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K S V INSTRUMENTS LTD

Application Note #100

P.O. Box 128, 00381 Helsinki, Finland E-mail: [email protected] www.ksvltd.fi

This application note provides a brief introduction why it is important to measure contact angles and surface tensions of different surfaces and liquids. Additionally, a brief comparison to other surface characterization techniques is given. Finally, a list of application areas and industries where these properties are of importance with short example will be presented. More about what contact angle and surface tension are and how they can be measured can be found in the other application notes AN #101AN #106.

Why measure contact angle and surface tension? The large influence of surface chemistry has only recently been acknowledged in many industrial fields. In order to meet the challenges of the increased competition and the challenges of the feature in improving our quality of life and protecting our environment, the development of new surface-engineered materials and products are the focus of many fields of today´s scientific research. The precise characterization and knowledge of the properties of surface engineered materials and liquids are of utmost importance in the development of new, better performing products with improved qualities. The knowledge of the surface properties of raw materials and how to manipulate these properties of the material to fit a certain application has become a key role this process. Measurement of contact angles and surface tensions provides a better understanding of the interactions between solids and liquids or liquids/liquids. These interactions play a key role in understanding adhesion, material wettability, biocompatibility, lubricity of solid surfaces as well as the wetting, washability, spreading and adsorption of liquids. Contact angle and surface tension measurements provide the information needed for development and modification of liquids and solid surfaces using today´s sophisticated surface engineering techniques. Hence, almost any solid or liquid surface can be modified to fit an application.

How come contact angle and surface tension are so important ? Molecules inside (bulk) a liquid/solid are in every direction affected by equal attraction forces, whereas the molecules at the surface lack a neighbor towards the air phase and therefore they have larger attraction forces towards the liquid/solid than air (see figure below). This leads to a situation where the interface has excess free energy. This excess free energy is characteristic for any liquid or solid. For liquids a spontaneous contraction of the surface will take place due to this free energy and the Surface Tension of a liquid is a direct measure of it. In the case of solids a contraction is hardly ever seen, but still this free energy is present at the interface of a solid. However, now it is called Surface Free Energy (instead of Surface Tension as for liquids) and can be accessed by measuring the contact angle of a series known liquids placed on the solid surface. The dimension of Surface Tension and Surface Free Energy is mN/m.

The unbalance of interactions at an air/liquid interface Due to the unbalance of forces at the surface/interface the structure and composition of the surface/interface is different than in the bulk. Interactions at surfaces/interfaces therefore result in special orientations of molecules, accumulation of certain types of molecules at the interface, separation of positive and negative charges. This often results in a situation where the surface/interface properties are completely different from the properties of the bulk material. The knowledge of the properties of the outermost layer of a material is therefore very important in many cases. Combined analysis of surface tension and contact angle gives information about the properties of the outermost layer of a surface in a simple way.

Contact Anngle and Surface Characterization Techniques ?

Tension

measurements

vs.

Other

Surface

Methods based on Tensiometry and Goniometry to measure contact angles and surface tensions are very useful techniques for surface and liquid characterizations due to their university, accuracy and simplicity. KSV Instrument manufactures tensiometers (Sigma 70 series) and Goniometers (CAM 200 and CAM 100) for educational purposes, R&D and industrial applications. A large range of other surface characterization methods such as X-ray Photoelectron Spectroscopy (XPS or ESCA), Secondary Ion Mass Spectrometry (SIMS), Small Angle X-ray Scattering (SAXS), Raman and IR spectrometry also exists. However, these techniques penetrates more or less into the surface, while Tensiometry and Goniometry are the only techniques to access the properties of the outermost layer of the surface/interface. Furthermore, the techniques mentioned above are very expensive (hundreds of thousands of dollars), while a Tensiometer or Goniometer can be obtained for 10-20 000 USD. Of course, Tensiometry and Goniometry techniques do not give exact information about the chemical composition of the surface/interface such as XPS, SIMS and IR spectrometry does, but they are fast, simple and accurate techniques for quick checks of for example surface treatments of solids or liquid formulations. Imaging tools such as Transmission and Scanning Electron Microscopy (TEM and SEM), Scanning Tunneling Microscopy (STM) and Scanning Probe Microscopy (SPM) are also widely used for surface characterization. Apart from being very expensive techniques these are laborious techniques to work with. Consequently, Tensiometry and Goniometry can be seen as complementary techniques giving supporting information to other surface characterization methods and/or techniques that can be used to guideline the researcher in the right direction before a more elaborate analysis with a much more expensive surface characterization tool is started.

Application areas Biomaterials Advances in medical and dental technology have been at the forefront of the materials revolution. These new and surface-modified biomaterials are being employed to create disposable contact lenses, IOL's, catheters, dental prosthetics and biocompatible implants. The artificial materials need to have certain wettability properties (Surface Free Energy) to avoid rejection of the human body for example for body implants, and to ensure a good adhesion between the tooth and embodiment in the case of dental surgery. The wettability of contact lenses of cleaning solutions can be examined by contact angle measurements. The effectiveness of the cleaning solution formulation for contact lenses can be improved, by optimizing the surface free energy of the lens, changing the formulation of the cleaning liquid and measuring contact angles. Pharmaceuticals By applying special surface treatments to pharmaceutical powders the distribution and dissolving behavior of the powder can largely be influenced. The controlled release, wettability and dissolving behavior of an orally ingested pharmaceutical powder, tablet and capsule, or transdermally applied drug product in various body fluids can be improved with the help of contact angle and surface tension measurements. This enables the development of more safe and effective time-released pharmaceuticals. Composites "New-age materials" constructed of structural reinforcing fibers and resin matrix systems have replaced many of the traditional metals and other heavier and weaker materials. Composites are found in a wide range of products used in aerospace, automotive and sporting good applications. The adhesion between the fiber and resin matrix system can be optimized, by finding the right formulation of the resin matrix with proper wetting properties against the fiber. Semiconductors The cleanliness of semiconductor surfaces is of utmost importance in the semiconductor industry. The surface free energies of clean and treated surfaces (mainly nitrides and oxides) used in semiconductor industry are directly correlated to the cleanliness and surface composition of the surface. Therefore, contact angle measurements are a highly suitable quality control method in this field. Paper, Film & Ink Products The advent of new environmentally friendly water-based inks has sparked new research in coating processes in the paper industry to improve ink adhesion and support recycling efforts. Adhesion of inks and protective coatings to polymer film products used in the production of photographic and electronic media and in the packaging of food and electronic products are also benefiting from surface chemistry technology. For example, all materials involved in an offset printing process (paper, metal and rubber surfaces, printing ink and dampening solution) need to have a certain surface free energy or surface tension in order to obtain an optimum printing quality. A container for juice requires different surface energies on the outside (printability) than on the inside (liquid-resistant), while a sheet of newspaper should have the same properties on both sides. Paints & Coatings With the increased use of plastic and composite materials in automotive applications and new water-based paints with special surfactant mixtures, drives for new preparation methods for obtaining stable emulsions for storage shelf life and long-lasting adhesion between the coating and substrate surfaces (paper, metal, wood, plastic etc.). Therefore, the optimization of the interfacial tension between the particle and carrier for example in the paint formulation and measurement of interaction, i.e. contact angle, between coating and substrate are critical in the coating processes. In addition, new and improved coating formulations such as the anti-weather polymer sealant covering the car body or the preservative covering the stain on your wood deck are all important products that provide a protective barrier from the environment. The

effectiveness of the coating formulation and the coating process of for example a car body coating can be accessed by measuring the hydrophobicity (i.e. contact angle) of the lacquer surface. Adhesives Nowadays a large range of material combinations are more easily and durably connected by using adhesives, instead of using soldering, welding and mechanical connection as in the past. The adhesion between different composite structures (glass-metal, leather-fabrics, wood-paper) and the wetting of the adhesive on the substrate can be accessed by contact angle measurements. Cosmetics Shampoos, conditioners or other cleaning products are only effective when the surface of the hair or the cleaned surface is receptive to the product. The cleaning process of shampoos and the effectiveness of cleaning solutions can be followed by measuring the contact angle. The surface tension is a measure of the effectiveness of surfactant solutions. The interfacial tension plays an important role when developing time and temperature stable emulsions for various cosmetics products such as suntan, toothpaste and body creams and lotions. For example, a stable dispersion of Titanium Oxide used in suntan can be obtained by optimizing the surface free energy of the solid particles and the surface tension of the liquid carrier. Superabsorbent personal hygiene products such as baby diapers have been studied with the contact angle method to improve absorbency and provide protection against wetness. Textiles The application of water and stain or static-resistant coatings is widespread in the textile industry. Everything from carpet fibers to surgical gowns to cross-country ski apparel involves surface treatments applied to a textile material providing important barriers for protection from the elements. The wettability of single fibers or fabrics as well as the hydrophobicity and homogeneity of the coating can be checked by contact angle measurements. Environment Environmental issues have surfaced in almost every form of manufacturing and in many instances the focus of product improvement research involves surface chemistry measurement technologies. Pesticides that adhere to a leaf and are less likely to wash into the soil obviously need to be replaced less often. The surface tension of the pesticide or fertilizer formulations directly affects their spreading on plant leaves or in soil, which influence the effectiveness of the formulation. The contact angle of different pesticide formulations on plant leaves can be used for finding the optimum composition of the pesticide formulation. Oil polluted areas can be treated by surfactant solutions, and the cleaning process can be followed by contact angle measurements on the treated samples. And Many More The examples of application areas given above are only the most important ones that can be covered by contact angles and surface tension. Do not hesitate to contact us with any questions concerning any application areas.