Mixed monolayers

Fig. 8 (top) Liposome with polydiacetylene linked monolayer mixed with ligand for receptor detection. (bottom) Colorimetric detection of influenza virus using polymerized liposomes to which have been added increasing amounts of influenza virus from left to right. [18]... 

Exposure of a gold slide to a mixture of two different thiols results in the formation of a mixed monolayer, in which both thiolate moieties are present148. The composition of such a monolayer (usually determined with XPS) is generally different from the composition of solution149. Coadsorption of different thiols provides an easy way to prepare mixed layers with variable controlled concentration of the components, and this method is routinely used to tailor the properties of monolayers. Mixed monolayers with the gradient of two thiols can be prepared using Sephadex gel to prevent diffusion150,151. 

Ci BEDT-TTF monolayer mixed with 10% CnHasOC-TCNAQ (OC = oxycarbonyl, TCNAQ = tetracyanoanthraquino-dimethane) in aqueous FeC solution was deposited to form a conductive LB film. In this case, the acceptor molecule was added to increase the stability of the monol er. The conductivity of the LB films thus obtained was 2 S/cm [127]. The structure of the film examined by STM has been reported [128]. 

Zhao, J., Abe, K., Akiyama, H., Liu, Z., and Nakanishi, F., Photoinduced dimerization of a p-phenylenediacrylic acid derivative in a Langmuir monolayer mixed with stearic-d35 acid on a water surface, Langmuir, 15, 2543, 1999. 

Fig. IV-21. Surface pressure versus area for monolayers of immiscible components a monolayer of pure cadmium arachidate (curve 1) and monolayers of mixed merocyanine dye, MC2, and cadmium arachidate of molar ratio r = 1 10 (curve 2) 1 5 (curve 3), 1 2 (curve 4), and pure MC2 (curve 5). The subphase is 2.5 x 0 M CdC, pH = 5.5 at 20°C. Curve 3a (O) was calculated from curves 1 and 5 using Eq. IV-44. (From Ref. [116].)...

Still another manifestation of mixed-film formation is the absorption of organic vapors by films. Stearic acid monolayers strongly absorb hexane up to a limiting ratio of 1 1 [272], and data reminiscent of adsorption isotherms for gases on solids are obtained, with the surface density of the monolayer constituting an added variable. 

Fig. IV 23. Penetration of cholesterol monolayers by CTAB (hexadecyl-trimethylam-monium bromide. [From D. M. Alexander, G. T. Barnes, M. A. McGregor, and K. Walker, Phenomena in Mixed Surfactant Systems, in J. F. Scamehom, ed., ACS Symposium Series 311, p. 133, 1986 (Ref. 269). Copyright 1986, American Chemical Society.]...

Barnes and co-workers have studied mixed-monolayer systems [278,281,283,284] and found some striking nonidealities. Mixed films of octadecanol and cholesterol, for example, show little evaporation resistance if only 10% cholesterol is present [278] apparently due to an uneven granular microstructure in films with cholesterol [284]. Another study of cellulose decanoate films showed no correlation between holes in the monolayer and permeation rate [285]. Polymerized surfactants make relatively poor water evaporation retarders when compared to octadecanol [286]. There are problems in obtaining reproducible values for r [287] due to impurities in the monolayer material or in the spreading solvent. 

Photodegradation as well as fluorescence quenching has been observed in chlorophyll monolayers [302,316]. Whitten [317] observed a substantial decrease in the area of mixed films of tripalmitin and a ci5-thioindigo dye as isomerization to the trti 5-thioindigo dye occurred on irradiation with UV light. 

The energetics and kinetics of film formation appear to be especially important when two or more solutes are present, since now the matter of monolayer penetration or complex formation enters the picture (see Section IV-7). Schul-man and co-workers [77, 78], in particular, noted that especially stable emulsions result when the adsorbed film of surfactant material forms strong penetration complexes with a species present in the oil phase. The stabilizing effect of such mixed films may lie in their slow desorption or elevated viscosity. The dynamic effects of surfactant transport have been investigated by Shah and coworkers [22] who show the correlation between micellar lifetime and droplet size. More stable micelles are unable to rapidly transport surfactant from the bulk to the surface, and hence they support emulsions containing larger droplets. 

Mixing fatty acids with fatty bases can dissolve films as the resulting complexes become water-soluble however, in some cases the mixed Langmuir film is stabilized [128]. The application of an electric field to a mixed lipid monolayer can drive phase separation [129]. 

Chemical properties of deposited monolayers have been studied in various ways. The degree of ionization of a substituted coumarin film deposited on quartz was determined as a function of the pH of a solution in contact with the film, from which comparison with Gouy-Chapman theory (see Section V-2) could be made [151]. Several studies have been made of the UV-induced polymerization of monolayers (as well as of multilayers) of diacetylene amphiphiles (see Refs. 168, 169). Excitation energy transfer has been observed in a mixed monolayer of donor and acceptor molecules in stearic acid [170]. Electrical properties have been of interest, particularly the possibility that a suitably asymmetric film might be a unidirectional conductor, that is, a rectifier (see Refs. 171, 172). Optical properties of interest include the ability to make planar optical waveguides of thick LB films [173, 174]. 

Straniok S J, Parikh A N, Tao Y-T, Allara D L and Weiss P S 1994 Phase separation of mixed-oomposition self-assembled monolayers into nanometer soale moleoular domains J. Phys. Chem. 98 7636... 

Sagiv J 1980 Organized monolayers by adsorption. 1. Formation and struoture of oleophobio mixed monolayers on solid surfaoes J. Am. Chem. Soc. 102 92-8... 

Bertilsson L and Liedberg B 1993 Infrared study of thiol monolayers assemblies on gold—preparation, oharaoterization, and funotionalization of mixed monolayers Langmuir 3 141-9... 

Koutsos V, van der Vegte E W, Grim PCM and Hadziioannou G Isolated polymer chains via mixed self-assembled monolayers morphology and friction studied by scanning force microscopy Macromolecules 116-23... 

The adsorbed layer at G—L or S—L surfaces ia practical surfactant systems may have a complex composition. The adsorbed molecules or ions may be close-packed forming almost a condensed film with solvent molecules virtually excluded from the surface, or widely spaced and behave somewhat like a two-dimensional gas. The adsorbed film may be multilayer rather than monolayer. Counterions are sometimes present with the surfactant ia the adsorbed layer. Mixed moaolayers are known that iavolve molecular complexes, eg, oae-to-oae complexes of fatty alcohol sulfates with fatty alcohols (10), as well as complexes betweea fatty acids and fatty acid soaps (11). Competitive or preferential adsorption between multiple solutes at G—L and L—L iaterfaces is an important effect ia foaming, foam stabiLizatioa, and defoaming (see Defoamers). 

Based on this low surface tension feature and the commonly observed insolubiUty of defoamers, two related antifoam mechanisms have been introduced (29) (/) The agent dispersed in the form of fine drops enters the Hquid film between bubbles and spreads as a duplex film. The tensions created by this Spreading lead to the mpture of the original Hquid film. (2) A droplet of the agent enters the Hquid film between bubbles, but rather than spreading produces a mixed monolayer on the surface. This monolayer, if of less coherence than the original film-stabilizing monolayer, causes destabilization of the film. 

Screen-printed gold electrodes were, firstly, modified with a mixed monolayer of a 25-mer thiol-tethered DNA probe and a spacer thiol, 6-mercapto-1 -hexanol (MCH). The DNA probe sequence was internal to the sequence of the 35S promoter, which sequence is inserted in the genome of GMOs regulating tire transgene expression. 

The (I)-(III)-samples sorption ability investigation for cationic dyes microamounts has shown that for DG the maximum rate of extraction is within 70-90 % at pH 3. The isotherm of S-type proves the physical character of solution process and a seeming ionic exchange. Maximal rate of F extraction for all samples was 40-60 % at pH 8 due to electrostatic forces. The anionic dyes have more significant affinity to surface researching Al Oj-samples comparatively with cationic. The forms of obtained soi ption isotherms atpH have mixed character of H,F-type chemosorption mechanism of fonuation of a primary monolayer with the further bilayers formation due to H-bonds and hydrophobic interactions. The different values of pH p for sorbents and dyes confirm their multifunctional character and distinctions in the acid-base properties of adsoi ption centers. 

The inner structure of monolayers and bilayers, the liquid/gel transition, tilt transitions, mixed layers, etc. 

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