Colloidal Structure and Physical Properties of Gel Networks Containing Anionic Surfactant and Fatty Alcohol Mixture

Surface freezing of cationic surfactant-adsorbed films at the oil-water interface: Impact on oil-in-water emulsion and pickering emulsion stability

When n-alkanes or n-alcohols coexist with surfactants that have similar chain lengths, they can form mixed surface-frozen films at the oil-water interface. In this review, we first explain the basic characteristics of this surface freezing transition mainly from a thermodynamic viewpoint. Then, we discussed the effect of surface freezing of a cationic surfactant (cetyltrimethylammonium chloride: CTAC) with tetradecane, hexadecane, or hexadecanol on the kinetic stability of the oil-in-water (O/W) emulsions. We show that the surface frozen film not only increases the kinetic stability of the O/W emulsions but also stably encapsulates coexisting organic molecules in the oil core. Finally, we will introduce one of our recent works in which we observed that the exchange between silica nanoparticles and CTAC molecules occurs at the surface of Pickering emulsions when the oil-water interfacial tension is lowered by the surface freezing. The resulting detachment of silica particles from the oil-water interface broke the Pickering emulsion. The advantages of controlling the stability of O/W emulsions via the use of surface-frozen film are discussed in comparison with normal surfactant emulsifiers in the conclusion part of the review.

Urea as hydrogelator of surfactants

HYPOTHESIS

The formation of adducts via urea interaction with distinct classes of surfactants (cationic, anionic, nonionic, and zwitterionic), leading to their assembly into lamellar structures and subsequent formation of hydrogels. The characteristics of these hydrogels are associated with both, the length of the alkyl chain, and the specific head group of the surfactant molecules.

EXPERIMENTS

Characterization of adduct formation was conducted using Wide-Angle X-ray Scattering (WAXS), while Small-Angle X-ray Scattering (SAXS) was employed to probe the subsequent assembly into lamellar structures. The kinetics of hydrogel formation were assessed through rheological measurements and observed thermal transitions utilizing Differential Scanning Calorimetry (DSC).

FINDINGS

The investigation revealed a universal propensity for hydrogel formation across all surfactant classes. The formation arises from the interactions between urea molecules via hydrogen bonding, forming adducts around the surfactant chains. In sequence, the adducts self-assemble in lamellae. This process constructs the intricate three-dimensional network characteristic of the hydrogel. Furthermore, the kinetics of hydrogel formation, and their rheological properties under equilibrated conditions, were found to be significantly influenced by the nature of the polar head group of the surfactant molecules. This is the first evidence on the formation of adducts of urea with classes of surfactants. As they are common components in cosmetic, supramolecular hydrogels have high potential to be used in formulations.

The Effect of Process Parameters on the Microstructure, Stability, and Sensorial Properties of an Emulsion Cream Formulation

A classical emulsion formulation based on petrolatum and mineral oil as the internal phase with emulsifier wax as a typical topical emulsion cream was investigated for the effect of process parameters on drug product quality and performance attributes. The Initial Design of Experiment (DoE) suggested that an oil phase above 15%, coupled with less than 10% emulsifying wax, resulted in less stable emulsions. Different processing parameters such as homogenization speed, duration, cooling rate, and final temperature showed minimal influence on properties and failed to improve stability. The final DoE suggested that the optimal emulsion stability was achieved by introducing a holding period midway through the cooling stage after solvent addition. Within the studied holding temperature range (25-35 °C), a higher holding temperature correlated with increased emulsion stability. However, the application of shear during the holding period, using a paddle mixer, adversely affected stability by disrupting the emulsion microstructure. IVRT studies revealed that the release of lidocaine was higher in the most stable emulsion produced at a holding temperature of 35 °C compared to the least stable emulsion produced at a holding temperature of 25 °C. This suggests that a holding temperature of 35 °C improves both the stability and active release performance. It appears that a slightly higher holding temperature, 35 °C, allows a more flexible and stable emulsifying agent film around the droplets facilitating stabilization of the emulsion. This study offers valuable insights into the relationship between process parameters at various stages of manufacture, microstructure, and various quality attributes of emulsion cream systems. The knowledge gained will facilitate improved design and optimization of robust manufacturing processes, ensuring the production of the formulations with the desired critical quality attributes.

Urea induces (unexpected) formation of lamellar gel-phase in low concentration of cationic surfactants

HYPOTHESIS

The unexpected formation of a lamellar structure with concomitant gelation in solutions containing high urea concentration (40 wt%) and relatively low amount of cationic surfactant (3 wt%), indicates that a hierarchically structured complex is formed by both molecules.

EXPERIMENTS

Gels formed by combination of aqueous solutions of urea and C₁₂TAB, C₁₄TAB or C₁₆TAB were prepared in different proportions and their structures at microscopic and mesoscopic levels were investigated using XRD and SAXS, respectively. The elastic and viscous moduli and yield stress of the samples were determined and correlated with the composition and structuration of the gels. The lamellar structure is reversibly thermically destroyed and this process was investigated using DSC.

FINDINGS

XRD revealed that, at microscopic scale, the gels are formed through crystallization of adducts containing surfactant molecules loaded into the cavities of honeycomb-like urea assemblies. Such crystalline phase arranges itself in lamellae with interplanar distance around ∼20-30 nm, which were observed by SAXS. This hierarchical structure is independent of the chain length of the cationic surfactants. The blocks of lamellae dispersed in the continuous phase form a three-dimensional rigid particulate network structure, giving the characteristic rheological behavior of a hydrogel. DSC revealed a reversible thermal transition at around 20-25 °C, beyond which the adducts and the lamellar phase are destroyed and micelles are formed. The characteristic transition temperature is independent of the chain length of the surfactant, and thus, it is not associated with their Krafft temperatures. The structures of the gels indicate that they resemble alpha-gels formed by fatty-alcohols and surfactants, although they self-assemble by different driving forces.

Effect of Adding Lecithin and Nonionic Surfactant on α-Gels Based on a Cationic Surfactant-Fatty Alcohol Mixture

α-Gels are often used as base materials for cosmetics and hair conditioners. α-Gel-based commercial products typically contain many types of additives, such as polymers, electrolytes, oily components, and other surfactants, in addition to the three basic components. However, few systematic studies have been conducted on the effect of such additives on α-gels. In this study, we chose surfactant as an example to initiate the effect of such additives on the structure and rheological properties of α-gel samples formulated using cetyl alcohol (C₁₆OH) and cetyltrimethylammonium chloride (CTAC). Optical microscopy analysis demonstrated that the size of the vesicles in the α-gel samples in this study was decreased via the addition of hydrogenated soybean lecithin (HSL) and penta(oxyethylene) cetyl ether (C₁₆EO₅), a nonionic surfactant, to them. Rheological measurements revealed that at high C₁₆OH/CTAC ratios, the viscosity and yield stress of the α-gel samples decreased owing to the addition of surfactants to them. Conversely, at low C₁₆OH/CTAC ratios, the opposite tendency was observed. Small-angle X-ray scattering analysis indicated that for the α-gel samples with high C₁₆OH/CTAC ratios, the addition of HSL or C₁₆EO₅ to them decreased the interlayer spacing of their lamellar bilayer stack, which led to the changes in the rheological properties of the α-gel samples.

The Influence of Co-Surfactants on Lamellar Liquid Crystal Structures Formed in Creams

It is well-established that oil-in-water creams can be stabilised through the formation of lamellar liquid crystal structures in the continuous phase, achieved by adding (emulsifier) mixtures comprising surfactant(s) combined (of necessity) with one or more co-surfactants. There is little molecular-level understanding, however, of how the microstructure of a cream is modulated by changes in co-surfactant and of the ramifications of such changes on cream properties. We investigate here the molecular architectures of oil-free, ternary formulations of water and emulsifiers comprising sodium dodecyl sulfate and one or both of the co-surfactants hexadecanol and octadecanol, using microscopy, small-angle and wide-angle X-ray scattering and small-angle neutron scattering. We then deploy these techniques to determine how the structures of the systems change when liquid paraffin oil is added to convert them to creams, and establish how the structure, rheology, and stability of the creams is modified by changing the co-surfactant. The ternary systems and their corresponding creams are shown to contain co-surfactant lamellae that are subtly different and exhibit different thermotropic behaviours. The lamellae within the creams and the layers surrounding their oil droplets are shown to vary with co-surfactant chain length. Those containing a single fatty alcohol co-surfactant are found to contain crystallites, and by comparison with the cream containing both alcohols suffer adverse changes in their rheology and stability.

Progressing Towards the Sustainable Development of Cream Formulations

This work aims at providing the assumptions to assist the sustainable development of cream formulations. Specifically, it envisions to rationalize and predict the effect of formulation and process variability on a 1% hydrocortisone cream quality profile, interplaying microstructure properties with product performance and stability. This tripartite analysis was supported by a Quality by Design approach, considering a three-factor, three-level Box-Behnken design. Critical material attributes and process parameters were identified from a failure mode, effects, and criticality analysis. The impact of glycerol monostearate amount, isopropyl myristate amount, and homogenization rate on relevant quality attributes was estimated crosswise. The significant variability in product droplet size, viscosity, thixotropic behavior, and viscoelastic properties demonstrated a noteworthy influence on hydrocortisone release profile (112 ± 2–196 ± 7 μg/cm²/√h) and permeation behavior (0.16 ± 0.03–0.97 ± 0.08 μg/cm²/h), and on the assay, instability index and creaming rate, with values ranging from 81.9 to 120.5%, 0.031 ± 0.012 to 0.28 ± 0.13 and from 0.009 ± 0.000 to 0.38 ± 0.07 μm/s, respectively. The release patterns were not straightforwardly correlated with the permeation behavior. Monitoring the microstructural parameters, through the balanced adjustment of formulation and process variables, is herein highlighted as the key enabler to predict cream performance and stability. Finally, based on quality targets and response constraints, optimal working conditions were successfully attained through the establishment of a design space.