1.1 The Central Role of Formulation in Surfactant Science
The field of surfactant science is intrinsically linked to the concept of "formulation." Formulation encompasses the accumulated knowledge and practical experience involved in the meticulous selection, mixing, association, and conditioning of various ingredients to create a well-defined product capable of meeting a pre-established set of requirements.1 This definition indicates that formulation transcends simple mixing; it is a scientific discipline rooted in understanding the complex physicochemical interactions between components within a system.1
"Formulation... [is] the set of knowledge and know-how concerning the selection, mixing, association and conditioning of various matters, in order to attain a well defined product able to meet a pre-established set of requirements." - J. L. Salager 1
The historical trajectory of formulation evolves from empirical art, evident in ancient practices like soap making and metallurgy, through alchemical explorations, to its establishment as a science grounded in chemistry and physics.1 Modern formulation, particularly in surfactants applications, relies heavily on understanding concepts such as the Hydrophilic-Lipophilic Deviation (HLD), which provides a quantitative framework for prediction and control. The significance of formulation is vast, leveraging countless products and processes across diverse industries, including petroleum production (especially Enhanced Oil Recovery - EOR), food science, pharmaceuticals, cosmetics, detergency, paints, fuels, wastewater treatment, and many others encountered in daily life.1
Within this context, the HLD concept is not merely another theory but a the foundational principle of modern, rational surfactant formulation science. Salager's work aimed to elevate formulation from a practice reliant on extensive trial-and-error and empirical rules to a predictive engineering discipline. This elevation is achieved because HLD provides a quantitative, multivariable framework that integrates the effects of surfactant structure, oil type, aqueous phase salinity, temperature, pressure, and cosurfactants into a single, coherent model.1, 2 By explicitly contrasting HLD and its normalized successor, HLDN, with older, less comprehensive approaches such as the Hydrophilic-Lipophilic Balance (HLB) number2, it has been highlighted HLD's superior ability to predict phase behavior, interfacial tension, and emulsion properties across a wide range of conditions. This predictive power transforms formulation into a more systematic and efficient process, enabling the targeted design of surfactant systems for specific, demanding applications.1
1.2 Beyond HLB: Winsor's R Ratio and the Need for a Quantitative Framework
Before the advent of the HLD concept, attempts to rationalize surfactant behavior often relied on the Hydrophilic-Lipophilic Balance (HLB) system, introduced by Griffin.2 However, the HLB system suffers from significant limitations. It primarily considers the surfactant's structure (originally based on the weight percentage of the hydrophilic ethylene oxide portion) and largely ignores the crucial influence of other formulation variables such as the nature of the oil phase, the salinity of the aqueous phase, and temperature.2 Consequently, HLB values provide only a rough guideline and often fail to accurately predict phase behavior or optimal performance, especially in complex systems or under varying conditions.2
A significant conceptual advance was made by Winsor, who introduced the idea of interaction energies between the surfactant adsorbed at the oil-water interface and the bulk oil (O) and water (W) phases.4 Winsor defined the R ratio as the ratio of the surfactant's interaction energy with the oil phase (ACO) to its interaction energy with the water phase (ACW), R = ACO / ACW.4 These interaction energies were further broken down into contributing terms representing favorable interactions (like the lipophilic tail in oil, ALCO, or the hydrophilic head in water, AHCW) and unfavorable interactions (e.g., the hydrophilic head in oil, AHCO, or lipophilic tail in water, ALCW), as well as cohesive interactions within the phases and lateral interactions within the surfactant layer.4
Winsor's R Ratio: R = ACO / ACW
Balanced State: R ≈ 1
Winsor's critical contribution was the link established between the balance of these interactions (specifically, R ≈ 1) and unique physicochemical phenomena: the formation of a three-phase system (Winsor Type III) containing a surfactant-rich middle phase, and the occurrence of very low interfacial tension (IFT) between the oil and water phases.1 This established the fundamental principle that a specific balance of affinities leads to desirable properties for applications like EOR.
However, Winsor's R ratio remained a qualitative concept. Calculating the individual interaction energies (ACO, ACW) from first principles was impractical for complex systems, preventing the R ratio from being used as a quantitative predictive tool.4 This gap – the need for a practical, quantitative, multivariable framework that could predict the conditions for achieving the balanced state (R=1 or equivalent) and the resulting phase behavior across different formulations – was precisely what the HLD concept was developed to address. Winsor provided the crucial conceptual understanding of why balanced interactions were important, but HLD provided the practical means to calculate how to achieve that balance by manipulating experimentally accessible variables.1
1.3 The HLD: Quantifying Surfactant Affinity
Building upon Winsor's foundation, the Hydrophilic-Lipophilic Deviation (HLD) concept was developed.1 HLD provides a numerical measure of how far a given surfactant-oil-water (SOW) system deviates from the ideal state of "optimum formulation," where the surfactant's interactions with the oil and water phases are perfectly balanced.6
Conceptually, HLD is related to the Surfactant Affinity Difference (SAD), which represents the difference in the chemical potential or free energy of the surfactant between the oil and water environments. The relationship is often expressed as:
HLD = SAD / RT
where R is the ideal gas constant and T is the absolute temperature, rendering HLD a dimensionless quantity.4
A key feature of the HLD framework is its integration of multiple formulation variables into a single equation. Unlike HLB, which focuses solely on the surfactant, HLD explicitly accounts for the contributions of the surfactant's head and tail structure, the nature of the oil phase (characterized by ACN or EACN), the salinity of the aqueous phase (S), temperature (T), pressure (P), and the presence of cosurfactants or alcohols (f(A)).1, 2 This multivariable approach allows for a much more comprehensive and accurate description of real-world SOW systems.
The conceptual shift from Winsor's ratio (R) to HLD or difference (SAD) is significant. While Winsor focused on the condition R=14, the HLD framework quantifies the degree of imbalance. By assigning numerical HLD values to non-optimal states (HLD < 0 or HLD > 0), the framework allows for the prediction of specific phase behaviors (Winsor Types I and II) associated with these imbalances.4 This predictive capability across the entire formulation spectrum, not just at the optimum point, makes HLD a far more practical tool for designing and controlling SOW systems for various applications that may require specific emulsion types or stability characteristics, rather than just the properties found at the exact optimum.1, 2
1.4 Optimum Formulation (HLD=0): The Core Concept
The cornerstone of the HLD framework is the concept of "optimum formulation." This is rigorously defined as the specific set of conditions (surfactant type, oil type, salinity, temperature, etc.) under which the Hydrophilic-Lipophilic Deviation is exactly zero (HLD = 0), or equivalently, the Surfactant Affinity Difference is zero (SAD = 0).1 This condition signifies a perfect balance: the surfactant molecules adsorbed at the oil-water interface exhibit precisely equal affinity or interaction strength for both the oil and water phases in their immediate vicinity.1
Optimum Formulation: HLD = 0 (Balanced Affinity)
This state of balanced interactions (HLD=0) is associated with a unique and characteristic set of observable phenomena 1:
- Minimum Interfacial Tension (IFT): The tension between the bulk oil and water phases reaches a distinct minimum, often achieving ultralow values (e.g., < 0.001 mN/m).
- Winsor III Phase Behavior: The system typically separates into three coexisting phases: an excess oil phase, an excess water phase, and a surfactant-rich middle phase (often a bicontinuous microemulsion) that contains significant amounts of both solubilized oil and water.
- Maximum Solubilization: The middle phase microemulsion exhibits its maximum capacity to solubilize both oil and water at the optimum formulation.
- Minimum Emulsion Stability: Macroemulsions (O/W or W/O) prepared at or very near the optimum formulation are characteristically unstable and tend to break or phase-separate rapidly.
- Phase Inversion Point: For emulsified systems, HLD=0 often coincides with the phase inversion point, where the emulsion type transitions between oil-in-water (O/W) and water-in-oil (W/O).
The term "optimum" arises because this specific set of properties is highly desirable for certain applications. For Enhanced Oil Recovery (EOR), the ultralow IFT achieved at HLD=0 is crucial for mobilizing trapped oil globules by overcoming capillary forces.1 Conversely, for processes like crude oil demulsification or dehydration, the minimum emulsion stability at HLD=0 is exploited to efficiently break unwanted stable emulsions.1
Therefore, HLD=0 serves as the central organizing principle within the HLD framework. It is not merely one point among many but the fundamental reference state around which the entire phase behavior and property map of a SOW system is structured. Understanding the combination of variables that leads to HLD=0 provides the key to predicting and controlling the system's behavior across the full spectrum of possible formulations. The HLD equation itself is fundamentally designed to identify these HLD=0 conditions11, and transitions between different phase behaviors (e.g., Winsor I to III to II) are understood as occurring precisely when the system's HLD value passes through zero.4 Properties like IFT and stability display their extreme values at this critical balance point.10
1.5 Relating HLD to Winsor Phase Behavior (Types I, II, III, IV)
A crucial aspect of the HLD framework is its ability to directly predict the equilibrium phase behavior of a surfactant-oil-water system, typically classified according to the Winsor nomenclature (Types I, II, III, and IV). Multiple works consistently demonstrate a clear correlation between the calculated HLD value (or SAD) and the observed Winsor type1:
- HLD < 0 (Negative Deviation): Surfactant interactions with water dominate (more hydrophilic). Leads to Winsor Type I (WI) behavior: O/W microemulsion + excess oil (2 phases, denoted 2).4
- HLD > 0 (Positive Deviation): Surfactant interactions with oil dominate (more lipophilic). Leads to Winsor Type II (WII) behavior: W/O microemulsion + excess water (2 phases, denoted 2).4
- HLD ≈ 0 (Near Optimum): Balanced interactions. Leads to Winsor Type III (WIII) behavior: Middle phase microemulsion + excess oil + excess water (3 phases, denoted 3).4
- HLD ≈ 0 (Higher Surfactant Concentration): May form Winsor Type IV (WIV): Single-phase microemulsion (1 phase, denoted 1ϕ).16
These transitions (WI ↔ WIII ↔ WII or WI ↔ WIV ↔ WII) can be systematically observed by performing a formulation scan, where a single variable (like salinity, temperature, or ACN) is changed progressively, effectively sweeping the HLD value from negative to positive or vice versa. Diagrams illustrating these transitions, often depicted as test tubes showing the phase volumes or as regions on phase diagrams, are central to Salager's pedagogical approach1 Fig 5A, 30 Fig 2, 10 Fig 2, 33 Fig 8, 16 Fig 1. It is worth noting Salager's caution regarding the term "microemulsion," which has sometimes been confusingly applied to different phases within these systems23.
The power of the HLD framework lies in its ability to provide a unified explanation for phase transitions induced by any formulation variable. Whether one changes salinity, temperature, oil ACN, surfactant tail length, or head group hydrophilicity, the effect is mediated through a change in the overall HLD value of the system. This predictably moves the system along the Winsor sequence (I → III → II or the reverse). The HLD equation4 incorporates terms for all these variables, and experimental scans confirm that manipulating any one of them can drive the transitions.1 This demonstrates that HLD acts as the underlying parameter governing the phase behavior, offering a single, powerful explanatory and predictive tool for diverse SOW systems.
Interactive HLD Explorer
Adjust the parameters below to see how they influence the HLD value and the resulting Winsor phase behavior. (Note: Uses a simplified illustrative HLD equation (anionic surfactant) for demonstration).
Balanced system with middle phase