# Zein Monolayers: Characterization and Interaction with (Bio)surfactants

## TL;DR

This study explores how zein proteins form layers on water and how different surfactants affect these layers.

## Contribution

The novel contribution is the characterization of zein monolayers and their interactions with various (bio)surfactants using neutron reflectivity.

## Key findings

- Zein monolayers can be reversibly compressed up to ∼47 mN/m.
- Synthetic surfactants like SDS and Triton X-100 remove precompressed zein layers through different mechanisms.
- Layers cast from acetic acid show slightly higher mechanical strength than those from aqueous ethanol.

## Abstract

Zein is the main protein of corn seeds, which is often
employed
in food packaging and as a model of keratin. In this study, zein monolayers
were deposited from nonconventional solvents: aqueous ethanol and
acetic acid, on pure water that was later exchanged for 1% (bio)­surfactant
solutions: SDS, CTAB, Triton X-100, and the saponin-rich plant extracts
of soapwort ( L.)
and cowherb ( [P.
Mill.] Rauschert), as well as Quillaja bark saponins (QBS). The monolayers
on pure water could be reversibly compressed up to ∼47 mN/m.
On the basis of neutron reflectivity (NR) results, the liquid expanded–liquid
expanded (LE-LE) transition observed at π ≈ 30 mN/m was
assigned to an expulsion of the well-packed monolayer initially located
on the air side of the interface, toward the aqueous side. The phase
transition was accompanied by an increase in the layer thickness (from
∼1 to ∼6 nm) and the adsorbed amount (from ∼1.7
to ∼5.0 mg/m2). In contrast to the saponin-rich
solutions, the synthetic surfactants introduced to the subphase easily
removed the zein monolayer precompressed to π0 =
30 mN/m, although the mechanism was different for the ionic (continuous
displacement) and for the nonionic (orogenic-like). The zein layers
at Si/water and their resistance to the detergent activity of SDS
and QBS were assessed using NR, proving that the layers cast from
acetic acid showed slightly higher mechanical strength than those
cast from aqueous ethanol.