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

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 α ) 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 hydrophobic-chain 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.