Surfactant affinity difference

Rgure 1.6. (a) Schematic formulation-composition map. SAD is the surfactant affinity difference it is positive for a lipophilic surfactant and negative for a hydrophilic one. The gray zones are abnormal, (b) Schematic representation of the proposed mechanism for a transitional inversion, (c) Schematic representation of the proposed mechanism for a catastrophic phase inversion. 

The property of interest to characterize a surfactant or a mixture of surfactants is its hydrophilic-lipophilic tendency, which has been expressed in many different ways through a variety of concepts such as the hydrophiUc-lipophilic balance (HLB), the phase inversion temperature (PIT), the cohesive energy ratio (CER), the surfactant affinity difference (SAD) or the hydrophilic-lipophilic deviation (HLD) [1], which were found to be more or less satisfactory depending on the case. In the next section, the quantification of the effects of the different compounds involved in the formulation of surfactant-oil-water systems will be discussed in details to extract the concept of characteristic parameter of the surfactant, as a way to quantify its hydrophilic-lipophilic property independently of the nature of the physicochemical environment. 

Later these correlations were reported to be identical to the expression of the surfactant affinity difference (SAD) [9], i.e., the variation of Gibbs free energy when a surfactant molecule passes from oil to water ... 

A closer relationship between foam stability and HLB has been reported for two- or three-phase systems surfactant solution-oil or oil-surfactant phase-water [60,109-111]. The effect of various parameters changing HLB on the stability of foams and emulsions has been studied in [111]. These were the concentration of amyl alcohol and sodium chloride, the number of the ethylene oxide groups in the molecule of the oxyethylated octylphenol. As a general parameter of HLB the authors used the surfactant affinity difference concept (SAD) which is an empirical generalised formulation. It measures the deviation from the optimum formulation for three phase behaviour. For anionic surfactants... 

A decade after the empirical determination of the correlations for three-phase behaviour and the corroboration that the linearity and generality could not be coincidental, a simple interpretation was found through the so-called surfactant affinity difference (SAD) concept discussed next. When a simple ternary surfactant-oil-water system exhibits three-phase behaviour, the chemical potential p. of the surfactant is equal in the three phases (oil, water and microemulsion) at equilibrium referred to by subscripts O, W and M. It holds... 

By definition, HLD = 0 at optimum formulation, and away from it, it is the expression of the relative surfactant affinity difference from it. Since the partition coefficient may be measured with a proper analytical technique, its variation with respect to formulation variables such as surfactant EON, oil ACN, water salinity S etc. may be determined. Such studies have been carried extensively with polyethoxylated surfactants and have shown that HLD dependence on the formulation variables has the same expression than the correlation for three-phase behaviour [35, 36]. It holds... 

It was a century ago that researchers started to study the factors affecting the behaviour of water-oil-surfactant systems but it is only with the introduction of Winsor s R theory (1954) that the formulation effects could be interpreted. Winsor s R theory was the first qualitative description of the formulation, paving the way to an understanding of how intermolecular interactions among the different chemical species present in a system are related to its behaviour. Throughout the following decades, several empirical experimental correlations such as the phase inversion temperature (PIT), semiempirical ones such as the cohesive energy ratio (CER), and models based on thermodynamics such as the surfactant affinity difference (SAD) or the hydrophilic-lipophilic deviation (HLD) [15, 143, 144] led... 

The surfactant affinity difference (SAD) is used to model emulsion phase behaviour based on the chemical potentials of surfactant in aqueous and oil phases. SAD is the negative of the free energy of transfer of a surfactant molecule from an oil to a water phase (see the papers by Salager et al. [132, 142, 143]). 

This kind of correlation was first attained from empirical experimentation, and a few years later it was recognized to be an expression of the surfactant affinity difference SAD (affinity is the negative of the chemical potential), i.e., the free energy of transfer of a surfactant molecule from the excess water to the excess oil in a three-phase system, which is related to the partitioning coefficient [32-34]. 

E. Surfactant Affinity Difference Generalized Formulation Concept... 

At the light of the current formulation state of the art. it can be said that the PIT was a clever and precursory technique. Today the PIT can be viewed as the optimum temperature in the multivariani surfactant affinity difference (SAD) concept to he discussed later on. 

Enhanced oil recovery research in the 1970s led to the development of empirical correlations that numerically describe the conditions for attaining ultra-low interfacial tension and maximum oil mobilization. The correlation, the surfactant affinity difference (SAD), is a measure of the difference between the standard chemical potentials or the Gibbs free energy of surfactant in the oil and water phase, as follows ... 

If HLB < 7, the surfactant is lipophilic and is likely to produce systems related to a R> 1 situation, in particular W/0 emulsions, and conversely if HLB > 13. Between 7 and 13, the actual effect might depend on the values of the other formulation variables and the temperature, so that the formulator should be very cautious. In this range, which is quite wide, the experimentalist is advised to use a more precise characterization method that includes other variables, such as the surfactant affinity difference (SAD) scale that... 

Empirical relationship are useful in practice, but their utilization could be made safer if they were related to some theoretical framework. The correlations are the numerical translation of the R = 1 concept in Winsor terminology. They are linear forms of all the formulation variables with no crossed term, so that the different variable effects are independent, which may seem to be too much of a coincidence. A few years after the correlations were empirically found, it was shown that they were actually a numerical expression for the surfactant affinity difference (SAD)-the difference between the negative of the standard chemical potential of the surfactant in the oil phase and the corresponding term for the water phase—in a three-phase optimum system [27,42,129] ... 

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