MONOMOLECULAR FILM VISCOSITIES AND MICROMORPHOLOGIES OF FATTY-ACIDS AND THEIR ANALOGS AT AIR WATER INTERFACES - RELATIONS TO MOLECULAR-STRUCTURES

The monomolecular film viscosities and micromorphologies for five amphiphiles were investigated to obtain molecular structures yielding uniform monolayers and good-quality Langmuir-Blodgett films. The molecules employed were a monoalkyl acid (stearic acid, I), a steroid acid (cholanic acid, II), and three dialkyl acids: 3-((dioctadecylamino)carbonyl)propionic acid (N,N-dialkylamide acid), III; 3-heptadecyle-icosanoic acid (dialkyl acid), IV; and 3,5-bis(octadecyloxy)benzoic acid (dialkyl benzoic acid), V. The viscosities of I and II were determined to be ca. 10~3 g/s at 10 dyn/cm on pure water at 18°C. The micromorphologies of I and II observed with a fluorescence microscope consisted of liquid-like fluid domains with round boundaries. The dialkyl acids, IV and V, produced viscous films with larger viscosities of 0.30 and 1.8 g/s, respectively, and showed solid-like angular domains, which appeared difficult to fuse into a homogeneous film. The tyiV-dialkylamide acid containing a large hydrophilic group (III) produced a less viscous film (0.11 g/s) and a soft microdomain. Cadmium chloride added to water caused the viscosities to increase and made the microdomains less fluid, while sodium carbonate caused the viscosities to decrease.The monomolecular film viscosities and micromorphologies can be ascribed to the hydrophobic and hydrophilic interactions of the molecules. Each hydrophobic cohesion of the molecules was estimated by measuring the enthalpies of melting for the amphiphile powders: 1,14.1; II, 5.4; III, 21.7; IV, 21.1; and V, 20.7 kcal/mol. The bulkiness of the hydrophilic group for I or in should generate the repulsion, resulting in lower viscosity and fluid or soft microdomains. A key for designing the molecular structure producing good-quality multilayers onto solid substrates was discussed. © 1990, American Chemical Society. All rights reserved.