Alkaline subphases

Tn 1922 Adam (I) published the third paper in his extraordinary series on surface film structure. He observed that fatty acid monolayers greatly expanded on alkaline subphases. He also suggested that fatty acid anions desorbed or dissolved from the monolayer into the alkaline subphase. In 1933 he and Miller (2) showed that the composition of the subphase buffer significantly affected the monolayer thus palmitic and stearic acid monolayers were more condensed on 2N sodium hydroxide than on 2N potassium hydroxide. The expansion, desorption, and cation selectivity of ionizing monolayers are the subjects of this investigation. 

Subphase buffers at pH 9-11 and 0.1 to 0.2 ionic strength were prepared with sodium or potassium bicarbonate-carbonate mixtures (8, 26). Subphase buffers at pH 8-11 were prepared by adding 0.01M tris to 0.1M or 1.0M sodium chloride and adjusting the pH with concentrated hydrochloric acid or concentrated sodium hydroxide. Another alkaline subphase contained 0.1 N sodium hydroxide (pH 12.7). The acid subphase contained 0.01N hydrochloric acid and 0.1M sodium chloride (pH 2.1). All subphases contained 0.1mM EDTA. 

Figure 2. Schematic of the relationships between ionization and surface area, A, in a monolayer solid line, moderately alkaline subphase dashed line, strongly alkaline subphase X and Z, complete ionization in expanded region of tt-A isotherm Y, partial ionization in plateau region of 7r-A isotherm...

The initial observation of Sears and Schulman (3) that Na+ > K+ selectivity on strongly alkaline subphases reversed on a weakly alkaline subphase was confirmed in several studies. Desorption experiments (Figure 11) showed that palmitic acid monolayers with essentially complete ionization—strong fields—were condensed more by Na+ than by K+. At lower pH, the partially ionized monolayers—weaker fields—were condensed the same amount by Na+ and K+. 

Mohwald and co-workers conducted similar research on arachidic acid monolayer on the subphase of different pHs in the presence of sodium ions [93]. The fitted peak intensities are related to surface concentrations of single and double hydrogen bonded and non-hydrogen bonded carbonyl and carboxylate. From the change of the concentrations with the pH, tentative conclusions are drawn on the bonding situation of fatty acids in monolayers on water and alkaline solution. 

Figure 14. it-A isotherms for stearic acid (SA) spread on subphases containing different alkaline earth ions (0.2mM) at pH 9. H20 isotherm is for pure water (17). 

Other properties of fatty acid monolayers such as the phase transition temperature are consistent with the field strength theory stearic acid monolayers formed rigid films on alkaline earth subphases (17, 35). The temperature of the phase transition from rigid to fluid monolayers, estimated by the Devaux talc test, was a function of pH and buffer composition (Figure 17). Thus transition temperature at pH 6 decreased in a weak field sequence (I in Table III) while transition temperatures at pH 8 decreased in an intermediate field sequence (III or IV in Table III). Since variables such as tt are not controlled in these experiments, it is surprising that transition temperature data followed these sequences. 

Figure 18. Stearate content, determined by infrared spectroscopy of monolayers spread on alkaline earth (0.2 mM) subphases plotted as a function of pH (17)...

Manipulation of the subphase composition during fabrication. For instance, the use of high ionic strength subphases furnishes highly fiiermostable alkaline phosphatase LB films [56],... 

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