XIX-th ARS SEPARATORIA – Złoty Potok, Poland 2004
MICELLAR LUBRICATION: INTERACTIONS OF SOFT-CORE
REVERSE MICELLES WITH OIL ADDITIVES AND ENGINE
SURFACES
Zenon PAWLAK1) and Doug HARGREAVES2)
1)
Faculty of Technology and Chemical Engineering,
University of Technology and Agriculture,
85-326 Bydgoszcz, Seminaryjna 3
and Utah Department of Health, Environmental Chemistry,
Salt Lake City, 46 Medical Drive, UT 84113, USA
2)
Queensland University of Technology,
School of Mechanical, Manufacturing and Medical Engineering,
Brisbane, 4001, Australia
The primary objectives of lubrication of reciprocating engines are the
prevention of wear and the maintenance of power-producing ability and
efficiency. These objectives require that lubricants function effectively to
lubricate, seal, control frictional properties, prevent excessive wear and
seizure of moving parts, protect against corrosion, keep surfaces and oil
ways clear, cool and permit operation at temperature extremes.
Engine oil formulations consist of (a) base oil and viscosity improver
(72-96%), and (b) additive package (4-28%). The composition of a typical
engine oil is shown in Table 1.
Table 1. Concentration range of main additives used in the formulation of engine
oils [1]
Material
Weight (%)
Material
Weight (%)
SAE 30 or 40 base oil
71.5 to 96.2
Antioxidant/Wear
0.1 to 2.0
Metallic detergent
2 to 10
Friction modifier
0.1 to 3.0
Ashless dispersant
1 to 9
Anti-foam agent
2 to 15 ppm
Zinc dithiophosphate
0.5 to 3.0
Pour point depressant
0.1 to 1.5
The additive mixtures interact in a variety of ways, both in the bulk oil
and on surfaces. Surfactant molecules, when dissolved in base oil, are
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capable of self-organization to form aggregates such as soft-core reverse
micelles (RMs). The formation of RMs, due to dipole-dipole interactions
between the polar heads of the surfactant molecules, may play an important
role in oil formulation. The polar or charged head groups of these molecules
with the counter ions form the interior of the micelle (core) and the
hydrocarbon chains made up its external shell. The most important factor
governing the tribochemical reactions under boundary lubrication involves
the action of soft - reverse micelles [2], see Fig.1.
The nature of micelles, whether normal or reverse, depends on the
polarity of the medium. Several interesting features have been obtained with
respect to the aggregation and micelle structure with changing polarity of the
medium. If the medium polarity decreases progressively as, for instance, in
the series (dielectric constant): water (80.1), monoethylene glycol (38.7), and
chloroform (4.8), the surfactant initially existing as a normal micelle (M)
with a non-polar core, gradually changes to an intermediate monomeric state
(n) to form a reverse micelle (RM) with a polar core [3]. The free-energy
changes of micellization (∆Gomic) of the surfactants are also schematically
shown in Fig. 1.
∆G o
mic
RM
n
M
a
r
b
i
t
r
a
r
y
u
n
i
t
s
low-polar
intermediate
high-polar
Dielectric constant ( ε)
Fig. 1. Dependence of free-energy changes (∆ Go ) of micellization of surfactants
in high, intermediate, and low-polar solvents. Normal micelle (M) to reverse micelle
(RM) transition of polarity (ε) of the medium. Surfactants in the medium
of optimum dielectric constants 38 to 41 do not aggregate but remain
in the monomeric state (n) [2].
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XIX-th ARS SEPARATORIA – Złoty Potok, Poland 2004
The micelle formation process and structure can be described by
thermodynamic functions: free energy, ∆Gomic, enthalpy, ∆Homic, and
entropy,∆Somic, physical parameters (surface tension, conductivity, refractive
index) or by using techniques such NMR spectroscopy, fluorescence
spectroscopy, small-angle neutron scattering and positron annihilation.
Experimental data show that the dependence of the aggregate nature,
whether normal or reverse micelle is formed, depends on the dielectric
constant of the medium [3,4]. The thermodynamic functions for
micellization of some surfactants are presented in Table 2.
Table 2. Thermodynamic functions for micellization of surfactants at 25°C [2]
Surfactanta
Thermodynamic function
Free energy, ∆Gmic (kJ mol-1)
Enthalpy, ∆Hmic (kJ mol-1)
Entropy, ∆Smic (J mol-1 K-1)
A
B
C
D
Reverse micelle (RM)
-60.7 -63.8 -15.9 -87.0
-79.5 -105.4 -50.2 -116
-63.8 -143.1 -115.1 96.7
E
Normal micelle (M)
-335 to -502
-167 to +586
+837 to +2,51
a
The following surfactants have been tested: (A) Potassium benzenesulfonate in
heptane, (B) Tri-n-dodecylammonium bromide in cyclohexane (C) Tri-ndodecylammonium bromide in benzene, (D) Tri-n-dodecylammonium
tetrachloroferrate in benzene [3,4], (E) Most surfactants in aqueous solutions [5].
Micellization occurs in order to reach minimum free energy (∆Gomic)
in the system, once the process of adsorption at the interface has reached
saturation. Thermodynamic measurements in dilute low-polar solvents
surfactant solutions give values for the enthalpy change on micellization
(∆Homic) in the range of (-50.2 to -119.9 kJ mol-1) and the associated entropy
change ( ∆Somic) in the range (-63.8 to -143.1 J mol-1 K-1), see Table 2. The
free energy change (∆Gomic) comprise enthalpy and entropy contributions
according to the equation 1:
∆Gomic =
∆Homic - T∆So mic
(1)
and are negative in the range of (-15.9 to -87.0 kJ mol-1) which can indicate
self-association processes.
The enthalpy and entropy changes of micellization have been
calculated for benzenesulfonate and alkylammonium salts in low-polar
solvents suggesting that micellization is essentially an enthalpy-driven
effect. The aggregation can take place at low concentrations of surfactant
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and can have different aggregation numbers. The absence of well-defined
critical micelle concentrations (CMC) for some systems in the low-polar
solvents was observed [4].
In aqueous solutions, the resulting free energy changes (∆Gomic) are
negative (in the range of -502 to -335 kJ mol-1) indicating spontaneous
processes. The entropy changes are dominant, contributing most to the
overall free energy changes as a result of changes in the water structure. In
water, formation of micelles is an entropy-driven process which favors the
shielding of the amphiphilic hydrocarbon moiety from the solvent [5].
Tribochemical nature of tribofilm. RM's soft-core interactions with
ZDDP has been considered as a reason for reduced antiwear performance
[9,10,11]. The tribofilm formation is strongly influenced by detergents, e.g.,
a calcium phenate (basic) detergent affects the film formation at low
concentrations, and calcium sulfonate (neutral) detergent affects the film
generation only at high concentrations (over 2%). The polyisobutylene
succinic anhydride polyamide dispersant (PIBS-PAM) in formulated engine
oil interacts strongly with ZDDP. The polyphosphate tribofilms formed in
the presence of the these additives have shorter chain-length compared with
ZDDP alone and contain less decomposed ZDDP [9]. A layered structure of
polyphosphate tribofilms the surface (50 Angstroms) and bulk (500
Angstroms) spectra were recorded and identified by (XANES) spectroscopy.
The additives compete with the adsorption of the ZDDP on the surface [9,
10]. If less ZDDP is adsorbed, a greater % of the adsorbed ZDDP will
decompose to polyphosphate, and a greater % of the polyphosphate will give
a short-chain phosphate. Using a XANES spectroscopy, investigators have
observed for the first time direct processes of the detergents and dispersants
interactions with ZDDP on the surface. These interactions manifest themselves
in three ways as far as the micellar ZDDP adsorption and tribofilm formation is
concerned:
- at low concentrations micellar additives, c < 2 % tribofilm was free from
unreacted ZDDP,
- tribofilms generated in the presence of micellar additives are thinner than
tribofilms produced in the absence of them, and
- polyphosphates film formed in the presence of micelles are of shorter chain
length.
Two mechanisms of action of reverse micelles are obvious. These are as
follows:
(1) Prevention of agglomeration of insoluble oxidation products and sludge
particles generated by the combustion of diesel fuels,
(2) Solubilization of oxidation products, e.g., organic acids (HA) and some
additives (ZDDP) by soft-core reverse micelles and inorganic acids
(HS) by hard-core reverse micelles in oil formulations.
Micellar Solubilization in Lubrication. Several mechanisms of action
of RMs are generally recognized: solubilization of organic acids, solubilization
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of guest molecules e.g., ZDDP, steric and electrostatic stabilization of
contaminants. The solubilized compounds go into micelles interior cavity
which consists of ionic or polar groups. The RMs interface solubilize or
deactivate polar compounds, such as water, catalytic metal ions, organic acids
and sludge [2,6,7]. The 3-nitro- and 3,4-dinitrophenol were solubilized by
sodium dialkyl sulfosuccinate (AOT)/heptane/water RM's system [13]. The
questions of the localization of small substituted phenol molecules
inside/outside of water pool size (WP = [H2O]/[surfactant]) and size of RM
should receive a definitive answer. p-Nitrophenol in bulk water has pKa(H2O)
of 7.14 and is normally 95% ionized at pH 8.4. In the micellar
AOT/heptane/H2O (Wo = 25.8) p-nitrophenolate ion is not produced until the
pH exceeds 11.5. The acidity of p-nitrophenol in micelle water pool lowers
ionization by 4.5 pKa units. This is attributed to the adsorption at the pool
interface where phenolic hydroxyl are held in proximity to anionic surfactant
groups. The acidity of pool-incorporated p-nitrophenol is not sensitive to water
pool (Wo) size (2 to 10 for small pools and 30 to 50 for large pools).
There are some possible residence sites for the hydrophilic solutes in
reverse micelles. Acidic molecules can be adsorbed internally (a), (b) and
externally (c), (Fig. 2) [2,13].
...ROH
(H2O) n
ROH aq
(a)
(b)
...HOR
(c)
Fig. 2. The anionic type soft-core reverse micelles with p-nitrophenol (ROH)
located: (a) on the internal wall of the anionic group, there is no "free" water,
Wo<10; (b) internally hydrated by "free water" in the water pool unless, Wo > 10;
(c) the acidic molecules can be adsorbed externally by soft-core RMs at the interface
of an detergent-stabilized water pool, Wo > 30.
The pKa of acidic substances in micelles core differs considerably from
that of the dilute aqueous solution. The pKa changes are about 0.5 to 2.5 units
higher pH indicating a lower acidity in the micelle than in bulk water:
pKa(ROH, in soft-core RM's) > pKa (ROH, in bulk water)
Traces of strong mineral acids, such as sulfuric, halogen or nitric acid, are
neutralized by the alkaline components of the carbonate-salicylate RMs [12].
The water solubilized by soft-core carboxylate or sulfonate RMs can
subsequently solubilize formed salts which were originally oil-insoluble. This
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phenomenon is known as "secondary solubilization" [14]. Metallic detergents
such a sulfonates, phenolates and salicylates enhance the solubilizing properties
of succinimide additives. For small amount about 0.25% w/w sulfonate
and 2.8% succinimide, a synergistic effect is observed, but for high sulfonate
concentrations the amount of acid solubilized is less than that observed for
succinimide alone.
A different mechanism exists for the solubilization of weak acids and
for strong acids. The different interaction between the amine groups in the polar
head of a succinimide micelles and a weak acids (WH) and a strong acids (SH)
[8]:
Weak acid (WH) : R3N + mWH = R3N...(HW)m, where (m = 2 to 6),
Strong acid (SH) : R3N + SH = R3NH+Swith the resulting adduct R3N...(HW)m being much less polar than the R3NH+Sadduct. Solubilized sulfuric acid, can make the succinimide salt more polar and
thus less soluble. The molar ratio of M(acid)/M(additive), is 2 to 6 times higher
than that for a strong mineral acid.
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