Interactions in 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...

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. 

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. 

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. 

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],... 

In the presence of anionic surfactants, it is reasonable to expect that the hydrophobic groups of the poljrmer and of the surfactant would combine to form a mixed film at the liquid-air interface. The interactions between the cationic groups of the polymer and the anionic groups of the surfactant would further strengthen the interactions in the monolayer. These effects can be expected to increase the surface and sub-solution viscosity in lamellae and in turn enhance their stability. 

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]. 

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... 

Differences in the molecular area are of obvious relevance in mixed monolayers, where larger molecules have a larger partial molar area than smaller ones. Such differences lead to a situation where the smaller molecules cire increasingly preferentially adsorbed with increasing surface pressure, even in the absence of any surface interactions [16]. In adsorption layers consisting of a single surface-active compound similar effects can occur if, due to the asymmetry in different adsorption states the molecules can occupy different areas [3, 4, 15, 19, 21, 22], The fraction of molecules which are in the state characterised by a particular partial molar area depends on the surface pressure. In a thermodynamic study by Joos and Serrien [21] it was shown that if the molecule possesses, say, the two modifications 1 and 2, with different partial molar surface areas coi and o)2 (in absence of intermolecular interactions) their ratio in the surface layer obeys the equation... 

GlvcoDhorin-Lioid Monolayers at the Air-Water Interface. Further to the study of pure glycophorin monolayers we investigated the interaction between the glycophorin and dipalmitoyl-phosphatidylcholine (DPPC) in mixed monolayers at the air-water interface (27). Pure DPPC undergoes the characteristic liquid expanded (L ) to intermediate state (I) transition in monolayers at temperatures below the chain-melting temperature (- 42 C) of... 

Zhou, Q Rosen, M. J. Molecular Interactions of Surfactants in Mixed Monolayers at the Air/ Aqueous Solution Interface and in Mixed Micelles in Aqueous Media The Regular Solution Approach. Langmuir 2003, 19, 4555-4562. 

Since Rubingh [9] proposed the so-called P" parameter to evaluate interactions between surfactants in a mixed micelle, Rosen and co-workers have done extensive work on the interaction between surfactants in a binary-surfactant system and introduced a new parameter, p [10] (pii for liquid-liquid interface [11], pis for liquid-solid interface [12]) to measure the interaction between surfactants in mixed monolayer at the surface of an aqueous solution. 

Funasaki and coworkers [12] studied intermolecular interactions in mixed micelle formation of polyoxyethylene (15) dihydrocholesterol (DHC-E015) with polyoxyethylene (7) dodecanol (C12E07), by means of volumetric methods. The objective of their study was to investigate the condensing effect of cholesterol on phospholipid monolayers and the membrane-thickening action of cholesterol on lecithin bilayers. The volume... 

In mixed monolayer, cholesterol and phosphatide molecules are in close contact, and therefore, should interact at least part of the time. The question whether such interactions lead to complexes, i.e., associations of appreciably longer half-life than associations between like molecules, would seem to depend on whether the van der Waals interactions generated by complex formation are appreciably higher than those generated between like molecules. Hence the half-life of complexes may be altered by the nature of acyl chains (length, position, unsaturation). 

The simplest situation is the symmetrical one (NA = NB), with the solvent equally good for both blocks. We imagine that the excluded volume interactions of A and B are stronger than the A-B repulsive interactions so that the overall structure of the layer is like that of a single component in other words, both components are equally stretched. The issue is whether or not they are homogeneously mixed with one another in the monolayer. This is essentially a two-dimensional random mixing process. In that spirit, we write the free energy... 

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