Emollients contribute to the moisturizing, lubricating, protecting, conditioning, and softening performance of cosmetic formulations. Though the chemical structures of emollients are well-defined, the relationship between these structures and such performance features can be strengthened through quantifying fundamental properties. In this work, properties including contact angle, interfacial tension, coefficient of friction, and permittivity are measured. These properties are related to spreadability, ease of emulsification, skin feel, and polarity, respectively. Since there are numerous emollients available, the goal in developing such relationships is to facilitate the selection of emollients based on the specific application and desired performance.
Summary
The purpose of this study is to measure various fundamental performance related properties of emollients. The focus is primarily on esters, but other frequently used cosmetic emollients (e.g. silicones, hydrocarbons, synthetic and natural oils) are included as reference materials. The properties measured are interfacial tension, permittivity, lubricity, and contact angle. These properties relate to ease of emulsification, polarity as a solvent, skin feel, and spreading, respectively.
A number of structure-property relationships are made using the measured values. Several functional groups and structural elements are identified that are important to emollient properties. The availability of such relationships should facilitate the formulator in choosing raw materials.
Materials and methods
Permittivity is a measure of polarity; emollients with high permittivity are more polar, with greater affinity for water. Permittivity measurements are particularly valuable in the formulation of multi-component color cosmetics. By matching the polarity of oils, waxes, solvents, and pigments, the formulator can insure that these components are stabilized properly. In this work, dielectric analysis was performed on a Novocontrol GmbH bench top impedance/dielectric analyzer at 25ºC using gold plated parallel plate electrodes. Though permittivity data was collected over a range of frequencies, permittivity at 1MHz was reported.
The contact angle (CA) is the angle formed by a liquid droplet on a solid surface at the air-solid-liquid contact point. It is a measure of how well a liquid wets or spreads on a solid. A CA of 0º indicates complete wetting, while a 180º CA indicates complete non-wetting. Using a DataPhysics OCA20, a droplet of emollient was placed on Vitro-skin® (IMS, Milford, CT). Images of the spreading droplet were captured with a high-speed digital camera and analyzed by computer to determine CA. Because viscosity is a key factor in spreading, it was assessed using ASTM method D445 to measure kinematic viscosity, and an Anton Paar DMA48 densitometer to measure density. Emollients with little spreading are well-suited for applications that require targeted delivery or cohesive film formation, such as sunscreens, lip treatments and eye area products.
Interfacial tension (IFT) indicates how easily emulsions are formed and is key to formulating sunscreens, water-proof cosmetics, moisturizers, and creams. Using the DataPhysics OCA20, a water droplet was suspended in the emollient. Its shape is determined by two forces: gravity, which elongates the drop, and IFT, which pulls the droplet into a sphere. Knowing the density of the two phases, IFT can be calculated by measuring the curvature of the droplet. In this case, image analysis software was used to carry out the curvature measurement [1].
Lubricity, which affects skin feel, was quantified by measuring the coefficient of friction between two surfaces with the emollient of interest between them. One surface was 3Mtm Transporetm Tape mounted on steel, while the other was a Teflon® blade. The Teflon blade was reciprocated across the surface at room temperature with a stroke length of 13.5mm at 1Hz, with an applied load of 50N.
The following materials were studied: petrolatum (Penreco, Karns City, PA), castor oil (Caschem Inc., Bayonne, NJ), hydrogenated polyisobutene (Lipo Chemicals, Patterson, NJ), cyclomethicone, dimethicone, dimethicone/dimethiconol (Dow Corning, Midland, MI), capric/caprylic triglyceride (Protameen Chemicals, Totowa, NJ), isoparaffin (ExxonMobil Chemical, Baytown, TX), isohexadecane (Presperse, Somerset, NJ), dimethicone/polysilicone-11 gel (Grant Industries, Elmwood Park, NJ), mineral oil (Crompton, Tarrytown, NY), isododecane (A&E Connock, Fordingbridge, Hampshire, UK), and various ester emollients (The Lubrizol Corporation, Clifton, NJ).
Results
Dielectric permittivity (Figure 1) depended heavily on hydrocarbon chain branching and chain length, and the presence of aromatic and oxygen-containing groups. Because oxygen atoms increase polarity, esters have higher dielectric permittivity than hydrocarbons. Castor oil, a triglyceride that contains hydroxyl groups, also has a higher permittivity than hydrocarbons.
The contact angle presented in Figure 2 is the CA of the emollients on Vitro-skin one second after initial contact. Materials which spread quickly have a low CA.
Because the emollients spread out after being deposited on the Vitro-skin, viscosity was a major factor affecting contact angle. Figure 3 shows the correlation of the contact angle of the materials on vitro-skin at 1 second post contact and viscosity at 25ºC. The solid line is a logarithmic fit of the data.
Materials which have a lower contact angle than would be predicted by viscosity alone fall below the solid line on the plot above. Such enhanced spreading can be explained by two factors: the interaction of the fluid with air (surface tension) and the interaction of the fluid with the solid substrate.
As expected, chemicals containing more polar oxygen groups have a lower IFT against water (Figure 4).
Polar groups interact more favorably with water, through hydrogen bonding and other polar interactions. Lauryl lactate had the lowest IFT because of interactions between water molecules and its lone hydroxyl group. Hydrocarbons, which all have high IFT's against water, have no polar groups to interact with water.
The coefficient of friction (COF, Figure 5) is a parameter that accounts for complex interactions at the fluid-surface interfaces and in the bulk fluid. Also, the measured COF will depend on the type of experiment (rolling or sliding surfaces, etc).
The COF of the esters were between 0.174 and 0.059. This range is broad as compared to other emollients, which had COFs between 0.086 and 0.062.
Discussion
Several structure-performance observations can be made from the data above.
Adding ester groups to a hydrocarbon has several effects on the bulk properties. Isododecane and diisopropyl adipate have similar structures (Figure 6), with the exception of the ester groups in diisopropyl adipate. Comparing isododecane to diisopropyl adipate, adding ester groups increases polarity, which is reflected in a higher permittivity and lower IFT. Intermolecular interactions due to the ester groups also cause an increase in viscosity, which leads to a decrease in spreading (indicated by an increase in the CA).
The effect of adding a hydroxyl group was investigated using diisostearyl fumarate and diisostearyl malate (Figure 7).
The addition of a hydroxyl group allows for hydrogen bonding interactions between the emollient and other more polar materials, as well as between emollient molecules. This is evident in lower IFT and higher viscosity of diisostearyl malate as compared to diisostearyl fumarate. Again, the change in CA can be attributed to an increase in viscosity. The higher permittivity of diisostearyl malate also indicates that hydroxyl group contributes to polarity.
The length of the hydrocarbon chains attached to the ester groups can also affect the properties of an ester.
Comparing diisopropyl adipate to diisopropyl sebacate (Figure 8), longer hydrocarbon chain length between ester groups gives higher IFT and lower permittivity due to the overall lower polarity. Viscosity is higher due to the increase in molecular weight, while other measured factors remain essentially unaffected.
Another factor affecting bulk behavior of esters is the degree of branching on the hydrocarbon moieties.
As shown by the two neopentyl glycol esters (Figure 9), the branched ester has a higher interfacial tension than its straight chain counterpart. Straight chain esters may be more able to rearrange their structure at the ester-water interface, pointing their hydrophobic tails away from the water. Branched chain esters are bulkier, which hinders their motion and may lead to closer contact between their hydrophobic tails and water. This point is further elucidated by the time dependence of IFT of the two esters (Figure 10).
The two esters have similar initial IFTs; however, the dramatic decrease in IFT over time for neopentyl glycol dicaprate indicates adsorption and molecular rearrangement at the interface. The IFT of neopentyl glycol diethylhexanoate decreases by only 1.3 mN/m over 10 minutes, implying less absorption and rearrangement.
The replacement of the branched hydrocarbon group in cetyl ethylhexanoate with an aromatic group leads to several notable differences.
First, the aromatic group increases IFT. Similar to the branched vs. straight chain example above, this may be due to hindered motion of the aromatic group close to the ester-water interface. Although viscosity does not change dramatically, there is a significant change in the contact angle. C12-15 alkyl benzoate has a higher surface tension, which contributes to this effect. The aromatic group is also more polar, so it increases permittivity (Figure 11).
Conclusion
In this work, fundamental properties that relate to the spreading, solvency, and emulsification behavior of emollients have been measured. More specifically, the interfacial tensions, surface tensions, contact angles, permittivities, coefficients of friction, and viscosities have been determined for various esters, hydrocarbons, silicone fluids, castor oils, and other commonly used emollients. Esters, in particular, offer the most versatility in these properties relative to other types of emollients studied. Furthermore, the above properties have been linked to aspects of the chemical structure of emollients. These structure-property relationships may facilitate formulators in selecting molecules best suited for their applications.