Spreading solvent

The material of interest is dissolved in a volatile solvent, spread on the surface and allowed to evaporate. As the sweep moves across, compressing the surface, the pressure is measured providing t versus the area per molecule, a. Care must be taken to ensure complete evaporation [1] and the film structure may depend on the nature of the spreading solvent [78]. When the trough area is used to calculate a, one must account for the area due to the meniscus [79]. Barnes and Sharp [80] have introduced a remotely operated barrier drive mechanism for cleaning the water surface while maintaining a closed environment. 

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. 

The first step in the preparation of an LB film is the successful spreading of a monolayer of the material of interest, which may be molecular, polymeric, or particulate and need not be amphiphilic in the traditional sense. This is accomplished by depositing drops of a dilute solution of the material in an appropriate spreading solvent onto the water surface. The concentration is generally millimolar or less, and the solvent selected should be one that will spread across the surface rapidly and evaporate without remaining at the surface or dissolving into the subphase. Common spreading solvents include chloroform, benzene,... 

This is clearly a complex system, for it is pointed out that the calixarene-limiting area depends on the spreading solvent and is subject to a memory effect. In another study, mixed monolayers and the resulting LB films of C60 with two derivatized calix[4]resorci-nolarenes and with p-tert-butylcalix[6]arene and p-tcrt-butylcalix[8]arene were examined... 

Materials. The nearly monodisperse atactic PMMA, which was used for the electron beam lithography and fluorescence spectroscopy studies, was obtained from Pressure Chemical. It has a weight average molecular weight (Mw) of 188,100 and Mw/Mn< 1.08. Pyrenedodecanoic acid (PDA) used in the fluorescence studies was obtained from Molecular Probes and used as supplied. Spectroscopic grade benzene purchased from J.T. Baker was used as the spreading solvent in the PMMA and PMMA/PDA solutions. 

The novolac sample, which was provided by Kodak, was synthesized from pure meta-cresol and formaldehyde. It has a weight average molecular weight of 13,000 and a very broad polydispersity of 8.5. The polymer was purified by two precipitations from tetrahydrofuran into hexane. The PAC was a naphthoquinone-1,2-(diazide-2-)-5-sulfonyl ester provided by Fairmount Chemical (Positive Sensitizer 1010). A hydroxyl substituted benzophenone is attached to the sulfonyl ester. The spreading solvent was isopropyl acetate, which was obtained from Aldrich Chemicals and used as received. 

Keywords Interface Langmuir monolayer Adsorption process Spreading solvent Surface tension Surface isotherm Langmuir-Blodgett film Collapse pressure Amphiphilic polymer... 

It is very well known that different macromolecular arrangements may be induced either by changing the nature of the subphase [44, 68, 69] or by changing the spreading solvent [70], However, only a few studies describing these effects on polymer monolayer surface pressure behavior have been reported [71-73]. 

Baglioni et al. [72] showed that poly (y - methyl L - glutamate) may form a monolayer in the a helice or (3 sheet conformation according to whether the spreading solvent did or did not contain pyridine. 

The properties of poly(D, L-lactic acid) monolayers spread at the air - water interface were also shown to be strongly dependent of the nature of the spreading solvent. In this case, the monolayers spread from acetone and tetrahydrofuran exhibited typical reversible collapse behavior in the compression - expansion cycle with a quasi - plateau at large areas followed by a steep rise in the surface pressure at small areas. Conversely, the monolayers spread from chloroform, dichloromethane,... 

Dimethyldioctadecylammonium bromide was obtained from Kodak and used without further purification. Chloroform was used as the spreading solvent and was redistill before use. All other chemicals were of analytical grade and used as received. The water used as a subphase was of Milli-Q grade with a resistivity higher than 18 M cm. 

Spreading may take place directly by depositing either a crystal of the Insoluble surfactant, or a drop of the solution of such a surfactant in a volatile solvent at the Interface. Spreading requires the spontaneous breakdown of the three-dimensional structure of the solid or the liquid, respectively, and the creation of the two-dimensional monolayer. Figure 3.1 illustrates this process for a liquid. Most monolayers, however, are obtained by dissolving the material to be spread in a volatile solvent that will spread spontaneously over the interface and disappear soon thereafter by evaporation (GL interfaces) or by dissolution in one of the liquid phases (LL interfaces). Obviously, the spreading solvent should satisfy the following conditions ... 

Several examples can be found in the literature where the authors attempted to trace the origin(s) of method-dependent monolayers. For instance, Mtngins et al. reported the influence of the solvent on n(A) curves for octadecyltrimethyl ammonium bromide spread from five different solvent mixtures or from crystals on a substrate of 0.1 M NaCl in which the surfactant is insoluble. At a given area k was found to differ between the various spreading solvents by a variation of 0.3 mNm" at low F to 3 mN m" at high F. Iwahashi et al. ) found that n A] curves obtained for the spreading of fatty acid crystals depended on the sizes and shapes of the crystals, apparently caused by irregular dissolution rates. For a comprehensive discussion see ref. ). 

The method of Trumit avoids problems that may arise with a spreading solvent. However, a factor that was not studied was the effect of the electric charge carried by the protein and the accompanying electrical potential barrier produced at the interface, as discussed in Section III,C. For this reason, it is preferable, when using the Trurnit method, to have the protein at a pH close to its isoelectric point to ensure complete spreading. 

Methods. Using an Agla microsyringe, alkyl alcohol solution (0.025 ml) was spread on the subsolution of 0.01 M HC1. A time interval of 5 minutes was allowed for spreading solvents to evaporate or diffuse in the subsolution from the monolayers. The monolayer was compressed at a constant rate by an electrically operated motor. The surface pressure area curve for a monolayer was recorded automatically by x-y recorder. Three to five mono-layers of each mixture were studied and the results reported are average values. The reproducibility of data was + 0.15 8 /molecule. The detailed discussion of the apparatus is given elsewhere (17). 

A dilute solution of a preformed polymer can be separately cast onto water, preferably from a surface active spreading solvent such as cyclohexanone. The thin film is then laminated onto a porous support layer. This approach was originally developed by Camell and Cassidy (40) and was first utilized by Cadotte and Francis in 1964 to coat microporous CA membranes. Petersen also utilized this approach to cast thin films of ethyl xellulose perfluorobutyrate which were then laminated to CelgartT and Tyvek supports for use as blood oxygenation membranes (41). 

With the aim of simulating biogenic sea slicks, we generated PME slicks at the sea surface by distributing the film-forming material with the help of different spreading solvents, i.e., by the application of ethanol or n-hexane. These man-made sea slicks were overflown by a helicopter carry-... 

C (45) taken in conjunction with Figure 4, the Davies term can explain the difference in n on 0.1 and 0.01M NaCl for A > 2,000 A2/ molecule but fails on the change from 0.01 to 0.001M NaCl until an area of 5000 A2/molecule is reached. A similar picture at 20°C emerges from a comparison of the less precise results in Figure 2 for the A/W interface obtained before our spreading solvent problems were resolved. 

In Figure lb the same two conducting phases are opposed, but the surface of phase /3 now includes a defined surface density of molecules of an insoluble monolayer. The reference electrode, phase a, is assumed unchanged by the spreading of the monolayer on (3. In practice this means waiting until the spreading solvents have fully evaporated from the region... 

Chloroform and hexane were the spreading solvents and were purified by being passed through an activated silica gel column (Florosil). Surface active impurities were detected by spreading 0.1 cc of the solvent on the surface of the millidyne film balance and monitoring the surface pressure after the solvent had evaporated (1-5 min). No residue was detected with the purified solvents as indicated by the absence of surface pressure after evaporation. If non-purified solvents were used, appreciable amounts of surface active impurities were detected. All solvents were stored under nitrogen to avoid oxidation the lipid solutions were... 

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