262
characteristics, partly derived from the specificity of siloxanes, others come from
the combination of those with a variety of hydrophilic molecules. For example, silicone surfactants exhibit both hydrophobic and oleophobic properties, thus considerably reducing the surface tension of water and organic media, which is due to the
high flexibility and low cohesive energy of the siloxane bond (Noll 1968; Voronkov
et al. 1978). The effect of siloxane-based compounds as surface active materials is
comparable to that of polyfluorinated compounds, but the latter are potentially bioaccumulative and toxic. Chen et al. (2018) estimated that more than one billion
pounds of organosilicon surfactants are produced worldwide per year, and those
containing hydrophilic polyalkoxylate (ethoxylate and propoxylate) chain are the
best known. In agriculture, siloxane surfactants are used as adjuvants to improve
herbicide absorption (https://brandt.co/), being the most super-spreading and penetrating adjuvants available.
A wide range of organic moieties, namely nonionic, anionic, cationic and zwitterionic groups, have been linked to polysiloxane backbones, as previously reviewed
(Hill 2002; Racles et al. 2010b). A characteristic of siloxane surfactants is their
lower critical aggregation concentration and the corresponding surface tension
compared to traditional hydrocarbon surfactants. The flexibility of the siloxane
bond favors the chain conformations exposing the methylene groups towards the
interface or surface, thus reducing the surface tension to values close to the pure
PDMS. Another rather general aspect of siloxane surfactants is their propensity for
self-assembly in vesicles (Guoyong et al. 2010; Racles 2010; Racles et al. 2014d;
Zhou and Zhang 2016). The formation of aggregates with less curvature has been
attributed to the bulkiness of the hydrophobic dimethylsiloxane tails, which has an
important impact on the critical packing parameter. Self-assembled spherical vesicles open potential applications as drug delivery systems in microspheres and as
biomembrane models (Guoyong et al. 2010).
The surface properties and aggregation behavior of tetrasiloxane Gemini imidazolium surfactants were studied by Zhao et al. (2016) compared with the corresponding monomer and the hydrocarbon-based Gemini imidazolium surfactant.
These authors observed that the former has the greatest capacity to form micelles,
but less efficiency to reduce surface tension, while the CMC value was about an
order of magnitude less than that of the monomer. On the other hand, the
tetrasiloxane- based surfactants have a greater capacity to reduce the surface tension
than the hydrocarbon-based surfactant.
Carbohydrate-modified siloxane surfactants have been synthesized and their surfactant behavior has been extensively investigated (Wagner et al. 1996a, b; Racles
and Hamaide 2005; Racles et al. 2006b; Berson et al. 2008; Halila et al. 2008;
Guoyong et al. 2010; Racles et al. 2010b; Wang et al. 2011; Zhou and Zhang 2016).
The polysiloxane functionalization with carbohydrates is a challenge, mainly
because of the completely different solubility of the reagents. Several methods have
been reported, which may or may not require protection-deprotection steps of carbohydrates (Fitremann et al. 2008). Selected examples refer to the hydrosilylation
of allylglycosides (Jonas and Stadler 1994; Wagner et al. 1996a; Akimoto et al.
2000; Racles and Hamaide 2005; Racles et al. 2006b; Berson et al. 2008), ring
opening reaction between an amino-functional siloxane and a carbohydrate lactone
C. Racles et al.
characteristics, partly derived from the specificity of siloxanes, others come from
the combination of those with a variety of hydrophilic molecules. For example, silicone surfactants exhibit both hydrophobic and oleophobic properties, thus considerably reducing the surface tension of water and organic media, which is due to the
high flexibility and low cohesive energy of the siloxane bond (Noll 1968; Voronkov
et al. 1978). The effect of siloxane-based compounds as surface active materials is
comparable to that of polyfluorinated compounds, but the latter are potentially bioaccumulative and toxic. Chen et al. (2018) estimated that more than one billion
pounds of organosilicon surfactants are produced worldwide per year, and those
containing hydrophilic polyalkoxylate (ethoxylate and propoxylate) chain are the
best known. In agriculture, siloxane surfactants are used as adjuvants to improve
herbicide absorption (https://brandt.co/), being the most super-spreading and penetrating adjuvants available.
A wide range of organic moieties, namely nonionic, anionic, cationic and zwitterionic groups, have been linked to polysiloxane backbones, as previously reviewed
(Hill 2002; Racles et al. 2010b). A characteristic of siloxane surfactants is their
lower critical aggregation concentration and the corresponding surface tension
compared to traditional hydrocarbon surfactants. The flexibility of the siloxane
bond favors the chain conformations exposing the methylene groups towards the
interface or surface, thus reducing the surface tension to values close to the pure
PDMS. Another rather general aspect of siloxane surfactants is their propensity for
self-assembly in vesicles (Guoyong et al. 2010; Racles 2010; Racles et al. 2014d;
Zhou and Zhang 2016). The formation of aggregates with less curvature has been
attributed to the bulkiness of the hydrophobic dimethylsiloxane tails, which has an
important impact on the critical packing parameter. Self-assembled spherical vesicles open potential applications as drug delivery systems in microspheres and as
biomembrane models (Guoyong et al. 2010).
The surface properties and aggregation behavior of tetrasiloxane Gemini imidazolium surfactants were studied by Zhao et al. (2016) compared with the corresponding monomer and the hydrocarbon-based Gemini imidazolium surfactant.
These authors observed that the former has the greatest capacity to form micelles,
but less efficiency to reduce surface tension, while the CMC value was about an
order of magnitude less than that of the monomer. On the other hand, the
tetrasiloxane- based surfactants have a greater capacity to reduce the surface tension
than the hydrocarbon-based surfactant.
Carbohydrate-modified siloxane surfactants have been synthesized and their surfactant behavior has been extensively investigated (Wagner et al. 1996a, b; Racles
and Hamaide 2005; Racles et al. 2006b; Berson et al. 2008; Halila et al. 2008;
Guoyong et al. 2010; Racles et al. 2010b; Wang et al. 2011; Zhou and Zhang 2016).
The polysiloxane functionalization with carbohydrates is a challenge, mainly
because of the completely different solubility of the reagents. Several methods have
been reported, which may or may not require protection-deprotection steps of carbohydrates (Fitremann et al. 2008). Selected examples refer to the hydrosilylation
of allylglycosides (Jonas and Stadler 1994; Wagner et al. 1996a; Akimoto et al.
2000; Racles and Hamaide 2005; Racles et al. 2006b; Berson et al. 2008), ring
opening reaction between an amino-functional siloxane and a carbohydrate lactone
C. Racles et al.