Surfactant Type and Concentration

The type and concentradon of a surfactant molecule affect the analytical separations of the analytes in MEEKC. High concentrations of a surfactant increase the stability of the microemulsion, but it enlarges the size of microdroplets and lengthen the retention time of the analytes. 

an anionic tensioaclive and with single-chain structure, is the surfactant mostly employed in the development of a microonulsion at concentrations in the range between 50 and lOOmM. Selectivity can be modified by mixing SDS with another surfactant like a bile salt (sodium cholate) or polyoxyethylaieglycol ethers [43], Cationic surfactants have been used in MEEKC resulting in the reverse of polarity [19], 

It must be noticed that resolution of compounds with different and high hydrophobicity using SDS as surfactant in MEEKC sometimes may be unsuccessful [44], Double-chain structure surfactants such as phosphatydUchoUne achieved a better selectivity compared to the traditional MEEKC-SDS system [7,22]. 

Another double-chain, anionic, and hydrophobic surfactant is sodium bis(2-ethylhexyl) sulfosuccinate (AOT) used in the development of a novel microemulsion in a MEEKC system for the separation of estrogens with different hydrophobicity in the same run [45]. 

Chiral surfactants used in MEEKC have also been like dodecoxycarbonylvaline (DDCV) for the separation of different enantiomeric analytes [46,47]. Chiral polymeric surfactant has been also employed in the enantiomeric resolution of barbiturate, binaphthyl, and paveroUne [48], 

---

Oil/water interfacial tensions were measured for a number of heavy crude oils at temperatures up to 200°C using the spinning drop technique. The influences of spinning rate, surfactant type and concentration, NaCI and CaCI2 concentrations, and temperature were studied. The heavy oil type and pH (in the presence of surfactant) had little effect on interfacial tensions. Instead, interfacial tensions depended strongly on the surfactant type, temperature, and NaCI and CaCL concentrations. Low interfacial tensions (<0.1 mN/m) were difficult to achieve at elevated temperatures. 

The amount of water solubilized in a reverse micelle solution is commonly referred to as W, the molar ratio of water to surfactant, and this is also a good qualitative indicator of micelle size. This is an extremely important parameter since it will determine the number of surfactant molecules per micelle and is the main factor affecting micelle size. For an (AOT)/iso-octane/H20 system, the maximum Wq is around 60 [16], and above this value the transparent reverse micelle solution becomes a turbid emulsion, and phase separation may occur. The effect of salt type and concentration on water solubilization is important. Cations with a smaller hydration size, but the same ionic charge, result in less solubilization than cations with a large hydration size [17,18]. Micelle size depends on the salt type and concentration, solvent, surfactant type and concentration, and also temperature. 

Effect of surfactant type and concentration An increase in surfactant concentration results in an increase in the number of micelles rather than any substantial change in size, and this enhances the capacity of the reverse micelle phase to solubilize proteins. Woll and Hatton [24] observed increasing protein solubilization in the reverse micelle phase with increasing surfactant concentration. In contrast, Jarudilokkul et al. [25] found that at low minimal concentrations (6-20 mmol dm AOT), reverse mieelles eould be highly seleetive in separating very similar proteins from... 

There are two classes of parameters that influence the efficiency of back extraction first, the parameters that govern the forward extraction such as pH, salt type and concentration, surfactant type and concentration, and protein type and concentration and second, the pH, salt type and concentration of stripping solutions, and extraction temperature. 

Fig. 8.20 Solubilization rate of trichloroethylene as affected by surfactant type and concentration Cs and C denote surfactant concentration and initial surfactant concentration, respectively. Reprinted from Zhong L, Mayer AS, Pope GA (2003) The effects of surfactant formulation on nonequUibrium NAPL solubilization. J Contam Hydrol 60 55-75. Copyright 2003 with permission of Elsevier.

While the quahty of the foam was not discussed, changes in surfactant type and concentration were the primary determinants of cell size, distribution, and type and doubtless affected the cell effectiveness and retention of cells in the foams. 

In terrestrial systems, numerous factors have been recognized as essential in determining a herbivore s gut environment, and, hence, its response to tannins. In marine systems, many of these same factors may impact the activity of phlorotannins in the herbivore gut. These include gut morphology 44 pH,25 redox potential,58 124 166 enzyme composition and activity,33 57 surfactant type and concentration,70 160 167 cation type and concentration,70 124168 proteins or amino acids,169-175 gut microbial activity,49 50 67 and nutritional status.33... 

As already mentioned (see Chapter 3), at the instant of foam formation the films and borders are in non-equilibrium state. The films thin mainly due to the capillary pressure, while the borders thin due to gravity or a pressure drop (when the foam is dried by the Foam Pressure Drop Technique [21-23]). The surfactant adsorption layers decrease the flow rate through the borders and films and the process of thinning becomes similar to the flow in thin gaps with solid surfaces. As indicated in Sections 3.2.1 and 5.3 the degree of retardation of the flow depends on the surfactant type and concentration as well as on the film type. A complete immobility at the film and border surfaces usually is not reached. 

The stability of emulsion and foam films have also been found dependent upon the micellar microstructure within the film. Electrolyte concentration, and surfactant type and concentration have been shown to directly influence this microstructure stabilizing mechanism. The effect of oil solubilization has also been discussed. The preceding stabilizing/destabilizing mechanisms for three phase foam systems have been shown to predict the effectiveness of aqueous foam systems for displacing oil in enhanced oil recovery experiments in Berea Sandstone cores. 

Micelles are approximately spherical aggregates of surfactant molecules with their nonpolar tails in the interior and their hydrophilic ends oriented towards the aqueous medium. They are some 50-100 A in diameter. The bulk concentration of surfactant is usually around 0.1 M and this corresponds to approximately I o micelles per milliliter of aqueous phase, since there are typically about 50-100 emulsifier molecules per micelle. The apparent water solubility of organic molecules is enhanced by micellar surfactants, because the organic molecules are absorbed into the micelle interiors. The extent of this solubilization of organic molecules depends on the surfactant type and concentration, the nature of the solubilized organic substance, and the concentration of electrolytes in the aqueous phase. As an example, there will be about an equal number of styrene molecules and potassium hexadecanoate (palmitate) molecules in a micelle of the latter material. In this case about half the volume of the micelle interior is occupied by solubilized monomer, and the concentration of styrene is approximately 4.5 M at this site. Thus radical polymerization starts very rapidly in the interior of a micelle once it is initiated there. 

To stabilize emulsions, a surfactant, which increases the repulsive force between oil droplets, is used. Nonionic surfactants are the preferred type because they are effective in brines, are generally cheaper, and often form less viscous emulsions than do ionic surfactants. In addition, their emulsions are easier to break, and they do not introduce inorganic residues that might lead to refinery problems. They are chemically stable at oil reservoir temperatures and are noncorrosive and nontoxic. The surfactant type and concentration required for a particular situation can be determined by conducting laboratory tests. A typical concentration of 0.1 lb of surfactant per barrel of oil is used for emulsions containing about 50-70% oil (2). 

Effect of Surfactant Type and Concentration. Surfactant concentration and type is of great importance for the stability of thin liquid films and for emulsion stability. Type and concentration of surfactants are responsible for the degree of lowering the interfacial tension and for the viscoelastic properties of droplet surface, as well as for the film thickness between two droplets. 

HLD is a generalised formulation yardstick that is some kind of extended HLB, which is function of all formulation variables (surfactant characteristics, co-surfactant type and concentration, temperature, oil nature, salinity. ..) and it may be numerically estimated or measured with a much better accuracy than HLB, roughly equivalent to one-tenth of an HLB unit. From the physical chemistry point of view, it has a strong foundation, since it represents the change in standard chemical potential when a surfactant molecule passes from oil to water in the conditions of experiments. 

The surfactant also lowers the interfacial tension, thereby facilitating droplet breakup. The effective y value during breakup depends on surfactant type and concentration and on the rate of transport to the drop surface. Approximate equations are available for this rate, and also for the stresses acting on a drop, the drop size resulting from breakup, and the frequency at which the drops encounter each other. 

Those factors that were previously mentioned that produce finer-textured foams also produce more stable foams. Factors such as surfactant type, concentration, increasing pressure, and higher inputs of mechanical energy generate more stable foams. For higher temperatures such as those that exist downhole, dynamic foam stability relies upon surfactant type and concentration rather than the addition of thickeners (polymer stabilizers). It is not known what rates are necessary to maintain dynamic stability in fractures, or whether those conditions typically exist. 

Certain methods of calculating friction pressures involve the use of pressure charts that are in the form of pressure drop per length of pipe versus the flow rate of the foam. These types of charts incorporate foam quality and tubular geometry but may neglect consideration of one or more of the parameters such as temperature, pressure, foam texture, polymer or surfactant type, and concentration. Each of these parameters is often unspecified but drastically affects the friction pressures of foams. Information derived from such charts should be taken only as a guideline. 

Assuming that the model system is a valid model for a MHAP prepolymerization solution, these results for the model system can be applied to the MHAP. The model system results suggest that the micelle solution copolymerization process can be a means of producing multiblock copolymers in which hydrophobic and hydrophilic blocks alternate. At constant monomer concentration, the number of blocks in the copolymer molecule is inversely related to the number of monomers in the block. Thus, factors that increase the sequence length of a block decrease the number of blocks. The monomer concentrations and the polymer MW are also factors governing the sequence length and number of blocks in the copolymer. Polymerization conditions (e.g., surfactant type and concentration) can be used to control the block size to some extent. 

The pore size of mesoporous carbon is of importance with respect to practical applications. When mesoporous carbon is synthesised via soft-template methods, the self-assembly of organic-organic species and pore size can be influenced by synthesis conditions, including surfactant type and concentration, and synthesis temperature. For example, Meng et al. observed that the pore size of mesoporous carbon derived from soft-templated mesoporous polymer composites decreased from 7.4 to 5.9 nm when the pyrolysis temperature increased from 400 to 800 How-... 

In order to gain more industrial relevance, the feasibility of preparing high solids latexes via polymerization of miniemulsions was investigated in terpolymerizations of styrene/2-ethylhexylacrylatehnethactylic add (45/45/10 mole ratio). Variables included the surfactant (type and concentration), initiator (concentration), solids content and addition mode (batch [10] and semi-continuous [13]). 

Parameters that may be used to tune the phase behavior of microemulsions include salinity, surfactant type and concentration, cosolvent type and concentration, pH, oil composition, temperature, and pressure. As salinity increases, there is a steady progression from lower phase to middle phase to upper phase microemulsions. This reflects a continuous evolution of the preferred curvature of the surfactant film and corresponds to an increase in hydrophobicity with added electrolyte such as NaCl. At low salinity the droplet size in the water-continuous lower phase increases with increasing salinity. This corresponds to an increase in the solubilization of oil and is reflected in increased light scattering. As salinity increases further, the middle phase appears and is initially water-continuous. 

During formulation of a suitable microemulsion liquid membrane, the researcher must be able to incorporate a certain amount of additives such as the liquid ion exchanger for metal ion separations. This additive will affect the microemulsion phase behavior and require a screening of surfactant types and concentrations to obtain the desired microemulsion properties discussed in Sec. III. A. In some cases (e.g., quaternary amines), the additive itself is so interfacially active that a microemulsion cannot form. 

Synthesis of high molecular weight random copolymers of acrylamide and alkylacrylamides required a novel aqueous surfactant micellar solution polymerization. The surfactant type and concentration were chosen to provide solubilization of the hydrophobic monomer with preferably one or at most a few hydrophobic monomer groups per micelle. 

In these studies, polymeric nanocapsules with encapsulated dsDNA (790 base pairs) were produced via anionic polymerization of n-butylcyanoacrylate (BCA) carried out at the interface of homogeneously distributed aqueous droplets in inverse miniemulsion which are in a second step then redispersed in an aqueous continuous phase. The obtained capsules were characterized in terms of size, size distribution, morphology, polymer molecular weight, and encapsulation efficiency of DNA. The effects of surfactant type and concentration, viscosity of the continuous phase, monomer amount, and water-to-oil ratio were investigated and results are discussed in this paper. 

Three different stabilizing systems were chosen for the experiments presented in this paper (i) surfactants for inverse (water-in-oil) systems possessing low HLB values (Span 80, HLB 4.3) (ii) surfactants for oil-in-water systems with high HLB values (Tween 80, HLB 15), and (iii) a mixture of surfactants for both systems (Tween 80 and Span 80 in the ratio of 3 2 w/w). Ligure 2a shows the average diameter of the miniemulsion droplets in Miglyol 812N, plotted as a function of the surfactant type and concentration. 

Fig. 2 Average miniemulsion droplet sizes in Miglyol 812N (a) or cyclohexane (b) as a function of surfactant type and concentration. The concentration of surfactant is given in wt.%, corresponding to the oil phase...

---