Strain rate and temperature dependence of collapse pressure in Langmuir monolayer of cholesteryl dimers

Abstract

We have synthesized two amphiphilic cholesteryl dimers, bis(cholesterylsuccinamidoethyl) ether [(NHSCE)2] and bis(cholesterylphthalamidopropyl) ether [(NHPCE)2]. They form stable Langmuir monolayer films at air-water interface. (NHSCE)2 exhibits only liquid condensed phase, whereas (NHPCE)2 exhibits both liquid expanded and liquid condensed phases. The collapse phenomenon of a Langmuir monolayer has relevance to biological processes like control of surface tension in lung alveolar and so on. Our studies show that the collapse pressure (πc) of (NHPCE)2 Langmuir monolayer is invariant with the strain rate of compression and temperature. Interestingly, the πc of (NHSCE)2 Langmuir monolayer depends on the strain rate of compression and temperature. A power law relation is used for analysis of the strain rate dependence of πc. On varying the temperature, initially we find an increase in πc with increase in temperature, a behavior known as alpha (α) flow. On further increase in temperature, we find decrease in πc, a behavior known as kappa (κ) flow. The α- flow is very unusual and has not been reported in literature for dimers. An analogy is drawn between the observed α- flow in (NHSCE)2 dimer and the yield strength anomaly reported for alloys.