Trisiloxane and hydrocarbon surfactants

FTIR ATR Study of Adsorption of Trisiloxanes and Hydrocarbon Surfactants at Hydrophobic Solids from Aqueous Solutions... 

FTIR-ATR spectroscopy was used to study the absorbance kinetics of trisiloxane and hydrocarbon surfactants on different hydrophobic solids from aqueous solutions. The surface of AMTIR (the internal reflection element) was used as low hydrophobic surface, and Teflon AF coated AMTIR surface was chosen as a highly hydrophobic surface. It has been shown that the absorbance of all surfactants studied on Teflon AF is considerably higher than that on AMTIR surface. Irrespectively of the surface energy of solid substrates the absorbance of trisiloxanes increased over 10 minutes of the experiment, while the absorbance of hydrocarbon surfactants reached equilibrium in seconds on AMTIR and in a few minutes on Teflon AF surfaces. The continuous growth of absorbance with increasing bulk concentration of surfactants has been detected in case of trisiloxanes on both substrates even at C > CWC while hydrocarbon surfactants... 

Figure 1 The typical spectra in (a) high wavenumbers and (b) low wavenumbCTs areas of trisiloxane solutions at ICAC (L-77, ciuves 1,2) and hydrocarbon surfactant solutions at CAC (C12EO5, curves 3,4) on AMTIR surface (2, 3) and Teflon AF surface (1,4). Spectra 3 and 4 are shifted relative to the base line for better discerning peaks...

It is a common misunderstanding that silicones and silicone surfactants are incompatible with hydrocarbon oils this is only partly correct. Small silicone surfactants, such as the trisiloxanes, are very compatible with organic oils. For example, aqueous solutions of the trisiloxane surfactants give very low interfacial tension against alkane oils. The incompatibility between polymeric silicones and some hydrocarbon oils is due more to the polymeric nature of the silicone block rather than to strong phobicity such as that between fluorocarbon and hydrocarbon groups. The compatibility between two species, such as a polymer and a... 

Silicone surfactants in aqueous solutions show the same general behavior as conventional hydrocarbon surfactants - the surface tension decreases with increasing concentration until a densely packed film is formed at the surface. Above this concentration, the surface tension becomes constant. The concentration at the transition is called the critical micelle concentration (CMC) or critical aggregation concentration (CAC). The surface and interfacial activity of silicone surfactants was reviewed by Hoffmann and Ulbricht [27]. Useful discussions of the dependence of the surface activity of polymeric silicone surfactants on molecular weight and structure are given by Vick [28] and for the trisiloxane surfactants by Gentle and Snow [29]. 

The efficiency of the nonionic trisiloxane surfactants is comparable to nonionic hydrocarbon surfactants with a linear dodecyl hydrophobe. The surface properties of a homologous series of trisiloxane surfactants M(DE OH)M with n = 4—20 show that the CAC, the surface tension at the CAC and the area per molecule each vary with molecular structure in a way that is consistent with an umbrella model for the shape of the trisiloxane hydrophobe at the air/water interface [29]. The log(CAC) and the surface tension at the CAC both increased linearly with EO chain length. 

Trisiloxane surfactants containing polyethyleneoxide chains of different lengths, known as superwetters, have been studied by soft-contact AFM imaging and direct force measurements at the solid-liquid interface, using different substrates [50]. The surface aggregate structures for these siloxane surfactants correlate with those of their hydrocarbon-based equivalents and resemble bulk structures. 

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