Monolayers, microbubble surfactant

As can be seen from Fig. 6.3, it was found that the partially purified, microbubble-surfactant mixture does in fact form stable monomolecular films at an air/(distilled) water interface. During the first compression-expansion cycle a minor degree of hysteresis was observed, but this effect was essentially absent during recompression (Fig. 6.3) and is probably due to the presence of various contaminants in the microbubble-surfactant mixture (see Section 6.3). It was further found that these microbubble-surfactant monolayers remain quite insoluble (cf. Section 6.1.3) when highly compressed, i.e., up to measured surface pressures of 24 dyne/cm. 

Fig. 6.3. Surface pressure-area ( II-A ) curves for a microbubble-surfactant monolayer spread at the air/(distilled) water interface. (Note that area is expressed as m2/mg protein in this figure and Figs. 6.4 and 6.5, which is also equivalent to m2/110 mg of microbubble-surfactant mixture. See text for further discussion. Taken from ref. 361.)...

Fig. 6.4. Surface pressure-area ( II-A ) curves for microbubble-surfactant monolayers spread on various aqueous subphases. (Taken from ref. 361.)...

BONDING WITHIN COMPRESSED MICROBUBBLE-SURFACTANT MONOLAYERS... 

Fig. 6.4 shows the IT-A curves obtained for microbubble-surfactant monolayers on a variety of aqueous subphases. In order to compare Il-A measurements for monolayers which would contain essentially only glycopeptide-acyl lipid complexes, the data plotted in Fig. 6.4 include only Il-A measurements made during the expansion phase (following an initial compression to at least 23 dyne/cm). Judging from the fact that subphases of either distilled water, 0.1 M HC1 (pH 1.1), 0.1 M NaOH (pH 12.3), or 0.1 M NaF... 

Interestingly, the FIA-FI plots for microbubble-surfactant monolayers on subphases other than distilled water provide indirect evidence for induced association of the glycopeptide-acyl lipid complexes themselves. It can be seen from Fig. 6.5 that the monolayer data obtained on subphases of 0.1 M NaF, 0.1 M HC1,... 

In the series of microbubble experiments (ref. 394) included in this chapter, the actual film material, contained in compressed microbubble-surfactant monolayers, was collected for structural determinations using H-nuclear magnetic resonance (NMR) spectroscopy. The resulting spectrum is then compared to the H-NMR spectrum which was obtained beforehand from the partially purified, microbubble surfactant mixture prior to monolayer formation and compression. 

Fig. 7.1 shows a typical H-NMR spectrum obtained with the partially purified, microbubble surfactant mixture prior to monolayer formation. For comparison, Table 7.1 gives the chemical-shift data for the proton resonances that can be readily identified in the 1 H-NMR spectra of long-chain acyl lipids (ref. 395-401). 

For the collection of monolayers, the microbubble surfactant mixture was spread (see above) and compressed to at least 24 dyne/cm. Thereafter, a rectangular piece of stainless steel mesh (4 cm x 1 cm, 30 mesh) was introduced vertically through the surface of the subphase, by means of a small handle, and alternately raised and lowered so that the air/water interface remained within the area of the mesh. This slow, vertical oscillation was coupled with a horizontal movement of the mesh along the surface of the subphase. The procedure resulted in a continual drop in the measured surface pressure (see above) as monolayer material gradually accumulated over the surface of the mesh. The surfactant-coated mesh was then transferred to and shaken in a glass vial containing high-purity ethanol. Once rinsed, the... 

The collected microbubble-surfactant monolayer material was subsequently lyophilized and redissolved in CD3OD. As with the preceding NMR measurements (see Section 7.1), H-NMR spectra at 270 MHz were obtained with the same Bruker Hx-270 NMR spectrometer. In this case, approximately 0.5% solutions (w/v in CD3OD) of the microbubble-surfactant monolayer material were employed in the measurements (all taken at 26°C). Tetra-methylsilane was used as an internal reference standard (ref. 394). 

Fig. 7.2 shows a typical H-NMR spectrum obtained with microbubble-surfactant material collected from compressed mono-layers. Comparison of this spectrum with Fig. 7.1 clearly demonstrates that the characteristic family of peaks originating from carbohydrate material (ref. 402,403), between 8 values of 3.4-3.8 ppm, is markedly reduced following monolayer formation and compression. (Note, for example, the relative area and maximum height of the terminal methyl group (8 = 0.9 ppm) versus the carbohydrate family of peaks (8 = 3.4-3.8 ppm) within each of the two spectra (Figs. 7.1 and 7.2).) Contrariwise, the characteristic peaks (cf. Table 7.1) for methylene chains (8 = 1.3 ppm) and terminal methyl groups (8 = 0.9 ppm) of long-chain lipids remain essentially unaltered (Fig. 7.2). 

However, the peaks usually associated (cf. Table 7.1) with CH3 and CH2 attached to olefinic carbon (8 = 1.6 and 2.0, respectively) show a relative reduction in height and area. This finding suggests that compression of the microbubble-surfactant monolayer results in the ejection of some unsaturated lipids, as well as most of the carbohydrate material, from the monolayer. Such a conclusion is consistent with the frequently mentioned... 

CHEMICAL SIMILARITIES BETWEEN MICROBUBBLE-SURFACTANT MONOLAYERS AND LIPID SURFACE FILMS AT THE AIR/SEA INTERFACE... 

J.S. D Arrigo, Surface properties of microbubble-surfactant monolayers at the air/water interface, J. Colloid Interface Sci. 100 (1984) 106-111. 

MOLECULAR PACKING WITHIN THE MICROBUBBLE S SURFACTANT MONOLAYER... 

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