Two-Dimensional Surface Properties of 2-Methoxy
Ethyl Oleate at the Air/Water Interface
Geetha Baskar*, S. Venkatesan, Aruna Dhathathreyan, and A.B. Mandal*
Industrial Chemistry Division, Central Leather Research Institute, Adyar, Madras, 6000 20, India
ABSTRACT: Methoxy ethyl oleate, a nonionic surfactant, has acid molecules has been demonstrated in recent studies (9). It
is inferred that trivalent ion, (Tb+3), affects maximum con-
been investigated at the air/water interface for various surface
properties by employing the Langmuir film balance technique.
The ester forms an expanded isotherm at the air/water interface.
The minimum area of packing (A0), initial area of increase of
surface pressure (Ai), collapse pressure (πc), and area/molecule
at collapse pressure (Ac) have been estimated from the isotherm
curve. The higher compressibility coefficient (K) suggests that
the ester forms a more expanded liquid film than the parent
oleic acid. Besides, the ester film is fairly stable as suggested by
only about 30% loss in area over a period of 20 min. The relax-
ation rates of the ester film at different surface pressures of 10,
densation of stearic acid at the air/water interface. Among
various fatty acids, unsaturated fatty acids, such as oleic and
linoleic acids, assume immense significance owing to their
tremendous potential in various formulations, such as corro-
sion inhibitors, extreme-pressure lubricants, pharmaceuticals,
paints, food, and cosmetics. Furthermore, these fatty acids en-
able introduction of functional groups at the double bond
through chemical modification processes, such as polymer-
ization (10,11), sulfation, and chlorination (12). These unsat-
15, and 20 mN·m−1 have been estimated from changes in the urated fatty acids and their derivatives can be considered as
surface area/molecule with time. Interestingly, the surface area
(54.2 Å2/molecule) that corresponds to a minimized structure
projected for the ester, calculated theoretically, agrees reason-
ably well with the experimental value (57.2 Å2/molecule).
Paper no. J9031 in JAOCS 76, 853–858 (July 1999).
reactive surfactant systems, capable of promoting chemical
reactions inside the respective organized assemblies. The ad-
vantages of such reactions in exhibiting control over the mi-
crostructure and molecular properties of the ultimate product
have been well documented (13,14). To cite an example,
polymerization inside aqueous micellar assemblies influences
the polydispersity and microstructure of the homopolymer of
methoxy poly(ethylene glycol) acrylate (15). To exploit the
advantages of chemical reactions inside the organized assem-
blies involving oleic acid or its ester, it is necessary to inves-
tigate the surface properties in detail. The influence of the cis
KEY WORDS: Collapse pressure, compressibility coefficient,
isotherm, Langmuir film balance, methoxy ethyl oleate, relax-
ation rate.
Fatty acids and their derivatives, such as esters and amines, double bond on the organization behavior of oleic acid at the
exhibit surface activity by virtue of their amphipathic struc- air/water interface has been reported in a few studies (16,17).
tures, and are capable of forming organized assemblies rang- In this study, 2-methoxy ethyl oleate has been selected be-
ing from monolayers and micelles to membranes akin to clas- cause it has extended polar groups at the hydrophilic end.
sic surfactant materials (1,2). It is well known that the glyc- Some of the significant parameters pertaining to packing be-
eryl esters of fatty acids form the main component of lipids havior of the tail group, orientation of the head group, and sta-
present in cell membranes. Organized assemblies of fatty bility of the ester film at the air/water interface have been de-
acids and their esters thus provide useful models for under- termined by the LFB technique.
standing various membrane-mediated processes (3–5). The
orientation and conformation of surface-active molecules in-
fluence some important surface and interfacial properties,
EXPERIMENTAL PROCEDURES
such as packing density, hydrophilicity, and free energy. The Materials. Cis-octadecenoic acid (oleic acid), 98% pure, was
Langmuir film balance (LFB) technique provides a simple purchased from Sigma Laboratories (St. Louis, MO), and 2-
tool for drawing useful information that compares well with methoxy ethanol (99% pure) was from Glaxo Laboratories,
theoretical modeling studies (6). Extensive studies have been India. Analar para-toluene sulfonic acid with a purity >98%
carried out on the monolayer properties of fatty acids with the (S.D. Fine, India) was used as catalyst in the synthesis of the
LFB technique (7,8). The influence of pH and multivalent ester. Chloroform used in the monolayer experiments was of
metallic ions present in the subphase on the packing of fatty high-performance liquid chromatography grade. Distilled
water was purified with a Milli Q (Millipore, Milford, MA)
filtering system for the monolayer experiments. The surface
*To whom correspondence should be addressed.
E-mail: clrimd@md2.vsnl.net.in
Copyright © 1999 by AOCS Press
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JAOCS, Vol. 76, no. 7 (1999)