In mixed monolayers

In the present review, first we will describe how to fabricate artificial photosynthetic reaction center in nanometer scales by making use of phase separation in mixed monolayers of hydrocarbon (HC) and fluorocarbon (FC) amphiphiles [2,5,20-26] as shown in Fig. 2b [3]. The phase separated structures were studied by SPMs such as AFM, SSPM, and scanning near-field optical/atomic force microscopy (SNOAM) [27-33] as well as a conventional local surface analysis by SIMS [3,5], The model anionic and cationic HC amphiphilic... 

Fig. 9.11 a) Chemical structures of MMB and TFMB. b) Possibilities to reduce the total dipole moment in a SAM of mercaptobiphenyls by interactions with a polar solvent, tilt or assembly of opposite dipoles in mixed monolayers, c) Surface versus solution composition found for MMB and TFMB mixed systems in polar (ethanol) and less polar (toluene) solutions illustrating the effect of the assembling dipoles, d) The opposite molecular dipoles of MMB and TFMB proved to be sufficient to induce ligand exchange in order to reach an equilibrium situation in the surface composition (modified from ref [96]). 

Non-ideal solution theory is used to calculate the value of a parameter, S, that measures the interaction between two surfactants in mixed monolayer or mixed micelle formation. The value of this parameter, together with the values of relevant properties of the individual, pure surfactants, determines whether synergism will exist in a mixture of two surfactants in aqueous solution. 

Our data, to date, show that molecular interaction between two surfactants, both in mixed monolayers at the aqueous solution/air interface and in mixed micelles in aqueous solution, increases in the order POE nonionic-POE-nonionic < POE nonionic-betaine < betaine-cationic < POE nonionic-ionic (cationic, anionic) betaine-anionic cationic-anionic. The greatest probability of synergism exists, therefore, in cationic-anionic mixtures, followed by betaine-anionic mixtures. Synergism can exist in POE nonionic-ionic mixtures only if the surfactants involved have the proper structures. 

The different behavior of 7 and 8 is probably due to the charged head group in 7. Phase separation to form enriched domains of this lipid in mixed monolayers would be inhibited by electrostatic repulsion. Interestingly, mono-layer films of 7 mixed with the biologically important molecule cholesterol did exhibit phase separation at all compositions provided the temperature was maintained below the Tm of 7. Presumably the significantly different shapes of the two molecules promotes the phase separation and overcomes the electrostatic barrier. 

Surface Potential. Shah and Schulman have proposed that interaction between dipoles of uncharged lipids in mixed monolayers should result in a change in surface potential, AV. Linearity of the relation of AV to composition of the lecithin-cholesterol monolayer was taken to indicate absence of interaction (17). We do not agree with Shah and Schulman, since surface potential does not appear to be a valid criterion for assaying interaction between dipoles of uncharged lipids. Except for the speculations of Shah and Schulman (17, 18), there is neither theoretical nor experimental evidence that dipole-dipole interactions have... 

Concept of Intermolecular Cavities in Mixed Monolayers. In mixed monolayers a deviation in average area per molecule occurs if one component forms expanded and the other condensed monolayers. This reduction in average area per molecule has been attributed by previous workers to an interaction between components in the mixed monolayer. However, this need not be true in all cases where condensation occurs. In several instances the condensation can be explained on the basis of steric considerations in the mixed monolayers. Although the following discussion is based on lecithin-cholesterol monolayers, it is equally applicable to other mixed monolayers. 

Surface Pressure, Potential, and Fluidity Characteristics for Various Interactions in Mixed Monolayers. It is possible to distinguish various types of interactions which occur in mixed monolayers by measuring the surface pressure, surface potential, and surface fluidity of the monolayers. Deviation from the additivity rule of molecular areas indicates either an interaction between components or the intermolecular cavity effect in mixed monolayers. 

Figure 5. Interactions in mixed monolayers and their surface pressure, potential, and fluidity characteristics...

Ion-Ion or Ion-Dipole Interaction. Figure 5b, shows the general characteristics of mixed monolayers in which ion-ion or ion-dipole interaction takes place—e.g., alkyl phosphate-alkyl trimethylammonium, or steric acid—octadecanol monolayers. The average area per molecule may or may not show a deviation from the additivity rule line, depending upon whether the two components form expanded or condensed mono-layers. However, surface potential per molecule must show a deviation from the additivity line since ion-ion or ion-dipole interactions reduce the average surface dipole of the molecules in mixed monolayers (31, 42). These interactions result in a negative deviation in the plot of log < vs. mole fraction (6). 

Since cholesterol is an important component of many biological membranes mixtures of polymerizable lipids with this sterol are of great interest. In mixed monolayers of natural lipids with cholesterol a pronounced condensation effect , i.e. a reduction of the mean area per molecule of phospholipid is observed68. This influence of cholesterol on diacetylenic lecithin (18, n = 12), however, is not very significant (Fig. 32). Photopolymerization indicates phase separation in this system. Apparently due to the large hydrophobic interactions between the long hydrocarbon chains of... 

The importance of selecting gravimetry instead of volumetry to measure liquid amounts in the pharmaceutical industry of liquid dosage forms is well illustrated by the volume contraction of water-ethanol and volume expansion of ethyl acetate-carbon disulfide liquid mixtures as well as a CS2-ethyl acetate system. The National Formulary (NF) diluted alcohol is a typical example of the volume nonadditivity of liquid mixtures [29], This solution is prepared by mixing equal volumes of alcohol [U.S. Pharmacopeia (USP)] USP and purified water (USP). The final volume of this solution is about 3% less than the sum of the individual volumes because of the contraction due to the mixing phenomenon [1], In addition, molecular interactions of surfactants in mixed monolayers at the air-aqueous solution interface and in mixed micelles in aqueous media also cause some contraction of volume upon mixing [30],... 

The possibility of Azone forming a separate phase within the DPPC has been poorly investigated to date, although it can be inferred that, as with OA, it is only likely to occur at high surface pressures (from observations of squeeze-out behavior in mixed monolayers). This would account for the observed expansion from ideality (Lewis and Hadgraft, 1990) at low surface pressures and the composition-dependent nature (Street and Hadgraft, 1992) of the pressure at which squeeze-out begins. 

Arnold and co-workers attempted to prepare imprinted metal-coordinating polymers for proteins [25]. For this purpose, efforts were made to prepare metalcoordinating molecularly patterned surfaces in mixed monolayers spread at the air-water interface or liposomes. This approach was termed as molecular printing and is illustrated in Fig. 6.6. In this process, a protein template is introduced into the aqueous phase, which imposes a pattern of functional amphiphiles in the surfactant monolayer via strong interactions with metal-chelating surfactant head groups. The pattern is then fixed by polymerising the surfactant tails. The technique has also been employed for two dimensional crystallisation of proteins [26]. 

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... 

Cyanine dyes (513) containing a benzothiazole ring form aggregates in mixed monolayers with methyl arachidate and -hexadecane <86JPC6144>. The changes in the orientation of that type of... 

As will be described in Chapter 11, the interaction of two different types of surfactants with each other, either in mixed monolayers at an interface or in mixed... 

This much stronger interaction of geminis with other surfactants in mixed monolayers than in mixed micelles means that there is a strong possibility of... 

Figure 1 shows the surface pressure—area curve of stearyl alcohol monolayer at pH 2.0 and 25 °C, where Ae is the area/molecule in the expanded state at zero surface pressure, Ac is the area/molecule in the condensed state at zero surface pressure, and A2o is the area/molecule at surface pressure of 20dynes/cm. Figure 2 shows the values of Ae, Ac, and A2o for various alkyl alcohols. From this diagram the average area per molecule in mixed monolayers was calculated using simple additivity rule. Figure 3 shows the excess molecular area when Ci6 alkyl alcohol... 

Philips et al. (18) reported that in mixed monolayers of dioleoylleci-thin—distearoyllecithin, the area/molecule showed expansion from the additivity rule because the unsaturated fatty acid chains increased the kinetic motion of saturated chains. However, in contrast to the studies here on Ci6 + Ci8 alcohols, they found no expansion in the mixed mono-layers of dipalmitoyllecithin-distearoyllecithin. 

Significant interaction in mixed monolayers containing poly (methyl methacrylate) (as evidenced by non-additivity of film areas) was noted in certain cases by Wu and Huntsberger (20). However, there have been a number of reports (21, 22) which suggest that the monolayer characteristics of this polymer depend appreciably on both molecular structure details, such as tacticity, and experimental procedure, notably compression rate. Accordingly, a more detailed examination of this effect is beyond the scope of the present work. 

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