An AFM Study of Lipid Monolayers. 1. Pressure-Induced Phase Behavior of Single and Mixed Fatty Acids

Abstract

Monolayers of palmitic (C16:0) and lignoceric acid (C24:0) and their equimolar mixture were transferred to a hydrophilic mica substrate at various surface pressures and investigated by means of atomic force microscopy (AFM) in contact and lateral force modes. The first-order transition of lignoceric acid gives a plateau region, representing a liquid expanded to liquid condensed phase transition in the pressure-area isotherm. This was visualized by AFM as stripes of a condensed phase within the expanded phase, exhibiting a small height difference but a significant difference in friction. The corresponding phase transition of the palmitic acid was continuous, and no changes of the Langmuir-Blodgett films with respect to pressure were observed with AFM. Both the surface pressure-area isotherms and the direct observations of domains of irregular size and shape using the AFM showed that lignoceric and palmitic acid were immiscible. The height difference between the domains was 1.1 nm corresponding to the difference in hydrocarbon chain length of the two fatty acids.

Influence of environmental conditions on the interfacial organisation of fengycin, a bioactive lipopeptide produced by Bacillus subtilis

“…When the surface pressure values of phase transitions in mixed monolayers are similar to the ones observed for the pure component monolayers, this indicates immiscibility between the two components at the air-water interface [29]. In this study, it has been shown that the surface pressure of the characteristic transition of DPPC and the surface pressure corresponding to the final plateau of fengycin monolayers are higher when these two compounds are mixed at the air-water interface.…”

Lamellar subcomponents of the cuticular cell membrane complex of mammalian keratin fibres show friction and hardness contrast by AFM

“…3), even with further magnification. This was surprising because height differences of c. 1.5 nm have been observed for fatty acids on mica surfaces (Ekelund et al ., 1999). However, in the present case, the outer β‐layer is maintained on a softer and more uneven support.…”

Human stratum corneum lipid organization as observed by atomic force microscopy on Langmuir–Blodgett films

“…The deposited lipid Wlm was left for 25 min for the solvent to evaporate, and thereafter compressed to 22 mN/m at a speed of 1 mN/m per minute, with a maximum barrier speed of 10 mm/min (cf. Ekelund et al, 1999;Sparr et al, 1999). Three isotherm cycles were performed to ensure reproducibility.…”