Surfactants-The Chemical Workhorse: Polymeric surfactants

Excerpt: The repulsive barriers of the polymer chain prevent coalescence and agglomeration as compared to monomeric surfactants both in aqueous and non aqueous suspension

Polymeric surfactants

During the past three decades, safety considerations have been driving the substitution of solvent driven formulations to aqueous systems in all areas of surfactant applications. The process has gained further momentum from the European Union setting limits on to release of volatile compounds in the environment. In areas like cotton cultivation, concentrated emulsions have displaced the emulsifiable concentrates. In the field of emulsion polymers, new colloidal species have been developed. There has been considerable interest in recent years in the field of polymeric surfactants. These polymeric surfactants can covalently bind to the dispersed phase and as such have a distinct advantage over conventional surfactants that are only physically adsorbed and can be displaced from the interface by shear or phase changes resulting in emulsion instability. The binding of polymeric surfactants to the dispersed phase makes these surfactants an integral part of the finished product and prevents the release of surfactants in the effluent, thus reducing the environmental impact of the intermediate product and commercial formulations.

The repulsive barriers of the polymer chain prevent coalescence and agglomeration as compared to monomeric surfactants both in aqueous and non aqueous suspension thus making it an an excellent choice for emulsion polymerization, where surfactants {via micelle formation} control the molecular weight distribution. The word polymeric suggest high molecular weight with a large number of repeating molecular units. Most polymeric surfactants have only a few repeating units. Polymeric surfactants probably offer excellent opportunities in terms of flexibility, diversity and functionality. This new polymer chemistry has enabled synthetic polymer chemists to make new, well-defined amphiphilic block copolymers, many of which exhibit interesting surfactant behavior. This is especially true in the light of recent advances in controlled radical polymerization chemistry One of the salient features of Polymeric surfactants is its ability to impart excellent particle stability. The multiple anchoring points of polymeric surfactants facilitate excellent interaction between the surfactant and the substrate. Combined with steric stabilization, this results in a highly robust and stable finish product formulations. Polymeric surfactants can stabilize very high load suspensions and still maintain the viscosity of the product to acceptable levels. A class of polymeric surfactants which has multiple end applications are Ethylene oxide – Propylene co polymers, acrylic styrene co polymers – methacrylic acid co polymers – Alkyd PEG resin derivatives. Polyacrylic acid of low molecular weight < 15000}, condensates of naphthalene sulphonate with formaldehyde and sulphonated styrene maleic anhydride co polymers are also included in the family of polymeric surfactants. Amongst the natural class, alginates – pectins and protein based products too figure in the list of polymeric surfactants. Polymeric surfactants exhibit poor surfactant properties with respect to surface tension reduction in aqueous solution and the formation of micelles. Polymeric surfactants exhibit super-low critical micelle concentration {CMC}. The right combination of polymer and ordinary surfactant can lead to a remarkable reduction in CMC. The right polymer blend can attract the surfactant to itself, leading to an early "virtual micellation" and, therefore, a low critical micel concentration {CMC}. In film forming polymers the water resistance of polymeric surfactant is far superior compared to conventional surfactants.

The majority of developments on polymeric surfactants are the area of hydrophilichydrophobic diblock copolymers, where the hydrophobic block is permanently hydrophobic. Recently some attention has been given to hydrophilic hydrophilic diblocks, in which the less hydrophilic block can be tuned to become hydrophobic by changing the external solution conditions, e.g. the solution pH, temperature or electrolyte concentration. For example, to polymerize, vinyl monomers such as styrene or methacrylate, where an alkyl halide is based initiator in combination with a transition metal catalyst, usually, Cu has been used. The propagating intermediate is believed to be radical-based, thus this chemistry is particularly tolerant of functional monomers and can be performed in protic solvents, including water.

Polymeric surfactants can be tailor-made to suit many diverse areas, namely preparation of new colloidal emulsions, novel latex stabilizers and emulsifiers.

Author Details

C.N. Sivaramakrishnan