Surfactants molecular structures

The assessment of surfactant structures and optimal mixtures for potential use in tertiary flooding strategies in North Sea fields has been examined from fundamental investigations using pure oils. The present study furthermore addresses the physico-chemical problems associated with reservoir oils and how the phase performance of these systems may be correlated with model oils, including the use of toluene and cyclohexane in stock tank oils to produce synthetic live reservoir crudes. Any dependence of surfactant molecular structure on the observed phase properties of proposed oils of equivalent alkane carbon number (EACN) would render simulated live oils as unrepresentative. 

The EACN values for toluene were found to vary depending upon the nature of the surfactant molecular structure, but sulphonate systems confirm an EACN equivalence of 0. 

Nagarajan R. Theory of micelle formation quantitative approach to predicting micellar properties from surfactant molecular structure. Surface Sci Ser 1997 70 1-81. 

Table 2 shows a decrease in Foo when the location of the sulfate group in the surfactant molecular structure is shifted towards the middle of the hydrocarbon tail. The adsorbed species actually acquires two (shorter) hydrophobic tails thereby increasing the area per molecule at the interface as pictorially shown in Fig. 5. The decrease in Foo is explained by the increase in the area... 

The results presented in this section indicate that all surfactants may not be suitable for surfactant-enhanced desorption. Multiple factors may influence the suitability of a surfactant for surfactant-enhanced desorption. These factors have been discussed previously. It is clear that additional research is needed to better understand how surfactant molecular structure and soil composition/chemistry affect the rate of solute desorption. However, the results presented in this chapter and in other studies indicate that surfactant-enhanced remediation of aquifers is a promising technology that needs to be explored. 

Variables identified as important in the achievement of the low IFT in a W/O/S/electrolyte system are the surfactant average MW and MW distribution, surfactant molecular structure, surfactant concentration, electrolyte concentration and type, oil phase average MW and structure, temperature, and the age of the system. Salager et al. (1979b) classified the variables that affect surfactant phase behavior in three groups (1) formulation variables those factors related to the components of the system-surfactant structure, oil carbon number, salinity, and alcohol type and concentration (2) external variables temperature and pressure (3) two-position variables surfactant concentration and water/oil ratio. Some of the factors affecting IFT-related parameters are briefly discussed in this section. Some other factors, such as cosolvent, salinity, and divalent, are discussed in Section 7.4 on phase behavior. Healy et al. (1976) presented experimental results on the effects of a number of parameters. 

Kabin, J.A. et al.. Removal of organic films from rotating disks using aqueous solutions of nonionic surfactants effect of surfactant molecular structure, J. Colloid Interface ScL, 206, 102, 1998. 

Several toxicity, irritation, and sensitization phenomena will be summarized in this section with special reference to hair care products that contain surface active agents. Skin irritation by surfactants will be covered in some depth providing a few fundamental principles and useful relationships of skin irritation to surfactant molecular structure to provide guidance for formulating milder hair care products. A few important regulatory statutes will be summarized and referenced for further follow up as needed. 

In aqueous solutions of surfactants at concentrations above the critical micelle concentration (CMC), the molecules self-assemble to form micelles, vesicles, or other colloidal aggregates. These may vary in size and shape depending on solution conditions. In addition to surfactant molecular structure, the effects of concentration, pH, other additives, cosolvents, temperature, and shear affect the nanostructure of the micelles. The presence of TLMs or cylindrical, rodlike, or wormlike micelles at concentrations > CMCii are generally believed to be necessary for surfactant solutions to be drag reducing [Zakin et al., 2007]. 

A typical spherical micelle in aqueous solution has an average number of surfactant molecules, or aggregation number, of 40-100. The diameter of a spherical or cylindrical micelle is around 4-6 nm for typical surfactants. The shape of the micelle depends on surfactant molecular structure, solution ionic strength, temperature, and presence of organic solutes in solution, among other factors. 

The actual structure also depends on the surfactant molecular structure. For instance, dual-tail amphiphiles such as sulfosuccinate surfactants are more likely to produce W/O-type miniemulsions and microemulsions with water core islands. If too much water is present, because of the inability of this surfactant to accommodate its branched double tails in an oily core, it would result in more complex structures such as vesicles, in which a surfactant bilayer closes on itself, as shown in Fig. 4. 

For a given surfactant concentration, an increase in the concentration of water led to a decrease in particle size this trend is consistent with Figs. 5e and 6b. On the other hand, when the water concentration was kept constant, the particle size went through a minimum (at i = [H20]/[NP-4]= 1.9) with an increase in surfactant concentration. The effects of surfactant molecular structure and type of oil were also investigated by Chang and Fogler [75]. Three different polyoxyethylene-type surfactants were used, i.e., NP-4, NP-5, and DP-6. The particle size of the silica particles followed the order NP-5 > NP-4 > DP-6. The particle size was found to be sensitive to the type of oil, with the size decreasing in the order heptane > heptane/cyclohexane (50/50 v/o) > cyclohexane. 

The sequence in which reversed phases occur is much more complicated than that for the normal phases and is not yet understood in terms of surfactant molecular structures. The main reason is (probably) that there is no limitation on the radius of the inverse micelles such as that imposed by the length of the paraffin chain on normal micelles. Water could swell the micelles indefinitely (this does not happen because the size and shape of inverse micelles is controlled by limits on surfactant packing on a curved surface). Figure 21.12 (or something similar) is often employed to describe the general pattern of mesophase behaviour as a function of surfactant (water) concentration. In the description... 

The dependence of film drainage rate on silicone surfactant molecular structure was also systematically investigated. In order to understand this correlation, three physical parameters of the film affected by surfactant structure must be considered. These parameters are the surface partition coefficient, the surfactant molecular diffusion coefficient and the degree of intermolecular cohesion within the surface layer. Specifically, as the length of the polyether (solvophilic) portion of the surfactant increased, the surface partition coefficient decreased, the diffusion coefficient decreased, and the degree of cohesion increased. This resulted, at constant surfactant concentration, in a complex effect on the film drainage rate. 

The Effect of Surfactant Molecular Structure on Film Drainage Rate... 

Figure 1 Examples of surfactant molecular structure evidencing the wide range of both hydrophilic and hydrophobic moieties. (Reproduced from Ref. 2, Detergents and Soaps.com.)...

The presence of a chromophore group in the hydrophilic or hydrophobic moieties in the surfactant molecular structure makes it sensitive to different physical responses, in particular, for the control of physicochemical parameters of colloidal systems such as surface activity, aggregation structure, viscosity, microemulsion separation, and solubilization. 

It has been found that surfactants, when aggregating in their lamellar liquid crystalline form, provide good lubrication of surfaces. This contribution discusses the conditions for obtaining lamellar liquid crystalline phases at surfaces. Beside the surfactant molecular structure itself, other parameter of importance are co-surfac-tants, CO- and counter-ions and the presence of solubilizates. 

FIGURE 16.1 Surfactant molecules self-assemble into various aggregate shapes, depending on the surfactant molecular structure as described by the critical packing parameter (CPP), which is the ratio of the molecular volume (v) divided by its length (/) times the cross-sectional area of the head group (a) v/al. 

The ability to form a lamellar liquid crystalline film depends on the spontaneous curvature of the surfactant aggregates, or the CPP, which is a convenient and intuitive description of the surfactant molecular structure. Kabalnov and Wennerstrbm [16] have shown that, for the formation of a water bridge between two water droplets, a large free energy is required for a surfactant with a high CPP, while the free energy required for a surfactant with a low CPP is lower. Hence, the stability of a surfactant double layer increases with an increase of the CPP of the surfactant. 

Lamellar liquid crystalline phases should be formed at the surface in order to attain good lubrication of a surfactant system. Surfactant molecular structure is one of the critical variables affecting the shape of the surfactant aggregates and, hence, the lubrication properties of surfactant systems. The aggregate shape is also determined by other factors, such as hydrophilic/hydrophobic additives, salt, solubilizates, and temperature. In addition, the aggregates present at the surface are also sensitive to surface properties as well as to co-ions. 

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