Winsor R ratio

Three-phase behavior takes place exactly when the surfactant exhibits an equal affinity for both phases, which is indicated by a unit value of Winsor R ratio. This situation has been called optimum formulation because it is associated with an ultra low minimum of the interfacial tension, which en-... 

The method developed originally for microemulsion formulation (Section II above) has been adapted (Salager, 1983, 2000) to macroemulsion formation. In this method, the value of the left-hand side of equation 8.10 or 8.11 is called the hydrophilic-lipophilic deviation (HLD). When the value equals zero, as in Section II, a microemulsion is formed when the value is positive, a W/O macroemulsion is preferentially formed when it is negative, an O/W macroemulsion is preferentially formed. The HLD is similar in nature to the Winsor R ratio (equation 5.2) in that when the HLD is larger than, smaller than, or equal to 0, R is larger than, smaller than, or equal to 1. The value of the HLD method is that, on a qualitative basis, it takes into consideration the other components of the system (salinity, cosurfactant, alkane chain length, temperature, and hydrophilic and hydrophobic groups of the surfactant). On the other hand, on a quantitative basis, it requires the experimental evaluation of a number of empirical constants. 

III. PHASE BEHAVIOR AND FORMULATION CONCEPTS A. Winsor R Ratio Approach... 

Table 1 Effect of Formulation VariaMes on Winsor R Ratio...

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

The Winsor R ratio is an excellent pedagogical concept, but it lacks, in practice, the quantification of interaction energies [55] that is desirable for carrying out comparisons and developing new formulations. 

The HLD is equivalent to the Winsor R ratio in the sense that it gathers all formulation effects in a single generalized formulation variahle. When the HLD is equal to, larger, or smaller than zero, R is equal to, larger, or smaller than unity. However, and unlike R, the HLD can he calculated numerically when it is different from zero. 

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]. 

Because the lipophilic tails are oriented in the oil phase, Ahco in Eq. 7.3 may be neglected in many cases. Similarly, because the hydrophilic heads are oriented to the water phase, Alcw in Eq. 7.4 may be neglected. Then the surfactant affinity to oil or water phase may be described, as proposed initially by Winsor (1948), by the R-ratio ... 

The proposed explanation is based upon the Winsor R concept (Chapter 5, Section III), with the interaction between the surfactants producing a pseudononionic complex that decreases the Acw value in the denominator of the R ratio, with consequent decrease in the solubilization power (Bourrel, 1984). 

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... 

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. 

Figure 5 Phase behavior, R ratio, and types of structures according to Winsor notation. (From Ref. 11.)...

It was postulated by Winsor that the ratio of Aco to Acw would provide a way to determine the convexity of the C layer, that is, the phase behavior situation. The original R ratio was written as Aco/Acw, and it was suggested later that a better ratio would be... 

The phase behavior observed in surfactant systems can be viewed from several theoretical perspectives. One is the qualitative Winsor R theory. In its simplest form, the Winsor R parameter is the ratio of the forces acting on the oil side of the surfactant interface to those on the water side. This definition has been extended to include other interactions that tend to oppose surfactant aggregation [34]. When R>, the forces on the oil side of the interface are the strongest, so the interface curves about water, resulting in a 2 system. When i < 1, the water-side forces are dominant, which causes the formation of a 2 system. When R = U the forces at the interface are balanced, which leads to a bicontinuous microemulsion... 

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... 

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. 

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. 

A concept equivalent to spontaneous curvature is used in the petroleum industry, i.e. the / -ratio quantifying the hydrophilic-lipophilic balance (HLB), as introduced by Winsor (9). The R-ratio is arbitrarily set to 1 when the spontaneous curvature is zero. Then, by using differential values, the R-ratio for a given water-oil-surfactant combination is given by the following ... 

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