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394 Lubricant Additives: Chemistry and Applications
chosen for a system based on laboratory trials, it is imperative to con rm the laboratory results
with a eld trial.
15.7.6 BIOCIDE HANDLING
It is important to remember that biocide products are designed to control and arrest microbiologi-
cal growth and, therefore, have toxic properties. Special precautions should always be taken when
handling biocides. A product’s MSDS is the best resource to consult, as it will present comprehen-
sive information on the proper handling methods for that particular chemical hazard. The product
label should also be consulted, as Federal law requires that registered pesticide labels provide
detailed safety procedures and use instructions. Although the recommended types of protection
will vary depending on biocide chemistry, in the interest of good chemical hygiene, protective
gloves, aprons, and eye and face protection, at a minimum, are recommended when handling any
biocide.
15.7.7 PHYSICAL AND NONCHEMICAL CONTROL METHODS
Although chemical methods are by far the most common means of controlling microbiologi-
cal growth in metalworking uids, several nonchemical methods have received some attention.
These include heat treatment or pasteurization, UV treatment or irradiation, and ltration. Com-
pared to chemical treatments, these technologies share some common disadvantages. Because
these methods treat metalworking uids at a single point in a system, microbiological growth
that may not be circulating in the uid, or is held up in dead areas of the system or in bio lms,
is not treated.
Pasteurization of metalworking uids heats the uid to a temperature high enough to destroy
the microorganisms responsible for uid deterioration, but temperate enough not to damage the
functional properties of the uid. Although many microorganisms are particularly susceptible to
heat treatment, others are more thermotolerant and can survive this treatment, leaving the remain-
ing microorganisms to degrade the uid. This process is also energy intensive, requiring uids to
remain at ∼142°F (63°C) for periods >30 min. Consequently, the success of this method depends
on the availability of an affordable energy source and the feasibility of heating the uid in an entire
system. Although Elsmore and Hill [24] obtained good results with heat treatment, they found that
heat-resistant populations developed upon intermittent pasteurization. However, the use of uid
heating together with biocidal treatment is more effective in microbial control than either method
alone [25].
Several types of radiation technology might appear to prove useful in controlling microbial
growth in metalworking uid systems, including UV irradiation, high-energy electron irradiation,
and gamma irradiation. With all these methods, uids ow through an irradiation bank, contained
in a thin- lm layer. The radiation causes lethal mutations or kills organisms by direct ionizing
effects. All three methods have at least some degree of dif culty penetrating opaque materials such
as metalworking uids. UV technology is probably the least effective due to this challenge, followed
by high-energy electron radiation, which is also energy intensive, making it less cost-effective.
Gamma irradiation offers the most potential in penetrating uids and killing organisms; however,
handling of this radiation source is costly and requires well-trained personnel [26].
Filtration systems are designed to remove particulate material from uid streams; however,
they do not remove microbial cells directly. Microbial populations tend to grow on particulate
surfaces rather than in free- owing uids. By removing these particles, ltration systems can keep
metalworking uids cleaner and, therefore, less vulnerable to microbial growth. Naturally, cleaner
systems often require less biocide. It is important to remember, however, that lter media can also
harbor microorganisms and may actually add to contamination problems.
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Antimicrobial Additives for Metalworking Lubricants 395
Currently, nonchemical treatment methods remain largely unpopular for metalworking uid
applications because of their signi cant capital expenditures and energy costs, and the scarcity of
well-documented technical success stories from the eld. However, as pressure to remove toxic
chemicals from uid formulations increases, there may be a greater impetus for the industry to
investigate more of these nonchemical treatment technologies.
15.7.8 ENHANCEMENT OF FLUID PROTECTION
Today’s metalworking operations emphasize extending uid life as long as possible. Not only does
this reduce waste disposal costs, but customers also wish to eliminate or reduce the concentration of
uid components viewed as toxic, such as biocides. In an attempt to reach these goals, considerable
effort has been spent on trying to formulate uids with greater inherent biological stability.
One way to attack the problem is to incorporate nonbiocidal materials that will enhance biocide
activity. The best-known example of such technology is the use of chelating agents such as ethylene-
diaminetetraacetic acid (EDTA) salts. Chelants are ordinarily used as conditioners in water treat-
ment to soften hard water by complexing calcium and magnesium cations out of solution. However,
EDTA has also been known for a long time as a potentiator of antimicrobial agent activity [27]. This
is due to its effect in disrupting the cell wall of gram-negative bacteria (e.g., P. aeruginosa), render-
ing the cells more susceptible to damage from biocidal chemistries. Thus, the presence of EDTA in
a use-diluted metalworking uid could lower the level of actual biocide required for microbial con-
trol and lengthen the useful life of a working uid. However, as is often the case in the development
of chemical formulations, a balance must be maintained between desired properties and detrimen-
tal ones imparted by an ingredient. Too much EDTA or other chelating agent can lead to excessive
uid foaming and increased potential for corrosion of metal parts and machine tools.
Another approach is the use of bioresistant functional ingredients in the uid. A bioresistant
material is one that, although not actually killing microorganisms, is not readily decomposed by
microbial attack. Bioresistant materials do not provide a readily available food source for microor-
ganisms, but at the same time these materials cannot stand up to repeated insults of heavy microbial
contamination without degradation. Bioresistant products are not subject to the requirements for
U.S. EPA registration under FIFRA as long as no claims of antimicrobial ef cacy are made. A met-
alworking uid’s overall potential for microbial degradation depends on the susceptibility of each of
its components to microbial attack. A bioresistant component remains unchanged in structure and
functional properties in the presence of a microbial population. Fluids with greater bioresistance
do not necessarily show lower counts of microorganisms, and inhibition of microbial activity or
growth is not ordinarily a property of a bioresistant raw material.
One metalworking uid component chemistry that has received considerable attention in this
regard is the alkanolamines, employed to maintain an alkaline pH and assist in corrosion protection.
Although all alkanolamines eventually become susceptible to biodegradation, their speci c biore-
sistance characteristics can vary [28,29]. In general, 2-amino-2-methyl-1-propanol (AMP, CAS#
124-68-5) appears more bioresistant than do 2-(2-aminoethoxy)ethanol (CAS# 929-06-6), MEA
(CAS# 141-43-5), or triethanolamine (TEA, CAS# 102-71-6). However, differences exist in the
bioresistance of these alkanolamines, depending on the uid formulation and the situation in which
the uid is being used. At least some of the bioresistance attributed to alkanolamines appears to be
related to pH. Rossmoore [30] showed that high pH attributed to alkanolamines provides a degree
of bioresistance. However, in some uids, bioresistance was maintained when pH was lowered,
whereas in others bioresistance was lost. Sandin et al. [31] reported that bioresistance of metalwork-
ing uids was directly proportional to pH and alkyl chain length of the particular alkanolamine
tested. Other alkanolamines providing bioresistance include N-butylethanolamine (CAS# 111-75-1)
and N-butyldiethanolamine (CAS# 102-79-4).
Amine reaction products with boric acid can be used as water-soluble inhibitors for corrosion of
ferrous metals. A major additional bene t of these materials is that they resist microbial attack very
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396 Lubricant Additives: Chemistry and Applications
effectively [32]. However, on drying, they may tend to form hard or sticky residues that are dif cult
to remove from machines and parts [33].
15.8 THE FUTURE OF MICROBIOLOGICAL CONTROL
Although the metalworking uid industry is not viewed as an aggressively expanding market, it is
lled with impending changes and challenges. Metalworking operations and uid technologies will
continue to become more sophisticated. As many experts predict, this will drive the use of syn-
thetic and semisynthetic uids, increasing the demand for innovative and effective microbial control
strategies. Environmental restrictions regarding uid disposal are also likely to escalate, increasing
disposal costs and creating even more pressure to extend uid life. The popular option of simply
adding more biocide to increase uid utility will likely see counterpressure as safety- conscious
workers demand the use of lower-toxicity uids. As environmental and exposure concerns increase,
the industry’s biocide options will decrease, as more biocidal substances come under regulatory
pressure. In addition, microbial resistance will continue to reduce the ef cacy of current biocides.
New chemistries coming to market are also anticipated to slow due to the high cost of registering
new active ingredients. Furthermore, recent regulatory schemes such as the European law known as
Registration, Evaluation and Authorization of Chemicals (REACH, Regulation 1907/2006/EC) may
make it economically unattractive to either develop or continue to market some specialty-chemical
materials, including biocide enhancers and bioresistant ingredients. At the same time, a recent trend
in “green” chemistry is to replace at least a part of a hydrocarbon lubricant material with a vegetable
oil or a vegetable-oil-derived ester or triglyceride. Although such materials offer greater lubricity
per se than does mineral oil, their inherently biodegradable nature can provide new opportuni-
ties for microbiological attack in metalworking uids. These are the industry’s challenges, ghting
more battles with fewer and more expensive weapons, but they will also serve as the impetus for the
advancement of the industry and evolution of the next generation of microbial control strategies.
REFERENCES
1. White, E.M. The low down on the slime—bio lm control in metalworking uid distribution systems.
Compoundings 54(9): 22–23, 2004.
2. Passman, F.J. Microbial problems in metalworking uids. Trib Lubr Technol 60(4): 24–27, 2004.
3. Buers, K.L., E.L. Prince, C.J. Knowles. The ability of selected bacterial isolates to utilize components
of synthetic metalworking uids as sole sources of carbon and nitrogen for growth. Biotechnol Lett 19:
791–794, 1997.
4. Abanto, M., J. Byers, H. Noble. The effect of tramp oil on biocide performance in standard
metalworking uids. Lubr Eng 50(9): 732–737, 1994.
5. Almen, R.G. et al. Application of high-performance liquid chromatography to the study of the effect of
microorganisms in emulsi able oils. Lubr Eng 38(2): 99–103, 1982.
6. Rossmoore, H.W., L.A. Rossmoore. Effect of microbial growth products on biocide activity in
metalworking uids. Int Biodetn 27: 145–156, 1991.
7. Scott, P.B.J. et al. Expert consensus on MIC—prevention and monitoring, Part 1. Mater Perf 43(3): 2–6,
2004.
8. Rossmoore, H.W., L. Rossmoore, D. Bassett. Life and death of mycobacteria in the metalworking
environment. Lubes’N’Greases 21–27, April 2004.
9. Passman, F.J. Metalworking uid microbes: What we need to know to successfully understand cause-
and-effect relationships. Tribol Trans 51(1): 107–117, 2008.
10. ASTM E2564-07, Standard test method for enumeration of mycobacteria in metalworking uids by
direct microscopic counting (DMC) method, Annual Book of Standards, vol 11.03, Philadelphia, 2007.
11. ASTM E2250-02, Standard method for determination of endotoxin concentration in water miscible
metal working uids, Annual Book of Standards, vol 11.03, Philadelphia, 2002.
12. Rossmoore, H.W., ed. Handbook of Biocide and Preservative Use. Blackie Academic & Professional,
Glasgow, 1995.
13. Ash, M., I. Ash. The Index of Antimicrobials. Gower, Aldershot, U.K., 1996.
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Antimicrobial Additives for Metalworking Lubricants 397
14. Paulus, W., ed. Directory of Microbicides for the Protection of Materials—A Handbook. Springer,
Berlin, 2005.
15. Sondossi, M., H.W. Rossmoore, J.W. Wireman. Observations of resistance and cross-resistance to
formaldehyde and a formaldehyde condensate biocide in Pseudomonas aeruginosa. Int Biodetn 21:
105–106, 1985.
16. Sondossi, M., H.W. Rossmoore, R. Williams. Relative formaldehyde resistance among bacterial
survivors of biocide-treated metalworking uids. Int Biodetn 25: 423–437, 1989.
17. Heenan, D.F. et al. Isothiazolinone microbiocide-mediated steel corrosion and its control in aluminum
hot rolling emulsions. Lubr Eng 47(7): 545–548, 1991.
18. Passman, F.J., J. Summer eld, J. Sweeney. Field evaluation of a newly registered metalworking uid
biocide. Lubr Eng 56(10): 26–32, 2000.
19. Sondossi, M., H.W. Rossmoore, E.S. Lashen. In uence of biocide treatment regimen on resistance
development to methylchloro-/methylisothiazolinone in Pseudomonas aeruginosa. Int Biodeterior
Biodegrad 43: 85–92, 1999.
20. Law, A.B., E.S. Lashen. Microbiocidal ef cacy of a methylchloro-/methylisothiazolinone/copper
preservative in metalworking uids. Lubr Eng 47(1): 25–30, 1991.
21. Sondossi, M. et al. Factors involved in bacterial activities of formaldehyde and formaldehyde condensate/
isothiazolone mixtures. Int Biodetn 32: 243–261, 1993.
22. ASTM E2169-01, Standard practice for selecting antimicrobial pesticides for use in water-miscible
metalworking uids, Annual Book of Standards, vol 11.03, Philadelphia, 2007.
23. ASTM E2275-03E01, Standard practice for evaluating water-miscible metalworking uid bioresistance
and antimicrobial pesticide performance, Annual Book of Standards, vol 11.05, Philadelphia, 2003.
24. Elsmore, R., E.C. Hill. The ecology of pasteurized metalworking uids. Int Biodetn 22: l0l–120, 1986.
25. Rossmoore, L.A. Magnets & magic wands—exploring the myths of metalworking uid microbiology.
Compoundings 55(9): 19–21, 2005.
26. Heinrichs, T.F., H.W. Rossmoore. Effect of heat, chemicals and radiation on cutting uids. Dev Ind
Microbiol 12: 341–345, 1971.
27. Brown, M.R.W., R.M.E. Richards. Effect of ethylenediamine tetraacetate on the resistance of
Pseudomonas aeruginosa to antibacterial agents. Nature 20: 1391–1393, 1965.
28. Auman, U., P. Brutto, A. Eachus. Boost your resistance: metalworking uid life can depend on
alkanolamine choices. Lubes’N’Greases 22–26, June 2000.
29. Aumann, U. Comparative study of the role of alkanolamines with regard to metalworking uid
longevity-laboratory and eld evaluation. Proceedings of 12th International Colloquium Tribology,
10.8: 787–794, Technische Akademie Esslingen, Germany, 2000.
30. Rossmoore, H.W. Biostatic uids, friendly bacteria, and other myths in metalworking microbiology.
Lubr Eng 49(4): 253–260, 1993.
31. Sandin, M., I. Mattsby-Baltzer, L. Edebo. Control of microbial growth in water-based metalworking
uids. Int Biodetn 27: 61–74, 1991.
32. Watanabe, S. et al. Antimicrobial properties of the products from the reaction of various aminoalcohols
and boric anhydride. Mater Chem Phys 19: 191–195, 1988.
33. Byers, J.P., ed. Metalworking Fluids, 2nd ed. CRC Press, New York, 2006.
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399
16
Surfactants in Lubrication
*
Girma Biresaw
CONTENTS
16.1 Introduction .........................................................................................................................399
16.2 Structure of Surfactants .......................................................................................................400
16.3 Solubility of Surfactants ......................................................................................................400
16.4 Organized Surfactant Assemblies .......................................................................................402
16.5 Functions of Surfactants in Lubrication ..............................................................................404
16.6 Solubilization with Surfactants ...........................................................................................405
16.7 Boundary Lubrication Properties of Surfactants ................................................................407
16.7.1 Boundary Friction of Vegetable Oils in Metal/Metal Contact ..............................408
16.7.2 Boundary Friction of Vegetable Oils
in Starch/Metal Contact ......................................................................................... 410
16.7.3 Mechanism of Boundary Lubrication of Vegetable Oils ....................................... 411
16.8 Conclusion ........................................................................................................................... 417
Acknowledgment ........................................................................................................................... 418
References ...................................................................................................................................... 418
16.1 INTRODUCTION
Surfactants are one of the most widely applied materials [1–9]. The application of surfactants
varies from everyday mundane tasks such as cleaning to highly complex processes involving the
formulation of pharmaceuticals, foods, pesticides, lubricants, etc. Although surfactants have been
known and applied for centuries, they continue to be the subject of vigorous research and devel-
opment effort. This is because of the continued development of new and complex applications
(e.g., nanotechnology) that require new and improved surfactants and a deeper understanding of
their performance.
The objective of this chapter is to provide an overview of the applications of surfactants in
lubrication. However, before delving into the lubrication aspects of surfactants, the chapter provides
basic information about surfactants. This basic information is critical to understanding the role of
surfactants in lubrication discussed in the subsequent sections.
The chapter begins with a review of the basic information about the chemical structure of sur-
factants. Although there are thousands of surfactants being used in thousands of applications, they
all have similar basic structures.
Another important property of surfactants is their ability to dissolve in both polar and nonpolar
solvents. A brief description of this phenomenon is provided followed by a discussion of surfactant-
organized assemblies. The ability of surfactants to spontaneously form various types of microstruc-
tures or organized assemblies is responsible for many of their applications [1–9].
* Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the stan-
dard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others
that may also be suitable.
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400 Lubricant Additives: Chemistry and Applications
Surfactants are used in lubrication to provide a number of desirable outcomes (e.g., lower fric-
tion, lower wear, better ltration, lower emission, and better cooling). The list of surfactant functions
in lubrication is numerous, and a brief description of some examples is given. In addition, an in-depth
review of two of the most important functions of surfactants in lubrication is highlighted. These two
functions are solubilization and boundary lubrication.
Solubilization deals with the ability of surfactants to solubilize various polar and nonpolar
ingredients [1–13]. This property of surfactants makes it possible to formulate the various water-
and oil-based lubricants currently in use for various applications.
In boundary lubrication, friction surfaces are pressed together at very high load and rub
against each other at low speeds. Such a process leads to the generation of high friction and
wear. Surfactants adsorb on friction surfaces and prevent direct contact between the friction sur-
faces, thereby lowering both friction and wear. This property of surfactants is illustrated from
a review of an extensive study into the boundary lubrication properties of agriculture-based
surfactants [14–32].
16.2 STRUCTURE OF SURFACTANTS
The term surfactant is used to describe organic molecules that comprise two dissimilar groups in
the same molecule. One group is a hydrocarbon chain of varying structures and is soluble only in
nonpolar solvents. Examples of nonpolar solvents include benzene, toluene, and hexanes. Because
it is soluble only in nonpolar solvents, the group is also referred to as the lipophilic segment or the
lipophilic tail of the surfactant.
The second group in the surfactant is composed of polar groups that readily associate with or
dissolve in polar solvents such as water, glycerol, formamide, and other similar solvents. Because
of its strong af nity for polar solvents, the group is also referred to as the hydrophilic segment or
hydrophilic head of the surfactant.
Figure 16.1 displays schematics of surfactant structure of varying complexity. The simplest
surfactants comprise a linear or branched lipophilic chain directly attached to the hydrophilic com-
ponent (Figures 16.1a and 16.1b). More complex structures include more than one lipophilic chain
attached to one or more hydrophilic component (Figures 16.1c through 16.1e) or polymeric surfac-
tants with various repeating hydrophilic and lipophilic components (Figure 16.1f).
Depending on the nature of the hydrophilic component, surfactants are classi ed into four cat-
egories: anionic, cationic, nonionic, and zwitterionic. Anionic surfactants are negatively charged
and contain one or more positively charged organic or inorganic counterion. Similarly, cationic sur-
factants are positively charged with negatively charged organic or inorganic counterions. Nonionic
surfactants have no charge, and zwitterionic surfactants have both positive and negative charges
with no counterions. Examples of ionic and nonionic surfactants of varying hydrophilic and lipo-
philic structures are given in Table 16.1, along with their common names.
16.3 SOLUBILITY OF SURFACTANTS
Almost all ionic (anionic, cationic, and zwitterionic) surfactants display good solubility in water but
poor solubility in nonpolar solvents. The reverse is true for most nonionic surfactants, except for
ethoxylated surfactants (C
12
EO
6
in Table 16.1). In the latter case, the solubility of the surfactant in
hydrophilic (e.g., water) versus lipophilic (e.g., hexanes) solvents depends on the relative “strength”
of the hydrophilic versus the lipophilic segments of the surfactant. Becher [9] developed the follow-
ing empirical equation to quantify the net strengths, or the hydrophilic–lipophilic balance (HLB) of
such nonionic surfactants.
HLB
MW EO)
MW
⫽
⫻⫺20 (
(16.1)
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Surfactants in Lubrication 401
(L = lipophilic, H = hydrophilic)
H
L
H
1
H
2
L
H
(c)
(a)
(b)
L
1
L
1
L
2
L
1
L
2
L
2
L
1
L
3
L
3
L
4
(d)
H
H
H
2
H
1
(e)
(
f
)
(
(
)
)
FIGURE 16.1 Schematics of surfactant structure of varying complexity. Hydrophilic and lipophilic seg-
ments are indicated by H and L, respectively.
TABLE 16.1
Examples of Surfactants of Varying Hydrophilic and Lipophilic Structures
General Structure: RXY
Type R X Y Name
Anionic C
11
H
23
–
COO
−
K
+
Potassium laurate (soap)
C
12
H
25
–
SO
4
–
Na
+
Sodium dodecyl sulfate
(SDS)
Cationic C
16
H
33
–
(CH
3
)
3
N
+
Br
−
Cetyltrimethyl
ammonium bromide
C
8
H
17
–
(C
8
H
17
)
2
CH
3
N
+
Cl
−
Tri(n-octyl)-methyl
ammonium chloride
Zwitterionic C
12
H
25
CH– (CH
3
)
2
N
+
–CH
2
CO
2
–
Laurylbetaine
Nonionic C
12
H
25
– OH Lauryl alcohol
C
17
H
33
– COOH Oleic acid
C
12
H
25
O– (CH
2
CH
2
O)
6
HC
12
EO
6
, ethoxylated
lauryl alcohol
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402 Lubricant Additives: Chemistry and Applications
where MW – EO is the molecular weight of the ethoxylated segments of the surfactant and MW is
the molecular weight of the surfactant.
To illustrate the application of Equation 16.1, let us examine the HLB of a hypothetical eth-
oxylated nonionic surfactant shown in Figure 16.2. According to Equation 16.1, the maximum
value of HLB is 20, and it is for a nonionic surfactant without a lipophilic segment, that is, x = 0
in Figure 16.2. Using a similar argument, the minimum value of HLB is zero and is for a nonionic
surfactant with no hydrophilic segments, that is, y = 0 in Figure 16.2. The general relationship
between HLB and solubility of ethoxylated nonionic surfactants is as follows [9,10]: nonionic
surfactants with high HLB (>11) are soluble in water and, hence, are good candidates for formu-
lating oil-in-water (o/w) emulsions. On the contrary, nonionic surfactants with low HLB (<9) are
soluble in oil and are good candidates for formulating water-in-oil (w/o) emulsions.
16.4 ORGANIZED SURFACTANT ASSEMBLIES
Surfactants form various types of structures when dissolved in hydrophilic and lipophilic solvents.
The types of structures formed depend on various factors including the nature of the surfactant,
type of solvent, surfactant concentration, temperature, and nature and concentration of cosolutes
(e.g., salts and other surfactants).
At low concentrations, surfactants adsorb on the surface of the solvent, with one segment dis-
solved in the solvent and the other segment in contact with air. Which segment contacts the solvent
depends on whether the solvent is hydrophilic or lipophilic. In hydrophilic solvents, the polar group
is in contact with the solvent, whereas the lipophilic chain is in contact with air. The structure is
reversed in a lipophilic solvent. These structures are depicted in Figures 16.3a and 16.3b.
When the concentration of the surfactant reaches a certain critical value, the surfactant mol-
ecules spontaneously form micelles or aggregate. The concentration at which such microstructures
spontaneously form is called critical micelle concentration (CMC) or critical aggregate concentra-
tion (CAC). In hydrophilic solvents, normal micelles or aggregates are formed. In such systems, the
lipophilic chains of the surfactants form the core of the spherical structures, away from the hydro-
philic solvent, whereas the hydrophilic heads are in contact with the hydrophilic solvent. In lipo-
philic solvents, reverse micelles and aggregates are formed where the hydrophilic heads are buried
in the core and away from the lipophilic solvent, whereas the lipophilic tails are in contact with the
solvent. The structures of normal and reverse micelles are depicted in Figures 16.3c and 16.3d.
Micelles differ from aggregates in that they have a uniform size and a constant aggregation number,
that is, they comprise the same number of surfactant molecules per micelle. Aggregates, on the con-
trary, have no xed size or aggregation number but a range of sizes and aggregation numbers [11].
Incorporation of a cosurfactant (e.g., short-chain alcohol) allows normal micelles and reverse
micelles to solubilize lipophiles (e.g., oils) and hydrophiles (e.g., water) in their cores, respec-
tively. The resulting micelles have bigger diameters and are referred to as swollen micelles. Nor-
mal micelles with large volumes of solubilized oil are called o/w microemulsions, whereas reverse
micelles with large volumes of solubilized water are called w/o microemulsions. The structure of
o/w and w/o microemulsions are illustrated in Figures 16.3e and 16.3f.
The diameters of micelles and microemulsions are smaller than the wavelength of light. As a
result, solutions with micelles and microemulsions are clear and transparent and remain so inde -
nitely since they are thermodynamically stable.
In the absence of cosurfactants, normal micelles dissolve little or no oil. As a result, mixing oil
in micelle solutions will result in the formation of emulsions. Emulsions differ from micelles and o/w
FIGURE 16.2 General structure of a polyethoxylated nonionic surfactant.
H−(C
x
H
2x
O)−(CH
2
CH
2
O)
y
−H
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Surfactants in Lubrication 403
microemulsions in that they comprise oil droplets with diameters larger than the wavelength of light.
As a result, they re ect visible light and have a milky appearance. Emulsions are also thermodynami-
cally unstable; as a result, they separate into their respective oil and water phases over time.
This distinction between micelles, o/w microemulsions, and o/w emulsion is very important
in lubrication. In water-based lubricants, these three uids are referred to by different names by
lubricant suppliers and users [12]. Generally, emulsions are referred to as soluble oils because they
comprise relatively large quantities of solubilized oils. Microemulsions are referred to as semisyn-
thetic lubricants because they comprise relatively smaller quantity of solubilized oils. Micelles are
called synthetic lubricants because they compise no oil, and lubrication is achieved by synthetic
ingredients dissolved in the water phase of the micelle. More detail about water-based lubricants
is given in Section 16.6.
Increasing the concentration of the surfactant after the formation of micelles results in the
formation of lamellar liquid crystals. These are three-dimensional surfactant multilayer structures.
The exact orientation of the surfactants in these structures depend on whether the solvent is polar or
nonpolar, as illustrated in Figures 16.4a and 16.4b.
Further increase in the concentration of surfactant leads to the formation of normal and reverse
hexagonal liquid crystal structures, which are depicted in Figures 16.4c and 16.4d. Other surfactant-
based microstructures include vesicles (Figure 16.4e) and foams (Figure 16.4f). Control of foam is
a major problem in lubrication [12].
FIGURE 16.3 Microstructures of surfactant assemblies in solution: (a,b) monolayers, (c) normal micelles,
(d) reverse micelles, (e) o/w microemulsions, and (f) w/o microemulsions.
Air Air
Lipophilic
solvent
Hydrophilic
solvent
Monolayers
Micelles
(a)
(c)
(b)
(d)
Normal
Normal
o/w microemulsion
Reverse
Reverse
w/o microemulsion
Microemulsion
Oil
H
2
O
Surfactant
Cosurfactant
(
e
)(
f
)
CRC_59645_Ch016.indd 403CRC_59645_Ch016.indd 403 12/6/2008 9:23:00 AM12/6/2008 9:23:00 AM