Effect of Surface Energy on Freezing Temperature of Water - ACS

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Effect of Surface Energy on Freezing Temperature of Water Yu Zhang, Emmanuel Anim-Danso, Selemon Bekele, and Ali Dhinojwala* Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States S Supporting Information *

ABSTRACT: Previous studies have found that superhydrophobic surfaces are effective in delaying freezing of water droplets. However, the freezing process of water droplets on superhydrophobic surfaces depends on factors such as droplet size, surface area, roughness, and cooling rate. The role of surface energy, independent of any other parameters, in delaying freezing of water is not understood. Here, we have used infrared-visible sum frequency generation spectroscopy (SFG) to study the freezing of water next to solid substrates with water contact angles varying from 5° to 110°. We find that the freezing temperature of water decreases with increasing surface hydrophobicity only when the sample volume is small (∼10 μL). For a larger volume of water (∼300 μL), the freezing temperature is independent of surface energy. For water next to the surfaces with contact angle ≥54°, we observe a strong SFG peak associated with highly coordinated water. This research sheds new light on understanding the key factors in designing new anti-icing coatings. KEYWORDS: hydrophobicity, surface energy, surface modification, water, ice, freezing

1. INTRODUCTION Controlling the heterogeneous nucleation of ice next to solid surfaces is of both scientific and technological importance in many areas, including cloud formation,1,2 snow making, and improving the freezing tolerance of both crops and animal tissues,3 as well as for the design of ice-repellent coatings for wind turbine blades, power lines, and aircrafts.4−6 On the basis of classical nucleation theory, the rate of heterogeneous nucleation depends on several factors including the volume of water, surface area, surface energy, heat of melting, and temperature. The majority of studies reported in the literature are measuring freezing of water droplets on substrates with different chemical composition, surface energy, and roughness.2,7−13 Although this is an easier experiment to conduct, the interpretation of the data to understand the effect of surface energy is more difficult. For example, comparing a droplet of same volume on a hydrophobic and hydrophilic surface will also involve different interfacial areas (solid−liquid and liquid− air) and this will influence both the rates of surface nucleation and heat transfer. Therefore, one of the most fundamental questions of how surface energy influences the freezing temperature and nucleation rate has not been unambiguously answered and in some cases the conclusions have been contradictory. For example, the ice nucleation rate on a hydrophobic surface is reported to be higher than that on a hydrophilic surface.14,15 However, molecular dynamics simulations have shown that surface roughness dominates over surface hydrophilicity in predicting ice heterogeneous nucleation ability.7,16 In contrast, there have been reports where water droplets have lower freezing temperatures,17,18 longer freezing time,9,10,19 or slower growth rates20 next to surfaces with higher hydrophobicity. In previous droplet experiments, the contact area changes with changes in surface energy, which complicates the analysis. © XXXX American Chemical Society

Here, in our new experimental geometry, water is confined in sample cells (10 and 300 μL) with a well-defined surface area and thickness, which are independent of the surface energy of the substrate. We have systematically studied the freezing temperature of water next to surfaces with water contact angles from 5° to 110° in this sample cell consisting of the same volume and solid−liquid surface area. A surface-sensitive sum frequency generation (SFG) technique is used to measure the structure of interfacial water and ice as a function of surface energy. In addition we have measured the freezing temperature of water as a function of surface energy, which provides important information on how the structure of interfacial water affects nucleation kinetics. This research provides new insight on the mechanism behind lower freezing temperatures of water next to hydrophobic surfaces, understanding of which is crucial in designing ice-repellent surfaces.

2. EXPERIMENTAL SECTION 2.1. Materials. Epoxy-silane ((3-glycidoxypropyl)trimethoxysilane, ≥98%, Mw = 236.34 g/mol), PETS-silane (trichloro(phenethyl)silane, ≥95%, Mw = 239.60 g/mol) and FAS-silane (trichloro(1H,1H,2H,2Hperfluorooctyl)silane, ≥97%, Mw = 481.54 g/mol) were purchased from Sigma-Aldrich and used as received. PEG-silane (2-[methoxy(polyethyleneoxy)9−12 propyl]trimethoxysilane, ≥90%, Mw = 591−723 g/mol) was purchased from Gelest and used as received. PVNODC (poly(vinyl n-octadecyl carbamate-co-vinyl acetate)) was provided by 3 M Corp. and was used as received (Mw = 70 kg/mol and Mw/Mn ≈ 3.0). In PVNODC, the ratio of the mole fraction of n-octadecyl carbamate units to the vinyl acetate side chains was ≈90%, as confirmed by NMR. Received: February 19, 2016 Accepted: June 17, 2016

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DOI: 10.1021/acsami.6b02094 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Chemical structures of silanes and polymer used to modify sapphire substrates. The static water contact angles are measured using the procedure described in the text, shown as black fonts in the figure. The values shown as green fonts are the root-mean-square roughness of the surfaces, measured by atomic force microscopy (AFM). 2.2. Preparation and Characterization of the Self-Assembled Monolayers. We have used 60° equilateral sapphire prisms (15 mm × 15 mm × 15 mm × 10 mm, Meller Optics, Inc.) as substrates. Each prism was cut with the c-axis parallel to the plane of incidence to avoid any changes in the polarization of the incident and reflected laser beams. The sapphire prisms were heated to 700 °C for 3 h in an oven and then exposed to air plasma (Harrick Plasma, PDC-32G) for 4 min to remove organic contaminants. The prisms were then immersed in a 90 °C piranha solution (3:1 concentrated sulfuric acid/30 vol % hydrogen peroxide) for 30 min, then rinsed multiple times with ultrapure water (18.2 MΩ/cm from a Millipore filtration system). After rinsing, the sapphire substrates were dried under a stream of dry nitrogen, then used directly in the bare sapphire/water experiments, or as substrates to prepare self-assembled monolayers (SAMs). The clean sapphire substrates were immersed in the following silane solution (1 vol % PEG-silane, Epoxy-silane, PETS-silane in anhydrous toluene for 16 h, 2 mM FAS in anhydrous n-hexanes for 3 h) to form the SAMs in nitrogen-filled reaction flasks. The substrates were rinsed several times with toluene, n-hexane and ethanol, then placed in ethanol for 10 min in an ultrasonic bath to remove any excess silanes. The PEG-SAM formed was dried overnight at ambient conditions before measurements. The other SAMs samples were annealed at 130 °C for 2 h. After preparation, samples were stored in a vacuum desiccator. The SAMs were characterized using X-ray photoelectron spectroscopy (XPS) and the scans are provided in Figures S1−S9 in the Supporting Information. We have also characterized the surfaces by measuring water contact angles before and after the freezing experiments. Ultrapure water (18.2 MΩ/cm) was used for the contact angle measurements (Ramé-Hart Instruments Advanced Goniometer model). A droplet of 10−12 μL of water was deposited on the surface and the static contact angle was measured after 5 s. A clean sapphire surface was used as a control for the ≤5° water contact angle sample and the contact angle images for the surfaces have been presented in the Supporting Information (Figure S10). A minimum of five

individual contact angle measurements were taken for each sample and the average and ±1 standard deviation are presented in Figure 1. The roughness of all the surface types were characterized using atomic force microscopy (AFM). The root-mean-square roughness of the samples is shown as the green fonts in Figure 1. The AFM images and further information are provided in the Supporting Information section 2, Figures S11 and S12. By using t test (setting reliability level as 5%), we find the root-mean-square roughness for the uncoated, SAM-coated and PVNODC-coated sapphire surfaces are similar. 2.3. Spin Coating of PVNODC Sample. The flat surface of a sapphire prism was spin coated with a 4 wt % solution of PVNODC in toluene at a speed of 3000 rpm for 60 s at 35 °C. The polymer films were annealed in vacuum for 3−4 h at 110 °C. A film thickness of 300−400 nm was measured on a silicon substrate using the same spin coating and annealing conditions by a spectroscopic ellipsometer (VASER Spectroscopic Ellipsometer manufactured by J. A. Woolam Co). 2.4. SFG Measurements. The details of the SFG system used in this work have been described in a previous publication.21 In brief, a picosecond Spectra Physics laser system with tunable ∼3.5 μJ IR beam (2000−3800 cm−1, 1 ps pulse width, 1 kHz repetition rate and a diameter of 100−200 μm) is overlapped with a ∼70 μJ visible beam (800 nm in wavelength, 1 ps pulse width, 1 kHz repetition rate, and a diameter of 1 mm). A home-built computer-controlled motorized delay stage is used to maintain the temporal delay as the IR beam is scanned from 2700 to 3800 cm−1. The SFG signals were acquired using a 5 s averaging time and at intervals of 10 cm−1. Due to the low IR beam energy used in the water experiments, laser heating of ice was negligible. The melting temperature of the ice was close to 0 °C. The temperature of the sample cell was calibrated based on the procedure described in our previous publication22 and SI (section 9). The SFG experiments were performed using equilateral 60° angle sapphire prisms at or near total internal reflection. A photomultiplier tube connected to a 0.5 m long spectrometer was used to collect the SFG signals. The spectra were collected at IR incidence angles of 16°, B


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Figure 2. Selected SFG spectra collected in SSP polarization for water (open circles, red) and ice (open triangles, black) collected using a 10 μL cell next to bare sapphire (a), PEG-SAM (c), epoxy-SAM (e), PETS-SAM (g), FAS-SAM (i) and PVNODC (k), respectively. The solid lines are fits using the Lorentzian equation and the results from the fits are summarized in Tables S2−S7 (Supporting Information). The SFG intensity at ∼3100 cm−1 in SSP polarization during heating (open circles, red) and cooling cycles (open triangles, black). The panels b, d, f, h, j and l correspond to sapphire, PEG, epoxy, PETS, FAS and PVNODC, respectively. The transition temperature is determined as a midpoint between a sharp increase and decrease in SFG intensity, same way as our previous publication.26 laser. The polarization reported in this work is SSP (s-polarized SFG output, s-polarized visible input and p-polarized IR input). A model to interpret SSP polarization results in internal reflection geometry has been provided in a previous publication.23

18° and 42°, with respect to the surface normal of one of the prism face, to probe the H2O/(SAMs coated) sapphire, H2O/PVNODC and air/polymer interfaces, respectively. The incident angle of the visible laser beam was ∼1.5° lower than the incident angles for the tunable IR C


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ACS Applied Materials & Interfaces We have used a Lorentzian fitting function to fit our data:24

ISFG ∝ χeff,NR +

∑

Aq ωIR − ωq − i Γq

sapphire.28 The water spectrum (red curve, open circles) shows three peaks around 3200, 3450 and 3700 cm−1. The two broad peaks at ∼3200 and ∼3450 cm−1 have been assigned to the strongly tetrahedrally coordinated hydrogen-bond stretch and loosely coordinated hydrogen-bond stretch, respectively.29−33 An alternate assignment for the water peaks has been provided in the published literature.34,35 For water/sapphire interfaces, the sharp peak located at ∼3700 cm−1 has been assigned to the surface hydroxy groups on the sapphire surfaces.24,28,36 Recently, part of this ∼3700 cm−1 peak has also been attributed to a free OH stretch of interfacial water molecules at the water/ Al2O3 interface.37 The black curve with open triangle markers shows the ice spectra with a sharp peak located at ∼3100 cm−1, which is assigned to highly ordered hydrogen-bonded OH stretching modes in ice Ih crystals.28,30,38 Compared to the water spectra, the ice peak next to bare sapphire is red-shifted and has higher intensity, indicating the hydrogen bonding in ice is stronger than that of liquid water at the interface. 3.1.2. H2O/Coated Sapphire Substrates. Figure 2c,e,g,i,k shows the spectra in SSP polarization for interfacial water and ice next to coated sapphire substrates. These spectra include the CH and OH stretching regions. The peak assignments for these stretching modes are summarized in Table 1. No clear peaks

2

(1)

where Aq, ωq and Γq are the amplitude, angular frequency and damping constant of the qth vibrational resonance. χeff,NR describes the nonresonant contribution. The use of total internal reflection geometry and scanning the IR beam over a broad range of wavelength could result in Fresnel factors that are a function of wavelength.23,25 The changes in SFG intensity as a function of wavenumber at a constant incident angle are provided in Figures S13 and S14 (Supporting Information). The SFG data reported here are not corrected for the changes in Fresnel factors and this variation may affect the magnitude of the 3700 cm−1 peak in comparison to the 3200 and 3400 cm−1 peaks. The refractive indices of ice and water are slightly different and this could also lead to differences in the SFG intensity due to freezing/melting, in addition to the changes due to the structure of interfacial water molecules. We estimated that the Aq of water could be ∼1.4 times higher than the Aq of ice due to differences in Fresnel factors alone between water and ice (Supporting Information, Table S1). 2.5. SFG Sample Holder and Temperature Measurements. Sample Holder. Glassware was cleaned in a base bath and treated for 4 min by air plasma (Harrick Plasma, PDC-32G). The cells (material: 316 stainless steel) were sequentially sonicated for 1 h in different solvents (toluene, acetone, methanol, and deionized water) to remove nonpolar and polar contaminants. The cells were then dried under a stream of nitrogen and cleaned by air plasma for 2 min before the experiments. Two kinds of cell have been designed. For the small volume cell (Supporting Information, Figure S15A), ultrapure water (18.2 MΩ/cm from a Millipore filtration system, pH 6−7) was sealed between the cell and the desired face of the sapphire prism with a Teflon gasket as a spacer (Teflon PTFE from McMaster-Carr Supply Co., thickness ∼0.015 ± 0.002″, area ∼1.2 cm2). A fixed force (7.5 N) was applied to the Teflon spacer for sealing. On the basis of the modulus (≈5 × 108 N/m2) of the Teflon spacer, the thickness of the water film was calculated to be ∼0.04 cm. The surface area of the cell is ∼0.3 cm2. For the small volume cell, we applied one layer of Teflon spacer, which allows for a water volume of ∼10 μL which is similar to the volume of a typical droplet studied in previous freezing studies.9,10,14 The volume of water was increased to ∼300 μL by increasing the thickness of the water layer (by milling a slot in the cell and adding two Teflon spacers). Temperature Measurements. Water condensation is one of the biggest challenges in low temperature measurements, and we have designed a vacuum chamber (pressure of 0.13 Torr) to house the liquid cell to reduce water condensation (Supporting Information, Figure S15B). A customized temperature stage (MK1000 series from Instec Inc.) was used to control the temperature of the liquid cell. A steel dome was designed with CaF2 windows to allow the passage of the visible and IR input beams and the SFG output beam. The cooling and heating experiments were conducted using a rate of 0.3 °C/min. The water and ice spectra were collected after equilibrating the system for 20 min at the specified temperature. We also recorded SFG intensity during the cooling and heating cycles and the transition temperatures were defined as the midpoint of a sharp change in SFG intensity during the cooling and heating cycles.26 At least four individual measurements were taken for each substrate, and the average value and ±1 standard deviation were calculated to estimate the error bars in these measurements. The details for the procedure used to calibrate the temperature are provided in our previous paper27 and the SI (section 9).

Table 1. Relevant Peak Assignments for CH and OH Stretching Mode at the Interfaces peak position [cm−1]

origin CH2 symmetric strecha, d+ CH2 Fermi resonanceb, d+FR CH3 symmetric strechc, r+ CH3 Fermi resonanced, r+FR terminal phenyl groupe highly ordered ice Ih crystalf ice structure next to PETS- and FAS-SAM surfacesg strongly tetrahedrally coordinated hydrogen-bond stretchh loosely coordinated hydrogen-bond stretchi dangling OHj a,c Reference 39. b,dReference 40. eReferences 41,42. 29−32. jReferences 24,28,36.

2855 2920 2875 2950 3050 3100 3180−3200 3100−3200 3450 3600−3720 f,g,h,i

References

were observed in the CH stretching region for the hydrophilic PEG-SAM (Figure 2c) and epoxy-SAM (Figure 2e) interfaces in contact with water. In contrast, sharp and narrow CH peaks were observed at the air/PEG-SAM and air/epoxy-SAM interfaces (Supporting Information, Figures S16 and S17). The absence of the CH peaks in the water/PEG-SAM and water/epoxy-SAM interfaces is consistent with the hypothesis that these hydrophilic monolayers are solvated by water and disorder when in contact with water.43,44 For hydrophobic surfaces in contact with water, we observe sharp peaks associated with CH vibrations (Figure 2g,i,k). The spectra in the hydrocarbon region are very similar for both air and water interfaces (Supporting Information, Figures S18− S20), suggesting that these hydrophobic chains do not disorder when in contact with water. A sharp peak was observed between 3040 and 3060 cm−1 in the SFG spectra for PETSSAM in contact with water (Figure 2g), which is in agreement with the dominant SFG peak in SSP polarization for the phenylsiloxane monolayer and polystyrene observed previously41,42 and corresponding to highly ordered phenyl groups at the interface. We also observed a shoulder between 2850 and

3. RESULTS AND DISCUSSION 3.1. SFG Spectra of Water and Ice Next to Surfaces. 3.1.1. H2O/Sapphire Substrate. Figure 2a shows the SFG spectra in SSP polarization for ice and water next to bare D


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ACS Applied Materials & Interfaces 3000 cm−1, corresponding to the CH2,Sym groups of the alkyl chains. The SFG intensity of the hydrocarbon peaks drop upon freezing (even after correcting the SFG intensity for the changes in Fresnel factor due to phase change), suggesting either disordering or tilting of the phenyl groups toward the surface plane. Figure 2i shows a peak at ∼2930 cm−1 assigned to CH2 vibration for the water/FAS-SAM interface. The peak location is consistent with the SFG spectra for FDS-SAM reported previously45 and the SFG intensity corresponding to CH peaks decreases upon freezing. For water/PVNODC interface (Figure 2k), two major peaks around 2875 and 2940 cm−1 corresponding to the symmetric stretch and the Fermi resonance of CH3 were observed. A weak shoulder near 2850 cm−1 is assigned to CH2,Sym stretch, which is due to gauche defects that are formed when the polymer surface comes in contact with liquid water.40 After freezing to ice, we observe that the intensity of the hydrocarbon peaks decreases and the relative intensity of the CH2 peak versus CH3 peak increases, which suggests increase in gauche defects resulting in higher methylene signals compared to methyl groups. For water/sapphire and water/PEG-SAM interfaces (Figures 2a,c), we observe two broad peaks at around 3200 and 3450 cm−1. For epoxy, PETS, FAS and PVNODC (contact angle >45°), we observe a dominant red-shifted broad peak (3100− 3180 cm−1) (Figure 2e,g,i,k). A strong peak ∼3100−3180 cm−1 has also been observed in the SFG spectra for water in contact with hydrophobic surfaces in previous studies.30,33,35,45−47 The red-shifted water peak corresponds to the strongly coordinated water network due to enhanced ordering of the water molecules next to a hydrophobic surface. There are several hypothesis for the origin of enhanced ordering next to hydrophobic surfaces. Shen’s group suggested that this enhanced ordering was due to the rigid hydrophobic wall, forcing water molecules into a more ordered strongly coordinated water structure.30 In another study, they explained that this peak was due to the negative charges at the octadecyltrichlorosilane (OTS) interface.33 This negative charge was due to adsorption of OH− and is responsible for ordering water molecules.33 Recently, Hopkins et al. also observed ordering of water molecules next to OTS- and FDSSAM on silica surface.45 They assigned this peak to water interacting with the charged silica underneath of the monolayer.45 Moreover, the Bakker group studied water/ PDMS interface and they explained that the highly ordered water structure was caused by the corrugated interface between water and hydrophobic CH groups, which could act as a template for the water network to fold around.35 Here, the similarity in the OH peak between the SAMs and PVNODC surface indicates that this effect is not due to the sapphire interface and is an inherent property of water in contact with hydrophobic surfaces. The red-shifted free OH peak near 3680 cm−1 (Figure 2i,k) has been attributed to the van der Waals interactions between water and the hydrophobic layer.33,35 In a similar experiment on SiO2, Hopkins et al. interpreted this red-shifted peak to the water interactions with the underlying negatively charged SiO2 substrate.45 However, in our case (Figure 2a), we do not observe any shift in the surface free OH of sapphire when it is in contact with water. Therefore, the shift observed here suggests that water is mainly interacting with the hydrophobic layer. 3.2. Freezing Temperatures as a Function of Water Contact Angles. Figure 2b,d,f,h,j,l shows the changes in the

SFG intensity at ∼3100 cm−1 during the cooling and heating cycles (collected using a rate of 0.3 °C/min). The transition temperatures are defined as temperatures at which the SFG intensity is midway between the intensity for ice and water. Using multiple cooling and heating experiments, we have determined the freezing transitions as a function of surface energy (summarized in Table 2). We found that the freezing Table 2. Surface Contact Angles and the Effective Freezing Temperaturesa sample bare sapphire PEG-SAM epoxy-SAM PETS-SAM FAS-SAM PVNODC-spin coating a

static contact angle [deg] 5 34 54 85 106 107

± ± ± ± ± ±

5 3 2 3 3 2

ΔT [°C](10 μL) −9.4 −9.6 −9.6 −13.3 −16.6 −17.2

± ± ± ± ± ±

0.9 0.4 1.0 0.2 0.7 0.2

ΔT [°C](300 μL) −6.9 ± 0.2

−6.7 ± 0.1

ΔT = Tf − Tm.

temperatures decrease with increasing hydrophobicity of surfaces (or increase in water contact angles). We also show the results collected for SFG cell with ∼300 μL volume (Figure 3) and in this case the freezing and melting transition temperatures are not affected by surface energy. For the large volume cell, the surface effect is not obvious and nucleation in the bulk dominates the freezing transition temperature. The effect of heterogeneous nucleation is only observed in the small volume cell (∼10 μL). The convention to define a transition point between hydrophobic and hydrophilic nature of a substrate is a water contact angle of 90°. There is no fundamental reason behind this definition, and it was suggested recently on the basis of molecular dynamics simulation results that this boundary should be around 45°. This conclusion was reached by comparing the energy required to form a cavity next to surfaces.48 Interestingly, we have observed some clear trends based on the SFG spectra as a function of contact angle. The structure of water shows two broad OH peaks for hydrophilic surfaces (PEG-SAM) and shows one dominant peak associated with strongly coordinated water for epoxy-SAM. Therefore, the hydrophilic−hydrophobic boundary between PEG-SAM and epoxy-SAM with a contact angle between 34−55°, is consistent with the boundary between hydrophilic and hydrophobic surfaces found using MD simulation.48 Also, we would like to make some qualitative comparison between the SFG intensity of ice and water. The SFG signals for ice are higher than water for hydrophilic surfaces (Figure 2a,c). For hydrophobic surfaces we observe either a small difference (Figure 2k for PVNODC) in SFG intensity between ice and water or a decrease in SFG intensity (Figure 2e,g,i) upon freezing. This is only a qualitative comparison because the SFG intensity of ice is also expected to be lower in these experiments due to differences in the Fresnel factors between water and ice. 3.3. Comparison of the Experimental Results with the Predictions from Classical Nucleation Model. To explain the trend in freezing transitions, we present here a stochastic model based on classical nucleation theory. In classical nucleation theory, the freezing of supercooled water occurs when an ice nucleus forms either in the bulk or at the interface. E


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Figure 3. SFG spectra collected for PEG-SAM (a) and FAS-SAM (b) using a 300 μL cell. The changes in SFG intensity at 3100 cm−1 in SSP polarization are shown during heating (open circles, red) and cooling (open triangles, black). The SFG spectra in SSP polarization for PEG-SAM and FAS-SAM are shown as inset. The water spectra were collected during the cooling cycles at 5 °C (water) and −11/−14 °C (ice).

For heterogeneous nucleation of water film in our cell, the temperature dependent nucleation rate can be written as9,49 Jtotal = Js × S + Jv × V

(2)

where Jtotal, Js and Jv are the total ice nucleation rate, the surface nucleation rate of the (modified) substrate and the bulk nucleation rate, respectively. S and V are the surface area and bulk volume. Equation 2 suggests that the total nucleation rate is affected by both volume (V) and surface area (S). To model the freezing temperature, we used the expression for Jv, Js, and the parameters from previous literature8,49 (further information provided in the Supporting Information, section 7). To predict the effective freezing temperature vs contact angle, we integrated Jtotal using the Poisson process:50

∫T

Tf

0

Jtotal dT /C = Ntotal

(3)

where Jtotal is the number of nucleation sites generated per second in the cell, T0 is the melting temperature for ice (273.15 K), Tf is the undercooling freezing temperature, C = 0.3 K/min is the cooling rate used here and Ntotal is the total number of nucleation sites generated in the cell; a value of ∼1 was used for fitting the data. By performing the integration in eq 3 combined with eq 2, and solving for Tf when the integration is equal to Ntotal, we fitted the effective freezing temperature as a function of water contact angle (shown as a solid line in Figure 4). There are two main effects that also need to be discussed. First, the freezing temperature depends on the volume of the water used in these experiments and the model predicts that the effect of the surface energy should be reduced with increase in the volume of the water in the cell. We find that the effective freezing temperatures for both the hydrophilic and hydrophobic surfaces are around −7 °C in experiments with the ∼300 μL cell, which indicates the freezing temperature does not depend on the surface energy for larger volume of water. This could be explained by eq 2, the total nucleation rate is determined by both the surface term (Js × S) and bulk term (Jv × V). Here, when we increase the volume (V), the contribution of the surface term will be overwhelmed by the bulk term, and the total nucleation will not depend on the surface energy. The second parameter that could influence the results is the Teflon spacer or steel surface used in the construction of the SFG cell. If the nucleation is occurring at the steel-water interface or the Teflon-water interface, then we should not have observed any trends with surface energy in these experiments. Since we have the same stainless cell and Teflon spacer for both the large and the small volume cell, we can conclude that the surface energy

Figure 4. Freezing temperatures next to surfaces with water contact angles ranging from 5° to 110° using a cooling rate of 0.3 °C/min. The open red circles are for 10 μL cell and open red squares are for 300 μL cell. The error bars represent ±1 standard deviation. At least three measurements were taken for each surface. The black solid curve is a fit using the classical nucleation model (Supporting Information, section 7). The black dashed line represents T ∼ −7.0 °C, which is the value of the average freezing temperature of the 300 μL cells.

for the 10 μL cell is the main driving force in reducing the freezing transition temperature and not the stainless surface or Teflon spacer. It is interesting to compare the predictions of the nucleation model to understand the freezing transition temperatures reported in earlier publications. We have chosen the surfaces with water contact angle of 33° to illustrate the influence of cooling rates on the freezing temperature of a water film with our geometry (Supporting Information, Figure S21), and the influence of volume and surface area on the freezing temperatures of a water droplet (Supporting Information, Figure S22). As expected from the nucleation model, the freezing temperatures are reduced dramatically if we decrease the volume and surface area and increase the cooling rate. From calculation, the homogeneous nucleation dominated freezing when temperature is below −33 °C, which is similar to that reported in previous literature.49 Most studies that involve F


ACS Applied Materials & Interfaces freezing water droplets of small volume and surface area (3 × 10−7−4 × 10−3 μL and 1 × 10−6−3 × 10−4 cm2), report much lower freezing temperatures than reported here.2,14,51,52 Suzuki et al.17 reported freezing temperature of −22.7 °C for water droplets next to a FAS-SAM modified Si substrate (hydrophobic) compared to −16.3 °C next to an unmodified Si substrate (hydrophilic) using a cooling rate of 0.5 °C/min, this difference in freezing temperature between hydrophilic and hydrophobic surfaces is similar to what we have observed here. Using a faster cooling rate (5 °C/min), Li et al. found an opposite trend where the freezing probability of water next to a FAS-SAM modified Si surface (−38.9 °C) is higher than an uncoated Si surface (−39.3 °C).14 Because the transition temperatures they reported are very similar to those expected for homogeneous nucleation (−37 to −36 °C),49 the surface effects may not be dominant in influencing the freezing temperatures. Abdelmonem et al. have used a very similar experimental condition and they report a transition temperature of around −15 °C for water next to sapphire substrate,53 which is comparable to the freezing temperatures reported here.

ACKNOWLEDGMENTS

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REFERENCES

(1) Baker, M. B.; Peter, T. Small-Scale Cloud Processes and Climate. Nature 2008, 451, 299−300. (2) Atkinson, J. D.; Murray, B. J.; Woodhouse, M. T.; Whale, T. F.; Baustian, K. J.; Carslaw, K. S.; Dobbie, S.; O’Sullivan, D.; Malkin, T. L. The Importance of Feldspar for Ice Nucleation by Mineral Dust in Mixed-Phase Clouds. Nature 2013, 498, 355−358. (3) Lee, R. E., Jr; Warren, G. J.; Gusta, L. V. Biological Ice Nucleation and Its Applications; American Phytopathological Society, 1995. (4) Parent, O.; Ilinca, A. Anti-Icing and De-Icing Techniques for Wind Turbines: Critical Review. Cold Reg. Sci. Technol. 2011, 65, 88− 96. (5) Laforte, J.; Allaire, M.; Laflamme, J. State-of-the-Art on Power Line De-Icing. Atmos. Res. 1998, 46, 143−158. (6) Thomas, S. K.; Cassoni, R. P.; MacArthur, C. D. Aircraft AntiIcing and De-Icing Techniques and Modeling. J. Aircr. 1996, 33, 841− 854. (7) Lupi, L.; Hudait, A.; Molinero, V. Heterogeneous Nucleation of Ice on Carbon Surfaces. J. Am. Chem. Soc. 2014, 136, 3156−3164. (8) Zobrist, B.; Koop, T.; Luo, B. P.; Marcolli, C.; Peter, T. Heterogeneous Ice Nucleation Rate Coefficient of Water Droplets Coated by a Nonadecanol Monolayer. J. Phys. Chem. C 2007, 111, 2149−2155. (9) Alizadeh, A.; Yamada, M.; Li, R.; Shang, W.; Otta, S.; Zhong, S.; Ge, L.; Dhinojwala, A.; Conway, K. R.; Bahadur, V.; Vinciquerra, A. J.; Stephens, B.; Blohm, M. L. Dynamics of Ice Nucleation on Water Repellent Surfaces. Langmuir 2012, 28, 3180−3186. (10) Boinovich, L.; Emelyanenko, A. M.; Korolev, V. V.; Pashinin, A. S. Effect of Wettability on Sessile Drop Freezing: When Superhydrophobicity Stimulates an Extreme Freezing Delay. Langmuir 2014, 30, 1659−1668. (11) Guo, P.; Zheng, Y.; Wen, M.; Song, C.; Lin, Y.; Jiang, L. Icephobic/Anti-Icing Properties of Micro/Nanostructured Surfaces. Adv. Mater. 2012, 24, 2642−2648. (12) Heydari, G.; Thormann, E.; Järn, M.; Tyrode, E.; Claesson, P. M. Hydrophobic Surfaces: Topography Effects on Wetting by Supercooled Water and Freezing Delay. J. Phys. Chem. C 2013, 117, 21752−21762. (13) Hao, P.; Lv, C.; Zhang, X. Freezing of Sessile Water Droplets on Surfaces With Various Roughness and Wettability. Appl. Phys. Lett. 2014, 104, 161609. (14) Li, K.; Xu, S.; Shi, W.; He, M.; Li, H.; Li, S.; Zhou, X.; Wang, J.; Song, Y. Investigating the Effects of Solid Surfaces on Ice Nucleation. Langmuir 2012, 28, 10749−10754. (15) Li, K.; Xu, S.; Chen, J.; Zhang, Q.; Zhang, Y.; Cui, D.; Zhou, X.; Wang, J.; Song, Y. Viscosity of Interfacial Water Regulates Ice Nucleation. Appl. Phys. Lett. 2014, 104, 101605. (16) Lupi, L.; Molinero, V. Does Hydrophilicity of Carbon Particles Improve Their Ice Nucleation Ability? J. Phys. Chem. A 2014, 118, 7330−7337. (17) Suzuki, S.; Nakajima, A.; Yoshida, N.; Sakai, M.; Hashimoto, A.; Kameshima, Y.; Okada, K. Hydrophobicity and Freezing of a Water Droplet on Fluoroalkylsilane Coatings with Different Roughnesses. Langmuir 2007, 23, 8674−8677.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02094. XPS experiment method and spectra, contact angle images, AFM experiment method and images, Fresnel factors calculations, experimental setup, SFG spectra for polymer−air interfaces, equations for the theoretical model, fitting parameters (PDF).

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The authors thank Edward Laughlin, Anish Kurian, and Liehui Ge for their help in designing the temperature stage. We thank Zhorro Nikolov for XPS measurements. We also thank Mena Klittich, Saranshu Singla, Sukhmanjot Kaur, Dona Foster and He Zhu for helpful discussion. This work was funded by NSFDMR 1006764 (Y.Z., E.A.-D. and A.D.) and NSF-DMR 1410290 (S.B.). We also thank GE Global Research for providing support to fund the design of the temperature cell for SFG.

4. SUMMARY In this study, we have measured the effect of surface energy of solid surfaces on the structure and freezing temperature of water by keeping the other parameters that influence the ice nucleation rate constant. Sum frequency generation spectroscopy is used to monitor the changes in the interfacial structure during cooling and heating process. These experiments suggest that there is a depression in the freezing temperature with decrease in surface energy at conditions when surface nucleation rates dominate in comparison to bulk. The hydrophobic surfaces (water contact angles of 54° and higher) show highly ordered water structure. Especially, the epoxy-SAM surface which is at the transition from hydrophilic to hydrophobic, shows subtle C−H peaks in contact with water and the highly ordered water structure that is typical of the hydrophobic surfaces. The classical nucleation model with a Poisson distribution is adequate in capturing the depression in freezing transition as a function of surface energy. Understanding the influence of surface energy on depression in freezing temperature will help in designing novel anti-icing surfaces for industrial applications.

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Research Article

AUTHOR INFORMATION

Corresponding Author

*A. Dhinojwala. E-mail: [email protected]. Notes

The authors declare no competing financial interest. G


Research Article


SFG) at the Al2O3 (1120)/H2O Interface. J. Phys. Chem. Lett. 2014, 5, 528−533. (38) Wei, X.; Miranda, P. B.; Shen, Y. R. Surface Vibrational Spectroscopic Study of Surface Melting of Ice. Phys. Rev. Lett. 2001, 86, 1554. (39) Chen, Z.; Ward, R.; Tian, Y.; Baldelli, S.; Opdahl, A.; Shen, Y. R.; Somorjai, G. A. Detection of Hydrophobic End Groups on Polymer Surfaces by Sum-Frequency Generation Vibrational Spectroscopy. J. Am. Chem. Soc. 2000, 122, 10615−10620. (40) Rangwalla, H.; Schwab, A. D.; Yurdumakan, B.; Yablon, D. G.; Yeganeh, M. S.; Dhinojwala, A. Molecular Structure of an Alkyl-SideChain Polymer-Water Interface: Origins of Contact Angle Hysteresis. Langmuir 2004, 20, 8625−8633. (41) Gautam, K. S.; Schwab, A. D.; Dhinojwala, A.; Zhang, D.; Dougal, S. M.; Yeganeh, M. S. Molecular Structure of Polystyrene at Air/Polymer and Solid/Polymer Interfaces. Phys. Rev. Lett. 2000, 85, 3854−3857. (42) Briggman, K. A.; Stephenson, J. C.; Wallace, W. E.; Richter, L. J. Absolute Molecular Orientational Distribution of the Polystyrene Surface. J. Phys. Chem. B 2001, 105, 2785−2791. (43) Dreesen, L.; Humbert, C.; Hollander, P.; Mani, A.; Ataka, K.; Thiry, P.; Peremans, A. Study of the Water/Poly (Ethylene Glycol) Interface by IR-Visible Sum-Frequency Generation Spectroscopy. Chem. Phys. Lett. 2001, 333, 327−331. (44) Zolk, M.; Eisert, F.; Pipper, J.; Herrwerth, S.; Eck, W.; Buck, M.; Grunze, M. Solvation of Oligo (Ethylene Glycol)-Terminated SelfAssembled Monolayers Studied by Vibrational Sum Frequency Spectroscopy. Langmuir 2000, 16, 5849−5852. (45) Hopkins, A. J.; McFearin, C. L.; Richmond, G. L. SAMs under Water: The Impact of Ions on the Behavior of Water at Soft Hydrophobic Surfaces. J. Phys. Chem. C 2011, 115, 11192−11203. (46) Scatena, L. F.; Brown, M. G.; Richmond, G. L. Water at Hydrophobic Surfaces: Weak Hydrogen Bonding and Strong Orientation Effects. Science 2001, 292, 908−912. (47) Tyrode, E.; Liljeblad, J. F. D. Water Structure Next to Ordered and Disordered Hydrophobic Silane Monolayers: A Vibrational Sum Frequency Spectroscopy Study. J. Phys. Chem. C 2013, 117, 1780− 1790. (48) Godawat, R.; Jamadagni, S. N.; Garde, S. Characterizing Hydrophobicity of Interfaces by Using Cavity Formation, Solute Binding, and Water Correlations. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 15119−15124. (49) Pruppacher, H. R.; Klett, J. D.; Wang, P. K. Microphysics of Clouds and Precipitation; Taylor & Francis, 1998. (50) Shaw, R. A.; Durant, A. J.; Mi, Y. Heterogeneous Surface Crystallization Observed in Undercooled Water. J. Phys. Chem. B 2005, 109, 9865−9868. (51) Campbell, J. M.; Meldrum, F. C.; Christenson, H. K. Is Ice Nucleation From Supercooled Water Insensitive to Surface Roughness? J. Phys. Chem. C 2015, 119, 1164−1169. (52) Steinke, I. Ice Nucleation Properties of Mineral Dusts. Ph.D. Thesis, Heidelberg University, 2013. (53) Abdelmonem, A.; Lützenkirchen, J.; Leisner, T. Probing IceNucleation Processes on the Molecular Level Using Second Harmonic Generation Spectroscopy. Atmos. Meas. Tech. 2015, 8, 3519−3526.

(18) Van Dyke, A. S.; Collard, D.; Derby, M. M.; Betz, A. R. Droplet Coalescence and Freezing on Hydrophilic, Hydrophobic, and Biphilic Surfaces. Appl. Phys. Lett. 2015, 107, 141602. (19) Huang, L.; Liu, Z.; Liu, Y.; Gou, Y.; Wang, L. Effect of Contact Angle on Water Droplet Freezing Process on a Cold Flat Surfaces. Exp. Therm. Fluid Sci. 2012, 40, 74−80. (20) Rahimi, M.; Afshari, A.; Fojan, P.; Gurevich, L. The Effect of Surface Modification on Initial Ice Formation on Aluminum Surfaces. Appl. Surf. Sci. 2015, 355, 327−333. (21) Nanjundiah, K.; Hsu, P. Y.; Dhinojwala, A. Understanding Rubber Friction in the Presence of Water Using Sum-Frequency Generation Spectroscopy. J. Chem. Phys. 2009, 130, 024702. (22) Anim-Danso, E.; Zhang, Y.; Dhinojwala, A. Freezing and Melting of Salt Hydrates Next to Solid Surfaces Probed by InfraredVisible Sum Frequency Generation Spectroscopy. J. Am. Chem. Soc. 2013, 135, 8496−8499. (23) Li, G.; Dhinojwala, A.; Yeganeh, M. S. Interference Effect from Buried Interfaces Investigated by Angular-Dependent Infrared-Visible Sum Frequency Generation Technique. J. Phys. Chem. C 2011, 115, 7554−7561. (24) Zhang, L.; Tian, C.; Waychunas, G. A.; Shen, Y. R. Structures and Charging of α-Alumina (0001)/Water Interfaces Studied by SumFrequency Vibrational Spectroscopy. J. Am. Chem. Soc. 2008, 130, 7686−7694. (25) Liljeblad, J. F. D.; Tyrode, E. Vibrational Sum Frequency Spectroscopy Studies at Solid/Liquid Interfaces: Influence of the Experimental Geometry in the Spectral Shape and Enhancement. J. Phys. Chem. C 2012, 116, 22893−22903. (26) Harp, G. P.; Rangwalla, H.; Yeganeh, M. S.; Dhinojwala, A. Infrared-Visible Sum Frequency Generation Spectroscopic Study of Molecular Orientation at Polystyrene/Comb-Polymer Interfaces. J. Am. Chem. Soc. 2003, 125, 11283−11290. (27) Zhang, Y.; Anim-Danso, E.; Dhinojwala, A. The Effect of a Solid Surface on the Segregation and Melting of Salt Hydrates. J. Am. Chem. Soc. 2014, 136, 14811−14820. (28) Anim-Danso, E.; Zhang, Y.; Alizadeh, A.; Dhinojwala, A. Freezing of Water Next to Solid Surfaces Probed by Infrared-Visible Sum Frequency Generation Spectroscopy. J. Am. Chem. Soc. 2013, 135, 2734−2740. (29) Du, Q.; Superfine, R.; Freysz, E.; Shen, Y. Vibrational Spectroscopy of Water at the Vapor/Water Interface. Phys. Rev. Lett. 1993, 70, 2313. (30) Du, Q.; Freysz, E.; Shen, Y. R. Surface Vibrational Spectroscopic Studies of Hydrogen Bonding and Hydrophobicity. Science 1994, 264, 826−828. (31) Yeganeh, M. S.; Dougal, S. M.; Pink, H. S. Vibrational Spectroscopy of Water at Liquid/Solid Interfaces: Crossing the Isoelectric Point of a Solid Surface. Phys. Rev. Lett. 1999, 83, 1179− 1182. (32) Richmond, G. L. Molecular Bonding and Interactions at Aqueous Surfaces as Probed by Vibrational Sum Frequency Spectroscopy. Chem. Rev. 2002, 102, 2693−2724. (33) Tian, C. S.; Shen, Y. R. Structure and Charging of Hydrophobic Material/Water Interfaces Studied by Phase-Sensitive Sum-Frequency Vibrational Spectroscopy. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 15148−15153. (34) Sovago, M.; Campen, R. K.; Wurpel, G. W. H.; Müller, M.; Bakker, H. J.; Bonn, M. Vibrational Response of Hydrogen-Bonded Interfacial Water Is Dominated by Intramolecular Coupling. Phys. Rev. Lett. 2008, 100, 173901. (35) Strazdaite, S.; Versluis, J.; Backus, E. H. G.; Bakker, H. J. Enhanced Ordering of Water at Hydrophobic Surfaces. J. Chem. Phys. 2014, 140, 054711. (36) Sung, J.; Zhang, L.; Tian, C.; Shen, Y. R.; Waychunas, G. A. The Interfacial Structure of Water/Protonated α-Al2O3 112̅0) As a Function of pH. J. Phys.: Condens. Matter 2012, 24, 124101. (37) Boulesbaa, A.; Borguet, E. Vibrational Dynamics of Interfacial Water by Free Induction Decay Sum Frequency Generation (FIDH


Effect of Surface Energy on Freezing Temperature of Water - ACS

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