Surfactants experimental techniques

An excellent review of experimental techniques for measuring electrical resistivity in aqueous solutions is available [34], Separators used in nonaqueous systems can be characterized by wetting them with a surfactant and measuring the electrical resistivity in an aqueous solution. Then the resistivity in a nonaqueous membrane can be estimated from Eq. (2). 

An unusually extensive battery of experimental techniques was brought to bear on these comparisons of enantiomers with their racemic mixtures and of diastereomers with each other. A very sensitive Langmuir trough was constructed for the project, with temperature control from 15 to 40°C. In addition to the familiar force/area isotherms, which were used to compare all systems, measurements of surface potentials, surface shear viscosities, and dynamic suface tensions (for hysteresis only) were made on several systems with specially designed apparatus. Several microscopic techniques, epi-fluorescence optical microscopy, scanning tunneling microscopy, and electron microscopy, were applied to films of stearoylserine methyl ester, the most extensively investigated surfactant. 

In this experiment we will use various experimental techniques to attempt to identify the structures formed by a range of oil, water, surfactant and co-surfactant mixtures. The clues to this identification will come from ... 

The experimental techniques available to determine AWPCs and their limitations have been discussed by Staudinger and Roberts [2]. These authors also evaluated the effects of pH, compound hydration, compound concentration, cosolvent, cosolute, and salt effects, suspended solids, dissolved organic matter, and surfactants. The experimental data have been compiled by a number of different authors [2-11]. 

A number of experimental techniques by measurements of physical properties (interfacial tension, surface tension, osmotic pressure, conductivity, density change) applicable in aqueous systems suffer frequently from insufficient sensitivity at low CMC values in hydrocarbon solvents. Some surfactants in hydrocarbon solvents do not give an identifiable CMC the conventional properties of the hydrocarbon solvent solutions of surfactant compounds can be interpreted as a continuous aggregation from which the apparent aggregation number can be calculated. Other, quite successful, techniques (light scattering, solubilization, fluorescence indicator) were applied to a number of CMCs, e.g., alkylammonium salts, carboxylates, sulfonates and sodium bis(2-ethylhexyl)succinate (AOT) in hydrocarbon solvents, see Table 3.1 (Eicke, 1980 Kertes, 1977 Kertes and Gutman, 1976 Luisi and Straub, 1984 Preston, 1948). 

Much of the early studies of surfactant adsorption at the solid-solution interface were based on classical experimental techniques, such as solution depletion [1, 32], fluorescence spectroscopy [2], and measurements of the differential enthalpy of adsorption [2], Such methods have provided much of the basic initial understanding. However, they provide no direct structural information and are difficult to apply to mixtures [23, 34], However, when combined with other techniques, such as NMR and flow microcalorimetry, they provide some insight into the behaviour of mixtures. This was demonstrated by Thibaut et al. [33] on SDS/C10E5 mixtures adsorbed onto silica and by Colombie et al. [34] on the adsorption of SLS/Triton X-405 mixtures onto polystyrene particles. 

In conclusion it is worth noting that the method of equilibrium foam film proved to be very appropriate for the determination of the equilibrium diffuse electric layer potential at the solution/air interface. Though it is an indirect experimental technique, it provides reliable results about the appearance of a negative surface charge in the case of surfactant-free solutions as well as in the case of non-ionic surfactant solutions. The existence of an isoeletric point and the re-charging of the interface can be considered as a direct evidence. 

The experimental techniques used to obtain the properties necessary to derive the kinetic constants of interest from the ultrasonic relaxation times have been previously described in detail [2,3]. Briefly, the degree of micelle ionization (P) and the binding constant (Ka) of an alcohol to mixed micelles were obtained from specific conductivity measurements as a function of surfactant concentration at various fixed alcohol compositions. The binding constant was determined from the slopes of the curves above the cmc, as proposed by Abu-Hamdiyyah et al... 

We have developed a variety of experimental techniques to control the geometry, dimensionahty, topology, and functionality in surfactant membranes that can be directly useful in nanoscale network and device design [11-13]. Methods based on self-assembly, self-organization, and forced shape... 

After describing the experimental technique in the next section, we report our observations of intermediate phase formation and spontaneous emulsification in three parts corresponding to three types of equilibrium phase behavior found when equal volumes of oil and the surfactant-alcohol-brine mixtures are equilibrated. The three types are well known (8-9) and, in order of increasing salinity, are a "lower" phase, oil-in-water microemulsion in equilibrium with excess oil, a "surfactant" or "middle" phase, probably of varying structure, in equilibrium with both excess oil and excess brine, and an "upper" phase, water-inoil microemulsion in equilibrium with excess brine. 

In reviewing a vast quantity of literature on the subject, it is possible to encounter some studies which are of little value because insufficient attention has been given to the control of important physical factors, to the potentiality of the experimental technique, or to the limitations of particular theoretical approaches. Therefore, Section 2 is exclusively devoted to description of the materials and experimental methods used in measuring the adsorption of ionic surfactant onto mineral substrates. Some experimental problems, encountered in the everyday laboratory practice, will be pointed out. Among different experimental methods, the adsorption calorimetry particularly deserves to be noted. 

Experimental Techniques for Studying Polymeric Surfactant Adsorption... 

For a volatile solute, the vapor pressure can be measured. This can be done as a function of the solute concentration at constant surfactant concentration. The activity of the solute is P/P° where P° is the vapor pressure of the pure solute. Two sets of data are required, the activity (or vapor pressure) of the solute in water and the activity (or vapor pressure) of the solute in aqueous surfactant solution. The horizontal distance between these two curves is a direct measure of the solubilized solute. The experimental techniques used for this purpose are headspace chromatography as used by Hayase and Hayano and Spink and Colgan, ° or the final equilibrium pressure over a solution containing a known quantity of volatile liquid can be measured. The latter method has been developed by Tucker and Christian. This method has the added advantage of providing an easy... 

The experimental techniques vary in the sense that some will determine the partition coefficients directly and some will determine the fraction of solute solubilized in the micellar phase. The data are easily converted if the surfactant and solute concentrations are given. 

Evidently, the mapping of surfactant aggregation in nonaqueous polar solvents has grown to be very extensive, and investigations of many different combinations of surfactants and solvents are available in the literature. Furthermore, a wide range of experimental techniques have been used. The results from different studies are quite consistent and most of the authors agree on some basic trends. 

Applications of this nonlinear laser imaging technique holds the promise of directly studying the effect that capillary wave stresses exert upon the surfactant layer as well as those exerted by the surfactant on the water surface. The importance of capillary wave/surfactant dynamics at the ocean underscores the importance of these experiments and suggests that further efforts directed toward extending these experimental techniques to in situ ocean studies would be most useful. 

In this review we focus on polyelectrolyte-surfactant interactions at solid-liquid interfaces as studied with surface force measuring techniques. The last years have seen much progress in this area, and it is timely to recapitulate some main findings. It is, however, clear that in order to understand interfacial properties of polyelectrolyte-surfactant systems one needs to understand bulk association. Further, a multitude of experimental techniques needs to be applied. Recent advances have been made using ellipsometry [34,35], reflectometry [36,37], neutron reflectivity [38], and surface sensitive spectroscopic techniques [39,40], It is also our belief that the... 

The aim of this chapter is to present the fundamentals of adsorption at liquid interfaces and a selection of techniques, for their experimental investigation. The chapter will summarise the theoretical models that describe the dynamics of adsorption of surfactants, surfactant mixtures, polymers and polymer/surfactant mixtures. Besides analytical solutions, which are in part very complex and difficult to apply, approximate and asymptotic solutions are given and their range of application is demonstrated. For methods like the dynamic drop volume method, the maximum bubble pressure method, and harmonic or transient relaxation methods, specific initial and boundary conditions have to be considered in the theories. The chapter will end with the description of the background of several experimental technique and the discussion of data obtained with different methods. 

In this section a selection of experimental data is presented which demonstrate that different experimental technique partly overlap or complement each other in their range of application. Cases of partial overlap should not be interpreted as needless developments but more as additional resources to obtain information on dynamics of adsorption. On the one hand, experimental data of the same surfactant solution are performed to demonstrate the agreement between different experimental techniques. Many examples of experiments with different techniques are given in a recent paper by Miller et al. (1994b, d). 

More or less systematic studies have been carried out on nonionic surfactants below the CMC but there is lack of systematic studies on micellar and mixed surfactant solutions. Moreover, there is an almost complete lack of studies on ionic surfactants, as discussed in Chapter 7. It seems that comprehensive experiments on adsorption dynamics can be performed on the basis of the recent theories and considerably improved experimental technique in order to understand the formation and action of dynamic adsorption layers better. This of course applies unrestrictedly to proteins, and mixed surfactant/protein systems where the level of imderstanding is even lower than for surfactants solutions (de Feijter Benjamins 1987, Serrienetal. 1992). 

In this chapter specific theories and experimental set-ups for interfacial relaxation studies of soluble adsorption layers are presented. A general discussion of relaxation processes, in bulk and interfacial phases, was given in Chapter 3. After a short introduction, in which the important role of mechanical properties of adsorption layers and the exchange of matter for practical applications are discussed, the main differences between adsorption kinetics studies and relaxation investigations are explained. Then, general theories of exchange of matter and specific theories for different experimental techniques are presented. Finally, experimental setups, based on harmonic and transient interfacial area deformations, are described and results for surfactant and polymer adsorption layers discussed. 

The measurements of buoyant bubble velocity is not suitable for solving these problems and attention must be paid to other experimental techniques, li is clear that investigations of surfactant transfer in foams and of sedimentation potential measurements deserve more attention. 

V. EXPERIMENTAL TECHNIQUES USED IN THE STUDY OF PROTEIN-SURFACTANT INTERACTION... 

The range of experimental techniques that have been used in the study of protein-surfactant interactions is summarized in Table 1. The most important questions to be answered are (a) the extent of... 

The concentration at which micellization commences is called the critical micelle concentration, cmc. Any experimental technique sensitive to a solution property modified by micellization or sensitive to some probe (molecule or ion) properly modified by micellization is generally adequate to quantitatively estimate the onset of micellization. The determination of cmc is usually done by plotting the experimentally measured property or response as a function of the logarithm of the surfactant concentration. The intersection of asymptotes fitted to the experimental data or as a breakpoint in the experimental data denotes the cmc. A partial listing of experimental... 

Micellization is a second-order or continuous type phase transition. Therefore, one observes continuous changes over the course of micelle formation. Many experimental techniques are particularly well suited for examining properties of micelles and micellar solutions. Important micellar properties include micelle size and aggregation number, self-diffusion coefficient, molecular packing of surfactant in the micelle, extent of surfactant ionization and counterion binding affinity, micelle collision rates, and many others. 

The theory of IR (or FTIR) and Raman spectroscopy has been reviewed in several monographs (i-3) and various general references on Raman spectroscopy (3-6). The objective of this review is to survey the spectroscopic results obtained for various water-soluble polymers and to evaluate recent experimental techniques. In particular, this chapter will focus on the studies of selected water-soluble polymers and copolymers and their interactions with solvents and surfactants. 

There are a number of techniques available to measure the surface or interfacial tension of liquid systems, which together cover a wide range of time. In many cases, several methods are required in order to receive the complete surface tension time dependence of a surfactant system. One of the important points in this respect is that the data obtained from different experimental techniques have to be recalculated such that a common time scale results, i.e. one has to calculate the effective surface age from the experimental time, which is typically determined by the condition of the methods. For example, the maximum bubble pressure... 

It has been already indicated (Fig. 7) that micelles can lead to an essential acceleration of the adsorption process. Therefore, special experimental techniques are necessary for its investigation, allowing measurements of the dynamic surface tension in a time interval of milliseconds. The maximum bubble pressure method [78, 81, 83, 89,90,93] and the oscillating jet method [77, 82, 86, 87, 88, 90, 92, 93, 156] are most frequently used for these purposes. The inclined plate method [83, 89, 90, 93], the method of constant surface dilation [85] and the drop volume method [84] have been used also for slow adsorbing surfactants. 

There are many experimental techniques for the preparation of nanowires from the liquid phase. A considerable research effort has been expended in developing template-free methods for the deposition of one-dimensional nanostructures in a liquid environment the most important procedures are hydrothermal methods, electrospinning,sonochemical and surfactant assisted. 

Nuclear magnetic resonance relaxation is a useful experimental technique to study surfactant aggregation in liquid solutions and liquid crystals [2,50,51]. It yields information on the local dynamics and the conformational state of the surfactant hydrocarbon chain and has, for example, demonstrated the liquidlike interior of surfactant micelles. However, the aim of NMR relaxation studies of microemulsions is often to study properties such as the surfactant aggregate (droplet) size. 

To make the significance of the NMR technique as an experimental tool in surfactant science more apparent, it is important to compare the strengths and the weaknesses of the NMR relaxation technique in relation to other experimental techniques. In comparison with other experimental techniques to study, for example, microemulsion droplet size, the NMR relaxation technique has two major advantages, both of which are associated with the fact that it is reorientational motions that are measured. One is that the relaxation rate, i.e., R2, is sensitive to small variations in micellar size. For example, in the case of a sphere, the rotational correlation time is proportional to the cube of the radius. This can be compared with the translational self-diffusion coefficient, which varies linearly with the radius. The second, and perhaps the most important, advantage is the fact that the rotational diffusion of particles in solution is essentially independent of interparticle interactions (electrostatic and hydrodynamic). This is in contrast to most other techniques available to study surfactant systems or colloidal systems in general, such as viscosity, collective and self-diffusion, and scattered light intensity. A weakness of the NMR relaxation approach to aggregate size determinations, compared with form factor determinations, would be the difficulties in absolute calibration, since the transformation from information on dynamics to information on structure must be performed by means of a motional model. 

The experimental technique used to find an optimum formulation, known a.s untdimensional scan, goes on as follows. Series of surfactant-oil-water. systems are prepared in test tubes, all with identical composition, and with the same formulation with the exception of the scanned variable, that is in general the aqueous phase salinity for ionic systems, and the average number of ethylene oxide groups per molecule (EON) if the systems contain an ethoxylated nonionic surfactant mixture. 

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