresponding to the actual n-A curve in Fig. 2b. We have considered that the increase of the subphase temperature induces an increase in the critical surface pressure. In other words, the increase changes the shape of the nonlinear S -shape function. If we simulate the experimental results (Fig. 2c), it should be used as another appropriate function. In addition to this, it is clear that the rate coefficients (fcj and k-i) depend on the subphase temperature. In the present work, for convenience of calculation, we used the parameters, ki and k-i, as a constant, and estimated the value of k to be much larger than k-i which was arbitrarily determined as 1.0 X 10 . ...

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