Praktikum zur. Materialanalytik

Transcription

1 Praktikum zur Materialanalytik Functionalized Surfaces B510 Stand: Table of contents Introduction 2 Basics 2 Surface tension 2 From wettability to the contact angle 4 The Young equation 5 Wetting of real surfaces the Wenzel and Cassie-Baxter case 6 From contact angle measurement to surface tension 8 Validity limits for classical Young equation 8 Further measurement techniques 9 The Zisman Plot 10 Experimental procedure 11 Recommended Literature 12 References 12

2 Introduction Surface tension pertains to concepts of surface free energy, adhesion and wetting. It also demonstrates some fundamental properties and phenomena such as intermolecular interactions. Some of these forces can actually be visualized through the behavior of a drop of liquid on the surface of an analyte. The macroscopic shape of the drop is characterized by the socalled contact angle. Contact angle measurements date to the early days of the 19 th century, when Laplace and Young tried to formalize wetting behavior using the methods of differential calculus. Wetting is a key phenomenon in many technological fields. Gluing, painting, inking and washing are only few examples of practical situations where a good contact between a liquid and a solid is necessary. On the other hand, a further large number of technical operations require some kind of protection from too strong liquid-solid interaction: waterproofing and anti-sticking or anti-adherent coatings are probably the best known examples. Basics Surface tension The surface tension of a liquid is a measurable force existing in all liquid surfaces. The physical origin of surface tension is the unfavorable state of matter at interfaces. It arises directly from an imbalance of the cohesive forces that exist between the constituent molecules of the liquid and which prevent the liquid from becoming a vapor (Fig.1). In other words when atoms or molecules are exposed at an interface, they are no longer surrounded from every side by molecules of the same type. They must either lose some of the interaction energy in the case of an ideal surface against vacuum or share some of the interaction energy with the molecules of the surrounding medium. Consequentially the trust of liquids to minimize their surface area, or, in general, the occurring of capillary phenomena (capillarity is the collective name of phenomena occurring on surfaces that are mobile enough to modify their shape according to surface tension requirements) results. The concept of surface tension or especially the critical surface tension (this term later will be explained) can be used to characterize a wide variety of polymers and correlate these 2

3 energies with polymer structure. Fluorinated materials have low values of surface tension. Hydrogenated materials, such as polyethylene and polypropylene, have slightly higher values. For other substituents (Cl, O, N) the surface tension is a little higher still. Fig.1: Model of an interface between a solid or a liquid coexisting with a vapor. Mention the perpendicular to the surface and inside the sample resulting direction of the sum of intermolecular forces. Generally the term surface tension is introduced with the observation that work is required to extend a thin film of a liquid (Fig. 2). (Eq.1) dw rev F ds l 2 ds da W rev = required reversible work F = force which tempt to contract the liquid da = created surface while extending the film p, T = const. Fig. 2: Definition of the term surface tension by the work required to extend a still existing surface. 3

4 The terms surface tension and surface energy f are commonly used interchangeably, though they are not necessarily the same. Surface energy is the work necessary to form a unit of surface area by a process of division. Surface tension is the tangential stress (force per unit length) in the surface layer. The relationship between and f is given by the following equation: f (Eq. 2) f A A where A is the area of the surface. To understand the difference between these two terms, one consider the production of a new surface of a solid or liquid when its cleaved in the direction perpendicular to the surface in a two-step process. First, the solid or liquid is divided to produce two surfaces, but the atoms or molecules are kept in exactly the same positions that they occupied when they were in the bulk phase. Second, the atoms or molecules in the newly formed surfaces are allowed to rearrange to achieve their most stable equilibrium configuration. In a liquid system, these two steps occur simultaneously because of the mobility of liquid molecules. This means that the last term in Eq.2 is zero, so that = f. On the other hand, in a solid system, the second step will occur slowly because of the limited mobility of the solid surface region. The solid surface may therefore be stretched or compressed with no change in the number of atoms or molecules. In this case, f and the relationship between and f is governed by Eq.2. From wettability to the contact angle When a liquid droplet interacts with a solid surface, the droplet attains an equilibrium shape. The droplet can be characterized by an angle formed at its edge where the liquid contacts the solid surface. This angle is called the contact angle. Depending on the type of surface and liquid the droplet may take a variety of shapes as illustrated in figure 3. The contact angle is given by the angle between the interface of the droplet and the horizontal surface. The liquid is seemed non-wetting when and wetting when A contact angle = 0 corresponds to perfect wetting and the drop spreads forming a film on the surface. 4

5 Fig. 3: Droplets shapes and contact angles. Tab.1: Contact angle of liquids with Teflon. Liquid Mercury Water Methane Iodide Benzene Propanol Contact angle with Teflon In summary, for proper wetting the contact angle must be 90 or less. From the defining equation, it is expected that a low contact angle will be associated with high solid surface energy and low liquid surface energy. Teflon for instance, has a low energy surface, so liquids do not want to wet it, but as the surface energy of the liquid is reduced, the contact angle decreases (Tab. 1). Of course, the solid used also affects the contact angle. As seen above, liquid water beads up on Teflon. However, on clean gold, which has a high surface energy, water spreads. Water also spreads on glass, which also has a high surface energy. In these two cases, the contact angle is quite low. On a carbon surface, the contact angle is between 72 and 86. On polymers such as polyethylene and polypropylene, the contact angle with water exceeds 90. The Young equation The concept of the contact angle can be used quantitatively to measure interaction between any liquid and solid surface. The surface free energy components of the liquid and solid can be modelled as vectors. A definition for the contact angle at the three phase boundary of a liquid in equilibrium with a solid and vapor can be written from a vector sum (Thomas Young 1805): 5

6 (Eq.3) cos SV SL LV Y where ij is the interfacial tension (resp. surface tension, surface energy) between phases i and j, subscripts S, L and V refer to solid, liquid and vapor respectively and Y is the equilibrium contact angle or Young angle. Fig. 4: Drop on surface with graphic representation of the Young-equation The solid-vapor interfacial tension is linked to the intrinsic solid surface tension, or the surface tension in equilibrium with its own vapor or in vacuum, by the following relationship: (Eq.4) e SL SV and e is called the spreading pressure, which represents the decrease of solid surface tension due to the vapor adsorption. If the contact angle is larger than about 10, the spreading pressure can be neglected and the Young equation can be written in terms of the true or intrinsic solid surface tension. Wetting of real surfaces the Wenzel and Cassie-Baxter case In contrast to perfectly smooth and homogenous surface assumed for the derivation of Young equation, real surfaces often show roughness and chemical inhomogeneity. In this case the observed contact angle is called apparent contact angle *, as it is modified by the structural features of the real surface and deviates from the contact angle that a perfect surface of the same material would show. The Wenzel model, introduced in 1936, takes into account surface roughness and implies total penetration of a liquid into the surface grooves as shown in Fig.5. The surface is assumed to be chemically homogenous. The surface roughness increases the overall surface of the solid. This change can be incorporated into Eq.3 using a roughness parameter r, which is defined as the ratio of the real and the projected surface. 6

7 (Eq.5) * real surface r SV SL LV cos ; r ; r 1 projected surface (Eq.6) * cos r cos Y Fig. 5: Wetting of a real surface with surface roughness Consequently, the introduction of roughness will increase the inherit hydrophilicity or hydrophobicity of smooth materials. The Cassie-Baxter model, introduced in 1944, deals with the wetting of flat but chemically heterogeneous surfaces. The wetted area of the surface consists of various areas of components with different surface tensions. You may think of it as a polished cut through a eutectic alloy, in which you find alternating α and β phases. The resulting contact angle depends on the ideal contact angle of all components, weighted by the fraction of the area the respective component covers. In case of two different components, each with its area fraction f 1 and f 2, the apparent contact angle can be written as: * (Eq.7) cos f cos f cos 1 f cos f cos 1 Y,1 2 Y,2 2 Y,1 2 Y,2 A special case of the Cassie-Baxter model is depicted in Fig.6. The drop is in contact with both, the surface of the solid and air, which is trapped in cavities between the solid (with fraction f s ) and the drop. The drop however does not get in contact with the walls of the cavities. * (Eq.8) f cos 1 cos f cos s air s Y Assuming cos 1 Eq.8 transforms to: air * (Eq.9) cos cos 1 1 f s Y The Cassie-Baxter model in this particular form is typically used to explain superhydrophobicity in various materials. 7

8 Fig. 6: Wetting of a real surface with chemical inhomogeneity (substrate/air) From contact angle measurement to surface tension Unfortunately, unlike liquids, whose surface tension can be measured by direct methods, simply measuring the work required in order to create a new surface, solids must be probed by indirect methods. One main indirect method is the contact angle measurement. To find Young s contact angle for a liquid in contact with a surface, a drop of the liquid is placed on a horizontal surface. Both the drop and the material on which it is placed have to be enclosed inside an environmental chamber (100% humidity). The drop is placed on the surface of the material and oriented in such a way that the instruments crosshair is tangent to the cross-sectional curvature of the drop at a point where all three media (solid, liquid and vapor) meet. Two general conditions must be fulfilled in the process of contact angle goniometry: the analyzed surface cannot be reactive towards the analyzing liquid and the drop must be stable (not changing shape) while in contact with the surface. Validity limits for classical Young equation During the measurement of contact angles various problems may arise, most of which are related to the surface of the solid The contact angle changes because of surface roughness and (chemical) inhomogeneity or if the surface is not really flat. Its shape depends also on the size of the drop. If one does not use a chamber with 100% humidity a change of the shape of the drop will occur during the measurement because of evaporation and with it the contact angle will also change. 8

9 Further measurement techniques Due to the non-ideal nature of real surfaces there is a phenomenon called contact angle hysteresis: While, according to the Young equation, the vector sum of interfacial tension should yield only one contact angle, it is commonly observed that a drop of liquid on a solid surface exhibits a range of allowed angles. The observed range is usually characterized by the measurement of the maximum (advancing angle) and the minimum (receding angle) allowed value. The problem is, of course, which of the observed angles (a range of some tens of degree is not unusual) should be used in the Young equation. According to the hypothesis of the Young equation, solid surfaces should be homogeneous and smooth. Moreover the liquid-solid interaction should not result in swelling of the solid, liquid penetration or liquid-induced surface restructuring or deformation. In other words, the drop of the liquid should behave as a neutral probe, only characterized by its interfacial tension parameters and should interact with the smooth, homogeneous and rigid solid surface only by interfacial energetics. Clearly, real surfaces often do not fulfill these requirements. Contact angle hysteresis results from the relaxation of these constraints. Fig. 7: Measurement of contact angle hysteresis Contact angle hysteresis is measured by increasing or decreasing the drop volume until the three-phase boundary moves over the solid surface (Fig. 5). In this practical curse we measured a contact angle only by the so-called sessile drop technique, simply putting a drop of liquid on the surface, which is the static kind of measurements. Another main problem is the fact that the Young equation actually contains two unknowns: in fact, while the contact angle is experimentally measured, as described below, and the liquid surface tension can be measured by several direct means, neither of the two other terms can be measured independently. 9

10 The Zisman Plot Zisman s method for obtaining the materials surface tension based on the experimental finding that when a liquid spreads freely on an analyzed surface, its surface tension is lower than or equal to that of the surface upon which it is spreading. Zisman called the value of the surface tension of the liquid that is equal to that of the analyzed material c, the critical surface tension (as mentioned above). To obtain the c value, a series of contact angles is measured using liquids with progressively smaller surface tensions. The surface tension of these liquids is then plotted against the cosine value of the corresponding contact angle (c.f. Fig. 8). The solid line in Figure 8 represents a best fit for the measured points and is extrapolated to intersect with the value of cos = 1. At the point of the intersection a line (dashed line) is drawn perpendicular to the x axis and a value of c can be obtained. This protocol for obtaining c is typically repeated for a variety of liquids; and qualitatively at least, the critical surface tensions for homologous liquids, on the same surface, correlate. Fig. 8: Typical Zisman Plot. 10

11 Experimental procedure The students are provided with two samples, for which the critical surface tension has to be determined in this lab course. The binary system water-acetone is used to probe the sample surface. The corresponding surface tensions for various mole fractions of acetone are given in Table 2. The students prepare 6 different water-acetone mixtures and apply these to the respective sample surface. For observing the drop on the surface and measuring the contact angle a Dataphysics OCA15Plus contact angle measurement setup is used. The students record a sequence of contact angles by measuring minimum three drops per solid with each test liquid. The surface tension for each sample is to be determined using the Zisman method. Tab.2: Surface tensions of the binary system water-acetone for given mole fractions of acetone. (Enders et al., 2007) Surface tension / mn/m Mole fraction of acetone T = 15 C T = 25 C T = 35 C

12 Recommended Literature Polymer Surfaces From Physics to Technology Garbassi, F., Morra, M., Occhiello, E.; John Wiley & Sons, Chichester, New York (1998) Lehrbuch der physikalischen Chemie Wedler, G.; Wiley-VCH, 4. Aufl. Physikalische Chemie / Physical Chemistry Atkins, P.W., Beran, J.A.; Wiley-VCH Physikalische Grundlagen der Materialkunde Gottstein, G.; Springer Verlag Thin-Film Deposition, Principles & Pratice Smith, D.L.; Mc Graw Hill, Boston.(1995) Surface and Colloid Science Volume 1 Ed. E. Matijevic; John Wiley & Sons (1969) Surface and Colloid Science Volume 2, Experimental Methods Edrs. Good, R.J., Stromberg, R. R.; Plenum Press, New York and London, (1979) References S. Enders et al., (2007). Surface Tension of the Ternary System Water + Acetone + Toluene. J. Chem. Eng. Data. E. Bormashenko, (2016). Physics of solid liquid interfaces: From the Young equation to the superhydrophobicity. Low Temperature Physics 42,