Page 202 - Fundamentals of Enhanced Oil and Gas Recovery

190 Mohammad Ali Ahmadi

6.2.1.1.1 Nonionic Surfactant
In the case of nonionic surfactants, the hydrophile has no charge but it is water soluble
due to the presence of highly polar groups, such as polyoxyethylene
( CH 2 CH 2 O ) n H, where n is the number of ethylene oxide units, or polyols.
Nonionic surfactants include alcohol ethoxylates, alkylphenol ethoxylates, and
polysorbates [22 24]. Many of these surfactants are based on polyoxyethylene
and a typical example is, dodecyl hexaoxyethylene glycol monoether
[C 12 H 25 (OCH 2 CH 2 ) 6 OH], often abbreviated to C 12 E 6 to denote hydrocarbon chain
length and ethylene oxide chain length. It should also be noted that with this type of
compound the head-group is larger than the hydrocarbon chain [24]. Nonionic sur-
face active agents with small head-groups also exist, some examples being, dodecyl
sulphinyl ethanol (C 12 H 25 SOCH 2 CH 2 OH) and decyl dimethyl amine oxide [22 24].

6.2.1.1.2 Ionic Surfactant
The hydrophilic group acquires a negative charge (anion) when it is dissolved in
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water. Anionic hydrophilic groups include sulfates (ROSO ), sulfonates (RSO ),
2
2
carboxylates (RCOO ), and phosphates (RPO ). Soaps are sodium or potassium
4
fatty acid carboxylates, which have been implemented by man for more than 2000
years [25]. One of the common anionic surfactants is sodium dodecyl sulfate
2
1
(C 12 H 25 SO Na ), a synthetic material. The name is often abbreviated to sodium
4
1
2
dodecyl sulfate (SDS) [24]. Sodium dodecanoate (C 11 H 23 COO Na ), a material that
can be synthesized is another one. There is also a large group of materials of this type
formed from natural sources, e.g., the hydrolysis of triglycerides [22 24].
6.2.1.1.3 Cationic Surfactant
In this category, the charge of the hydrophilic group is positive (cation). Although cat-
ionic surfactants are more expensive than anionic ones, their economic importance
has increased greatly in recent years. Typical cationic surfactants studied in petroleum
industry are quaternary ammonium salts with one long alkyl chain (carbon number
from 8 to 22) and methyl or hydroxyethyl groups in the head group positions
1
[25]. Some usual examples of these quaternary ammonium salts (R 4 N )are dodecyl
1
2
trimethyl ammonium bromide (C 12 H 25 N Me 3 Br ), the name is often abbreviated to
2
1
C 12 TAB or DTAB and hexadecyl trimethyl ammonium bromide (C 16 H 25 N Me 3 Br )
abbreviated to C 16 TAB or HTAB [22 24].
6.2.1.1.4 Zwitterionic Surfactant
The hydrophile’s charge can be either positive or negative depending on the pH of
the solution, or it can have both charges simultaneously (these are also known as
amphoteric surfactants). Compounds of this type include imidazoline derivatives,
amino acid derivatives, and lecithins. Zwitterionic surfactants represent 1% or less of