Hybrid surfactants

Mixtures of fluorocarbon and hydrocarbon surfactants have unusual interfacial properties. The fluorocarbon surfactant reduces the surface tension very effectively, whereas the hydrocarbon surfactant lowers interfacial tension. However, the use of hydrocarbon-fluorocarbon surfactant mixtures is complicated by the demixing of the micelles formed in solutions of the mixture. To avoid this problem, surfactants have been synthesized which contain both fluorocarbon and hydrocarbon chains in the same molecule [204-207] (see Chapter 2). 

Guo et al. [206,208] synthesized hybrid surfactants with a fluorocarbon group (m = 6-8) and a hydrocarbon group (n = 1-9) 

In aqueous solutions, the surface tensions of these hybrid surfactants are about equal or slightly lower than that of the corresponding surfactant with a single hydrophobe. The cmc values are relatively low and are governed by the Kleven equation. The increase of the hydrocarbon-chain-length by a CH2 group decreases the cmc by about 35%. The increase of the fluorocarbon chain by each -CF2- group decreases the cmc by about 75%. Quantitative analysis of the F-NMR spectra has revealed that the residence time of the fluorocarbon chain in 

The hybrid surfactants synthesized by Guo et al. [206] hydrolyze in moist air and have to be stored in a desiccator. Yoshino et al. [207] synthesized hybrid surfactants which contain an aromatic ring C F2 +iC6H4C0CH(S03Na)-C, H2w+i, where n = 4 and 6, m = 2. 4, and 6, and C6H4 = p-phenylene. These surfactants are stable in the presence of moisture and can emulsify a ternary system consisting of mutually immiscible components hydrocarbon, water, and per-fluoroether oil. 

The Krafft point, the area occupied by a surfactant molecule at the air-water and octane-water interfaces, and the aggregation number of micelles increase with an increase in fluorocarbon and/or hydrocarbon chain length of these hybrid surfactants [209]. The cmc, surface tension, and octane-water interfacial tension at the cmc decrease with an increase in fluorocarbon and/or hydrocarbon chain length. The surfactants lower the surface tension, as well as the hydrocarbon oil-water and the water-fluorocarbon oil interfacial tension. Their solutions in water can float on hydrocarbon liquids, such as benzene, cyclohexane, and decane. The 10% solution of the surfactant with m = 6 and n = 4 chains exhibit rubberlike viscoelasticity [210]. 

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This strategy has resulted in entirely new formulations with remarkable benefits. Sustainable Earth (SE) cleaning products combine reagents determined to be safer for human and environmental health with a positively characterized hybrid surfactant system containing a stabilized oxidizing compound. This system eliminates conventional ingredients such as alkyl glycol ethers, alkali builders, alkylphenol ethoxylates, EDTA and ethanolamine. 

Flarrison K, Goveas J, Johnston KP, O Rear EA. Water-in-carbon dioxide microemulsions with a fluorocarbon-hydrocarbon hybrid surfactant. Langmuir... 

The size of w/c microemulsion droplets has been measured by neutron scattering for a di-chain hybrid surfactant (C7Hi5)-(C7Fi5)CHS04 Na [32], 667 g/mol PFPE-C00"NH4 [33], and for a partially fluorinated di-chain sodium sulfo-succinate surfactant [34]. For the PFPE-COO NH4 surfactant, the droplet radius increases from 20 A to 36 A for W o values of 14 and 35, respectively. For the di-chain sodium sulfosuccinate surfactant, droplet radius varied linearly from 12 to 36 A as Wo increased from 5 to 30. This linear relationship has also been shown for AOT reverse micelles in organic solvents [7]. In each of these studies for a one-phase microemulsion, droplet size and Wq were found to be only a weak function of pressure, unless the pressure is reduced to the phase boundary where droplets aggregate. This trend was calculated theoretically [6,23] and has been measured in AOT w/o microemulsions in supercritical propane [35,36]. 

Harrison et al. reported the first w/c microemulsion in 1994 (20). A hybrid surfactant, namely F7H7, made of respectively one hydrocarbon and one fluorocarbon chain attached onto the same sulfate head group, was able to stabilize a w/c microemulsion at 35 and 262 bar. For a surfactant concentration of 1.9 wt %, water up to a w = 32 value ([water]/surfactant]) could be dispersed. A spherical micellar structure was confirmed by small-angle neutron scattering (SANS) experiments (21). This surfactant was later the subject of dynamic molecular simulations (22, 23). The calculations were consistent with the SANS data and high diffusivity was predicted, highlighting this important feature of low-density and low-viscosity supercritical fluids (SCF). 

A fVo of 32 was attained using a two-tailed hydrocarbon-fluorocarbon hybrid surfactant C7Fi5CH(OS03 Na )C7Hi5 (designated F7H7) [53]. This high value of Wo was for a one-phase system at 25°C and 231 bar, far in excess of any other surfactant... 

Hybrid anionic surfactants contain a fluorocarbon chain, a hydrocarbon chain, and a hydrophilic head group in the same molecule. Guo et al, [149] synthesized hybrid surfactants by the reaction scheme shown in Fig. 2.8. The hydrophilic group... 

Fig. 2.8 Synthesis of hybrid surfactants. (Reproduced with permission from Ref. 149. Copyright 1992 by the American Chemical Society.)...

The hybrid surfactants synthesized by Yoshino et al. [150] contain an aromatic ring. The synthesis involves the following sequence of reactions shown in Fig. 2.9. The hydrophile is a sulfonate attached to the molecule through a —CSOsNa bond, which is stable to hydrolysis. 

Ito et al. [211] investigated the micellar aggregation of the hybrid surfactants (m = 4, 6 n = 2,4,6) by the pyrene fluorescence method and Raman spectroscopy. The results suggested that the surfactants FC6-HO/ and FC4-HC6 form a loosely packed hydrated micelle first, but a dehydrated micelle coexists above the second cmc. 

Ito et al. [73] examined the micellization of fluorocarbon-hydrocarbon hybrid surfactants by Raman spectroscopy. The spectrometer was equipped with a multireflection cell and an optical-fiber light-collecting device [74]. The Raman spectra were obtained at high surfactant concentrations above the cmc, because the Raman intensity below cmc was too weak to be measured. 

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