Detergents 125
4.2 DETERGENT TYPES
Detergents are the metal salts of organic acids. The acids normally used to synthesize these com-
pounds include arylsulfonic acids such as alkylbenzenesulfonic acids and alkylnaphthalenesulfonic
acids [8–11]; alkylphenols [12–16]; carboxylic acids such as fatty carboxylic acids, naphthenic acids,
and petroleum oxidates [17–20]; and alkenylphosphonic and alkenylthiophosphonic acids [21–23].
Sometimes, a mixture of different types of acids is also employed [24]. The reaction of these acids
with inorganic bases, such as metal oxides, metal hydroxides, and metal carbonates, results in the
formation of salts [7]. The quantity of the metal used may be equal to (stoichiometric amount) or
in excess of the exact amount necessary to completely neutralize the acid functionality. The pres-
ence of metal in stoichiometric amount results in the formation of the neutral salt, often referred
to as a neutral detergent or soap. If the metal is present in excess, the detergents are called basic,
overbased, or superbased [7,25]. It is important to note that basic detergents appear as clear homo-
geneous fl uids, the same as neutral detergents, because the excess metal is present in a colloidal
form [26]. The general formulas for metal sulfonates, metal phenates, and metal carboxylates are
presented in Figure 4.2.
The excess base in basic detergents may be present as metal hydroxide, metal carbonate, or
both. For neutral detergents, x and y in the formulas in Figure 4.2 are zero. For low overbased
detergents, such as those with a base number of about 50 or less, x may be zero and y may be a low
number, or both x and y may be low numbers. This implies that slightly overbased detergents are
either carbonate-free or contain a mixture of both the hydroxide and the carbonate. Highly over-
based detergents invariably have a large amount of carbonate as the reserve base. That is, in their
case, y is low and x is very high. In some cases, x can be as high as 20, or more. In summary, the
excess base per equivalent of acid in metal hydroxide–containing detergents is generally lower than
that in metal carbonate–containing detergents.
4.3 DETERGENT PARAMETERS
Detergents are described chemically in terms of their metal ratio, percent sulfated ash, degree of
overbasing or conversion, soap content, and total base number (TBN) [7].
The metal ratio is defi ned as the total equivalents of metal per equivalent of acid. The percent
sulfated ash is the ash produced when the detergent is treated with sulfuric acid and burned. All
organic material in the detergent burns, leaving behind the metal sulfate ash. Sulfate ash results
from the reaction of metal compound with sulfuric acid either directly, as with metal hydroxide and
metal carbonate, or through the oxidative degradation of the metal sulfonate. Detergents are not the
only additives that result in sulfate ash. Other metal-containing additives in the lubricant also con-
tribute toward it. Such additives include metal carboxylates and metal dialkyldithiophosphates such
as zinc dialkyldithiophosphate. The former compounds are sometimes used as friction modifi ers
and corrosion inhibitors, and the latter compounds are commonly used as oxidation inhibitors and
antiwear agents. Because the metal compounds can lead to the formation of the inorganic material
FIGURE 4.2 General formulas for detergents. (Adapted from Rizvi, S.Q.A., Additives and additive chemistry.,
ASTM Manual on Fuels and Lubricants.)
(RSO 3 )a M·XMb CO 3 ·yM(OH)c(RCOO)a M·XMb CO 3 ·yM(OH)c(RPhO)a M·XMb CO 3 ·yM(OH)cBasic sulfonatea and c = 1 and b = 2, if the metal M is monovalent; a and c = 2 and b =1, if the metal M is divalentBasic carboxylateBasic phenate