Winsor’s R ratio

Since the best solubilisation is attained for the Winsor type III case, the formulator always tries to keep the HLD generalised formulation as close to zero as possible. Nevertheless, the HLD expression contains many variables and hence several degrees of freedom are available to satisfy other constraints or other desirable features. The increase in solubilisation which has been discussed in the previous section is important, but not necessarily the most important in practice. On the other hand, it is known that changes maybe brought to the system without altering the solubilisation, as for instance two concomitant modifications in the numerator or in the denominator of Winsor s R ratio. This is particularly the case when surfactant mixtures are used. 

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... 

It can he said that Winsor s H ratio has the best pedagogical value of ail formulation concepts because it is simple to understand and at the same time allows quite good qualitative predictions. If it were amenable to numerical calcu-laiii>iis there would be no need for alternate formulation concepts. This is mtt tfk case, however, hccniise the molecular interact ions Ajo cannot bt calciilaied with siifhcieiii accuracy yet. The R ratio would thus be the choice insirumcm to cany out qualitative inferences and to predicts trends. However, a quantitative... 

In a tentative approach to attain a formulation concept with both the theoretical content of Winsor s R and the down-the-bench numerical data feature of the Hl-B. Beerbower and collaborators introduced the cohesive energy ratio (CER) approach in 1971 (53.. i4). From the conceptual point of view it was very similar to Winsor interaction energies ratio, but this time it was the ratio between the adhesion energy of the surfactant "layer" with the oil phase and the adhesion energy of the surfactant "layer" with the water phase, It must be recalled that the cohesion energy between molecules of a pure component system is calculated as ... 

The quantification of the formulation of SOW systems was studied in detail during the 1970 s when a considerable research drive was dedicated to enhanced oil recovery by surfactant flooding methods [2], The basic concepts came from Winsor s work on the phase behavior of SOW systems and its R ratio of interactions between the surfactant molecules adsorbed at interface and oil and water [3], which has been presented thoroughly in a review book [4]. 

The phase behaviour at equilibrium turned out to be the main property reported in Win-sor s work in the late 1940s. Winsor interpreted the phase behaviour through the so-called R ratio of molecular interaction energies at interface. The R ratio was a handy theoretical concept to understand the variations of the phase behaviour of surfactant-oil-water systems and somehow of the emulsion properties. It is essentially qualitative, but for the first time the phase behaviour was linked with a condition that depended on all formulation variables, but could be expressed as a single generalised variable, i.e. the R ratio [1]. The original R ratio was... 

A major drawback to this method, however, is that Winsor s so-called R ratio could not be numerically calculated as in the HLB method, which made it difficult to use for practical emulsion formulations. 

It was postulated by Winsor that the ratio of /tco (n would provide a rationale to determine the curvature of the C layer, which i.s directly linked with the phase behavior situation. The original R ratio was written as A(x>/i4cw. and it was suggested later that a better R ratio would be... 

Knigliakov (95-97) proposed a concept called hydrophile-lipophile ratio (HLR). which i.s the ratio of the energy of adsorption of the surfactant molecule from the water phase to its energy of adsorption from the oil phase. The HLR is a good alternative, but it suffers from (he same drawback that Winsor R ratio... 

R = Ratio of dispersing tendencies on lipophilic and hydrophilic faces of C respectively. (The S-band ises m an as far as possible random arrangement of the hydrocarbon chains in the C and O sections all types of micelle.) From Winsor [174] with permission. 

Each term can be viewed as an energetic contribution to the overall interaction balance, which is expressed as an algebraic sum instead of a ratio as in Winsor R. The positive contributions (including the alcohol functions for lipophilic alcohol) tend to produce the transition in one way (from <1 to S >1) while the negative term does it the other way. 

To stabilise an emulsion, the surfactant must be present at a concentration above the CMC hence, we shall be mainly concerned with systems of this sort. The phase in which the surfactant forms micelles is dependent on the surfactant s affinity for oil and water. The surfactant s affinity is controlled by a number of factors. Winsof first addressed the problem of describing surfactant affinity. He introduced the concept of interaction energies between surfactant molecules adsorbed at the interface and the oil and water phases. Salager identified different types of interactions. The ratio of the total interaction energies (per unit area of interface) of the surfactant for the oil and water phases is known as Winsor r (symbols are defined below) ... 

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