Molecular Restructuring Kinetics and Low-Velocity Wetting of n-Alkane

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Molecular Restructuring Kinetics and Low-Velocity Wetting of n-Alkane Rotator Phases by Polar Liquids Steven J. Severtson* and Michael J. Nowak Department of Wood and Paper Science, Kaufert Laboratory, University of Minnesota, 2004 Folwell Avenue, St. Paul, Minnesota 55108 Received May 20, 2002. In Final Form: August 21, 2002 Advancing and receding contact angles were measured on n-alkane surfaces at substrate speeds below 300 µm/s for dimethyl sulfoxide, formamide, ethylene glycol, 2,2′-thiodiethanol, and glycerol. The existence of plastic crystalline or rotator phases prior to the melting transition for n-alkane solids was utilized to provide a range of substrate molecular freedom. Contact angles for liquid-substrate combinations demonstrated velocity dependence for high interface velocities resulting in steady-state hysteresis. For measurements made on a commercial paraffin substrate in transition between the ordered crystalline and RII rotator phases corresponding to a high mechanical loss tangent, contact angles converged to apparent equilibrium values at low velocities. However, no such relaxation was observed for the ordered crystalline phase, and thus significant differences in relaxation behavior between tested liquids on the paraffin wax were not found for substrates possessing high and low molecular freedom. Differences in restructuring kinetics between liquids were observed on a C21-C23 binary n-alkane blend in the RI rotator phase with a moderate level of molecular freedom. The order of relaxation toward equilibrium contact angles was consistent with the order of liquid-phase molecular correlation times estimated from nuclear magnetic resonance spin-lattice relaxation, T1 inversion recovery measurements. The exception was glycerol, which had the longest correlation time but demonstrated greater convergence of measured advancing and receding contact angles than expected. We speculate that contributions originating from viscous forces due to glycerol’s high capillary number may explain this behavior. Results of this study also demonstrate that water provides for anomalous wetting on n-alkane surfaces, a topic that will be addressed in a future publication.

Introduction In dynamic wetting, the contact angle is often dependent on the velocity at which the liquid phase advances or recedes over the solid substrate. It is generally found that advancing angles increase while receding angles decrease relative to static values with increases in positive and negative substrate velocities, respectively.1 For low wetting velocities below which viscous forces are thought to contribute to observed behavior, induced increases in contact angle hysteresis are often attributed to the restructuring kinetics of either substrate or liquid phases.2-7 When the relaxation rate is slow relative to the rate at which a liquid is forced across a surface, it is argued that nonequilibrium interfacial conditions exist at the liquid front. In a previous publication, we presented dynamic wetting data for water on substrates composed of normal alkane blends.8 Measurements showed that kinetic hysteresis was significantly reduced in the more viscoelastic plastic crystalline states of the alkanes consistent with a substrate reconformation mechanism. In this paper, data are reported for the velocity dependence of dynamic contact angle values of various polar liquids * To whom correspondence should be addressed. E-mail: [email protected]. (1) Blake, T. D. In Wettability; Berg, J. C., Ed.; Marcel Dekker: New York, 1993; Vol. 49, p 270. (2) Andrade, J. D. Surface and Interfacial Aspects of Biomedical Polymers; Plenum Press: New York, 1985; Vol. 1, Chapter 2. (3) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley: New York, 1990; Chapter 10. (4) Chen, Y. L.; Helm, C. A.; Israelachvili, J. N. J. Phys. Chem. 1991, 95, 10736. (5) Nguyen, D. T. Colloids Surfaces 1996, 116, 145. (6) Sedev, R. V.; Budziak, C. J.; Petrov, J. G.; Neumann, A. W. J. Colloid Interface Sci. 1993, 159, 392. (7) De Crevoisier, G.; Fabre, P.; Corpart, J.; Leibler, L. Science 1999, 285, 1246. (8) Severtson, S. J.; Nowak, M. J. Langmuir 2001, 17, 4990.

on n-alkane surfaces. The liquids studied included glycerol, formamide, ethylene glycol, 2,2′-thiodiethanol, and dimethyl sulfoxide. Results again indicate that the freedom of substrate species strongly influence wetting dynamics. Differences were also found in the wetting kinetics of tested liquids and appear to be related to the reorientation kinetics of liquid-phase molecules. This was more apparent in plastic crystalline states where the substrates possess moderate viscoelasticity. As discussed in detail elsewhere, normal alkane blends possess novel phase behavior.9,10 Bracketed between their highly ordered crystalline phase and isotropic liquid state, there often exists a series of weakly structured mesophases in which molecules in the crystals have gained rotational freedom while retaining their positional order. These states are referred to as plastic crystalline or rotator phases. The presence of several rotator phases and phase transitions over a short temperature span can produce a rich dynamic mechanical spectrum in which the level of viscoelasticity oscillates.9 With changes in molecular freedom being at the root of these property shifts, normal alkanes provide an excellent substrate for studying the influence of substrate relaxation on dynamic contact angle measurements. Previously reported results showed a correlation between measures of substrate viscoelasticity as gauged by mechanical loss tangent values and observed contact angle relaxation behavior for movement of water over n-alkane blends. Dynamic wetting experiments for substrate velocities of 2-264 µm/s indicated that contact angle behavior ranged from little or no relaxation for low loss tangent values to complete relaxation to presumably (9) Nowak, M. J.; Severtson, S. J. J. Mater. Sci. 2001, 36, 4161. (10) Ungar, G.; Masic, N. J. Phys. Chem. 1985, 89, 1036.

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equilibrium for high values. Although these results are consistent with a substrate relaxation mechanism, measurements were made solely with water, and thus the possibility that the liquid-phase behavior contributes to observed contact angle relaxation was not examined. A number of studies have investigated the influence of liquid properties on dynamic wetting behavior.1,11-20 Of particular pertinence is the work by Phillips and Riddiford whom measured low-rate dynamic contact angles of water, formamide, glycerol, and methylene iodide on siliconed glass and polyethylene substrates.11 The authors advocate the concept of a natural displacement velocity above which the liquid molecules of the wetting line do not have sufficient time to order with respect to the changing interface. Thus, the induced kinetic hysteresis is attributed to nonequilibrium, solid-liquid interfacial energies resulting from the relaxation behavior of the liquid rather than reorientation of substrate species. The authors argue that natural displacement velocities are a property of the liquid-substrate combination with the substrate participating through its ability to influence liquid structure and interfacial relaxation kinetics. In their analysis, the substrate is considered a static entity in the wetting process. As will be presented here, dynamic contact angle data collected for a series of liquids on n-alkane surfaces indicate that both the liquid and substrate can contribute to observed wetting behavior. Results are presented for experiments examining variations in both correlation times of the liquid phase and the loss tangent of the substrate. These experiments were made possible by the broad range of viscoelasticities and relatively constant equilibrium contact angles found in the mesophase region of n-alkane blends. Also important is the lack of solubility of alkanes in the polar liquids tested reducing or eliminating the possibility of substrate swelling and liquidphase contamination in measurements. The combination of both substrate and liquid variation provides useful information for understanding low-velocity wetting on nonpolar substrates. Experimental Section Materials. The commercially available paraffin wax used in this study was Chevron Refined Wax (Chevron, San Francisco, CA), herein referred to as the commercial wax. The carbon number distribution measured using gas chromatography indicates the distribution ranges from C18 to C30 normal alkanes, with a majority of the carbon numbers falling between C22 and C24.9 Hydrocarbons n-heneicosane (C21H44) and n-tricosane (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 surface energies of the commercial wax and binary mixture were determined to be 25.8 and 25.4 mJ/m2, respectively, using the Fowkes approach. All nonaqueous liquids used for contact (11) Phillips, M. C.; Riddiford, A. C. J. Colloid Interface Sci. 1972, 41, 77. (12) Hayes, R. A.; Ralston, J. J. Colloid Interface Sci. 1993, 159, 429. (13) Kasemura, T.; Takahashi, S.; Nakane, N.; Maegawa, T. Polymer 1996, 37, 3659. (14) Petrov, P. G.; Petrov, J. G. Langmuir 1992, 8, 1762. (15) Schneemilch, M.; Hayes, R. A.; Petrov, J. G.; Ralston, J. Langmuir 1998, 14, 7047. (16) Blake, T. D.; DeConinck, J.; D’Ortona, U. Langmuir 1995, 11, 4588. (17) Hayes, R. A.; Ralston, J. Colloids Surf. A 1994, 93, 15. (18) Schull, K. R.; Karis, T. E. Langmuir 1994, 10, 334. (19) Tsekov, R.; Matsumura, H.; Kawasaki, K.; Kambara, M. J. Colloid Interface Sci. 2001, 233, 136. (20) Elliot, G. P.; Riddiford, A. C. J. Colloid Interface Sci. 1967, 23, 389.

Severtson and Nowak angle measurements were obtained with purities >99% from Aldrich Chemical Co. (Milwaukee, WI) including spectrophotometric grade glycerol, formamide, ethylene glycol, 2,2′-thiodiethanol, and dimethyl sulfoxide. The liquids were utilized without further purification. Water used in experiments was purified and had a pH of 5.85, conductivity of 0.3 µS, and a resistivity of 18.2 MΩ‚cm. Dynamic Mechanical Spectroscopy Measurements. The mechanical properties of wax were investigated using a PerkinElmer (Norwalk, CT) 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 equipped with 10 mm plates. All experiments were normalized to the dimensions of the sample (rectangular bar, ∼6 mm long × 2 mm wide × 2 mm thick). Nitrogen was used to purge the system at a flow rate of 30 mL/min. A frequency of 1.0 Hz, static stress of 2200 mN, and a dynamic stress of 2000 mN were programmed. The 1.0 Hz frequency provided the best resolution of phase transitions. The thermal locations of transitions were consistent with those found using differential scanning calorimetry and independent of frequency over the range available to the DMA (0.01-50 Hz). Contact Angle Measurements. Utilizing a technique based on the Wilhelmy balance principle, the dynamic contact angles of wax (advancing and receding) were measured using a Cahn Instruments (Madison, WI) DCA-322 system. The supplied WinDCA software automatically performed contact angle and surface tension analyses. Prior to testing, the liquid phase 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 liquid 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 wax surface was prepared by dipping a cleaned plate into a melt solution maintained approximately 10 °C above the melting point of wax. The plate was then carefully removed and allowed to cool in the inverted vertical position to form a thin film substrate. (Similar dip-coating approaches have been shown to produce wax surfaces for contact angle measurements with little or no hysteresis behavior.21) A new, coated plate was used for each temperature studied. Interface velocities were varied from 2 to 264 µm/s and the uncertainty in measured contact angles over this range was estimated to be (2°. It is also important to note that the dynamic wetting results were unaffected by substrate swelling. This was verified by running successive scans with the same plate as well as monitoring weight variations as a function of time via static Wilhelmy balance measurements and gravimetric determinations. The surface tension of the liquid phase was also determined after contact angle experiments to check for substrate solubility. No differences were detected for any of the liquids reported. Nuclear Magnetic Resonance Relaxation Measurements. Nuclear spin-lattice relaxation, T1 inversion recovery experiments were performed on all liquids tested for contact angle behavior using a 1.5 T, Apollo NMR spectrometer (Tecmag, Houston, TX). A standard π-τ-π/2 inversion recovery pulse sequence was utilized with the following acquisition parameters: pulse width of 484 µs, frequency of 63 MHz, acquisition process consisting of 1027 points at 416 µs/point, τ values ranging from 100 µs to 10 s, and a time to repeat (TR) of 20 s. Free induction decay (FID) signals were obtained 200 µs following the 90° pulse. Fourier transform analysis of the experimental FID signals and subsequent calculation of T1 relaxation times were completed using the supplied NTNMR software, version 2.2b. Temperature inside the NMR magnet was determined to be (21) Ray, B. R.; Bartell, F. E. J. Colloid Sci. 1953, 8, 214.

Wetting Kinetics of n-Alkane Rotator Phases

Figure 1. Differential scanning calorimetry heating curve and loss tangent as a function of temperature for the C21-C23 n-alkane blend. The RI-RII rotator phase transformation is marked by an arrow in DSC data. 22 °C. All liquids were placed in glass vials and sealed prior to measurement.

Results and Discussion Influence of Phase on Aqueous Wetting of nAlkane Substrates. Previous work found that dynamic contact angles for water on n-alkane compounds are strongly influenced by phase and phase transitions.8 This was demonstrated for a commercial paraffin wax containing a broad distribution of n-alkanes (C18 to C32) and a 79:21 (C21:C23) molar ratio, binary mixture. The phase behavior of these substrates was characterized using a combination of differential scanning calorimetry (DSC), X-ray diffraction (XRD), and dynamic mechanical spectroscopy (DMS). Simply stated, DMS measures the response of a material to small sinusoidal oscillatory strains to gauge viscoelastic properties. For this study, the most important of these properties is the loss tangent, which is the ratio between the loss modulus and storage modulus. These quantities indicate the ability of a material to dissipate and store deformation energy, respectively, and local maximums in their ratio are often used to identify regions of enhanced molecular freedom associated with phase transitions. A summary of thermal analysis results for the binary mixture and commercial wax are shown in Figures 1 and 2, respectively. The DSC heating thermograms and superimposed mechanical loss tangent curves indicate multiple phase transitions. At low temperatures, both n-alkane systems possess an orthorhombic crystal structure in which molecules primarily adopt a staggered conformation and the alkane chains are stacked in layers with their long axes arranged parallel to the layer normal. For the binary mixture, the first transition centered at 22 °C on the DSC curve involves the movement from the crystalline phase into the RI rotator phase. Rotator or plastic crystalline phases are characterized by increased rotational freedom without the loss of positional order. The RI phase possesses a face-centered orthorhombic lattice (often described as distorted-hexagonal) with a bilayer (ABA) stacking arrangement. Immediately prior to melting, the binary mixture undergoes a second, much less endothermic solid-solid phase transition (indicated by the arrow). This corresponds to the transformation into the RII rotator phase, where molecules are stacked in a trilayer (ABC) sequence with an average hexagonal

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Figure 2. Differential scanning calorimetry heating curve and loss tangent versus temperature for the commercial paraffin wax (RI-RII transition marked by arrow).

symmetry. The final transition indicated by the DSC thermogram and dynamic mechanical spectrum is the melting transition, which is centered at approximately 42 °C. Thermal analysis of the commercial wax indicates that, similar to the binary mixture, three transitions are demonstrated involving two solid-solid phase changes (crystalline-RI and RI-RII) and melting (RII-melt). These substrates were selected for study because they demonstrate unique mesophase behavior. Normal alkanes typically possess several different rotator phases over a short temperature span prior to melting and thus only exist in any one particular plastic crystalline phase for a few degrees. This makes the study of their properties difficult due to overlap with the surrounding transitions. As shown in Figures 1 and 2, these systems each possess a rotator phase that is stable over an extended temperature interval. (These are the RI and RII phases for the binary mixture and commercial wax, respectively.) This stability provides for a broad range of phase conditions on which to study the spreading behavior of a liquid. The rate at which water wets n-alkane surfaces appears to correlate with the loss tangent of the substrate. Figure 3 reviews results demonstrating this relationship where advancing contact angles for water measured at interface velocities of 20, 30, and 80 µm/s for the binary mixture are plotted as a function of temperature. Superimposed are mechanical loss tangent values for the substrate. The figure shows that dynamic contact angle values are minimized in regions where the loss tangent is maximized. These results suggest that the velocity dependence of the advancing contact angle is related to the same factors controlling the dynamic mechanical properties of the substrate, likely, the freedom of substrate species. It was also found that receding angles were independent of substrate velocities and fixed at their equilibrium values. Influence of Phase on Nonaqueous Wetting of n-Alkane Substrates. To investigate the impact of both substrate and liquid-phase properties on wetting behavior for paraffins, a variety of paraffin-liquid combinations were selected to perform velocity dependent, dynamic contact angle measurements. The substrates tested included the C21-C23 binary mixture in the RI rotator phase (30 °C, loss tangent ) 0.27) and the commercial wax in both the ordered crystalline phase (25 °C, loss tangent ) 0.10) as well as at the transition temperature between the ordered crystalline phase and RII rotator phase

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Figure 3. Velocity-dependent dynamic contact angle curves for the C21-C23 blend as a function of temperature. The loss tangent from DMS results is superimposed. Data indicate a progression toward a nearly constant equilibrium contact angle at low substrate velocities and within transition regions.

(39 °C, loss tangent ) 0.78). At the measurement temperatures, the substrates possess moderate, low and high levels of molecular freedom, respectively. The liquids tested included water, dimethyl sulfoxide, formamide, ethylene glycol, 2,2′-thiodiethanol, and glycerol. These liquids were selected to provide a range of properties believed to influence dynamic wetting (Table 1) and are limited to liquids that did not solubalize or swell paraffin wax. The values listed in Table 1 were acquired under ambient conditions unless otherwise noted. Figures 4 and 5 show velocity dependent contact angle data for the liquids in Table 1 all measured on the ordered phase and transition region of the commercial paraffin, respectively. The general behavior of the velocity dependent curves is consistent with a more rapid relaxation toward equilibrium contact angles associated with greater molecular freedom of the n-alkane solid. Results for the ordered, low loss tangent substrate are shown in Figure 4. For this substrate and temperature, glycerol is the only liquid to demonstrate a significant change in advancing or receding contact angles over the range of velocities tested. Other liquids demonstrate constant advancing and receding contact angles and thus a constant contact angle hysteresis for all substrate velocities. In contrast to this, Figure 5 shows that all of the liquids converge toward equilibrium contact angles at low substrate velocities for measurements made on the commercial paraffin in a phase transition region. (Equilibrium contact angles were determined using static sessile drop measurements and are in agreement with values reported in the literature for all wax-liquid combinations presented here.21,22-25) This region would be expected to provide significant molecular freedom given its thermal location as evidenced by a local maximum in the loss tangent (Figure 2). Thus, from the commercial wax dynamic wetting results it appears that for extremes in the freedom of substrate species it is difficult to draw out differences in relaxation behavior (22) Jan´czuk, B.; Białopiotrowicz, T.; Zdziennicka, A. J. Colloid Interface Sci. 1999, 211, 96. (23) Ablett, R. Philos. Mag. 1923, 46, 244. (24) 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. (25) Stro¨m, G.; Fredericsson, M.; Stenius, P. J. Colloid Interface Sci. 1987, 119, 352.

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due to variation in liquid-phase properties. Data indicate complete relaxation to equilibrium contact angles when the substrate possesses enhanced molecular freedom to the lack of significant movement toward equilibrium values for more ordered structures. It seems reasonable then that a surface with a moderate level of molecular freedom may provide intermediate relaxation behavior. However, the phase behavior for the commercial wax is such that a temperature does not exist in which the substrate possesses the desired mechanical properties while being far removed from phase transition regions. It is the unusually broad temperature stability of the RI rotator phase found for the strategic blend of C21 and C23 n-alkanes that makes this investigation possible. Figure 6 shows velocity dependent contact angles measured for the liquids on the C21-C23 binary mixture. Various levels of contact angle relaxation are observed for the different liquids ranging from complete relaxation of water at positive substrate velocities to the lack of any measurable relaxation found for ethylene glycol. Glycerol, formamide, and dimethyl sulfoxide all show convergence of advancing and receding contact angles toward values that are consistent with equilibrium for these systems, while 2,2′-thiodiethanol exhibits little relaxation over the velocities tested. Although surface tension and viscosity are commonly reported as parameters that control wetting behavior (e.g., capillary number),1,14,18,26-30 no dependency was found here. A qualitative relationship does appear to exist between contact angle relaxation and molecular correlation times. The correlation times reported in Table 1, denoted τc, are first-order approximations from measured nuclear spin-lattice, T1 relaxation constants.31 The correlation time for a liquid is a measure of the timeaveraged rotational molecular motion, with shorter τc values indicating a faster reorientation of molecules in solution.31-34 Thus, analogous to viewing the loss tangent as a measure of the freedom of substrate species to reorient in response to changing interfacial conditions, correlation times can be considered a measure of the freedom of liquidphase molecules to do the same. However, in both cases the quantities are bulk parameters and their use as an indication of interfacial behavior is not strictly correct. Here, we are assuming the correlation times are simply a first approximation to interfacial behavior, just as loss tangent values appear to indicate the interfacial freedom of substrate species. The liquids that demonstrate relaxation of measured contact angles in Figure 6, with the exception of glycerol, also possess the lowest correlation times, and the kinetics and extent of this relaxation process appear consistent with differences in τc values. For example, water, dimethyl sulfoxide and formamide have τc values of 0.25 × 10-11, 0.30 × 10-11, and 0.33 × 10-11 s, respectively, and water relaxes the fastest followed by the similar relaxation behavior of dimethyl sulfoxide and formamide. The short correlation time measured for water reflects its small (26) Hamraoui, A.; Thuresson, K.; Nylander, T.; Yaminsky, V. J. Colloid Interface Sci. 2000, 226, 199. (27) Cox, R. G. J. Fluid Mech. 1986, 168, 169. (28) Van Giessen, A. E.; Bukman, D. J.; Widom, B. J. Colloid Interface Sci. 1997, 192, 257. (29) Carre´, A.; Shanahan, E. R. J. Colloid Interface Sci. 1997, 191, 141. (30) Extrand, C. W.; Kumagai, Y. J. Colloid Interface Sci. 1997, 191, 378. (31) Hertz, H. G. Prog. Nucl. Magn. Reson. Spectrosc. 1967, 3, 159. (32) Frolov, V. V. In Nuclear Magnetic Resonance; Leningrad University Publishers: Leningrad, 1969; Vol. 3, Chapter 2. (33) Packer, K. J. Prog. Nucl. Magn. Reson. Spectrosc. 1967, 3, 87. (34) Bloembergen, N. Nuclear Magnetic Relaxation (Reprint Volume); W. A. Benjamin, Inc.: New York, 1961; Chapter 4.

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Table 1. Properties of the Polar Liquids Tested for Dynamic Contact Angle Behaviora liquid phase

M (g/mol)

F (g/cm3)

γlv (mN/m)

η (cP)

Ca × 104

T1 inversion recovery (s)

τc × 1011 (s)

water dimethyl sulfoxide formamide ethylene glycol 2,2′-thiodiethanol glycerol

18.00 78.13 45.04 62.07 122.2 92.09

1.00 1.10 1.13 1.11 1.22 1.26

72.8 44.0 58.7 47.8 54.7 63.8

0.89 1.99 3.34 16.1 0.65 1412

0.32 1.19 1.50 8.89 0.31 584

2.77 ( 0.03 ms 2.30 ( 0.08 ms 2.07 ( 0.04 ms 0.36 ( 0.02 ms 0.28 ( 0.01 ms 0.06 ( 0.00 ms

0.249 0.300 0.333 1.917 2.464 11.50

a The capillary number is defined as Ca ) ην/γ, where ν is the interface velocity of the wetting experiment. Liquid-phase capillary numbers were calculated using an interface velocity of 264 µm/s.

Figure 4. Velocity-dependent contact angles measured for a range of polar liquids on the commercial paraffin wax in the ordered crystalline phase (25 °C, loss tangent ) 0.10).

Figure 5. Dynamic contact angles versus interface velocity for the same liquids shown in Figure 4 measured at 39 °C, which corresponds to the plastic crystalline phase transition of the commercial paraffin wax (loss tangent ) 0.78).

molecular mass and low viscosity, which is consistent with the unusually fast relaxation to equilibrium contact angles of 98° found for interface velocities up to 20 µm/s. Excluding glycerol, the liquids with longer correlation times than formamide do not demonstrate significant contact angle relaxation. It is understood that molecular correlation times are quite fast and thus intuitively would not be expected to contribute to wetting kinetics. The differences in relaxation behavior between liquids found here and their apparent connection to correlation times may be a result of consecutive interactions between the substrate and liquid phase that are only evident under certain

Figure 6. Dynamic contact angle as a function of interface velocity for a series of polar liquids on the RI rotator phase of the alkane blend (30 °C, loss tangent ) 0.27).

substrate conditions. The wetting behavior of glycerol in Figures 4 and 6 is difficult to explain with the current data. Glycerol possesses a τc that is significantly longer than any of the other liquids tested, which would be expected from its high viscosity. This high viscosity also means that glycerol has a capillary number that is significantly larger, nearly two orders of magnitude so, than those of the other liquids (Table 1). In previous wetting studies it was found that the dynamic contact angle was independent of flow geometry for capillary numbers below about 5 × 10-3.35-39 This falls between values for glycerol and the other polar liquids tested for interface velocities as slow as 20 µm/s. Thus, it is possible that viscous forces, which are likely negligible for the other liquids, contribute to or even govern the observed relaxation behavior of glycerol. The data presented in Figures 4-6 indicate that differences in wetting behavior between liquids are not drawn out at low or high levels of molecular freedom for n-alkane substrates. It is only when the substrate possesses moderate molecular freedom that these differences become apparent. Although loss tangent has been used here to gauge the relative level of molecular freedom and the potential for contact angle relaxation in a given substrate, it would appear that the combination of this quantity with consideration of the phase and proximity to phase transitions is required for this assessment. For example, greater contact angle relaxation can be found for systems in their phase transition regions even at lower loss tangent values (Figure 3). In addition, while the (35) Ngan, C. G.; Dussan, E. B. J. Fluid Mech. 1982, 118, 27. (36) Ngan, C. G.; Dussan, E. B. J. Fluid Mech. 1989, 209, 191. (37) Kafka, F. Y.; Dussan, E. B. J. Fluid Mech. 1979, 95, 539. (38) Bach, P.; Hassager, O. J. Fluid Mech. 1985, 152, 173. (39) Legait, B.; Sourieau, P. J. Colloid Interface Sci. 1985, 107, 14.

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thermal locations of identified phase transitions are relatively independent of deformation frequency in DMS experiments, the magnitudes of extracted dynamic mechanical properties are not and, as would be predicted, loss tangent values decrease with increasing frequency. Finally, the data presented here indicate anomalous wetting of n-alkane surfaces by water relative to other polar liquids. Phillips and Riddiford reported dynamic advancing angle data for water on low-energy surfaces.11 In comparison to other liquids tested, the authors found that water produced the greatest dependency on wetting velocity as measured by the change in the advancing angle over the range of velocities tested. They also found that water was the only liquid that demonstrated an absence of velocity dependency for the slowest velocity region. From data presented in the previous section it is evident that water behaves in a similar fashion on n-alkane substrates. Furthermore, in our measurements water demonstrates

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asymmetric wetting behavior with the receding contact angle possessing no velocity dependency regardless of the substrate speeds and conditions. This is in contrast to the symmetric wetting behavior observed for all other liquids tested. Future work will examine if the wetting behavior for water is associated with its unique ability to undergo significant rearrangements in the presence of nonpolar solutes and macroscopic surfaces. Acknowledgment. The authors thank Bruce Hammer of the Center for Interdisciplinary Applications in Magnetic Resonance, University of Minnesota, for help with NMR relaxation measurements. This research is supported by the Minnesota Agricultural Experimental Station, Project No. MIN-43-050. LA020471U