Microbubbles distilled water

Experiments to distinguish between these two possibilities have often involved measurements of ultrasonic attenuation (ref. 5,9,31,32). The popularity of this approach derives in part from the fact that small impurities in liquids, such as suspended particles, have negligible influence on attenuation in comparison with even a very small concentration of microbubbles (ref. 9). (Microbubbles, in contrast to solid particles, appreciably increase the compressibility of a liquid, introducing forms of viscous losses and nonreversible energy exchanges that do not exist in the case of solid particles.) It is therefore of considerable interest that all fresh tap water samples measured by Turner (ref. 9) showed substantial and persistent abnormal (ultrasonic) attenuation, amounting to a minimum of 44% over that of distilled water it was concluded that this result stemmed from the presence of stabilized micron-sized bubbles. 

In line with discussions included in previous sections, ultrasonic experiments carried out on fresh water by different investigators indicate that the stabilization of gas microbubbles, acting as gas nuclei for ultrasonic cavitation, is always attributable to the presence of surface-active substances in the water (ref. 15-17,25). As a starting point, one should consider that laboratory tests with various tap waters, distilled waters, and salt solutions have shown that no water sample was ever encountered that did not contain at least traces of surface-active material (ref. 46). Sirotyuk (ref. 25) estimates that the content of surface-active substances in ordinary distilled water amounts to 10 7 mole/liter, and in tap water it is 10"6 mole/liter or higher. These values indicate the appreciable content of such substances in both cases (ref. 122), although they differ by roughly an order of magnitude in absolute value. It is essentially impossible to completely remove... 

The trough itself measured 20 x 12 cm, was milled from a block of Teflon, and held approximately 750 ml of liquid. Monomolecular films were prepared on a subphase of (ultrapure) distilled water or on aqueous subsolutions containing varying concentrations of either NaF, HC1, NaOH, thiourea, or dimethyl sulfoxide (DMSO). Aliquots, between 50 and 250 pi, of the ethanol-solubilized microbubble-surfactant mixture were applied slowly to the surface of the subsolution 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. 

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 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,... 

Fig. 9.1. Synthetic microbubble histograms determined by laser-light scattering in aqueous media (at 21°C) (A) saturated solution of Filmix 3 surfactant mixture (B) distilled water alone (C ) computed difference in above two histograms. (See text for further discussion.)...

An estimate of the actual concentration of synthetic microbubbles present in the (shaken) artificial-microbubble-surfactant solution, represented by Fig. 9.1(A), is given by the fact that 360-400 particles/sec were consistently detected at a flow rate of approximately 1 ml/30 min in order to produce the histogram shown. Therefore, the calculated approximate concentration of synthetic microbubbles in the sample is 7 x 105 microbubbles/ml. (A similar calculation for the distilled water sample shown in Fig. 9.1(B) results in an estimated concentration of only 5 x 103 microbubbles/ml.)... 

Fig. 10.1. Particle size distribution determined for artificial, surfactant-stabilized microbubbles (and micelles) in distilled water.

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