Orientation of Hydrophobic Polyoxyethylene Chain on Aggregate Structure for
Homogeneous Polyoxyethylene-Polyoxypropylene-type Nonionic Amphiphilic Compounds
using Small-Angle Neutron Scattering
Nara Women’s University, Tomokazu Yoshimura

1. Introduction

Polyoxyethylene (EO) alkyl ether type nonionic surfactants (CnEOx; n and x represent alkyl and EO chain
length, respectively), comprising an EO chain as the hydrophilic group, are generally safe, non-toxic, andnon-irritant. Because their hydrophilic/lipophilic balance (HLB, degree of surfactant affinity for water/oil)can be easily changed by altering both the alkyl and EO chain length, these surfactants have been employed
in a wide range of fields (e.g., detergents and cosmetics). Thus, much research has been dedicated towards
the design, development, and analyses of novel surfactant structures with improved properties and greater
functionality. The EO chain length distribution as well as the lengths of the alkyl and EO chains must also betaken into account when investigating the surface-active and aggregation properties of these surfactants inaqueous solution. Consequently, interpretation of these surfactant properties has proven to be difficult. Incontrast, because CnEOx surfactants with a single chain length have a homogeneous chain length distribution,
their properties can be determined precisely. Thus, in recent years, the development of homogeneous
surfactants has been a prime objective. PO-EO alkyl ether type nonionic surfactants (CnEOxPOy, y represents
PO chain length), synthesized from propylene oxide and CnEOx species with a distribution of chain lengths
have also been investigated. Narrowing the chain length distributions of both the EO and PO chains afforded
excellent foaming and antifoaming properties. CnEOxPOy surfactants with single EO and PO chain lengths
have not been investigated to date. The analyses of these surfactants would allow the precise determinationof their physicochemical properties. The small-angle X-ray scattering (SAXS) showed that the CnEOx
species formed small micelles in solution at low concentrations and the structures transformed to hexagonalliquid crystals as the surfactant concentration increased, conversely, CnEOxPOy maintained a micellar
structure even at high concentrations. Notably, the introduction of a PO chain into the CnEOx surfactant
controlled the formation of aggregates with a higher-order structure (hexagonal liquid crystals). However, it
is not clear the orientation of the PO chain in the surfactant micelles. We want to know the detail information
about the orientation of PO chain using SANS measurements.
The aim of this study is to evidence the detail nano-structure of aggregates formed by novel nonionicsurfactants in aqueous solution. Our final goal is to discuss the effects of alkyl chain length, oxyethylene andoxypropylene chain lengths, and distribution of polymerization degree of the nonionic surfactants on theaggregate formation. Because this is the first trial for TAIKAN measurements, we will measure some moresteels with typical microstructure and check reproducibility of profile shape and intensity by comparing
SAXS measurement in the future.

2. Experiment
The SANS measurement of the polyoxyethylene-polyoxypropylene-type novel nonionic surfactants with1

identical chain lengths in D2O was performed at room temperature by changing the alkyl chain length (10, 12,14, 16), oxyethylene chain length (4, 6, 8), oxypropylene chain length (3, 6), and the concentration (5 points
per 1 sample).

3. Results

Fig.1 shows the best-fitted theoretical scattering curves
with experimental SANS profiles for C12EO8POy (y = 0, 3)
at 100 mmol dm–3 in D2O. We tried to reproduce for SANS
profile from the results of analysis of the model-fitting
obtained by SAXS; y = 0 showed good agreement,
however, y = 3 did not. This is considered that micelle size
obtained by SAXS estimated to be smaller than the actualmicelle structure because the PO chain was not visible,
since the scattering length density of the PO chain for
X-ray almost agrees with that of water. On the other hand,in SANS, it is possible to evaluate accurately, because D2O
has sufficient contrast. The SANS results suggested thatthe size of micelle became slightly small and the thicknessof shell decreased, as the PO chain length increased. ThisFig.1 SANS profiles, and fitting curves for
C12EO8POy (y = 0, 3).

is considered that C12EO8POy form ellipsoidal micelles with the EO chain folded by the interaction between
the alkyl chain and the PO chain by the introduction of PO chain to the EO surfactant. The aggregation
number of the ellipsoidal micelles formed by the surfactants was calculated by using radius of core part,axial ratio of core, and volume of the core per a molecule obtained from the scattering length density. The
aggregation numbers decreased by the introduction of the PO chain (PO1, PO2, and PO3). This is resulted thatthe curvature of molecule became large due to form the micelle with the EO chain folded by the introductionof PO chain.
We are currently studying by model analysis using both SANS and SAXS.

4. Conclusion

It was clear that the homogeneous polyoxyethylene-polyoxypropylene-type novel nonionic surfactants
with identical chain lengths formed ellipsoidal micelles in D2O, and the EO chain was folded in the micelle.
This micelle formation was differed with those by the EO surfactants.
In the future, further detailed structure of the micelle will be clarified by model analysis.

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