Trough Measurements and Collection of Monolayers

For the next phase of this study, monomolecular films were prepared on a subphase of (ultrapure) distilled water. Aliquots, between 50-250 pi, of the ethanol-solubilized microbubble surfactant mixture were applied slowly to the surface of the subphase from a Hamilton microsyringe. It was found unnecessary to allow the films to stand for more than 2 min after spreading before taking measurements. Furthermore, following compression or expansion, the surface pressure was observed to remain constant for periods of up to at least 10 min. All measurements were made at 20.0 0.5°C. 

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). 

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 

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

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