Structure of surfactant and phospholipid monolayers at the air/water interface modeled from neutron reflectivity data

Abstract

Specular neutron reflectometry is a powerful technique to resolve interfacial compositions and structures in soft matter. Surprisingly however, even after several decades, a universal modeling approach for the treatment of data of surfactant and phospholipid monolayers at the air/water interface has not yet been established. To address this shortcoming, first a systematic evaluation of the suitability of different models is presented. The result is a comprehensive validation of an optimum model, which is evidently much needed in the field, and which we recommend as a starting point for future data treatment. While its limitations are openly discussed, consequences of failing to take into account various key aspects are critically examined and the systematic errors quantified. On the basis of this physical framework, we go on to show for the first time that neutron reflectometry can be used to quantify directly in situ at the air/water interface the extent of acyl chain compaction of phospholipid monolayers with respect to their phase. The achieved precision of this novel quantification is ∼10%. These advances together enhance significantly the potential for exploitation in future studies data from a broad range of systems including those involving synthetic polymers, proteins, DNA, nanoparticles and drugs.

Assessing molecular simulation for the analysis of lipid monolayer reflectometry

“…However, the values found in this work are substantially lower, at∼850 Å 3 . This reduction, of∼12 %, agrees well with the work of Campbell et al [40] and Small [56], which suggest that under the surface pressure investigated in this work a reduction of the tail volume of up to15 % may be observed. We believe that the model layer structure from the chemicallyconsistent method provides a satisfactory description of the monolayer structure.…”

“…Previous work has suggested that when only a single lipid type is present, the roughness between the layers should be conformal in nature, that is it should be carried uniformly through the layers [40,42]. However, from the investigation of the SLD profiles in figure 3(b) it appears that the roughness between the lipid tails and the air is dramatically different from that at the lipid head-water interface.…”

“…where, SLD s is the scattering length density of the subphase (water), and i indicates either the tail-or head-layer; it is assumed that the tail layer contains no solvent or air, i.e. f t =0 in agreement with the work of Campbell et al [40]. To ensure that the number density of the head components and pairs of tail components is the same, the following constraint was included in the model [41],…”

“…A single value for the interfacial roughness was fitted for all interfaces, which was limited to be no less than 3 Å, as there is only a single lipid type in each monolayer [40]. Therefore, any roughness at the air-water interface is carried equally through all the layers, in a conformal fashion [42].…”

Using Bayesian Model Selection to Advise Neutron Reflectometry Analysis from Langmuir-Blodgett Monolayers

“…This roughness is modelled with an error function [44], the width of which is taken as 2.9Å, in agreement with the work of Campbell et al. In this model, the volume fraction of the layer was kept constant at a value of 1 while the layer thickness, d was the single variable parameter with a uniform prior probability from [15,35)Å.…”