Phospholipid monolayers at non-polar oil/water interfaces. Part 1.—Phase transitions in distearoly-lecithin films at the n-heptane aqueous sodium chloride interface

Abstract

Using compression techniques, surface pressure (II) against area ( A ) isotherms are measured as a function of temperature for insoluble monolayers of 1,2-distearoyl-sn-glycero-3-phosphorylcholine spread at the n-heptane/aqueous sodium chloride interface. Problems of spreading are resolved and accurate results at moderate and high IT are presented. The results show an almost first order phase change which is very temperature sensitive. Various forms of the Clapeyron analysis are applied to the data to calculate the heats of the phase change and to assess variations arising from the choice of a close-packed area. These heats vary with temperature but not with salt concentration or pH over the range studied. Results on 1,2-dioleoyl-sn-glycero-3-phosphorylcholine show no such phase changes.

Molecular ordering and phase transitions in alkanol monolayers at the water–hexane interface

“…The values of 40.6 Å 3 for the -CH 2 OH volume ͑determined by bulk measurements just above the freezing point 60 ͒ and 30 Å 3 for the volume per bulk water molecule indicate that there is enough space for one water molecule for every 3 alkanol headgroups in layer 1. Including these additional 3 electrons per alkanol from the water that penetrates the headgroup region would raise the values of area per molecule calculated in the previous paragraph to 22.8 Ϯ2 , 23.0 Ϯ1 , 22.8 Ϯ1 , and 23.9 Ϯ1 Å 2 for C 20 OH, C 22 OH, C 24 OH, and C 30 OH. These values are still consistent with an area per alkanol molecule of 23.4 Å 2 .…”

“…These results are consistent with values measured by other groups using the pendant drop method to yield ⌬S a ϭ1.7 mJ/m 2 K for C 18 OH and ⌬S a ϭ2.0 mJ/m 2 K for C 20 OH at the water-hexane interface ͑the C 20 OH value was measured for concentrations of both 11.25 and 18.04 mol/kg͒. 17,55 The values of ⌬S a are much larger than the values reported previously for surface freezing at a pure alkanolvapor interface (⌬S a ϭ0.88, 1.15, 1.2, and 1.3 mJ/m 2 K for C 20 OH, C 22 OH, C 24 OH, and C 28 OH, respectively, where these values are half that for freezing of a bilayer in order to facilitate comparison with our monolayer͒. 56 Our values of ⌬S a are also much larger than the values of ⌬S for the bulk rotator-liquid transition ͑measured for the bulk, but here stated for an effective ''monolayer'' of bulk material, ⌬S a ϭ1.09, 1.2, and 1.35 mJ/m 2 K for C 20 OH, C 22 OH, and C 24 OH).…”

“…40 The latter approximation is appropriate for our data since Q z 2 2 Ӷ1 for our diffuse scattering measurements. Figure 3 shows our measurements of interfacial tension ␥ as a function of temperature for four alkanols (C 20 OH, C 22 OH, C 24 OH, and C 30 OH) at the water-hexane interface and for the pure water-hexane interface. 53 The sharp change in slope reveals an interfacial transition.…”

“…21,23 Pethica and co-workers discussed similar ideas in their studies of lipids at the water-oil interface. 24,25 Until recently, direct structural information on molecular ordering in monolayers at the water-oil interface was unavailable. Here, we mention the few recent nonlinear optical and X-ray scattering measurements that have been used to compare surfactant ordering at water-oil and water-vapor interfaces.…”

Analysis of adsorption of phospholipids at the 1,2-dichloroethane/water interface by electrochemical impedance spectroscopy: A study of the effect of the saturated alkyl chain

“…Pethica and co-workers [1] measured the surface pressure isotherms for a series of insoluble (distearoyl phosphatidylcholine-DSPC and dioleoyl phosphatidylcholine-DOPC) at the n-heptane/water interface using a Langmuir trough. These authors concluded that DSPC forms monolayers which were affected by the concentration of the aqueous electrolyte (NaCl which concentration is varied between 0.1 and 0.01 M) and by the pH (between 2 and 5).…”