Surfactants solutions

Surfactants are molecular or ionic species that have a polar (hydrophilic) moiety (head) attached to a long nonpolar (hydrophobic) moiety (tail). Examples are 

The mechanisms that affect heat transfer in single-phase and two-phase aqueous surfactant solutions is a conjugate problem involving the heater and liquid properties (viscosity, thermal conductivity, heat capacity, surface tension). Besides the effects of heater geometry, its surface characteristics, and wall heat flux level, the bulk concentration of surfactant and its chemistry (ionic nature and molecular weight), surface wetting, surfactant adsorption and desorption, and foaming should be considered. 

Surfactants have a unique long-chain molecular structure composed of a hydrophilic head and hydrophobic tail. Based on the nature of the hydrophilic part surfactants are generally categorized as anionic, non-ionic, cationic, and zwitter-ionic. They all have a natural tendency to adsorb at surfaces and interfaces when added in low concentration in water. Surfactant absorption/desorption at the vapor-liquid interface alters the surface tension, which decreases continually with increasing concentrations until the critical micelle concentration (CMC), at which micelles (colloid-sized clusters or aggregates of monomers) start to form is reached (Manglik et al. 2001 Hetsroni et al. 2003c). 

The change in surface wettability (measured by the contact angle) with concentration for the three surfactants is plotted in Fig. 2.54 (Zhang and Manglik 2005). The contact angle reaches a lower plateau around the CMC where bilayers start to form on the surface. Wettability of non-ionic surfactants in aqueous solutions shows that the contact angle data attains a constant value much below CMC. Direct interactions of their polar chain are generally weak in non-ionics, and it is possible for them to build and rebuild adsorption layers below CMC. The reduced contact an- 

The results were obtained at heat flux = 10 kW/m. For both liquids at f = 1 ms the contact angle is approximately of 0 = 60°, which is very close to the equilibrium surface tension of water. Throughout bubble growth this value decreases approxi- 

In Fig. 2.58 (Hetsroni et al. 2001b) the dependencies of the surface tension of the various surfactants a divided on the surface tension of water ow are shown. One can see that beginning from some particular value of surfactant concentration (which depends on the kind of surfactant), the value of the relative surface tension almost does not change with further increase in the surfactant concentration. It should be emphasized that the variation of the surface tension as a function of the solution concentration shows the same behavior for anionic, non-ionic, and cationic surfactants at various temperatures. 

C12EO6 Hexaoxyethylene dodecyl ether C12H25 - (O - CH2CH2)6 - OH 

Single-chained iipids (surfactants) with large head-group areas  

Single-chained lipids with small head-group areas  

Phosphatidyl choline (lecithin), phosphatidyl serine, phosphatidyl glycerol, phosphatidyl inositol, phosphatidic acid. 

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A zero or near-zero contact angle is necessary otherwise results will be low. This was found to be the case with surfactant solutions where adsorption on the ring changed its wetting characteristics, and where liquid-liquid interfacial tensions were measured. In such cases a Teflon or polyethylene ring may be used [47]. When used to study monolayers, it may be necessary to know the increase in area at detachment, and some calculations of this are available [48]. Finally, an alternative method obtains y from the slope of the plot of W versus z, the elevation of the ring above the liquid surface [49]. 

It was determined, for example, that the surface tension of water relaxes to its equilibrium value with a relaxation time of 0.6 msec [104]. The oscillating jet method has been useful in studying the surface tension of surfactant solutions. Figure 11-21 illustrates the usual observation that at small times the jet appears to have the surface tension of pure water. The slowness in attaining the equilibrium value may partly be due to the times required for surfactant to diffuse to the surface and partly due to chemical rate processes at the interface. See Ref. 105 for similar studies with heptanoic acid and Ref. 106 for some anomalous effects. 

A 1.5% by weight aqueous surfactant solution has a surface tension of 53.8 dyn/cm (or mN/m) at 20°C. (a) Calculate a, the area of surface containing one molecule. State any assumptions that must be made to make the calculation from the preceding data, (b) The additional information is now supplied that a 1.7% solution has a surface tension of 53.6 dyn/cm. If the surface-adsorbed film obeys the equation of state ir(o - 00) = kT, calculate from the combined data a value of 00, the actual area of a molecule. 

In detergency, for separation of an oily soil O from a solid fabric S just to occur in an aqueous surfactant solution W, the desired condition is 730 = 7wo+7sw. Use simple empirical surface tension relationships to infer whether the above condition might be met if (a) 73 = 7w. (6) 70 = 7W, or (c) 73 = 70. 

Ruch and Bartell [84], studying the aqueous decylamine-platinum system, combined direct estimates of the adsorption at the platinum-solution interface with contact angle data and the Young equation to determine a solid-vapor interfacial energy change of up to 40 ergs/cm due to decylamine adsorption. Healy (85) discusses an adsorption model for the contact angle in surfactant solutions and these aspects are discussed further in Ref. 86. 

Yaminsky and Yaminskaya [114] have used a Wilhelmy plate to directly measure the interfacial tension (and hence infer the contact angle) for a surfactant solution on... 

A drop of surfactant solution will, under certain conditions, undergo a fingering instability as it spreads on a surface [27, 28]. This instability is attributed to the Marongoni effect (Section IV-2D) where the process is driven by surface tension gradients. Pesach and Marmur have shown that Marongoni flow is also responsible for enhanced spreading... 

Fig. XIII-13. Concentrations of individual species in a surfactant solution.

A surfactant solution is a mixture of DTAC (dodecyltrimethylammonium chloride) and CPC (cetyl pyridinium chloride) the respective CMCs of the pure surfactants are 2 X lO M and 9 x IO M (Ref. 140). Make a plot of the CMC for mixtures of these surfactants versus the mole fraction of DTAC. 

F. M. Fowkes, in Solvent Properties of Surfactant Solutions, K. Shinoda, ed., Marcel Dekker, New York, 1967. 

Fig. XIV-16. A photomicrograph of a two-dimensional foam of a commercial ethox-ylated alcohol nonionic surfactant solution containing emulsified octane in which the oil drops have drained from the foam films into the Plateau borders. (From Ref. 234.)...

Rutland M W and Parker J L 1994 Surface forces between silica surfaces in cationic surfactant solutions adsorption and bilayer formation at normal and high pH Langmuir 0 1110-21... 

Xu Z H, Ducker W and Israelachvili J N 1996 Forces between crystalline alumina (sapphire) surfaces in aqueous sodium dodecyl sulfate surfactant solutions Langmuir 12 2263-70... 

Pashley R M and Israelachvili J N 1981 A comparison of surface forces and interfacial properties of mica in purified surfactant solutions Colloids Surf. 2 169-87... 

The Diels-Alder reaction provides us with a tool to probe its local reaction environment in the form of its endo-exo product ratio. Actually, even a solvent polarity parameter has been based on endo-exo ratios of Diels-Alder reactions of methyl acrylate with cyclopentadiene (see also section 1.2.3). Analogously we have determined the endo-exo ratio of the reaction between 5.1c and 5.2 in surfactant solution and in a mimber of different organic and acpieous media. These ratios are obtained from the H-NMR of the product mixtures, as has been described in Chapter 2. The results are summarised in Table 5.3, and clearly point towards a water-like environment for the Diels-Alder reaction in the presence of micelles, which is in line with literature observations. 

Table 5.3. Endo-exo product ratios of the Diels-Alder reaction of 5.1c with 5.2 in surfactant solution compared to water and organic solvents.

In all surfactant solutions 5.2 can be expected to prefer the nonpolar micellar environment over the aqueous phase. Consequently, those surfactant/dienophile combinations where the dienophile resides primarily in the aqueous phase show inhibition. This is the case for 5.If and S.lg in C12E7 solution and for S.lg in CTAB solution. On the other hand, when diene, dienophile and copper ion simultaneously bind to the micelle, as is the case for Cu(DS)2 solutions with all three dienophiles, efficient micellar catalysis is observed. An intermediate situation exists for 5.1c in CTAB or C12E7 solutions and particularly for 5.If in CTAB solution. Now the dienophile binds to the micelle and is slid elded from the copper ions that apparently prefer the aqueous phase. Tliis results in an overall retardation, despite the possible locally increased concentration of 5.2 in the micelle. 

With this picture in mind, let us consider what happens when monomer is stirred into a surfactant solution-which also contains a water-soluble initiator-above the cmc. 

The term microemulsion was introduced by Schulman, who studied surfactant solutions as eady as 1943 (22). At that time it was widely accepted that "oil and water do not mix," and Schulman understood that an emulsion scatters light because it contains droplets whose diameters are large compared to the wavelength of light (see Emulsions). Thus, the term y /mJemulsion implies a system which (like an emulsion) contains droplets of oil or water, but in which the droplets are too small to scatter light. 

Amphoteric Detergents. These surfactants, also known as ampholytics, have both cationic and anionic charged groups ki thek composition. The cationic groups are usually amino or quaternary forms while the anionic sites consist of carboxylates, sulfates, or sulfonates. Amphoterics have compatibihty with anionics, nonionics, and cationics. The pH of the surfactant solution determines the charge exhibited by the amphoteric under alkaline conditions it behaves anionically while ki an acidic condition it has a cationic behavior. Most amphoterics are derivatives of imidazoline or betaine. Sodium lauroamphoacetate [68647-44-9] has been recommended for use ki non-eye stinging shampoos (12). Combkiations of amphoterics with cationics have provided the basis for conditioning shampoos (13). 

Lignosulfonate has been reported to increase foam stabihty and function as a sacrificial adsorption agent (175). Addition of sodium carbonate or sodium bicarbonate to the surfactant solution reduces surfactant adsorption by increasing the aqueous-phase pH (176). 

P. Dunlop and co-workers, "Aqueous Surfactant Solutions Which Exhibit Ultra-Low Tensions at the Oil-Water Interface," Paper presented at the... 

Phenomena at Liquid Interfaces. The area of contact between two phases is called the interface three phases can have only aline of contact, and only a point of mutual contact is possible between four or more phases. Combinations of phases encountered in surfactant systems are L—G, L—L—G, L—S—G, L—S—S—G, L—L, L—L—L, L—S—S, L—L—S—S—G, L—S, L—L—S, and L—L—S—G, where G = gas, L = liquid, and S = solid. An example of an L—L—S—G system is an aqueous surfactant solution containing an emulsified oil, suspended soHd, and entrained air (see Emulsions Foams). This embodies several conditions common to practical surfactant systems. First, because the surface area of a phase iacreases as particle size decreases, the emulsion, suspension, and entrained gas each have large areas of contact with the surfactant solution. Next, because iaterfaces can only exist between two phases, analysis of phenomena ia the L—L—S—G system breaks down iato a series of analyses, ie, surfactant solution to the emulsion, soHd, and gas. It is also apparent that the surfactant must be stabilizing the system by preventing contact between the emulsified oil and dispersed soHd. FiaaHy, the dispersed phases are ia equiUbrium with each other through their common equiUbrium with the surfactant solution. 

Fig. 3. Schematic diagram of anionic surfactant solution at equiUbrium above its critical micelle concentration, where M = micelle and 0 are counterions ...

Physical and ionic adsorption may be either monolayer or multilayer (12). Capillary stmctures in which the diameters of the capillaries are small, ie, one to two molecular diameters, exhibit a marked hysteresis effect on desorption. Sorbed surfactant solutes do not necessarily cover ah. of a sohd iaterface and their presence does not preclude adsorption of solvent molecules. The strength of surfactant sorption generally foUows the order cationic > anionic > nonionic. Surfaces to which this rule apphes include metals, glass, plastics, textiles (13), paper, and many minerals. The pH is an important modifying factor in the adsorption of all ionic surfactants but especially for amphoteric surfactants which are least soluble at their isoelectric point. The speed and degree of adsorption are increased by the presence of dissolved inorganic salts in surfactant solutions (14). 

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