Surfactants structural properties

As indicated above, an important characteristic of a surfactant in solution is its solubility relative to the critical concentration at which thermodynamic considerations result in the onset of molecular aggregation or micelle formation. Since micelle formation is of critical importance to many surfactant applications, the understanding of the phenomenon relative to surfactant structures constitutes an important element in the overall understanding of surfactant structure-property relationships. 

Whereas microscopic models for bulk systems incorporate the amphiphihc character and often the orientational properties of the surfactants as basic ingredients, models for bilayers and monolayers are constructed to reproduce internal transitions, such as the gel-fluid transition, and therefore concentrate on rather different aspects of the surfactant structure. 

The use of AOS and other surfactants as steam-foaming agents has been studied by several oil companies in laboratories and in the field [55-62]. In the next section we will view olefinsulfonate structure-property relations [40] that have helped design optimum surfactants for enhanced oil recovery applications. 

The effect of surfactant structures and properties on emulsion polymerization have been investigated by numerous authors [82-89]. Efforts were made to study the effects of surfactants with different molecular weights on the rate of polymerization [82], swelling and solubilization effects [83], effects of alkyl chain length of homologous series on the rate of polymerization, particle size... 

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. 

Domain Where Physics, Chemistry, Biology, and Technology Meet (see above) p. 11. The paper, Use of quantitative structure-property relationships in predicting the Krafft point of anionic surfactants by M. Jalali-Heravi and E. Konouz, Internet Electronic Journal of Molecular Design, 2002, 1, 410, has a nice introduction and useful references. It can be downloaded at http //www.biochempress.com/av01 0410.html. 

Apart from the structural properties of the molecule, surfactant sorption is affected by physico-chemical parameters, such as temperature, pH, organic carbon content of sediment, suspended solids concentration, ionic strength, etc. The most relevant of these are discussed next. 

Alkyl ether sulfates are/after alkyl benzene sulfonates(LAS),the group of technically important anionic surfactants with the largest production voluJne and product value. They have in comparison with other anionic surfactants special properties which are based on the particular structure of the molecule. These are expressed,for example,in the general adsorption properties at different interfaces, and in the Krafft-Point. Alkyl ether sulfates may be used under conditions, at which the utilization of other surfactant classes is very limited. They possess particularly favorable interfacial and application properties in mixtures with other surfactants. The paper gives a review of all important mechanisms of action and properties of interest for application. 

Molecular Structure Effects and Detergency. The correlation of surfactant structure with interfacial and colloid properties is a poorly understood science. Much study in this area has been thermodynamic which has been a useful endeavor but which nevertheless fails to provide specific molecular structure/physical property correlations. The following study has also been largely thermodynamic to this point however, since the data has been collected on pure LAS homologs, it provides an opportunity to apply some of the quasi-thermodynamic treatments that have been proffered in the literature to date. 

It is neither feasible nor appropriate in a book like this to give a detailed presentation of biological membranes, which compartmentalize living matter and perform numerous cell functions as well. However, because of the impetus to the study of surfactants that the membrane-mimetic properties of surfactant structures have provided, it would be a mistake to exclude some mention of membranes in this chapter. We have already noted in connection with Figure 7.7 that a monolayer may collapse into a bilayer that leaves the surfactant in a tail-to-tail configuration. This is exactly the arrangement of molecules in the lipid portion of a cell... 

In tile application of surfactants, physical and use properties, precisely specified, are of primary concern. Chemical homogeneity is of little significance in practice. In fact, surfactants are generally polydisperse mixtures, such as the natural fats as precursors of fatty acid-derived surfactant structures e.g., coconut oil contains glycerol esters of Cc-Qa fatly acids. Nonionic surfactants of die alcohol edioxylate type are polydisperse not only with respect to the hydrophobe but also in the number of edivlene oxide units attached. 

The interfacial tension is a key property for describing the formation of emulsions and microemulsions (Aveyard et al., 1990), including those in supercritical fluids (da Rocha et al., 1999), as shown in Figure 8.3, where the v-axis represents a variety of formulation variables. A minimum in y is observed at the phase inversion point where the system is balanced with respect to the partitioning of the surfactant between the phases. Here, a middle-phase emulsion is present in equilibrium with excess C02-rich (top) and aqueous-rich (bottom) phases. Upon changing any of the formulation variables away from this point—for example, the hydrophilie/C02-philic balance (HCB) in the surfactant structure—the surfactant will migrate toward one of the phases. This phase usually becomes the external phase, according to the Bancroft rule. For example, a surfactant with a low HCB, such as PFPE COO NH4+ (2500 g/mol), favors the upper C02 phase and forms w/c microemulsions with an excess water phase. Likewise, a shift in formulation variable to the left would drive the surfactant toward water to form a c/w emulsion. Studies of y versus HCB for block copolymers of propylene oxide, and ethylene oxide, and polydimethylsiloxane (PDMS) and ethylene oxide, have been used to understand microemulsion and emulsion formation, curvature, and stability (da Rocha et al., 1999). 

The industrially produced surfactants used in detergents are applied in formulations containing mixtures of homologs, isomers, and oligomers. As a result, surfactants occur in the environment as mixtures of chemicals which, within a given class, are closely related. Historically, many properties of surfactants have been determined for such mixtures, most frequently without yielding information for the individual constituents. Therefore, much of the existing data, particularly on environmental behavior, does not lend itself for derivation of structure / property relationships and, in consequence, our selection of data here may appear sparse. For some properties, the scarcity reflects the available data. For other properties, we selected data to illustrate how surfactant structure influences environmentally relevant properties. 

Huibers, P.D.T., V.S. Lobanow, A.R. Katrizky, D.O. Shah, and M. Karelson. 1996. Prediction of critical micelle concentration using a quantitative structure-property relationsliip approach. 1. Nonionic surfactants. Langmuir, 12, 1462-1470. 

Leung, R. Hou, M.J. Shah, D.O. Microemulsions Formation, Structure, Properties, and Novel Applications in Surfactants in Chemi-cal/Process Engineering Wasan, D.T. Ginn, M.E. Shah, D.O. (Eds.), Marcel Dekker New York, 1988, pp. 315-367. 

The size and shape of micelles have been a subject of several debates. It is now generally accepted that three main shapes of micelles are present, depending on the surfactant structure and the environment in which they are dissolved, e.g., electrolyte concentration and type, pH, and presence of nonelectrolytes. The most common shape of micelles is a sphere with the following properties (i) an association unit with a radius approximately equal to the length of the hydrocarbon chain (for ionic micelles) (ii) an aggregation number of 50-100 surfactant monomers (iii) bound counterions for ionic surfactants (iv) a narrow range of concentrations at which micellization occurs and (v) a liquid interior of the micelle core. 

A concise account of the structure, properties and uses of gemini surfactants is given by Rosen [60]. 

In this paper we investigate the process of alternate adsorption of cationic polyelectrolyte and anionic surfactant, structure and properties of adsorbed layers depending on different factors (molecular weight of PE, concentration of polyelectrolyte and surfactant, adsorbed layer formation time, the flow rate of the solution) by measuring potential and streaming current using the capillary electrokinetic method. 

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