Estimation of the Mutual Orientation and Intermolecular Interaction
of C
12
E
x
from Molecular Dynamics Simulations
Maria Velinova, Yana Tsoneva, Anela Ivanova, and Alia Tadjer*
Laboratory of Quantum and Computational Chemistry, Department of Physical Chemistry, Faculty of Chemistry and Pharmacy,
University of Sofia, 1 James Bourchier Avenue, 1164 Sofia, Bulgaria
* S Supporting Information
ABSTRACT: Nonionic surfactants, such as poly(ethylene glycol) alkyl ethers
(abbreviated as C
y
E
x
) show a rich phase behavior in aqueous solution, i.e., they
form micellar, lamellar, cubic, and so forth phases depending on experimental
parameters such as the hydrophobic and hydrophilic chain lengths,
temperature, or concentration. The aim of the present study is to determine
the nature of the preaggregates, which are inferred to exist before the actual
self-assembly process in aqueous solution, and to assess the aptitude to their
formation. The target molecules are C
12
E
3
,C
12
E
4
and C
12
E
5
, surfactants of
moderate water solubility. Coarse-grained and all-atom molecular dynamics
simulations (NPT/293 K) of two molecules of each species with explicit water
in periodic boundary conditions are carried out to estimate the mutual
orientation and the interaction between the surfactants in their dimers. The force fields are MARTINI and Amber99, the latter
with self-derived parameters for the ether groups. The change in the orientation and distance between the molecules in the
dimers are discussed based on different structural parameters. In addition, the interaction between the surfactants is evaluated
from quantum chemistry calculations in terms of binding energy for the average structures from the cluster analysis. The
solvent-solute interaction is quantified by the mean number of hydrogen bonds formed between them. On the basis of
combined analysis, a series of different structures for subsequent study of the possible self-assembly patterns of C
12
E
3
,C
12
E
4
, and
C
12
E
5
is outlined.
■
INTRODUCTION
Poly(ethylene glycol) alkyl ethers are typical nonionic
surfactants with general chemical structure, H(CH
2
)
y
-
(OCH
2
CH
2
)
x
OH, which is simply denoted by C
y
E
x
, where y
corresponds to the number of carbon atoms in the hydrocarbon
tail, and x corresponds to the number of oxyethylene residues
in the head. C
y
E
x
with different numbers of ethylene glycol
units and various hydrophobic chain lengths form a chemically
stable homologue series, and their compounds are commer-
cially available. These surfactants exhibit high surface activity,
and their aqueous solutions show surface tension behavior
characteristic for aggregation in a very low-concentration range,
namely, from 10
-7
to 10
-6
mol·dm
-3
,
1
orders of magnitude
below their critical micelle concentration (CMC), which is on
the order of 10
-5
to 10
-4
mol·dm
-3
.
2
Beyond the CMC, this
class of surfactants forms a variety of neat and mesomorphic
phases depending on the chemical composition of the
surfactant and on the temperature.
3-10
The chemical structure
also allows tuning of the hydrophilic/hydrophobic balance by
varying the number of oxyethylene and methylene units
responsible for the solubilization in the respective liquid
phase. The CMC value of the various C
y
E
x
homologues
depends on the number of ether oxygens and grows with the
increase of this number.
2
In diluted aqueous solutions, such surfactants exist mainly in
the micellar or in the lamellar phases depending on the
chemical nature and molecular structure of the surface-active
species and on the thermodynamic conditions. In pure or
mixed solvents, these nonionic surfactants can produce a large
variety of self-assembled structures relevant to several fields of
application. In the form of emulsions, microemulsions, vesicles,
or liposomes, such systems are extensively used in detergency,
stabilizing and recovery from pollutants, or as nanotransport-
ers.
11-13
The propensity of C
y
E
x
to self-assemble in various
shapes and sizes can be used in certain novel materials, e.g.,
nanoporous structures with controlled dimensions of the pores,
for which C
y
E
x
micelles with different size are used as
templates.
14
Therefore, knowledge about the factors governing
the existence of the various phases of C
y
E
x
supramolecular
assemblies at the molecular level is valuable.
In this study, the target molecules are C
12
E
3
,C
12
E
4
, and
C
12
E
5
.C
12
E
3
is known to form only bilayers in the bulk, while
C
12
E
4
and C
12
E
5
are able to self-assemble in cylindrical micelles
in solution.
15
Recently, molecular dynamics (MD)
16
and Monte Carlo
17
simulations have been successfully applied to study poly-
(ethylene glycol) chains in water or nonionic surfactants in an
assortment of phases (at the air/water interface
18,19
and in
Received: December 14, 2011
Revised: March 3, 2012
Published: March 26, 2012
Article
pubs.acs.org/JPCB
© 2012 American Chemical Society 4879 dx.doi.org/10.1021/jp212047r | J. Phys. Chem. B 2012, 116, 4879-4888