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Dynamic Mechanical Properties and Low-Velocity Wetting Behavior of Plastic Crystalline States for n-Alkane Blends Steven J. Severtson and Michael J. Nowak* Department of Wood and Paper Science, University of Minnesota, Kaufert Laboratory, 2004 Folwell Avenue, St. Paul, Minnesota 55108 Received February 19, 2001. In Final Form: May 31, 2001 Data presented here demonstrate a correlation between viscoelastic properties and wetting behavior for an n-alkane, binary mixture, and several macrocrystalline paraffin waxes. Mesophases in the premelting region of n-alkane systems were identified using differential scanning calorimetry, and their level of viscoelasticity was characterized via dynamic mechanical spectroscopy (DMS). Thermal analysis indicates that transitions between crystalline, plastic crystalline, and the isotropic phases for these systems coincide with peaks in loss tangent values measured using DMS. The combination of a broad range of viscoelastic properties and a relatively constant equilibrium contact angle over the premelting region provides a unique opportunity to study the relationship between substrate mechanical properties and wetting behavior. Dynamic contact angle measurements were performed for water on these surfaces using low substrate velocity (2-264 µm/s) Wilhelmy plate tensiometry. Advancing dynamic contact angles were found to have a velocity dependence that was greatly enhanced far removed from phase transitions, while equilibrium values were observed for receding angles at all substrate velocities and temperatures. Correlations were identified between loss tangent values for the substrate, phase transitions, and the magnitude and relaxation kinetics of advancing dynamic contact angles. Results clearly demonstrate that differences exist between wetting and dewetting mechanisms for water on n-alkane substrates. Advancing angle data could not be fit with models attributing contact angle behavior to substrate deformation, adsorption/desorption rates at the three-phase line, or hydrodynamic considerations. We speculate that observed results are associated with enhanced molecular freedom, which can be gauged by the mechanical loss tangent. Findings from this study may provide insight to anomalous wetting behavior reported for other low-energy substrates.
Introduction Normal alkane oligomers in the carbon number range of 18-40 demonstrate novel phase behavior. Bracketed between their isotropic liquid state and highly ordered crystalline phase, there often exists a series of weakly structured mesophases in which molecules in the crystals have gained a rotational degree of freedom while retaining their positional order. These states are referred to as plastic crystalline or rotator phases. In this paper, differential scanning calorimetry (DSC) and dynamic mechanical spectroscopy (DMS) are used to locate and characterize rotator phases for normal alkane systems. These results are compared with dynamic contact angle data to determine the impact dynamic mechanical properties have on the velocity dependence of measurements. The results contribute to the discussion on the role of substrate properties in determining the low-velocity wetting behavior of organic solids. Two types of normal alkane systems are studied here, a 79:21 molar ratio of C21 and C23 n-alkanes, respectively, and paraffin waxes containing a broad range of carbon numbers. The C21-C23 blend is unusual in that the temperature region over which the solid exists in its plastic crystalline states is broad. Prior to melting, normal alkane systems will typically undergo several transitions over a narrow temperature span. This complicates the identification of the various rotator phases present and makes the use of DMS for characterization of the dynamic mechanical properties of these phases especially challenging. The broad nature of DMS peaks surrounding phase transitions and their close thermal proximity in typical n-alkane systems can mask properties of the actual rotator phases. The resulting dynamic mechanical spectra resemble that found for an amorphous polymer’s movement from a glassy to rubbery state. Minor but distinct
plateaus or breaks in property curves are the only indication of the existence of multiple transitions. For the C21-C23 blend, mesophases are stable over an extended temperature range of about 20 °C. This allows dynamic mechanical properties of the various states present to be isolated, producing a spectrum in which mechanical properties pass through three local extremes in a 20 °C span. For the paraffin waxes studied, the broad array of n-alkanes produces expanded rotator phases and rich dynamic mechanical spectra, but not to the same extent as that found for the C21-C23 blend. The relationship between dynamic mechanical properties and contact angle behavior is not well understood and, to the best of our knowledge, has only been discussed in reference to soft substrates.1,2 Soft materials are defined by many authors as those with a tensile modulus of less than 3 × 105 Pa.3,4 For these substrates, the vertical component of the three-phase or wetting line may deform the substrate surface, lifting a ridge that relaxes once the wetting line passes. For a sessile drop spreading on a viscoelastic substrate, this strain cycle would dissipate the energy driving the wetting process, slowing movement toward an equilibrium contact angle. The higher the loss tangent of the material, the greater the expected energy dissipation and wetting hysteresis in contact angle measurements. For n-alkane systems, the tensile modulus is typically above the 3 × 105 Pa limit even in its mesophase region,5,6 and there is no indication that the three-phase (1) Shanahan, M. E. R. J. Phys. D: Appl. Phys. 1988, 21, 981. (2) Carre´, A.; Shanahan, M. E. R. J. Colloid Interface Sci. 1997, 191, 141. (3) Andrade, J. D. Surface and Interfacial Aspects of Biomedical Polymers; Plenum Press: New York, 1985; Vol. 1, Chapter 7. (4) Lester, G. R. J. Colloid Sci. 1961, 16, 315. (5) Edwards, R. Tappi J. 1958, 41, 267.
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line is capable of deforming the interface. The significance of the dynamic mechanical properties for these systems is not necessarily in the mechanical changes that occur in the substrate but possibly in what these changes indicate about the freedom of molecular motions. Restructuring of surfaces in response to a solvent, even nonpolar surfaces, is an often-cited cause of kinetic hysteresis in contact angle measurements.3,7 Described as the minimization of interfacial free energy through molecular reorientation, the effect is likely promoted by enhanced molecular freedom. Although it is often argued that surface motions are different from those in the bulk, in all likelihood they are related. Temperature regions of increased molecular freedom in the bulk can be identified by local extremes in dynamic mechanical properties such as the loss tangent, indicating the enhanced ability of molecules to diminish an induced stress as heat through their relaxation. These regions also provide for greater movement of solvent molecules into the substrate. This has been demonstrated by various experimental techniques3,8,9 and was previously reported for the incorporation of melting point depressants into n-alkane structures.10 In addition, several authors have suggested the relationship between liquid adsorption on solid substrates and the velocity dependence of contact angles,11,12 a concept that is the basis of the molecular-kinetic model developed by Blake and Haynes.11 Although there are many reasons to believe that wetting behavior and changes in dynamic mechanical properties of the substrate are connected, few if any published studies exist demonstrating this. Here, a comparison is made between velocity-dependent contact angle behavior for water on n-alkane substrates and corresponding dynamic mechanical properties. Results are discussed with reference to existing theories on wetting. Experimental Section Materials. Hydrocarbons n-heneicosane (C21H44) and ntricosane (C23H48) were purchased from Roper Thermals (Clinton, CT). Purities were determined to be greater than 99% by gas chromatography. A C21-C23 solid solution (79:21 molar ratio) was prepared by cocrystallization from the melt. X-ray diffraction confirmed that the blend was homogeneous, with no existing phase separation. The paraffin wax utilized for a majority of this study was Chevron Refined Wax (Chevron, San Francisco, CA), herein referred to as the commercial wax. The sample arrived in a 1-lb block, which was ground, labeled, and stored in glass jars for use in testing. The carbon number distribution measured using gas chromatography-mass spectrometry (GC-MS) indicates that the distribution ranges from C18 to C30 normal alkanes, with a majority of the carbon numbers falling between C22 and C24.13 Two other waxes that were characterized and used for a portion of this study included Chevron Saturating Wax, with a melting point of 56 °C and composed primarily of C26-C28 n-alkanes, and a paraffin wax with a melting point of 54 °C purchased from Aldrich (Milwaukee, WI). Dynamic Contact Angle Measurements. Utilizing a technique based on the Wilhelmy balance principle, the dynamic contact angles of wax (i.e., advancing and receding) were measured using a Cahn Instruments (Madison, WI) DCA-322 (6) Freund, M.; Csikos, R.; Keszthelyi, S.; Mozes, G. In Paraffin Products: Properties, Technologies, Applications; Mozes, G., Ed.; Elsevier Scientific: Amsterdam, 1982; p 109. (7) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley: New York, 1990; Chapter 10. (8) Braun, J. M.; Guillet, J. E. Macromolecules 1975, 8, 882. (9) Leboeuf, E. J.; Weber, W. J. Environ. Sci. Technol. 1997, 31, 1697. (10) Severtson, S. J.; Coffey, M. J.; Nowak, M. J. Tappi J. 1999, 82, 67. (11) Blake, T. D. In Wettability; Berg, J. C., Ed.; Marcel Dekker: New York, 1993; Vol. 49, p 270. (12) Hayes, R. A.; Ralston, J. J. Colloid Interface Sci. 1993, 159, 429. (13) Nowak, M. J.; Severtson, S. J. J. Mater. Sci. 2001, 36, 4161.
Langmuir, Vol. 17, No. 16, 2001 4991 system. The supplied WinDCA software automatically performed contact angle and surface tension analyses. All experiments were run using double-distilled water as the liquid phase (pH ) 5.85, conductivity ) 0.3 µS, resistivity ) 18.2 MΩ cm), which was allowed to equilibrate at the selected temperature in a sterilized polystyrene Petri dish (Becton Dickinson, Franklin Lakes, NJ). Temperature in the sample dish was maintained using an inhouse stainless steel water jacket setup connected to a Neslab (Portsmouth, NH) model RTE-101 5 L circulator. Sample temperature was recorded before and after all measurements with a cleaned stainless steel probe with an estimated uncertainty of (1 °C. The change in water surface tension with temperature was determined prior to each contact angle measurement and accounted for. In addition, corrections were made for substrate buoyancy. A stainless steel plate (24 mm × 24 mm × 0.1 mm thick) was chosen as the coating substrate. The n-alkane surfaces were prepared by dipping a cleaned plate into a melt solution maintained approximately 10 °C above the melting point of the wax. The plate was then carefully removed and allowed to cool in the vertical position. (Similar dip-coating approaches have been shown to produce wax surfaces for contact angle measurements with little or no hysteresis behavior.14) A new coated plate was used for each temperature studied. Interface velocities were varied from 2 to 264 µm/s, and a velocity of 80 µm/s was utilized for a majority of the testing. Uncertainty in measured contact angles was estimated to be (2°. Dynamic Mechanical Spectroscopy Measurements. The mechanical properties of the n-alkane materials were investigated using a Perkin-Elmer (Norwalk, Canada) DMA 7e thermal analyzer system. Temperature calibration was performed with water, indium, and zinc standards at a scan rate of 5 °C/min. Pyris Series software was used to program the experiments and subsequently carry out data analysis. The method utilized for the DMS measurements involved a temperature scan from 10 to 60 °C at 5 °C/min using a parallel plate fixture in compression. All experiments were normalized to the dimensions of the sample (rectangular bar, ∼6 mm long × 2 mm wide × 2 mm thick). A frequency of 1.0 Hz, a static stress of 2200 mN, and a dynamic stress of 2000 mN were programmed. Nitrogen was used to purge the system at a flow rate of 30 mL/min. Differential Scanning Calorimetry Measurements. A Perkin-Elmer DSC 7 thermal analyzer system controlled with Pyris Series software was used for DSC measurements. The instrument was calibrated for heat flow and temperature at 5 °C/min using indium metal. An empty aluminum sample pan was used as the reference. The DSC scans for the n-alkane systems were measured by weighing approximately 8-10 mg of sample into an aluminum hermetically sealable pan and then placing the pan into the DSC’s sample tray. The wax was heated at 5 °C/min from 10 to 60 °C and then cooled back to 10 °C at 5 °C/min. The heating and cooling experiments were done under a nitrogen atmosphere (30 mL/min).
Results and Discussion Substrate Characterization. Phase, structure, and property information for the C21-C23 blend and commercial paraffin wax were previously established.13,15 Figure 1 shows DSC heating thermograms for both the blend and commercial wax. Paraffin materials, including most solid n-alkanes, demonstrate two major first-order thermal transitions. The first of these is a solid-solid transformation from the ordered crystalline state to a rotator phase, while the second transition corresponds to melting. The various phases present in the blend have been characterized with X-ray diffraction.15-17 Prior to the solid-solid transition, the blend has an orthorhombic structure involving lamellar close packing of the n-alkanes. Chain molecules mostly adopt a staggered conformation, with their long axes arranged parallel to the layer normal. In (14) Ray, B. R.; Bartell, F. E. J. Colloid Sci. 1953, 8, 214. (15) Ungar, G.; Masic, N. J. Phys. Chem. 1985, 89, 1036. (16) Sirota, E. B.; Singer, D. M. J. Chem. Phys. 1994, 101, 10873. (17) Ungar, G. J. Phys. Chem. 1983, 87, 689.
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Figure 1. Differential scanning calorimetry heating curves for the C21-C23 n-alkane blend and commercial wax showing two prominent first-order phase transitions. The first is a solidsolid transition between the orthorhombic crystal and RI rotator phase. The second is the melting transition. Arrows indicate a third, much weaker RI-RII rotator phase transformation.
this state, molecules possess both long-range positional and orientational order. The solid-solid transition marks the movement of the blend into its first rotator phase denoted as RI. Rotator phases have significant molecular disorder in which the molecules gain a rotational degree of freedom about their long axes while maintaining fixed positions in the crystal lattice. It is this increase in orientational disorder that is the namesake of the rotator phases. A rectangular or distorted-hexagonal lattice with bilayer (ABA) stacking characterizes RI in which the lamella structure is retained. Prior to melting at approximately 38 °C, a much less endothermic transition is apparent in the DSC thermogram of the blend (marked by an arrow in Figure 1). This is a transition into a second rotator phase designated as RII. Molecules in the RII rotator phase again retain the lamella structure but are packed in a lattice with average hexagonal symmetry possessing a trilayer (ABC) stacking sequence. The commercial paraffin wax consists primarily of n-alkanes in the C22C24 range. The same rotator phases present in the blend exist in the commercial wax. The three transitions can be seen from its DSC thermogram shown in Figure 1, which includes the solid-solid (crystalline-RI), rotator-rotator (RI-RII, indicated by the arrow), and the melting transition (RII-melt). Results of dynamic mechanical spectroscopy for the blend and commercial wax samples are shown in Figures 2 and 3, respectively. Figures include the storage modulus (E′), a measure of a material’s ability to store elastic energy, loss modulus (E′′), a measurement of its tendency to dissipate deformation energy through molecular motion, and loss tangent (E′′/E′), the balance between viscous and elastic behavior, all given as a function of sample temperature. By matching observed thermal transitions of the n-alkane systems from DSC analysis with mechanical property data, it is apparent that the dynamic moduli of these materials are strongly tied to rotator phase transitions. The mechanical properties of the blend and wax appear to move toward premesophase values after a transition, with the amount of recovery dependent on the stability of the rotator phase. As was discussed in the
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Figure 2. Dynamic mechanical heating curves for the C21C23 alkane blend.
Figure 3. Dynamic mechanical heating curves for the commercial wax.
Introduction, the thermal spacing between transitions in a typical n-alkane system is such that it may preclude the isolation of properties associated with the actual rotator phases. The C21-C23 blend and commercial wax were specifically selected for this study because of the extended stability of their rotator phases over a broad temperature span. This produces a range of dynamic mechanical properties over which to study the velocity dependence of contact angle measurements. Dynamic Contact Angles for Water on n-Alkane Surfaces. It is generally accepted that contact angle is dependent on the velocity of the wetting line. Experimental evidence indicates that dynamic advancing angles (θad) increase and dynamic receding angles (θrd) decrease with increases in positive and negative substrate velocities, respectively.11 Figure 4 shows a plot of advancing and receding contact angles as a function of interface velocity for the blend. The measurements were made at a temperature of 30 °C where the substrate is in the RI rotator phase. At the substrate velocities used, which range from 2 to 264 µm/s, the receding angle is relatively constant
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Figure 4. Dynamic contact angles as a function of interface velocity for the C21-C23 n-alkane blend and commercial wax. Measurements were made at 30 °C for the blend corresponding to the RI rotator phase (loss tangent ) 0.27), while contact angles for the commercial wax were measured at 25 °C (crystalline phase, loss tangent ) 0.10).
Figure 5. Dynamic contact angles as a function of interface velocity for Chevron Saturating and Aldrich paraffin waxes. Measurements were made at 25 °C where both materials are in their ordered crystalline phase. Loss tangents of Chevron Saturating and Aldrich paraffin were 0.18 and 0.09, respectively.
and unaffected by velocity, and the dynamic advancing angle converges to this value at low substrate speeds. In other words, the dynamic advancing contact angle extrapolated to zero substrate velocity is equal to the dynamic receding angle, which was constant for all velocities, that is, θa0 ) θr0 ) θrd ) 98°. This same value was found with a static sessile drop measurement. It is also important to note that results were repeatable and unaffected by substrate aging. This was tested using multiple scans at each temperature as well as using plates several days after the coating process. Velocity-dependent dynamic contact angles measured for the commercial wax at 25 °C corresponding to its ordered crystalline phase are also shown in Figure 4. Again, the dynamic receding angle is unchanged and is the same as values found using the static measurement (106°). However, for the commercial wax, dynamic advancing angles appear relatively constant, only demonstrating a slight decrease at the lowest velocities tested. These results contradict the conclusions reported in a previous study of paraffin wax wetting behavior.18 That study extrapolates data from measurements at significantly higher velocities than those used here and reports symmetric dynamic contact angle behavior with both the advancing and receding angles converging to a mean stationary value. Figure 5 is a plot of dynamic advancing and receding contact angles for two other paraffin waxes. These measurements were made at 25 °C where both waxes are in their crystalline state. The results shown in Figures 4 and 5 are representative of what was found for other n-alkane systems tested. All of the waxes had constant θrd values. This is consistent with the work of Hayes and Ralston who studied the wetting and dewetting processes of water on poly(ethylene terephthalate) (PET) surfaces.12 They report velocity-independent dynamic receding contact angles and velocity-dependent advancing angles. The authors conclude that dynamic contact angle behavior (wetting and dewetting) is not symmetric with respect to velocity for water on PET and emphasize the need for
further study of relaxation mechanisms. The reported data from Hayes and Ralston are similar to results found for water on wax for which the dynamic angles do not extrapolate at zero velocity to the same value (e.g., the commercial wax and Aldrich paraffin). This behavior appears to be one extreme for this system with the other being a complete relaxation of θad to the constant θrd value at positive velocities (as demonstrated by the blend and Chevron Saturating Wax). The existence of a “natural displacement velocity” at nonzero substrate speeds, which marks the onset of contact angle velocity dependency, has been previously reported for water on siliconed glass and polyethylene surfaces.19-22 Static contact angle measurements for water on all waxes tested using the sessile drop technique were similar to those reported in the literature14,18,23,24 and in close agreement with constant θrd values. It can be concluded from the results above that the demonstrated wetting behavior is a general result for water on wax, with the constant θrd likely being equal to the equilibrium angle, θ0, for the system. The sharp drop in θad to this equilibrium value with decreasing substrate speed ranges from being nonexistent, resulting in large differences between advancing and receding angles, to occurring at positive velocities. It is also observed that the higher the velocity at which relaxation occurs, the greater the θad values measured. Influence of Plastic Crystalline States on Wetting Behavior. In an attempt to relate mechanical properties of the various phases present in the blend to its wetting
(18) Ablett, R. Philos. Mag. 1923, 46, 244.
(19) Elliot, G. P.; Riddiford, A. C. J. Colloid Interface Sci. 1967, 23, 389. (20) Hansen, R. S.; Miotto, M. J. Am. Chem. Soc. 1957, 79, 1765. (21) Elliot, G. P.; Riddiford, A. C. Nature 1962, 195, 795. (22) Gaudin, A. M.; Witt, A. F. In Contact Angle, Wettability and Adhesion; Advances in Chemistry Series; American Chemical Society: Washington, DC, 1964; Vol. 43, p 202. (23) Johnson, R. E., Jr.; Dettree, R. H. In Contact Angle, Wettability and Adhesion; Advances in Chemistry Series; American Chemical Society: Washington, DC, 1964; Vol. 43, p 136. (24) Stro¨m, G.; Fredericsson, M.; Stenius, P. J. Colloid Interface Sci. 1987, 119, 352.
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Figure 6. Velocity-dependent dynamic contact angle curves for the C21-C23 blend as a function of temperature. Data indicate a progression toward a nearly constant angle of 98° at all temperatures.
Figure 7. Advancing contact angle and mechanical loss tangent versus temperature for the C21-C23 n-alkane blend. Contact angles were obtained at 80 µm/s.
behavior, dynamic contact angles were measured as a function of temperature. Results are shown in Figure 6 for substrate velocities of 20, 30, and 80 µm/s. The measurements cover a temperature range that begins in the material’s ordered crystalline phase, passes through its various plastic crystalline mesophases, and ends as the coated film begins to melt. Curves demonstrate the convergence with decreasing substrate velocity toward a nearly constant angle of 98° at all temperatures, the same value obtained via sessile drop measurements under ambient conditions and that found for dynamic receding angles at all substrate velocities and temperatures. It is evident from the figure that the relaxation of the contact angle to its apparent equilibrium value is dramatically enhanced in various temperature regions that correspond to phase transitions. This is further demonstrated in Figures 7 and 8, which show measured dynamic contact angles at 80 µm/s for water on the alkane blend and
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Figure 8. Advancing contact angle and mechanical loss tangent versus temperature for the commercial wax. Contact angles were obtained at 80 µm/s.
commercial wax substrates, respectively, as a function of temperature. Superimposed on these figures are the corresponding loss tangent curves for the substrates. Results indicate that regions where θad ≈ θrd correspond to local extremes in the loss tangent, which, according to DSC results, are associated with transitions between the various phases present. Given the preceding results, it appears that the velocity dependence of dynamic advancing angles correlates with the substrate’s loss tangent. To investigate this relationship further, temperatures corresponding to a range of loss tangent values surrounding the mesophase region of the commercial wax were selected to run dynamic contact angle measurements. (Due to the variability of loss tangent measurements from sample to sample, a temperature scan with the same material was preferred in order to give a good measure of relative changes in the loss tangent.) The commercial wax was chosen as the coating substrate for this particular study because it provides significant oscillations in dynamic mechanical properties over a short temperature span. Temperatures tested include 25, 39, 45, and 48 °C, which according to the DMS data shown in Figure 3 correspond to loss tangent values of 0.10, 0.78, 0.22, and 0.39, respectively. Figure 9 shows dynamic contact angle versus substrate velocity for the commercial wax at these various temperatures and loss tangent values. The results indicate that the higher the loss tangent, the higher the substrate velocity at which an advancing angle relaxes to its equilibrium value. This is confirmed in Figure 10, which plots the critical interface velocity at which the n-alkane system relaxes to θ0 (taken as the midpoint of the observed drop in measured dynamic contact angle) against loss tangent, producing a linear fit. A further demonstration of the role of loss tangent is found in Figures 4 and 5 where the most rapid contact angle relaxation is observed for the blend (loss tangent ) 0.27) followed by Chevron Saturating Wax (loss tangent ) 0.18), while the commercial wax (loss tangent ) 0.10) and Aldrich paraffin (loss tangent ) 0.09) demonstrate little relaxation. Given that the commercial wax, Aldrich paraffin, and Chevron Saturating Wax are all in their crystalline states, the significance of the loss tangent in resulting dynamic contact angle behavior seems evident. However, in some cases a more direct relationship is observed between contact angle relaxation and extremes in loss tangent
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Figure 9. Dynamic contact angle as a function of three-phase wetting line velocity for the commercial wax for a range of temperatures and dynamic mechanical properties.
Figure 10. Critical interface velocity (see text) as a function of loss tangent for the commercial wax.
values tied to phase transitions. It is also apparent that the magnitude of θad scales with the loss tangent of the substrate. The importance of the loss tangent in determining wetting behavior has been reported previously for studies involving elastomeric substrates. Shanahan and Carre´ have studied extensively spreading dynamics on viscoelastic solids.1,2,25-29 For these types of systems, it is argued that the vertical component of the three-phase or wetting line is capable of lifting a ridge that subsequently disappears as the three-phase line moves past. Because soft polymers usually demonstrate viscoelastic properties with a greater ability to dissipate deformation energy, it is suggested that the work required for ridge formation comes at the expense of flow energy slowing the movement (25) Shanahan, M. E. R.; Carre´, (26) Carre´, A.; Shanahan, M. E. (27) Shanahan, M. E. R.; Carre´, (28) Carre´, A.; Shanahan, M. E. (29) Shanahan, M. E. R.; Carre´,
A. R. A. R. A.
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of the three-phase line toward an equilibrium geometry. Several theoretical studies have argued the feasibility of this mechanism, and the existence of a wetting ridge structure has been reported for various soft polymeric substrates.2,25-31 Results presented here showing enhanced relaxation of the three-phase line with increasing loss tangent and significant differences in the wetting and dewetting mechanism would appear to contradict this theory. However, the polymeric substrates demonstrating slowed wetting were significantly softer than the paraffins tested here, and reported experiments examined the spreading of sessile drops. Thus, the mechanism described for sessile drops on elastomer surfaces may simply not be present in our water-paraffin systems. A theory that has been successfully used to describe velocity-dependent contact angles on low-energy surfaces is the molecular-kinetic model proposed by Blake and Haynes.11 The model describes the three-phase line as a region of rapid molecular exchange involving adsorption and desorption processes, the frequencies of these processes being balanced at equilibrium. In nonequilibrium situations, where there exists a difference between the cosines of the equilibrium and dynamic contact angle (i.e., a nonequilibrium force on the three-phase line), the molecular-kinetic theory attributes the stress with promoting greater adsorption for advancing angle measurements and enhanced desorption in the case of receding angles. This drives the three-phase line in the direction of the induced force toward its equilibrium geometry. Foregoing the mathematical details, the theory predicts that velocity dependency is more pronounced if strong interactions exist between the wetting liquid and solid substrate and/or there are a large number of adsorption sites for the liquid at the interface. The increase in specific volume associated with phase transition will, at the very least, increase the number of adsorption sites slowing the movement of the three-phase line,9 which is counter to the results presented above. Attempts to fit dynamic contact angle data with various forms of the three-phase line deformation model, molecular-kinetic theory, and the Cox theory,32 which attributes wetting behavior to hydrodynamic effects, were also unsuccessful. Discounting the above-mentioned theories, the idea of substrate relaxation, in part by process of elimination, appears to be the most feasible explanation for the observations presented here. A number of studies have demonstrated that surface-molecular rearrangements influence contact angle hysteresis,3,7,33-35 and a recent publication attributed significant differences in hysteresis values (i.e., a kinetic effect) to phase behavior or more specifically the greater freedom of molecules associated with a crystalline/liquid-crystalline transition.36 For nalkane systems, it is apparent from data presented here that an enhancement of stress relaxation at the threephase line is associated with increasing loss tangent values, which in basic terms can be taken as a measure of the ability of molecules in a material to dissipate forces through molecular motions or rearrangements. Often, this type of rearrangement is tied to specific functional groups (30) Extrand, C. W.; Kumagai, Y. J. Colloid Interface Sci. 1996, 184, 191. (31) Andrade, J. D.; King, R. N.; Gregonis, D. E.; Coleman, D. L. J. Polym. Sci., Polym. Symp. 1979, 66, 313. (32) Cox, R. G. J. Fluid Mech. 1986, 168, 169. (33) Chen, Y. L.; Helm, C. A.; Israelachvili, J. N. J. Phys. Chem. 1991, 95, 10736. (34) Nguyen, D. T. Colloids Surf. 1996, 116, 145. (35) Sedev, R. V.; Budziak, C. J.; Petrov, J. G.; Neumann, A. W. J. Colloid Interface Sci. 1993, 159, 392. (36) De Crevoisier, G.; Fabre, P.; Corpart, J.; Leibler, L. Science 1999, 285, 1246.
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on substrate molecules, including aliphatic groups,7,37,38 which arrange themselves in response to a spreading solvent in order to minimize interfacial energy. If this is the case, the extent and kinetics of substrate relaxation for a given surface should also be influenced by solvent properties, a concept that will be further examined in a future publication. Summary and Conclusions The interaction of water with paraffin structures (CnH2n+2) drives a number of behaviors that are important from both an industrial and scientific standpoint. Not only are hydrocarbon chains the primary component found in wax materials, they are also major constituents in many complex systems including liquid crystal phases, biological membranes, and surfactant molecules. Normal alkane systems in the carbon number range of 18-40 are of particular interest because they often possess several plastic crystalline or rotator states. In this paper, the various mesophases present in two n-alkane systems were identified and characterized via thermal and mechanical analysis. Dynamic mechanical spectroscopy measurements performed over temperatures surrounding these states indicate that significant molecular freedom, as determined by the loss tangent, exists within the transition regions. Corresponding dynamic advancing contact angles at low substrate velocities (2-264 µm/s) demonstrate that there exists little velocity dependence for measurements (37) Montgomery, M. E.; Green, M. A.; Wirth, M. J. Anal. Chem. 1992, 64, 1170. (38) Montgomery, M. E.; Wirth, M. J. Anal. Chem. 1994, 66, 680.
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within the transitions, where equilibrium values are rapidly achieved and consistent with static contact angles. However, in crystalline and rotator phase regions, a significant velocity dependency was observed. Thus, it appears that the velocity dependence of contact angles for n-alkane systems is influenced by phase behavior and that loss tangent gauges properties of the substrate that govern its ability to relax toward equilibrium contact angle values. Unlike the velocity-dependent trends measured for advancing angles, dynamic receding contact angles remained fixed at their equilibrium values at all temperatures and velocities tested. As discussed, similar differences between wetting and dewetting processes have been reported previously for water on low-energy polymeric surfaces. Attempts to model results presented here using theories that attribute wetting behavior to substrate deformation and fluid-substrate interactions proved unsuccessful. A hypothesis that does appear to be consistent with our findings concerns a molecular reorientation mechanism, which is likely enhanced by the increased freedom of molecules in transition regions thus reducing the barrier to equilibrium contact angles. A further investigation of how solvents of various pertinent properties affect the ability of n-alkane substrates to dissipate stress at the three-phase line will be the focus of a future publication. Acknowledgment. This research is supported by the Minnesota Agricultural Experimental Station, Project Number 1543-384-3650. LA010258I
