Effect of Hydrophilic- and Hydrophobic-Chain Lengths on the Phase Behavior of A−B-type Silicone Surfactants in Water

Abstract

The phase behavior of a long hydrophobic chain A-B-type silicone surfactant, Me 3 SiO-(Me 2 SiO) m-2 -Me 2 -SiCH 2 CH 2 CH 2 -O-(CH 2 CH 2 O) n H (Si m C 3 EO n ), in water was investigated by phase study and small-angle X-ray scattering (SAXS). The types of liquid crystals or self-organized structures are highly dependent on both EO-chain (n) and poly(dimethylsiloxane)-chain (m) lengths or the volume ratio of the EO chain to the total surfactant, nV EO /V S , which is related to the classical Griffin's HLB value. Reverse discontinuous cubic phase (I 2 ) for Si 14 C 3 EO 7.8 and Si 25 C 3 EO 7.8,12.2,15.8 , reverse hexagonal phase (H 2 ) for Si 14 C 3 EO 12 , lamellar (L R ) phase for Si 14 C 3 EO 15.8 and Si 25 C 3 EO 51.6 , and hexagonal (H 1 ) and discontinuous cubic (I 1 ) phases for Si 5.8 C 3 -EO 36.6,51.6 are formed. Hence, both hydrophobic and hydrophilic chains affect the surfactant layer curvature, but in an opposite way. On the other hand, the effective cross-sectional area per surfactant at the hydrophobic surface of self-organized structures, a S , increases with increasing m (or n) at constant n (or m). a S is related to the amphiphilicity of surfactant (surfactant size). Since the surfactant layer curvature changes from positive to negative with increasing m at constant n, the l eff /l max decreases with m, where l eff is the effective hydrophobicchain length and l max is the length of the chain in its fully extended form. Namely, the entropy loss of a long hydrophobic chain would be largely increased when it is stretched, and thus, long hydrophobic chain tends to be in a shrunk-bulky state. This causes the expansion of a S and the change in the surfactant layer curvature from positive to negative. In a similar mechanism, a S increases with increasing the EO-chain length, n, but the surfactant layer curvature changes from negative to positive.

Structure of Diglycerol Polyisostearate Nonionic Surfactant Micelles in Nonpolar Oil Hexadecane: A SAXS Study

“…Reverse micelles are mostly observed in the ternary mixtures of surfactant/water/oil, mostly in oil rich regions [11][12][13] . Furthermore, they have also been observed in aqueous systems of lipophilic surfactant in surfactant rich regions 14,15 . Only few reports exist that describes the formation of reverse micelles in organic solvents without water addition [16][17][18][19][20] .…”

Phase Behavior and Rheological Analysis of Reverse Liquid Crystals and W/I2 and W/H2 Gel Emulsions Using an Amphiphilic Block Copolymer

“…At lower temperatures, the viscosity is still high and the mixing would require a higher energy input in order to mix the excess water with the liquid crystal structure. According to Kunieda et al, 29 the melting temperature of the one-phase regions increases as water is solubilized in the reverse micelles until phase separation occurs, and it is maximum in the two-phase region, so higher temperatures are needed to form the gel emulsions. wt% water, and then increases at values higher than 0.9 and remains nearly constant all along the path.…”

Aqueous Phase Behavior of Ultra-Long Polyoxyethylene Surfactants

“…Hence, the longer the chain, the more bulky it is. Namely, even if the HLB value is the same, the surfactant layer curvature becomes less posi- tive due to the bulky hydrocarbon chain (14). This effect is not remarkable for C 8 -C 20 conventional surfactants, but the present C 30 surfactant appears to be more lipophilic than expected from its HLB value.…”