Layer surfactants

Calculate the friction force between the surfactant layers in air in Fig. XII-12 using the relationship in Eq. XII-19. How does this compare with the friction shown in Fig. XII-12 ... 

The earliest SFA experiments consisted of bringing the two mica sheets into contact m a controlled atmosphere (figure Bl.20.61 or (confined) liquid medium [14, 27, 73, 74 and 75]. Later, a variety of surfactant layers [76, 77], polymer surfaces [5, 9, fO, L3, 78], poly electrolytes [79], novel materials [ ] or... 

It is believed that solubilization is initiated when a biomolecule with charged surface groups approaches the bulk aqueous-lipophilic solvent interface, where the interactions cause the bulk interface s surfactant layer to bend, such that the protruding biomolecule becomes surrounded by the surfactant layer [105]. Ultimately, a filled w/o-ME forms, and partitions to the bulk organic phase [105]. 

It follows from the above that the mechanism for electrical potential oscillation across the octanol membrane in the presence of SDS would most likely be as follows dodecyl sulfate ions diffuse into the octanol phase (State I). Ethanol in phase w2 must be available for the transfer energy of DS ions from phase w2 to phase o to decrease and thus, facilitates the transfer of DS ions across this interface. DS ions reach interface o/wl (State II) and are adsorbed on it. When surfactant concentration at the interface reaches a critical value, a surfactant layer is formed at the interface (State III), whereupon, potential at interface o/wl suddenly shifts to more negative values, corresponding to the lower potential of oscillation. With change in interfacial tension of the interface, the transfer and adsorption of surfactant ions is facilitated, with consequent fluctuation in interface o/ wl and convection of phases o and wl (State IV). Surfactant concentration at this interface consequently decreased. Potential at interface o/wl thus takes on more positive values, corresponding to the upper potential of oscillation. Potential oscillation is induced by the repetitive formation and destruction of the DS ion layer adsorbed on interface o/wl (States III and IV). This mechanism should also be applicable to oscillation with CTAB. Potential oscillation across the octanol membrane with CTAB is induced by the repetitive formation and destruction of the cetyltrimethylammonium ion layer adsorbed on interface o/wl. Potential oscillation is induced at interface o/wl and thus drugs were previously added to phase wl so as to cause changes in oscillation mode in the present study. 

Greater durability of the colloidal Pd/C catalysts was also observed in this case. The catalytic activity was found to have declined much less than a conventionally manufactured Pd/C catalyst after recycling both catalysts 25 times under similar conditions. Obviously, the lipophilic (Oct)4NCl surfactant layer prevents the colloid particles from coagulating and being poisoned in the alkaline aqueous reaction medium. Shape-selective hydrocarbon oxidation catalysts have been described, where active Pt colloid particles are present exclusively in the pores of ultramicroscopic tungsten heteropoly compounds [162], Phosphine-free Suzuki and Heck reactions involving iodo-, bromo-or activated chloroatoms were performed catalytically with ammonium salt- or poly(vinylpyrroli-done)-stabilized palladium or palladium nickel colloids (Equation 3.9) [162, 163],... 

MCM-50 consists of stacks of silica and surfactant layers. Obviously, no pores are formed upon removal of the surfactant layers. The silica layers contact each other resulting in a nonporous silica. It is noteworthy to mention that materials of M41S type were probably already synthesized by Sylvania Electric Products in 1971 [32], However, at that time the high ordering of the materials was not realized [33], M41S-type materials are synthesized under basic reaction conditions. Scientists from the University of Santa Barbara developed an alternative synthesis procedure under acidic conditions. They also used alkyltrimethyl ammonium as the surfactant. The porous silica materials obtained (e.g., hexagonal SBA-3 Santa BArbara [SBA]) had thicker pore walls but smaller pore diameters. Furthermore, they developed materials with novel pore topologies, e g., the cubic SBA-1 with spherical pores. 

The use of neutron reflectivity at liquid interfaces, which is a method sensitive to both surface roughness and surfactant layer thickness, was reviewed with the examples of polydimethylsiloxane-surfactant layers.633 Sum-frequency generation (SFG) vibrational spectroscopy was applied to study surface restructuring behavior of PDMS in water in an attempt to understand antifouling properties of silicones.6 ... 

Figure 2.17 Pair potentials calculated for 50 nm polystyrene particles in 0.1 moldm 3 electrolyte and with a -potential of —30mV. Curve a is the result for a simple polystyrene surface and curve b was calculated from the model of a 1 nm surfactant layer so that the -potential is taken as occurring at the outer edge of the adsorbed layer. A maximum in the potential of 8kBT is insufficient to provide long-term stability and the curves clearly shows how electrosteric stabilisation can achieve this...

The situation is, however, different in the alveolar region of the lung where the respiratory gas exchange takes place. Its thin squamous epithelium is covered by the so-called alveolar surface liquid (ASL). Its outermost surface is covered by a mixture of phospholipids and proteins with a low surface tension, also often referred to as lung surfactant. For this surfactant layer only, Scarpelli et al. [74] reported a thickness between 7 and 70 nm in the human lung. For the thickness of an additional water layer in between the apical surface of alveolar epithelial cells and the surfactant film no conclusive data are available. Hence, the total thickness of the complete ASL layer is actually unknown, but is certainly thinner than 1 gm. 

A pulsed system, called Time-Clock System, has been developed. It comprises a solid dosage form coated with a hydrophobie surfactant layer to which a water-soluble polymer is attached to improve adhesion to the core [66]. The thickness of the outer layer determines the time required to disperse in an aqueous environment. Following the dispersion of the outer layer, the eore becomes available for dispersion. An advantage is that eommon pharmaceutical excipients can be used to manufacture this system. Studies performed on human volunteers showed that the lag time was not affeeted by gastrie residence time. Furthermore, the dispersion of the hydrophobic film was not influenced by the presence of intestinal digestive enzymes or by the mechanieal aetion of the stomach. 

Concerning the structure of dispersed CLAs, the model originally proposed by Sebba [57] of a spherical oil-core droplet surrounded by a thin aqueous film stabilized by the presence of three surfactant layers is, in our opinion, essentially correct. However, there is still little direct evidence for the microstructure of the surfactant interfaces. From an engineering point of view, however, there is now quantitative data on the stability of CLAs which, together with solute mass transfer kinetics, should enable the successful design and operation of a CLA extraction process. 

Rgure 2.30. Two adhesive emulsion droplets. A flat hquid fllm stabilized by the surfactant layers is located between the droplets. This fllm being very thin, it can be usually considered as a surfactant bilayer. Yf is the tension of the fllm and y nt the tension of single isolated interface. 

Assuming that the internal droplets experience a hard-sphere-like repulsion when surfactant layers come in contact, an estimation of the van der Waals interactions can be obtained from the average length of the surfactant tails (1 3 nm) [20]. The coalescence frequency is therefore the unique free pa-... 

The alveolar epithelium consists of so-called Type I and Type II cells. Type I cells cover over 90% of the alveolar surface, have a large surface, and are thin. Type II cells are larger in numbers but are small. Therefore, they cover only about 7% of the surface of the alveoli. Type II cells produce the phospholipids that make up the surfactant layer. 

Fig. 2. Microbubbles that are stabilized only with a mobile surfactant layer will fuse on contact and disappear - long term storage in the aqueous phase is not feasible...

The TLF may be regarded as a kind of condenser. The repulsion between the two surfactant layers (Figure 8.3) will be determined by the EDL. The effect of added ions to the solution is to make the EDL contract, and this leads to thin films. 

Stripped magnetite particles. Applications of the double surfactant layer. Principle in the preparation of water based magnetic fluids. J. Colloid Interface Sci. 149 98-104 Wuehn, M, JosephJ., Bagus, P.S. Niklewski.A., Puettner, R., Reis,S., Weiss,W., Martins, M.,... 

In case of lipases, one of the simplest methods to combine an enzyme with an organic solvent is to coat the lipase with a lipid or surfactant layer before lyophilisation. It is estimated that about 150 surfactant molecules are sufficient for encapsulating one lipase molecule. Following this route the surfactant coated lipase forms reverse micelles with a minimum of water concentration. The modified lipases are soluble in most organic solvents, and the reaction rates are increased compared to the suspended hpases due to the interfacial activation [59,60]. 

Figure 5.1 Illustration of the effect of an adsorbed surfactant layer on the interfacial energy between oil and water.

In the case of adsorption from solution, the surfactant layers are in equilibrium with the solution and will de-sorb on dilution. However, it would be very useful to produce adsorbed layers in both air and water, which will remain adsorbed. This can be achieved using the Langmuir-Blodgett deposition technique. The technique is based on the observation that if a surfactant, which is insoluble in water, is dissolved in a volatile, non-aqueous solvent and then spread on water, an insoluble monolayer of orientated surfactant molecules will remain at the air/solution interface. The effect of the spreading surfactant and its surface film pressure can be dramatically demonstrated by spreading hydrophobic talc powder on a clean water surface and then placing a... 

If water is emulsified into fuel as a water-in-oil emulsion, coalescence cannot affect the removal of water from the fuel. The outer oil or surfactant layer surrounding water will not permit water to hydrogen bond to the hydrophilic demulsifier sites. 

To break through this oil or surfactant layer and free the retained water, it is necessary to chemically disrupt the stability of this layer. Demulsifiers and dehazers can adsorb onto the protective film and subsequently interfere with the electrochemical forces which hold this outer layer together. 

Upon adsorption, demulsifier/dehazer compounds function to break the oil or surfactant layer thus releasing the contained water. Once free, the water can then coalesce into larger drops and be removed from the fuel. 

Andrews, E., and S. M. Larson, Effect of Surfactant Layers on the Size Changes of Aerosol Particles as a Function of Relative Humidity, Environ. Sci. Technol., 27, 857-865 (1993). 

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