Hydrophilic-lipophilic deviation HLD

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. 

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. 

In ionic surfactant systems, the partition coefficient between excess phases is often found to be unity, hence an optimum formulation is defined by SAD = 0. With polyethoxylated surfactants, the CMC in water is often extremely low, whereas the monomer solubility in many oils is high. Consequently, the assumption of unit activity coefficient is not valid anymore and the partition coefficient between excess phases is not unity. In such cases, the partition coefficient value at optimum formulation is taken as a reference, and the deviation from this reference, the so-called hydrophilic-lipophilic deviation (HLD) is defined by dividing by RT to make the yardstick dimensionless [34]. 

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... 

In summary, the HLB method is only an empirical approach which has made it possible to organize a great deal of rather messy information on emulsion preparation, in order to choose candidate surfactants in trial and error laboratory work for suitable emulsion selection. There are other surfactant selection methods such as the phase inversion temperature (PIT) and the hydrophilic-lipophilic deviation (HLD) methods used for the same purpose in the emulsion industry, but these are outside the scope of this book. 

The hydrophilic-lipophilic deviation (HLD) is a dimensionless representation of SAD, given by HLD = SAD/RT. Either SAD or HLD values can be used to determine composition regions for which macroemulsions or microemulsions are likely to be stable, break or invert. Negative SAD or HLD values refer to Winsor type 1 systems (O/W), positive SAD or HLD values refer to Winsor type II systems (W/O) and SAD = HLD = 0 refers to Winsor type III systems (most of the surfactant is in a middle phase with oil and water). Much of the use of SAD and HLD has been in developing surfactant formulations. 

More recently, these empirical equations were justified from a physicochemical point of view as representative of the surfactant affinity difference (SAD), i.e., the free energy of transfer of a surfactant molecule from the oil phase to the water phase [22], This free energy can be estimated from the measurement of the surfactant partitioning coefficient and from the way it changes with the different formulation variables [23,24], For simplicity, the relationship has been written as the hydrophilic-lipophilic deviation (HLD), which is the same concept as SAD but is related to a reference state [25], In the simple case of an ethoxylated nonionic surfactant, an n-alcohol cosurfactant, an n-alkane oil, and a sodium chloride brine, the HLD can be written as... 

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