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

Highlights: • Two amphiphilic cholesteryl dimers were synthesised. • Collapse pressure (π_c) of one of the cholesteryl dimers depends on strain rate. • The π_c dependency on strain rate was analysed using a power law. • One of the dimers shows an increase in π_c with increase in temperature (α- flow). • The α- flow is analogous to yield stress anomaly reported for alloys. 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 (π_c) of (NHPCE)₂ Langmuir monolayer is invariant with the strain rate of compression and temperature. Interestingly, the π_c 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 π_c. On varying the temperature, initially we find an increase in π_c with increase in temperature, a behavior known as alpha (α) flow. On further increase in temperature, we find decrease in π_c, 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.