The phase behavior of long hydrophobic A−B type silicone surfactants, Me3SiO−(Me2SiO)m-2−Me2SiCH2CH2CH2−O−(CH2CH2O)nH (SimC3EOn), in water and water + octamethylcyclotetrasiloxane (D4) was investigated by studying phase behavior and small-angle X-ray scattering. Si25C3EO15.8 forms a reverse micellar cubic phase (I2) in water and water + D4 systems. This cubic phase is highly thermally stable in a surfactant−water binary system. The thermal stability decreases monotonically with addition of silicone oil. Although the solubilization of water in the reverse cubic phase is low, a very large amount of excess water can be incorporated in a so-called reverse cubic phase based concentrated emulsion. The emulsion stability is enhanced upon addition of silicone oil. D4 molecules penetrate into the surfactant palisade layer in the reverse micelles forming the I2 phase and expand the effective cross-sectional area per surfactant, aS (penetration). The continuous penetration of oil destabilizes the I2 phase structure, and therefore the melting temperature of the phase decreases. The incorporation of D4 into the I2 phase in the aqueous mixtures of Si14C3EO7.8, Si25C3EO7.8, Si25C3EO12.2, and Si25C3EO15.8 varies with both the hydrophilic and lipophilic chain lengths of silicone surfactants.
A-B-type silicone copolymer (or surfactant), Me3SiO(Me2SiO)12-Me2SiCH2CH2CH2-O-(CH2CH2O)33.1H(Si14C3EO33.1), forms lamellar liquid crystal (Lα) in a pure state. In a binary water-Si14C3EO33.1 system, the Lα phase coexists with excess water in a dilute region, whereas the Lα phase changes to an isotropic solution (Wm) by replacing water with ethylene glycol. Although water enhances the segregation between hydrophilic and lipophilic chains of copolymer, it does not change the layer curvature of long lipophilic-chain copolymer to be positive. Since ethylene glycol is more soluble in the hydrophilic chain than water, it changes the curvature from zero to positive, where positive curvature means the copolymer-layer curvature is convex toward water or polar solvent. When ethylene glycol is replaced with PEG 300 (polyethylene glycol, Mn ca=300), the Lα-H1(normal hexagonal phase)-Wm transition takes place. With the further increase in the molecular weight of PEG, the Lα phase coexists with an excess PEG as well as the water-copolymer system and the solubilization of PEG in the Lα phase decreases. This change in the phase behavior may be attributed to the solubilization part of PEG in the hydrophilic chain of the silicone copolymer. On the other hand, non-polar solvent, octamethylcyclotetrasiloxane, D4, is soluble in the lipophilic chain, and the Lα phase changes to a reverse micellar solution (Om) via reverse hexagonal (H2) phase. With the increase in the molecular weight of oil, the Lα phase retreats to the concentrated region and the solubilization of oil (poly(dimethylsiloxane)) in the Lα phase becomes very low. As a result, PEG and silicone oil change the surfactant (or copolymer) layer curvature in the opposite way, but, the swelling of high-molecular-weight solvents or homopolymers in the Lα phase is very restricted. In ternary water/ Si14C3EO33.1/D4, ethylene glycol/ Si14C3EO33.1/D4, and PEG/Si14C3EO33.1/poly (dimethylsiloxane) systems, isotropic microemulsions are formed at an equal polar/non-polar solvent ratio. The relationship between microemulsions with water and oil or with hydrophilic and lipophilic homopolymers is also discussed.
We report a solubilization enhancing effect of A-B-type silicone surfactants in microemulsions. The effect of added long-chain silicone surfactants, Si25C3EO51.6 (extended length≈21.8 nm) and Si14C3EO15.8 (extended length≈8.5 nm) on the solubilization capacity of C12EO5 (extended length≈3 nm)/water/dodecane microemulsion was investigated at the hydrophile-lipophile balance temperature at which a microemulsion (surfactant) phase containing equal weights of oil and water touches the three-phase body. The addition of silicone surfactants exhibits an enormous increase of the swelling of the middle phase primarily with an associated increase in the structural length scale of the microemulsion. The solubilization power increases with increasing x2 (mole fraction of silicone surfactants to the total surfactant) and going through a maximum it decreases, since a lamellar liquid crystal introduces in the multiphase region at low surfactant concentrations. The solubilization capacity reaches at the maximum to an almost equal level for different x2 values, 0.02 for Si25C3EO51.6 and 0.09 for Si14C3EO15.8. The solubilization power of the lamellar phase shows a similar trend with lower magnitude.
Emulsifing ability of Polyoxyethylenemodified-polydimethylsiloxane (POES) in Silicone-Water system was investigated.9 kinds of POES were synthesized by addition reaction of SiH and CH2=CH-in the presence of Platinum catalyst. These were devided into 3 type by molecule structure, Polyoxyethylene (A)-Polydimethylsiloxane (B) linear block copolymer, A-B-A linear block copolymer and branched copolymer with side chain of A. Emulsifing ability of these POES was evaluated by observing the physical appearance of the mixture of each Silicone and Water with 4% of POES.Some of A-B and A-B-A linear copolymer showed higher emulsifing ability than branched copolymer. These copolymers are considered as promising emulsifier for silicone.
The phase behavior of a long hydrophobic chain A−B-type silicone surfactant, Me3SiO−(Me2SiO)m-2−Me2SiCH2CH2CH2−O−(CH2CH2O)nH (Sim C3EOn), 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, nVEO/VS, which is related to the classical Griffin's HLB value. Reverse discontinuous cubic phase (I2) for Si14C3EO7.8 and Si25C3EO7.8,12.2,15.8, reverse hexagonal phase (H2) for Si14C3EO12, lamellar (Lα) phase for Si14C3EO15.8 and Si25C3EO51.6, and hexagonal (H1) and discontinuous cubic (I1) phases for Si5.8C3EO36.6,51.6 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, aS, increases with increasing m (or n) at constant n (or m). aS 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 leff/lmax decreases with m, where leff is the effective hydrophobic-chain length and lmax 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 aS and the change in the surfactant layer curvature from positive to negative. In a similar mechanism, aS increases with increasing the EO-chain length, n, but the surfactant layer curvature changes from negative to positive.
Linear, long poly(oxyethylene) poly(dimethylsiloxane) surfactants, formula Me3SiO−(Me2SiO)m-2−Me2SiCH2CH2CH2−O−(CH2CH2O)nH (SimC3EOn), form reverse micelles in oil such as poly(dimethylsiloxane) and hydrocarbons. The critical micellar concentration (CMC) decreases dramatically with increasing the hydrophilic-chain length of the surfactant, whereas the difference in hydrophobic chain length has less influence on the CMC. Hence, the segregation of the poly(oxyethylene) (EO) chain from nonpolar medium is a main factor to form aggregates in oil. Since the lipophilic surfactants used in this study have very long hydrophilic and hydrophobic chains compared to conventional nonionic surfactants, they also form liquid crystals in nonpolar medium such as discontinuous reverse micellar cubic and reverse hexagonal phases at a high surfactant concentration and even in the absence of solvent. Judging from SAXS data, oil penetrates in the palisade layer of surfactant, increasing the preferred negative curvature and relaxing the packing restriction of the hydrophobic chain. Although a normal micellar cubic phase is always changed to a micellar solution upon dilution with water, the present reverse micellar phase coexists with oil in a wide range of composition in the squalane system.
A phase diagram of the polyoxyethylene trisiloxane surfactant−water system was constructed as a function of polyoxyethylene (EO) chain length at 25 °C. The HLB (hydrophile−lipophile balance) of surfactant corresponds to the volume ratio of the EO chain to that of surfactant molecule (φEO/φS), where φS and φEO indicate the volume fractions of surfactant and hydrophilic moiety in the system, respectively. Aqueous micellar (Wm), hexagonal liquid crystalline (H1), lamellar liquid crystalline (Lα), and reverse micellar (Om) phases are formed with decreasing φEO/φS. A sponge phase (D2) is also formed near the Lα phase region. The effective cross sectional area per one surfactant molecule, aS, in liquid crystals in the present systems depends only on their EO chain lengths and are the same as that in ordinary linear hydrocarbon surfactant systems. Since the maximum length of the trisiloxane moiety in its extended form is short, the Lα phase is formed at φEO/φS between 0.45 and 0.6 in the present system, whereas an ordinary linear-type nonionic surfactant forms the H1 phase in the same range of φEO/φS. The H1 phase, which was observed in a narrow range of φEO/φS, becomes stable upon addition of oil. Although the H1 phase in the absence of oil is considered to have the hexagonal structure confirmed by polarized optical microscopy, the calculated radius of the cylinder is much longer than the hydrophobic chain length in its extended form. Perhaps, the present H1 phase is different from an ordinary hexagonal liquid crystalline structure. The three-phase behaviors of microemulsions in the present systems were also examined.
The A−B-type silicone copolymer, Me3SiO−(Me2SiO)23−Me2SiCH2CH2CH2−O−(CH2CH2O)51.6H (Si25C3EO51.6), forms only lamellar liquid crystal (Lα) in water over a whole range of concentrations. Rich phase behavior was observed upon addition of conventional nonionic surfactant, C12EO5, whose molecular weight is ∼1/10 of Si25C3EO51.6. For both amphiphiles, f, the volume fraction of hydrophilic part in amphiphile, is the same, and f = 0.5. C12EO5 alone forms aqueous micellar solution (Wm), normal hexagonal (H1), bicontinuous cubic (V1), lamellar (Lα), and reverse micellar solution (Om) phases in water with increasing surfactant concentration. On the other hand, Wm, discontinuous micellar cubic (I1), Lα, reverse bicontinuous cubic (V2), and reverse hexagonal (H2) phases are successively formed in water at a 70/30 weight ratio of Si25C3EO51.6/C12EO5. A reduction in the effective cross-sectional area per amphiphile at hydrophobic surface (aS) takes place upon addition of C12EO5 in the mixed system. A small amphiphile, C12EO5, is dissolved in the copolymer Lα phase, and a relatively small amount of surfactant occupies a rather large area at an interface of aggregates, although the aS for Si25C3EO51.6 in the Lα phase is more than double that for C12EO5. Hence, the surfactant mainly dictates the morphology or amphiphile-layer curvature in the mixed system. The copolymer is practically insoluble in the Lα phase of C12EO5 due to the packing constraint. Hence, two Lα phases coexist in a surfactant-rich region at WS = 0.75, where WS is the weight fraction of total amphiphile in the system. On the other hand, the copolymer Lα phase changes to the I1 phase with increasing the surfactant mixing fraction at WS = 0.55. The spherical micelle in the I1 phase has a double-layer structure in which the surface is covered by surfactant and the core is pure poly(dimethylsiloxane) chain.
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