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icone polyether (SPE) surfactants. The SPE surfactants exhibit several unique properties (4): (1) they can lower the air/water interfacial tension to about 20 mN/m; (2) the siloxane chain remains flexible and in the liquid state up to high molecular weight; (3) they are surface active in both aqueous and nonaqueous media; and (4) they are suitable for cosmetic applications because they cause little skin irritation. The base of the SPE surfactants is polydimethylsiloxane (PDMS). PDMS with sufficient molecular weight, e.g., about 1000, is well known for its hydrophobicity as well as oleophobicity. In particular, the oleophobicity distinguishes the silicone-based surfactants from their hydrocarbon counterparts. The molecular structure of SPEs is either comblike, with the polyoxyalkylene chain side-grafted to a PDMS backbone, or linear diblock (AB) and triblock (ABA) types, with A representing the polyoxyalkylene hydrophile and B the PDMS hydrophobe. SPEs incorporating copolymers of polyoxyethylene (PEO) and polyoxypropylene (PPO) are widely used in polyurethane foam manufacture and personal care prod

The interaction forces provided by silicone polyether (SPE) surfactants, adsorbed at the interface of octadecyltrichlorosilan monolayer and aqueous surfactant solution, were measured by atomic force microscopy. Changes in the forces in the presence of ethanol were examined for a series of comb-type SPE surfactants. It was found that the steric repulsion provided by the SPE molecules persisted up to an 80% or higher ethanol level. Conversely, conventional hydrocarbon surfactants such as dodecyl polyoxyethylene ether lose their interfacial activity at much lower ethanol levels. The force measurement results were compared favorably with the findings from emulsion-stability and interfacial-tension measurements. The difference between the conventional hydrocarbon surfactants and the SPE surfactants is attributed to the polysiloxane moiety being hydrophobic but not oleophilic. The polyoxypropylene units in the polyether chain were found to add unique characteristics to the surface behavior of the SPE surfactants by modifying the hydrophile's solubility in the alcoholic media. The surface activity and colloidal stability can be fine-tuned by the ratio of polyoxypropylene to polyoxyethylene. In general, emulsion and colloidal products and processes that contain alcohol or other nonaqueous media will benefit from the unique properties of this special class of SPE surfactants.

The morphology and stability of small unilamellar egg yolk phosphatidylcholine (EggPC) liposomes modified with the Pluronic® copolymer (poly (oxyethylene)–poly (oxypropylene)–poly (oxyethylene) (PEO–PPO–PEO)) with different compositions on mica surface have been investigated using atomic force microscopy. Morphology studies reveal significant morphological changes of liposomes upon incorporating the Pluronic® copolymer. Bilayers are observed for Pluronic® with small hydrophilic (PEO) chain lengths such as L81 [(PEO)2(PPO)40(PEO)2] and L121 [(PEO)4(PPO)60(PEO)4]; bilayer and vesicle coexistence is observed for P85 [(PEO)26(PPO)39.5(PEO)26] and F87 [(PEO)61.1(PPO)39.7(PEO)61.1]; and stable vesicles are observed for F88 [(PEO)103.5(PPO)39.2(PEO)103.5], F127 [(PEO)100(PPO)65(PEO)100], and F108 [(PEO)132.6(PPO)50.3(PEO)132.6]. The micromechanical properties of Pluronic®-modified EggPC vesicles were studied by analyzing AFM approaching force curve. The bending modulus ( k c ) of the Pluronic®-modified EggPC vesicles increased several-fold compared with that of the pure EggPC vesicles. The significant difference is due to the enhanced rigidity of the EggPC vesicles as a result of the incorporation of PPO molecules and PEO chains. Based on the analysis of onset point by AFM and diameters of vesicles by light scattering, it was concluded that the favorable model to describe the polymer–bilayer interaction is the membrane-spanning model.

The micromechanical properties of pure and cholesterol modified egg yolk phosphatidylcholine (EggPC) vesicles prepared by sonication were studied by atomic force microscopy (AFM) on mica surface. The force curves between an AFM tip and an unruptured vesicle were obtained by contact mode. During approach, two repulsion regions with two breaks were observed. The slopes of the two repulsive force regimes for the pure EggPC vesicles are determined to be several times lower than that of EggPC/cholesterol vesicles. The elastic properties from force plot analysis based on the Hertzian model showed that Young's modulus (E) and the bending modulus (kc) of cholesterol-modified vesicles increased several-fold compared with pure EggPC vesicles. The significant difference is attributed to the enhanced rigidity of the EggPC vesicles as a result of the incorporation of cholesterol molecules. The behavior of cholesterol-modified vesicles upon adsorption is different from that in solution as revealed by mechanical properties. The results indicate that AFM can provide a direct method to measure the mechanical properties of adsorbed small liposomes and to detect the stability change of liposomes.