Since the advent of surfactants in the twentieth century, the use of surfactants has matured and there are now thousands of different kinds of surfactant products on the market for use in industry. The quality of our lives is closely related to the safe use of surfactants. Nowadays, surfactants play an important role in almost every chemical industry, including detergents, emulsions, paints, foaming agents, paper products, cosmetics, pharmaceuticals and insecticides.
Rosin is an important natural resource, whose main components are resin acids, and they have attracted great interest for use in surfactant synthesis and applications because of their special chemical structures and wide range of applications. There are three kinds of rosin: gum rosin, wood rosin and tall oil rosin. Gum rosin occupies about 60% of the industrial market, wood rosin about 5% and tall oil rosin about 35%. The total world annual production volume of rosin has remained at 1.1-1.2 million tons since the 1990s. Most pine resin acids belong to three basic skeletal classes: abietane, pimarane and isopimarane, and labdane. Rosin or modified rosin are widely used as sizes, adhesives, printing inks, emulsifiers, and these applications account for most of the rosin used in industry. Pine resin acids have been widely investigated, but the industrial use of them is low because of their high cost. With the development of science and technology, pure pine resin acids and their derivatives can be easily separated from commercial products on a large scale. For example, dehydroabietic acid (DAA) can be isolated by crystallisation of the 2-aminoethanol salt from disproportionate rosin, and dehydroabietylamine can be isolated by crystallisation of the acetic acid salt from commercial disproportionate rosin amine.
Surfactants are amphipathic molecules with both hydrophilic and hydrophobicmoieties. The amphipathic structure makes them capable of reducing surface andinterfacial tension, forming microemulsion and exhibiting some superficial orinterfacial activity in solvents.
Rosin and its derivatives are useful building blocks for the hydrophobic moiety of surfactants since they contain the tricyclic hydrophenanthrene structure, and hydrophilic groups can be introduced through reactions of carboxyl groups. The hydrophenanthrene can be obtained in enantiomerically pure form. Chiral surfactants from rosin can be used as chiral phase transfer catalysts and chiral separation agents. Surfactants with structures similar to derivatives of fatty acids, amines and alcohols, can be synthesised from rosin.
Classification of surfactants based on the charge characteristics of their polar (hydrophilic) head groups is commonly used. Like other surfactants, rosin-based surfactants can be classified into four groups: cationic, anionic, zwitterionic and nonionic.
The majority of rosin-based cationic surfactants are quaternary ammonium compounds, in which the nitrogen atom carries a positive charge. There are two kinds of rosin-based cationic surfactants. One is an ester quaternary ammonium surfactant and the other, a dehydroabietylamine-derived quaternary ammonium surfactant.
The most widely used starting materials for the synthesis of rosin-based quaternary ammonium compounds are rosin amine or dehydroabietylamine. Rosin amine or dehydroabietylamine can be used as starting materials to prepare tertiary amine in the presence of formaldehyde and formic acid, and then the rosin based quaternary ammonium salts can be prepared in a standard procedure called quaternisation. N,N-Dimethyldehydroabietylamine (DMDHA) is a very important intermediate for the synthesis of rosin-based cationic surfactants.
Rosin-based cationic surfactants can also be modified by incorporating polyethylene oxide chains. Dehydroabietylamine reacted with epoxy under pressure with a catalyst to form a tertiary amine, then the tertiary amine was quaternised with halide to form ethylene oxide quaternary ammonium salts.
Rosin, rosin amine or dehydroabietylamine, rosin hydroxyl ethyl amide and acrylic rosin can be ethoxylated by epoxy and the terminal hydroxyl group can be esterified by MA, after addition of sulfate to the double bond to form corresponding sulfonate anionic surfactants.
Rosin-based amino acids are widely investigated zwitterionic surfactants. Rosin acid and dehydroabietylamine can be used as raw materials for the synthesis of this kind of surfactant. Rosin acid chloride can react with an amino acid to form zwitterionic surfactants. This reaction can take place with different kind of amino acids to form different kinds of zwitterionic surfactants.
Rosin-based nonionic surfactants also exhibited anticorrosion activity. A series of N,N-polyoxyethylene dehydroabietylamines with different numbers of oxyethylene unitswere used as corrosion inhibitors for metals.
With the increasing amount of pine forestry that can be tapped, the output of rosin will increase more and more. As natural-based fine chemicals, rosin-based surfactants will attract more and more attention, not only because of the large quantity of product of raw material which is available, but also because of the special stereostructures which they contain.