The measurement and interpretation of low dynamic surface tensions of aqueous long -chain alcohols
Abstract
The tension behavior of three straight-chain alcohols, C14OH, C16OH, and C18OH, (sparingly soluble, having solubilities from 3 to 300 ppb at 25°C) was studied for fundamental studies and for relevance to lung surfactants. Tension measurements for dispersions with concentrations of 600 to 1500 ppm in water or saline were made with a bubble method at 25, 37, or 41°C at constant area or at pulsating area, at 1–80 cycles/min with area amplitude of 1.89. The dynamic surface densities Γ(t) were inferred from the dynamic surface tensions γ(t) by using the surface equation of state, which was obtained with a Langmuir trough. For concentrations of 1500 ppm, tensions much lower than the equilibrium values (22 ± 2 mN/m), and as low as 9 mN/m, were observed. These low tensions were inferred to be related to compressed monolayers rather than thicker films or a third phase at the air/water interface. These inferences were established also with direct probing of octadecanol layers at the air/water interface with IRRAS, ellipsometry, and a radio-tracer method using 14C-octadecanol. For ellipsometry experiments where a known amount of material was applied, calculated Γ-values were about 15% higher than expected, indicating a dense or anisotropic monolayer. Dynamic adsorbed densities from ellipsometry and the monolayer isotherm were in agreement. The IRRAS absorbance values for films of octadecanol with applied surface densities above 8 × 10−6 mol/m2 at the air/water interface were nearly the same, suggesting that extra material may form multilayer patches or crystallites which are either connected with the monolayer or lie below the surface. Results with a radio-tracer method support this inference. The above techniques were used to determine the thicknesses of Langmuir-Blodgett monolayers with Π = 10 to 50 mN/m. Thicknesses determined for the same films with FTIR (d1 = 7.5 to 8.5 Å) and ellipsometry (d1 = 6.6 to 11.0 Å) were similar but lower than expected either because of monolayer collapse or poor film quality. Radio-tracer studies indicated also that only 50–70% of the monolayer was deposited.
Degree
Ph.D.
Advisors
Franses, Purdue University.
Subject Area
Chemical engineering
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