1999 ANNUAL SCIENTIFIC MEETING 63 SURFACE SCIENCE METHODS FOR COSMETIC CHEMISTS Christopher Rulison, Ph.D. ThetaDyne Corporation, Charlotte, NC 28270 Introduction Shampoo, hairspray, hair color, moisturizer, lipstick, shaving creams and gels, toothpaste, sunscreen. Each of these products contain surface active molecules (small molecule surfactants and/or polymers) that are responsible for the product's ability to wet substrates, clean, form a coating, foam, or simply remain stable on the shelf as an emulsion or colloidal dispersion. As a result, a good understanding of surface science, and analytical methods in surface science, is key to every cosmetic chemist's success. We will review the traditional methods for studying both equilibrium and non-equilibrium surfaces and interfaces (the Wilhelmy and DuNouy methods, bubble pressure, drop volume and the like). We will also discuss the future for analytical methods in surface science - surface and interfacial theology. Traditional Surface Science Methods Definitions Surface science is the study of the boundaries between immiscible phases, be they gas/liquid, liquid/liquid, liquid/solid, or even gas/solid combinations. To a laymen, such interfaces would seem to be 2-dimnesional in nature. However, surface science is really the study of the transition region between two bulk phases. The breadth of this region varies from less than 10A, in the case of a monolayer of gas adsorbed on a solid, to more than 1000Jr for an oil-in-water emulsion system stabilized by an emulsifier. One of first things to learn about surface science is the conventional use of the terms "surface" and "interface". The term "surface" is traditionally is used to describe a gas/liquid boundary. The term "interface" is used to describe a liquid/liquid boundary. The term "surface tension" is therefore used to describe the tension that develops in the plane between a gas and a liquid. The term "interfacial tension" is used to describe the tension that develops in a liquid/liquid interface. Several methods exist to measure each of these properties. However, this distinction between surface and interfacial tension is somewhat critical. Ring and Plate Methods for Surface Equilibrium and Interfacial Tension Most people have their first experience measuring surface or interfacial tension with either a DuNouy ring or a Wilhelmy Plate. With the ring method, a ring of platinum wire is used to stretch a portion of the surface or interface. The force needed to accomplish this is measured. For the plate method, a small rectangle of platinum is placed exactly at the surface or interface to be measured, so that the edge of the plate is tangent to the surface or interface. The fbrce associated with the displacement of one of two phases from the plate, in favor of the other phase, is measured. Commercial instruments abound which will perform either of these measurements. However, the utility of these measurements is primarily to determine tension of a surface or interface that is in an equilibrium condition (i.e. with surface active additives (surfactants) already adsorbed to the surface or interface). Bubble Pressure and Drop Volume Methods for Non-Equilibrium Surface and Interfacial Tension Equilibrium surfaces and interfaces are not all that is of interest to a cosmetic chemist, whose products (be they sprays, creams, shampoos, or any of others listed above) are used and formulated under non-equilibrium conditions. Take hairspray as an example. It's effectiveness depends on its ability to wet and coat the surface of hair. The surface tension of a liquid determines its ability to wet substrates. Lower surface tension means better wetting. However, the surface tension of a hairspray is time dependent (i.e. not at equilibrium when it is applied to hair). As you spray the hairspray, you are creating a large amount of air/hairspray surface (in the form of small droplets flying through the air toward your hair). The surface actives in the hairspray start adsorbing to the newly created surfaces during the time of flight of the droplets. However, they do not have time to impart the equilibrium surface tension (that one might measure

64 JOURNAL OF COSMETIC SCIENCE by the ring of plate method for a hairspray formulation) on a hairspray before it contacts the hair. Therefore, a hairspray formulator must be concerned with non-equilibrium surface tension as well. Bubble pressure non-equilibrium surface tensiometry involves blowing bubbles of gas into a liquid and measuring the maximum pressure necessary to form each bubble. That pressure is directly proportional to surface tension. By blowing bubbles at different rates, non-equilibrium surface tension values can be determined at surface ages as low as 5 milliseconds. Drop volume tensiometry is an equivalent method for non-equilibrium interfacial tension. It involves measuring interfacial tension (liquid/liquid) by dispensing one liquid drop-wise into another liquid from a well defined capillary tip, and observing the balance of forces that exists between the adhesion force the drop has for the capillary tip and the buoyancy force the drop feels. Drop volume tensiometry is a non- equilibrium technique, because the drop can be formed and measured for interlhcial tension at various rates. Minimum measurable interface age is typically about 0.5 second. A major use of non-equilibrium interfacial tension is to study diffusion and adsorption rates of emulsifiers to liquid/liquid interfaces. This has great application to the formulation of emulsified products like sunscreens and lotions. Commercial instruments are also readily available for the study of non-equilibrium surface and interfacial tension, though manufacturers don't abound, as in the case for equilibrium surface and interfacial tension. Newly Developed Surface Science Methods The Pendant Drop Technique for Both Equilibrium and Non-Equilibrium Surface and Interfacial Tension It has long been known that either surface or interfacial tension can be determined simply by analyzing the shape of drop of liquid (or bubble of gas) that is pendant (attached to) to a capillary tip. The overriding principles are the Laplace equation (which equates the curvature of a liquid/gas or liquid/liquid interface to the tension that exists between the two phases) and buoyancy forces which cause a pendant drop to elongate due to the density difference between the two phases. However, despite the relative ease with which one might capture an image of a pendant drop for analysis (either photographically or digitally), this method was virtually unused by surface scientists until approximately 15 years ago. This was because the processing of an image into surface or interfacial tension data (i.e. the robust solution of the Laplace equation) was tedious. However, in the age of high-speed personal computers and high-speed digital image capture, that has changed. The method is now widely used for both surface and interfacial tension determinations. Non-equilibrium tensions are possible as per the rapid formation of a pendant drop and subsequent high-speed imagining. The pendant drop method is perhaps the most flexible of all surface and interfacial tension methods in terms of temperature, and even pressure, control. It is also one of the easiest techniques to keep free of contamination. Commercial instruments are readily available. The Oscillating Pendant Drop Technique for the Study of Surface and Interfacial Rheology Even more recently (within the last 5 years), a major breakthrough has made for surface scientists. A computer controlled oscillating pendant drop tensiometer has been commercialized which allows not only for determination surface and interfacial tension as a function of time as an interface equilibrates, but also enables the experimenter to controllably perturb a pre-equilibrated interface and monitor its response to the perturbation. This represents a radical departure from previous non-equilibrium surface and interfacial science techniques, which only focused on monitoring the progression of an interface toward equilibrium. Such "2-dimensional" rheological properties as: Gibbs' modulus, interfacial elasticity, surface elasticity, interfacial viscosity, and surface viscosity can be determined. The droplet's surface area is controlled by a precision pump and can be made to oscillate sinusoidally, linearly, step-wise, and/or by feedback control with respect to the real-time data. The ability to study what happens when an interface or surface at equilibrium is perturbed is critically important to the advancement of our uf•derstanding of emulsion stability and foam persistence. Yet, previously such measurements where not possilSle with commercial instrumentation (except in the case of Langmuir trough studies with insoluble monolayer systems). Oscillating Drop Tensiometers have been commercially available in the United States for approximately 1 year. Data directly related to emulsion stability and foam persistence will be discussed.