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Silicones as Surface-Active Agents

Feb 12, 2014

Silicones are highly surface active due to their low surface tension caused by the
large number of methyl groups and due to the small intermolecular attractions
between the siloxane hydrophobes.  The siloxane backbone of the molecule is highly flexible, which allows for maximum orientation of the attached groups at interfaces.

Silicone polyethers are non-ionic and have both a hydrophilic part (low-molecular-weight polymer of ethylene oxide or propylene oxide or both) and a hydrophobic part (the methylated siloxane moiety). The new carbinol-functional materials also incorporate a hydrophilic species on them and
function under the same principles in which the traditional silicone polyethers
do. The polyether groups are either ethylene oxide or propylene oxide, and are attached to a side chain of the siloxane backbone through a hydro-silylation or condensation process. They can form a rake-like, comb structure, or linear structure. Silicone polyethers are stable up to 320-256°F (160-180°C).

There is a great degree of flexibility in designing these types of polymers. A very wide variety of co-polymers is possible when the two chemistries are combined.  
They can be varied by molecular weight, molecular structure (pendant/linear) and the composition of the polyether chain (EO/PO), and the ratio of siloxane to
polyether. The molecular weight influences the rate of migration to the interface. The increased molecular weight of a polymer typically leads to an increased viscosity, but may also give greater substantivity to surfaces and improved shine level. Other variables include absence or presence of functionality or end groups on the polyether fragments.

Depending on the ratio of ethylene oxide to propylene oxide, these molecules can be water soluble, dispersible or insoluble; obviously, for efficient wetting, these surfactants need to have good solubility in solution. As surfactants, they have the ability to produce and stabilize foam depending on their structure. Conversely, if their solubility parameters are low, they behave as anti-foaming agents.

The different structures affect how the molecules can pack at an interface. With
pendant types in aqueous media, the silicone backbone aligns itself with the interface, leaving the polyalkylene oxide groups projecting into the water. Linear
types form a very flattened “W” alignment where the central silicone portion of the molecule aligns with the interface and the terminal groups are in the aqueous phase. The amounts to be added vary between 0.01% and 0.5% of the total formulation.

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