Thermal Behavior of Long-Chain Alcohols on Sapphire Substrate

Publication Date (Web): May 26, 2015. Copyright © 2015 American Chemical Society. *E-mail [email protected] (A.D.). Cite this:Langmuir 31, 23, 6306-631...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/Langmuir

Thermal Behavior of Long-Chain Alcohols on Sapphire Substrate He Zhu and Ali Dhinojwala* Department of Polymer Science, The University of Akron, Akron, Ohio 44325, United States S Supporting Information *

ABSTRACT: Structures of amphiphilic molecules at the liquid/solid and solid/solid interfaces are important in understanding lubrication, colloid stabilization, chromatography, and nucleation. Here, we have used interface-sensitive sum frequency generation (SFG) spectroscopy to characterize the interfacial structures of long-chain alcohols above and below the bulk melting temperature (Tm). The melting temperature of the ordered hexadecanol monolayer was measured to be around 30 °C above the bulk Tm, consistent with the transition temperature reported using X-ray reflectivity [Phys. Rev. Lett. 2011, 106, 137801]. The disruption of hydrogen bonds between the sapphire and the alcohol hydroxyl groups was directly measured as a function of temperature. The strength of this hydrogen-bonding interaction, which explained the monolayer thermal stability above Tm, was calculated using the Badger− Bauer equation. Below Tm, the ordered self-assembled monolayer influenced the structure of the interfacial crystalline layer, and the transition from the ordered monolayer to the bulk crystalline phases (α rotator phase, β crystalline phase, and γ crystalline phase) resulted in packing frustrations at the interface.

■

INTRODUCTION In the early 1940s, Zisman and co-workers1 reported that eicosyl alcohol (C20H41OH) in hexadecane, a nonpolar solvent, forms a monolayer on metal and glass surfaces. The monolayer, later referred to as a self-assembled monolayer (SAM), decreases the surface energy of polar substrates, and they become both hydrophobic and oleophobic. As a consequence, solutions and melts consisting of these amphiphilic molecules cannot wet high energy surfaces, a phenomenon called “autophobicity”.2 This self-assembly process is a result of the interactions between polar head groups with high energy surfaces and van der Waals interactions between the long alkyl chains.3 SAMs have been used extensively to modify surfaces for a variety of applications such as biomolecule recognition,4 surface patterning,5 electrode surface modification,6 and boundary lubrication.7 The structures and thermal behaviors of SAM-modified surfaces have been studied using a variety of experimental8−11 and theoretical techniques.12−14 Although Zisman’s observation of autophobicity has been known since the 1940s, understanding the structure of SAMs in contact with their own melts has been challenging because of the lack of experimental techniques to probe buried interfaces. Recently, Ocko and co-workers used a synchrotron X-ray source to study the structure and transition temperature of octadecanol SAM at the melt/sapphire interface.15 Surprisingly, they found that the thickness and electron density of these adsorbed monolayers do not change until 30−35 °C above the bulk Tm. They suggested that the near epitaxial molecular packing and the hydrogen-bonding interactions between the hydroxyl groups of octadecanol and the surface hydroxyl groups on sapphire (0001) are responsible for the higher melting temperature of these monolayers. Without the strong hydro© 2015 American Chemical Society

gen-bonding interactions, the Tm of hexadecanol molecules in contact with a polystyrene surface is similar to the bulk Tm.16 Here, we have taken advantage of surface sensitive infrared− visible sum frequency generation spectroscopy (SFG) to study the orientation of the long-chain alcohol monolayer in contact with its own melt and bulk crystal phases. SFG is a secondorder nonlinear optical technique, which under electric dipole approximation, provides direct information on the orientation and structure of the interfacial layer.17 This technique has been used to study SAMs at buried interfaces24−29 and in contact with air.11,18−23 SFG has not been used before to study the structure of long-chain alcohol SAMs in contact with its own melt (referred to as l−s interface). Since SFG can also monitor the position and shift of the surface hydroxyl peak, this technique can also be used to directly determine the strength of the hydrogen-bonding interaction and its influence on stabilizing the ordered monolayer above the bulk Tm.30,31 In addition to the structure of hexadecanol SAM in contact with its own melt, it is also intriguing to understand how the SAM influences the crystal structure upon freezing (referred to as s−s interface). It has been shown that long chain alcohols exhibit three polymorphic phases: the α rotator phase, β crystalline phase, and γ crystalline phase (Figure 1). In the α rotator phase, the molecule chains tilt perpendicular to the layer plane and rotate freely around the chain axes.32 In β crystalline bulk, molecular chains tilt perpendicular to the layer plane as in α phase but form gauche defects near the OH groups, which results in alternating gauche and trans chain packing.33 For γ Received: April 12, 2015 Revised: May 24, 2015 Published: May 26, 2015 6306

DOI: 10.1021/acs.langmuir.5b01330 Langmuir 2015, 31, 6306−6313

Article

Langmuir

autophobicity, a monolayer was formed on sapphire substrates.15 Film thickness measured by ellipsometry (J.A. Woolam Co. Inc. spectroscopic ellipsometer; control module: VB-400; monochromator: HS-190 high speed monochromator) confirmed the adsorption of a monolayer. The SFG spectra were collected using total internal reflection (TIR) geometry, where the incident angle can be adjusted to probe the s−v, s−l, and s−s interfaces.36 In this study, we have used the incident angle of 8° with respect to the surface normal of the prism to study the s−l and s−s interfaces. An incident angle of 42° was used to study the s−v interface. The SFG spectra have been taken in SSP (s-polarized SFG, s-polarized visible, p-polarized IR), SPS (s-polarized SFG, p-polarized visible, s-polarized IR), and PPP (p-polarized SFG, p-polarized visible, p-polarized IR) polarizations, and each spectrum was taken after equilibrating the system for 20 min at the specified temperature. For the phase transition study, the heating and cooling rates were 1 °C/ min above Tm and 0.3 °C/min near or below Tm. We have calibrated the temperature gauge of the SFG heating stage by measuring HeNe laser reflectivity from the hexadecanol/sapphire interface as a function of temperature. Details of the SFG experimental setup have been described in previous publications.37 The SFG spectra shown in this study were fitted using eq 1

Figure 1. Molecular packing of α, β, and γ phases for long-chain alcohols.

phase, the molecular chains pack in an all-trans conformation and tilt 30° away from the layer plane normal.33 The influence of an ordered autophobic monolayer on the interfacial molecular packing will be discussed based on SFG spectral observations.

■

Q

χijk (ω2) = χijkNR eiΦ +

EXPERIMENTAL METHODS

∑ q=1

A 99% purity sample of 1-hexadecanol was purchased from Aldrich Chemical Co. and purified by recrystallization in acetone. A 98% purity sample of 1-dodecanol was purchased from Aldrich Chemical Co. and was purified by distillation under vacuum. For hexadecanol, differential scanning calorimetric (DSC, TA Instruments Q2000) measurement at a cooling rate of 0.3 °C/min showed two phase transitions at 48.0 and 42.0 °C, which were the melt → α and α → γ transitions, respectively.33,34 In the heating cycle, the γ → α and α→ melt transitions were observed at 48.1 and 48.7 °C, respectively.35 For dodecanol, the melt → β and β → melt transitions were observed at 19.2 and 22.0 °C, respectively.33,34 These DSC measurements are summarized in Table S1 of the Supporting Information. We have used equilateral sapphire prisms (15 mm × 15 mm × 15 mm × 10 mm, c-axis ± 2° parallel to the prism face, Meller Optics, Inc.) as substrates for the SFG experiments. The sapphire prisms were sonicated for 20 min in chloroform (99.9%, certified ACS, Fisher Scientific) and then for 20 min in toluene (99.5%, AR ACS, Avantor Performance Materials). The prisms were dried with N2, and the remaining hydrocarbon residues were removed by a final 4 min plasma treatment (Harrick Plasma, PDC-32G). Clean prisms were mounted onto SFG cells and sealed using Teflon spacers. Melted alcohol was injected into the preheated SFG cells. For monolayer experiments, the sapphire prisms were peeled off the alcohol melt. Because of

Aijk , q ω2 − ωq + i Γq

(1)

where Aijk,q, ωq, Φ, and Γq are the amplitude strength, resonant frequency, relative phase of the nonresonant vibration to the resonant vibration, and damping constant for the qth normal mode vibration. χNR ijk is the nonresonant SFG susceptibility, which does not change with scanning wavenumber ω2. The parameter Q is the number of normal mode resonances observed in the range of wavenumbers scanned in the SFG experiments. If SFG spectra taken at different temperatures were shown in one figure, they were rescaled relatively based on the change of Fresnel’s factor with temperature. The refractive index of hexadecanol reported in the literature was used to calculate the Fresnel’s factors.38

■

RESULTS AND DISCUSSION Molecular Structure and Phase Transition above Tm. Figure 2 shows the SFG spectra in SSP and SPS polarizations at the s−l interface for hexadecanol at 65 and 120 °C, which are below and above the interfacial melting temperature (discussed later in this section), respectively. The solid lines in the figure are the fitting curves using eq 1. The parameters obtained from fitting are provided in Table S2 of the Supporting Information.

Figure 2. Panels A and B show the SFG spectra of hexadecanol at the s−l interface in SSP and SPS polarization, respectively. Data points taken at 65 °C are shown as ○, and the ones taken at 120 °C are shown as □. Spectra are offset vertically for better comparison. Solid lines are fitting curves using eq 1. SFG spectra at 120 °C were rescaled after the correction by Fresnel’s factor for direct comparison. 6307

DOI: 10.1021/acs.langmuir.5b01330 Langmuir 2015, 31, 6306−6313

Article

Langmuir At 65 °C (shown as ○), the SSP spectra show two major peaks: one near 2875 cm−1, assigned to methyl symmetric vibration (r+), and the other peak near 2945 cm−1, assigned to the combination of asymmetric methyl stretching (r−) and Fermi resonance (r+FR) of methyl symmetric stretching with overtones of its bending mode. In SPS polarization, the dominant peak is near 2960 cm−1 and is assigned to methyl asymmetric vibration (r−).39 The two nondegenerate in-plane (r−a ) and out-of-plane (r−b ) methyl asymmetric vibrations, usually observed around 2960 cm−1, cannot be resolved in this study. These spectra features at 65 °C indicate a well-ordered all-trans monolayer with close-packed alkyl chains.39,40 Previous X-ray reflectivity study also observed an ordered monolayer of long-chain alcohols on sapphire above the bulk Tm.15 Figure 2 also shows the SFG spectra at 120 °C (shown as □), which is far above the interfacial melting temperature. In addition to the symmetric methyl vibration (r+), methyl Fermi resonance (r+FR), and asymmetric methyl vibration (r−) at 65 °C, we have also observed peaks at 2850, 2895, and 2910 cm−1, which are assigned to the methylene symmetric stretching (d+), methylene asymmetric stretching (d−), and methylene Fermi (d+FR) resonance, respectively.41 The increase in the SFG signal intensity above 3000 cm−1 is due to hexadecanol OH vibrations between 3000 and 3400 cm−1 (discussed in the following section). To understand the changes in the molecular structure, it is necessary to analyze the changes in the amplitude strength of these peaks. The amplitude strength ratio of the symmetric methylene peak (d+) to the symmetric methyl peak (r+) in SSP spectra, |Ayyzd+/Ayyzr+|, which were calculated and provided in Table 1, can be used as an indicator of gauche conforma-

calculated using various width of the orientational distributions: 0° (δ distribution), 20°, 30°, and 40°. Figure 3A shows the calculated amplitude strength ratio as a function of average tilt angle. Experimental ratios are shown as dashed lines in black for 65 °C and dotted lines in red for 120 °C. At 65 °C, the ratio of |Ayyzr−/Ayzyr−| is 0.36, and the methyl end groups of the ordered monolayer are expected to have a narrow angular distribution. For an orientational distribution width of 0°, the methyl end groups tilted about 30° toward the surface normal. Since alkyl chains had all-trans conformations at this temperature, a tilt angle of less than 35° implies that the C−C−C axes of the alkyl chains tilted nearly parallel to the surface normal. In comparison, at 120 °C, the ratio of |Ayyzr−/Ayzyr−| intersects the calculated curves at 0.93, indicating a tilt angle of around 40°. However, this average tilt angle may not be accurate because for this ratio of Aq the intercept in Figure 3 is insensitive to the width of the orientational distribution (also referred as a magic angle).45 To determine the monolayer melting transition, we have measured the peak intensity of methyl symmetric vibration as a function of temperature (shown in Figure 4). The data shown in red were collected during the heating cycle and those shown in black during the cooling cycle. The peak intensity starts to drop more steeply from 75 to 85 °C, indicating that hexadecanol monolayer stayed stable up to around 30 °C above Tm. Upon cooling, the SFG intensity returned to the original value. This reversible process showed a hysteresis, which indicates a first-order transition. The temperature hysteresis could be a function of heating and cooling rates. As a comparison, similar experiments have been performed at the hexadecanol monolayer/sapphire interface in the air (s−v interface, Figures S1−S3 shown in the Supporting Information). Considering experimental errors, the molecular structure and the monolayer melting transitions at these two interfaces were similar. However, the melting transition at the s−v interface was not reversible during the cooling cycle. Since the disordering process is coupled with the desorption of molecules,15 our observation suggests that desorbed alcohol molecules were not able to reattach to the sapphire surface at the s−v interface. Unlike the alcohol monolayer at the s−l interface, the monolayer at the s−v interface has no reservoir to replenish the alcohol molecules. Interaction between Hexadecanol and the Sapphire Substrate. Next, we focus our discussion on the interaction between hexadecanol and the sapphire substrate. Even though we expect the alcohol OH groups to be hydrogen bonded with the sapphire surface OH groups, the strength of the hydrogen bonds has not been reported in published literature. SFG spectra collected at the s−l interface from 3000 to 3400 cm−1 and from 3400 to 3800 cm−1 in SSP and SPS polarization are shown in Figure 5. We show the SFG spectra at 65 °C, where an ordered monolayer adsorbed on the sapphire surface was in contact with the melted hexadecanol liquid. We have also shown the spectra taken at 120 °C, where the monolayer was disordered. There are several distinctive OH vibration bands observed in the SFG spectra. The broad peaks from 3100 to 3400 cm−1 are assigned to the OH groups of the alcohol molecules that are participating in hydrogen bonding.33 In SSP polarization, the alcohol OH peak is centered around 3200 cm−1, which suggests that the strength and orientation order of these hydrogen bonds resemble those in the crystalline state rather than liquid state.33 In SPS polarization, this OH peak shifts to a lower wavenumber around 3120 cm−1 and has an

Table 1. Peak Amplitude Ratios at 65 and 120 °C liquid T (°C)

|Ayyzd /Ayyzr |

|Ayyzr−/Ayzyr−|

65 120

0.22 ± 0.04

0.36 ± 0.06 0.93 ± 0.07

+

+

tion.24,39 At 65 °C, no methylene symmetric vibration was observed, which indicates an all-trans conformation of the long hydrophobic alkyl chains. However, the ratio increases to 0.22 at 120 °C, suggesting the formation of gauche defects at high temperature. The amplitude strength ratio between asymmetric methyl peaks (r−) in SSP and SPS spectra |Ayyzr−/Ayzyr−| can be used to interpret the average orientation of methyl end groups.42 For an all-trans conformation, the orientation of methyl end groups can then be used to deduce the orientation of C−C−C backbone of the alcohol molecule. Table 1 shows an increase in this ratio from 0.36 to 0.93 when temperature increases from 65 to 120 °C. To quantify the orientation of the methyl groups, we have also calculated |Ayyzr−/Ayzyr−| as a function of tilt angle θ using eq 2.43 Lyy(ω3)Lyy(ω1)Lzz(ω2) ⟨cos θ ⟩ − ⟨cos3 θ ⟩ Ayyz r− = Ayzyr− Lyy(ω3)Lzz(ω1)Lyy(ω2) ⟨cos3 θ ⟩ (2)

where L(ω) is the corresponding Fresnel’s factor for SFG (ω3), visible (ω1), and IR (ω2).44 The average angular distributions ⟨cos θ⟩ and ⟨cos3 θ⟩ can be calculated by assuming a Gaussian 2 2 distribution, f(θ) = (1/(σ(2π)1/2))e −(θ−θ0) /2σ , where σ is the angular distribution width. Then the |Ayyzr−/Ayzyr−| ratio was 6308

DOI: 10.1021/acs.langmuir.5b01330 Langmuir 2015, 31, 6306−6313

Article

Langmuir

Figure 3. (A) Theoretical and experimental measurements of the methyl peak amplitude strength ratio |Ayyzr−/Ayzyr−| at the s−l interface as a function of tilt angle. The theoretical predictions were calculated using Gaussian distributions with σ = 0°, 20°, 30°, and 40°. The data at 65 and 120 °C are shown as black dashed lines and red dotted lines, respectively. The shaded areas represent the range of measured experimental values. The proposed monolayer structure at 65 and 120 °C are shown in (B).

Figure 4. SFG intensity of methyl symmetric vibration (r+) as a function of temperature at the s−l interface. The heating and cooling cycles are plotted in red and black, respectively.

asymmetric shape. The assignment of this peak is still ambiguous. There are two possibilities: First, this 3120 cm−1 peak could originate from the OH head groups, which participated in stronger hydrogen bonding with the sapphire substrate. These two peaks at 3120 and 3200 cm−1 may also be due to the coupling of the adjacent OH groups.46 The hydroxyl peaks shown from 3400 to 3800 cm−1 are from the sapphire surface. The peak at 3707 cm−1 is assigned to the sapphire surface free OH groups, which have not formed acid− base interactions.30 Those between 3500 and 3700 cm−1 are assigned to the red-shifted vibrations of the OH groups, due to the acid−base interactions, including hydrogen bonds and dipole−dipole interactions.30,31 Upon heating, the intensity of the OH peaks (corrected for the changes in the Fresnel’s factors) decreased in both regions, as shown by the spectra in Figure 5. Thus, this decrease of peak intensity indicates the breakdown of the hydrogen bonding network, including the hydrogen bonds formed between alcohol OH groups and those between alcohol and sapphire OH groups. We have fitted the

Figure 5. SFG spectra in SSP and SPS polarization of alcohol OH peaks from 3000 to 3400 cm−1 are shown in (A) and (C). Those of sapphire OH peaks from 3400 to 3800 cm−1 are shown in (B) and (D). Data points taken at 65 °C are shown as ○, and the ones taken at 120 °C are shown as □. Spectra after correction by Fresnel’s factors were offset vertically for better comparison. Solid lines are fitting curves using eq 1.

SSP spectra at various temperatures below and above the monolayer melting temperature, and the amplitude strength was plotted against temperature shown as Figure S4 in the Supporting Information. A molecular picture of this hydrogenbonding network at 65 and 120 °C is shown in Figure 6. Interestingly, in SSP polarization, the 3645 cm−1 peak not only decreased in intensity but also shifted to lower wavenumber. It is possible that the weaker acid−base interactions were 6309

DOI: 10.1021/acs.langmuir.5b01330 Langmuir 2015, 31, 6306−6313

Article

Langmuir

measured using SFG are very close to the bulk transition temperatures measured using DSC. Thus, we have assigned each transition to the corresponding bulk phase transition. In Figure 7A, a sudden drop in peak intensity at 49.2 °C is due to the γ → melt transition. The two-step transition from γ → α and α → melt observed in DSC measurement was not resolved by SFG due to a sharp drop in signal intensity and a relatively narrow temperature window to observe the α phase. Figure 7C shows two transitions, at 48.5 and 41.5 °C, during the cooling cycle. The high temperature transition is from melt → α phase, and the second transition is a solid−solid transition from α → γ phase. The data for dodecanol during the heating and cooling cycles are shown in Figures 7B and 7D, respectively. Since dodecanol only has a β phase, the heating transition is from β → melt. Upon cooling, the transition is from melt → β phase. Since crystalline β and γ phases are monoclinic with space group P21/c and A2/a, respectively,48,49 the centrosymmetric bulk should be SFG inactive under dipole approximation for both these phases. In addition, we have calculated the Fresnel’s factor effect on SFG signal intensity upon crystallization. If the temperature decreases from 50 to 25 °Cfrom hexadecanol melt to γ phasethe SFG intensity should decrease by about two-thirds rather than increase as shown in Figure 7. Thus, the change of SFG signal intensity upon formation of different phases cannot be solely attributed to changes in the Fresnel’s factors. Instead, the different spectra features of the α rotator, β crystalline, and γ crystalline phases indicate distinctive interfacial molecular structures. Figure 8A shows the SFG spectra for the α phase in SSP and PPP polarizations. The SFG intensity in SPS polarization was weak, and no spectrum was collected. In the SSP spectra, the dominant peak is around 2840 cm−1, which is assigned to the symmetric vibration of the methylene group. Two additional weak peaks at 2865 and 2895 cm−1 are assigned to the symmetric methyl and asymmetric methylene vibrations, respectively. In the spectra collected using PPP polarization, the two dominant peaks around 2840 and 2960 cm−1 are assigned to symmetric methylene and asymmetric methyl vibrations. Since molecular chains in the bulk of α phase tilt perpendicular to the layer plane, this molecular packing structure could easily conform with the self-assembled monolayer next to sapphire surface as shown in Figure 9A. This uniform packing resulted in the cancellation of SFG

Figure 6. Hydrogen-bonding network for the ordered monolayer and disordered monolayer.

disrupted at lower temperatures compared to the stronger interactions. The correlation between the shift of OH peak and the strength of interactions can be quantified using the Badger− Bauer equation −ΔH = mΔν + C.30,47 If we assume that the hydrogen-bonding and van der Waals interactions between hexadecanol molecules in the self-assembled monolayer were the same as those in the solid bulk, the increased monolayer melting temperature should be a result of the acid−base interactions between alcohol molecules and sapphire surface OH groups. To calculate the strength of the acid−base interactions, which were disrupted during the monolayer melting transition, we have subtracted the portion of the acid−base interactions that existed at 120 °C from those at 65 °C. Using m = 0.0109 kcal/mol and C = 0.03 kcal/mol30 and assuming the change of entropy to be the same as that of bulk melting transition, ΔS = 24 cal/(mol °C),38 we have calculated the interaction energy to be ΔH = 0.67 ± 0.06 kcal/mol and the increased monolayer melting temperature to be approximately 28 ± 2 °C above Tm, which is very close to our experimentally measured value. Details of this calculation are provided in the Supporting Information. Molecular Structure at Transition Temperatures near or below Tm. In this final section, we discuss the interfacial molecular structures and transitions near or below the bulk Tm at the s−s interfaces. High-purity hexadecanol only has the α and γ phases below Tm,35 as per our DSC measurements. We used dodecanol, which forms a stable β phase below Tm, to compare all three solid phases of long-chain alcohols.33,34 Figure 7 shows the peak intensity of methyl symmetric stretching (r+) as a function of temperature upon heating and cooling at a rate of 0.3 °C/min. The transition temperatures

Figure 7. Changes in the peak intensity of methyl symmetric stretching (r+) during heating and cooling cycles. Panels A and C are for hexadecanol, and panels B and D are for dodecanol. Cooling and heating rates were controlled at 0.3 °C/min. 6310

DOI: 10.1021/acs.langmuir.5b01330 Langmuir 2015, 31, 6306−6313

Article

Langmuir

Figure 8. (A) SFG spectra of hexadecanol α phase in SSP and PPP polarization. (B) SFG spectra of hexadecanol γ phase in SSP, SPS, and PPP polarization. (C) SFG spectra of dodecanol β phase in SSP, SPS, and PPP polarization. Spectra were offset vertically and scaled relatively for better comparison. Solid lines are the fitting curves using eq 1.

tended to go through a tilting transition from 0° to 30° with respect to the surface normal (Figure 9B). This tilting transition might involve many layers, and the symmetry was broken near the head group and end group interfaces. This resulted in extremely strong vibrations of methylene next to the OH head groups and methyl end groups. A similar orientation transition over many layers has also been suggested for the nematic−isotropic interface of liquid crystals next to solid surfaces.51,52 The argument above also relies on the assumption that the self-assembled monolayer was intact during crystallization. We have separated the hexadecanol crystal from the sapphire prism and collected SFG spectra from both sides (Figure S5). The spectra for the sapphire prism surface were taken using internal reflection geometry, and the spectra features were identical to those of the self-assembled monolayer, which indicates that the hexadecanol monolayer was not disrupted during crystallization. For the hexadecanol crystal surface, the SFG spectra were collected using external geometry. Since SFG signal intensity in external geometry is 1− 2 orders of magnitude weaker than in internal reflection geometry,36 spectral features for the crystal surface after detaching from the sapphire substrate were similar as those of the γ phase/sapphire interface as shown in Figure 8B. To further confirm the importance of this self-assembled monolayer on the formation of the tilting transition, we have studied the crystallization of hexadecanol next to an OTS monolayer-coated sapphire substrate. In this case, SFG spectra were also similar as those of the γ phase/sapphire interface, with very high signal intensity (Figure S6). Comparatively, hexadecanol next to a polystyrene (PS) surface at 25 °C showed similar spectra features but much lower intensity.16 The difference between the hexadecanol/sapphire interface and the hexadecanol/PS interface was the presence of an ordered monolayer next to the substrate. Without the self-assembled ordered monolayer, there was no need for a gradual tilting transition. Finally, the SFG spectra for the dodecanol β phase are shown in Figure 8C. The spectra features of the β phase resemble those of the disordered monolayer and there are fours peaks in the SSP polarization, including methylene symmetric peak around 2840 cm−1, methyl symmetric peak around 2865 cm−1, methylene Fermi peak around 2910 cm−1, and methyl

Figure 9. Packing of hexadecanol on top of an ordered monolayer on sapphire substrate. The model is shown for both the α phase (A) and the γ phase (B).

signals between the adjacent layers. However, the weak methylene and methyl vibrations indicate that there were still a few conformational defects at the α phase/sapphire interface. The strong SFG signal, even stronger than a well-ordered monolayer, in the γ phase is surprising (Figure 8B). In the spectra collected using SSP polarization, the two dominant peaks at 2830 and 2865 cm−1 are assigned to the symmetric vibration of the methylene next to the OH head group and the symmetric methyl vibration, respectively. The shift of methylene vibration to a lower wavenumber is due to Bohlmann effect,50 which was observed for CH groups next to O and N atoms. In PPP polarization, the peak around 2880 cm−1 should be a combination of methyl symmetric peak and methylene asymmetric peak. In the SPS polarization, the weak peak at 2830 cm−1 and the stronger peak at 2895 cm−1 belong to the symmetric and asymmetric vibrations of the methylene next to the OH head group. As the crystalline bulk is centrosymmetric and SFG-inactive under dipole approximation, the strong signal should originate from the interface. Because molecular chains oriented with the C−C−C axis parallel to the surface normal in the self-assembled monolayer next to sapphire surface, while molecular chains tilted 30° from the surface normal in the bulk of γ crystalline phase, there would be a mismatched chain orientation between the ordered selfassembled monolayer and γ crystalline bulk. As a result, molecular chains in contact with the self-assembled monolayer 6311

DOI: 10.1021/acs.langmuir.5b01330 Langmuir 2015, 31, 6306−6313

Article

Langmuir asymmetric peak around 2960 cm−1. In both PPP and SPS spectra, the dominant peak is the 2960 cm−1 methyl asymmetric peak. Compared to the disordered monolayer, the β phase shows stronger methylene symmetric peak and methyl asymmetric peak, indicating more gauche defects. Although gauche conformation is formed near the OH head group in the bulk (Figure 1), the β phase has been determined to be centrosymmetric and SFG-inactive, which suggests the SFG signal should be from the solid/sapphire interface. For the β phase, molecular chain orientation is similar to the α phase shown in Figure 9A, but packing of alternating gauche and trans alkyl chains leads to jagged chain ends (Figure 1), which could not match with the well-ordered self-assembled monolayer next to sapphire substrate. Thus, the SFG signal originated from the extra gauche defects, which formed at the interface to ensure packing conformity. At all the three polymorphic phases/sapphire interfaces, the interfacial alcohol molecules were expected to pack similarly in an ordered monolayer with all-trans conformations next to the sapphire substrate. The different SFG spectra features were attributed to the varied chain packing structures between the ordered monolayer and the polymorphic phases.

SFG spectra of the γ phase/OTS sapphire interface, and the calculation of monolayer melting temperature. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01330.

■

Corresponding Author

*E-mail [email protected] (A.D.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We acknowledge the financial support from National Science Foundation (DMR-1105370). We thank Adrian Defante for preparing OTS-coated sapphire substrate, Wei Zhang for purifying 1-dodecanol, and Edward Laughlin, Dr. Anish Kurian, and Dr. Liehui Ge for the design of the temperature stage. We also acknowledge Dr. Benjamin M. Ocko, Dr. Yeneneh Y. Yimer, Emmanuel Anim-Danso, Yu Zhang, Adrian Defante, Nishad Dhopatkar, Dr. Tarak Nath Burai, and Saranshu Singla for the valuable discussions and their help in preparing this manuscript.

■

■

CONCLUSIONS In this study, we have determined the melting temperature of hexadecanol monolayer to be about 30 °C above the bulk Tm by both experiments and theoretical calculation based on sapphire hydroxyl peak shift. Above the monolayer melting temperature, there is a broadening of the angular distribution of methyl end groups and an increase in population of gauche defects. We observed reduction in the strength of acid−base interactions between the sapphire hydroxyl groups and the OH groups of hexadecanol as we heated the system above the monolayer melting temperature. The correlation between molecule−substrate interaction strength and monolayer thermal stability will be useful in designing monolayers with controlled transition temperatures. Below Tm, we detected different interfacial molecular structures for the three solid phases of long-chain alcohols. At the α phase/sapphire interface, the molecular chains oriented parallel to the surface normal with very few gauche defects and this generated weak SFG signal due to cancellation of the signals between the ordered monolayer and the bulk molecules. For the β phase, the molecular chains also oriented parallel to the surface normal, but we hypothesize that the SFG signals for methylene and methyl groups were a result of interfacial gauche defects formed due to a mismatch between the ordered monolayer and the jagged chain packing of the β phase. In comparison, the SFG signals were very strong for the γ phase/sapphire interface. We propose that molecular chains tilted from 0° to 30° toward the surface normal with a gradual tilting transition from the sapphire surface to crystalline bulk, which is reminiscent of liquid crystal orientation near the surface anchored layer. Complementary experiments and molecular dynamics simulations53,54 are necessary to verify the models proposed here for the crystal structure near the sapphire substrate.

■

AUTHOR INFORMATION

REFERENCES

(1) Bigelow, W. C.; Pickett, D. L.; Zisman, W. A. Oleophobic Monolayers: I. Films Adsorbed from Solution in Non-polar Liquids. J. Colloid Sci. 1946, 1, 513−538. (2) Hare, E. F.; Zisman, W. A. Autophobic Liquids and the Properties of their Adsorbed Films. J. Phys. Chem. 1955, 59, 335−340. (3) Schwartz, D. K. Mechanisms and Kinetics of Self-Assembled Monolayer Formation. Annu. Rev. Phys. Chem. 2001, 52, 107−137. (4) Schierbaum, K. D.; Weiss, T.; Thoden van Veizen, E. U.; Engbersen, J. F. J.; Reinhoudt, D. N.; Gö pel, W. Molecular Recognition by Self-Assembled Monolayers of Cavitand Receptors. Science 1994, 265, 1413−1415. (5) Manne, S.; Gaub, H. E. Molecular Organization of Surfactants at Solid-Liquid Interfaces. Science 1995, 270, 1480−1482. (6) Sabatani, E.; Rubinstein, I.; Maoz, R.; Sagiv, J. Organized SelfAssembling Monolayers on Electrodes: Part I. Octadecyl Derivatives on Gold. J. Electroanal. Chem. Interfacial Electrochem. 1987, 219, 365− 371. (7) Bhushan, B.; Kulkarni, A. V.; Koinkar, V. N.; Boehm, M.; Odoni, L.; Martelet, C.; Belin, M. Microtribological Characterization of SelfAssembled and Langmuir-Blodgett Monolayers by Atomic and Friction Force Microscopy. Langmuir 1995, 11, 3189−3198. (8) Naselli, C.; Rabolt, J. F.; Swalen, J. D. Order-Disorder Transitions in Langmuir-Blodgett Monolayers. I. Studies of Two-Dimensional Melting by Infrared Spectroscopy. J. Chem. Phys. 1985, 82, 2136− 2140. (9) Nuzzo, R. G.; Korenic, E. M.; Dubois, L. H. Studies of the Temperature Dependent Phase Behavior of Long Chain n-Alkyl Thiol Monolayers on Gold. J. Chem. Phys. 1990, 93, 767−773. (10) Prathima, N.; Harini, M.; Rai, N.; Chandrashekara, R. H.; Ayappa, K. G.; Sampath, S.; Biswas, S. K. Thermal Study of Accumulation of Conformational Disorders in the Self-Assembled Monolayers of C8 and C18 Alkanethiols on the Au(111) Surface. Langmuir 2005, 21, 2364−2374. (11) Weber, J.; Balgar, T.; Hasselbrink, E. Conformational Disorder in Alkylsiloxane Monolayers at Elevated Temperatures. J. Chem. Phys. 2013, 139, 244902. (12) Karaborni, S.; Toxvaerd, S. Molecular Dynamics Simulations of Langmuir Monolayers: A Study of Structure and Thermodynamics. J. Chem. Phys. 1992, 96, 5505−5515. (13) Rieu, J. P.; Vallade, M. Statistical Model of Short Alcohol Monolayers: Chain Conformations and Melting Transition. J. Chem. Phys. 1996, 104, 7729−7740.

ASSOCIATED CONTENT

* Supporting Information S

DSC measurements, SFG measurements at the s−v interface, fitting parameters for Figure 2, OH peak amplitude strength as a function of temperature, SFG spectra of the γ crystal surface in external geometry and sapphire surface in internal geometry, 6312

DOI: 10.1021/acs.langmuir.5b01330 Langmuir 2015, 31, 6306−6313

Article

Langmuir

Semi-microcalorimeter for Studies of Polymorphism. J. Chem. Thermodyn. 1974, 6, 477−487. (36) 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. (37) Kurian, A.; Prasad, S.; Dhinojwala, A. Unusual Surface Aging of Poly(dimethylsiloxane) Elastomers. Macromolecules 2010, 43, 2438− 2443. (38) Wilhoit, R. C.; Zwolinski, B. J. Physical and Thermodynamic Properties of Aliphatic Alcohols. J. Phys. Chem. Ref. Data 1973, 2 (Suppl. No. 1). (39) Guyot-Sionnest, P.; Hunt, J. H.; Shen, Y. R. Sum-Frequency Vibrational Spectroscopy of a Langmuir Film: Study of Molecular Orientation of a Two-Dimensional System. Phys. Rev. Lett. 1987, 59, 1597−1600. (40) Gautam, K. S.; Dhinojwala, A. Molecular Structure of Hydrophobic Alkyl Side Chains at Comb Polymer-Air Interface. Macromolecules 2001, 34, 1137−1139. (41) Lu, R.; Gan, W.; Wu, B.; Chen, H.; Wang, H. Vibrational Polarization Spectroscopy of CH Stretching Modes of the Methylene Group at the Vapor/Liquid Interfaces with Sum Frequency Generation. J. Phys. Chem. B 2004, 108, 7297−7306. (42) Lu, R.; Gan, W.; Wu, B.; Zhang, Z.; Guo, Y.; Wang, H. CH Stretching Vibrations of Methyl, Methylene and Methine Groups at the Vapor/Alcohol (n = 1−8) Interfaces. J. Phys. Chem. B 2005, 109, 14118−14129. (43) 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. (44) Hirose, C.; Akamatsu, N.; Domen, K. Formulas for the Analysis of the Surface SFG Spectrum and Transformation Coefficients of Cartesian SFG Tensor Components. Appl. Spectrosc. 1992, 46, 1051− 1072. (45) Simpson, G. J.; Rowlen, K. L. An SHG Magic Angle: Dependence of Second Harmonic Generation Orientation Measurements on the Width of the Orientation Distribution. J. Am. Chem. Soc. 1999, 121, 2635−2636. (46) Jakobsen, R.; Brasch, J.; Mikawa, Y. Hydrogen Bonding in Solid Alcohols. J. Mol. Struct. 1968, 1, 309−321. (47) Badger, R. M.; Bauer, S. H. Spectroscopic Studies of the Hydrogen Bond. II. The Shift of the O-H Vibrational Frequency in the Formation of the Hydrogen Bond. J. Chem. Phys. 1937, 5, 839−851. (48) Abrahamsson, S.; Larsson, G.; von Sydow, E. The Crystal Structure of the Monoclinic Form of n-Hexadecanol. Acta Crystallogr. 1960, 13, 770−774. (49) Ventolà, L.; Ramírez, M.; Calvet, T.; Solans, X.; Cuevas-Diarte, M.; Negrier, P.; Mondieig, D.; van Miltenburg, J.; Oonk, H. Polymorphism of n-Alkanols: 1-Heptadecanol, 1-Cctadecanol, 1Nonadecanol, and 1-Eicosanol. Chem. Mater. 2002, 14, 508−517. (50) Lii, J.-H.; Chen, K.-H.; Allinger, N. L. Alcohols, Ethers, Carbohydrates, and Related Compounds Part V.2 The Bohlmann Torsional Effect. J. Phys. Chem. A 2004, 108, 3006−3015. (51) Faetti, S.; Palleschi, V. Nematic-Isotropic Interface of Some Members of the Homologous Series of 4-Cyano-4′-(n-alkyl)biphenyl Liquid Crystals. Phys. Rev. A 1984, 30, 3241−3251. (52) Jerome, B. Surface Effects and Anchoring in Liquid Crystals. Rep. Prog. Phys. 1991, 54, 391. (53) Zhu, H.; Jha, K. C.; Bhatta, R. S.; Tsige, M.; Dhinojwala, A. Molecular Structure of Poly(methyl methacrylate) Surface. I. Combination of Interface-Sensitive Infrared-Visible Sum Frequency Generation, Molecular Dynamics Simulations, and ab Initio Calculations. Langmuir 2014, 30, 11609−11618. (54) Jha, K. C.; Zhu, H.; Dhinojwala, A.; Tsige, M. Molecular Structure of Poly(methyl methacrylate) Surface II: Effect of Stereoregularity Examined through All-Atom Molecular Dynamics. Langmuir 2014, 30, 12775−12785.

(14) Kaganer, V. M.; Möhwald, H.; Dutta, P. Structure and Phase Transitions in Langmuir Monolayers. Rev. Mod. Phys. 1999, 71, 779− 819. (15) Ocko, B.; Hlaing, H.; Jepsen, P.; Kewalramani, S.; Tkachenko, A.; Pontoni, D.; Reichert, H.; Deutsch, M. Unifying Interfacial SelfAssembly and Surface Freezing. Phys. Rev. Lett. 2011, 106, 137801. (16) Li, G.; Dhinojwala, A.; Yeganeh, M. S. Interfacial Structure and Melting Temperature of Alcohol and Alkane Molecules in Contact with Polystyrene Films. J. Phys. Chem. B 2009, 113, 2739−2749. (17) Shen, Y. Surface Properties Probed by Second-Harmonic and Sum-Frequency Generation. Nature 1989, 337, 519−525. (18) Wolfrum, K.; Laubereau, A. Vibrational Sum-Frequency Spectroscopy of an Adsorbed Monolayer of Hexadecanol on Water. Destructive Interference of Adjacent Lines. Chem. Phys. Lett. 1994, 228, 83−88. (19) Zhang, D.; Gutow, J.; Eisenthal, K. B. Vibrational Spectra, Orientations, and Phase Transitions in Long-Chain Amphiphiles at the Air/Water Interface: Probing the Head and Tail Groups by Sum Frequency Generation. J. Phys. Chem. 1994, 98, 13729−13734. (20) Sefler, G.; Du, Q.; Miranda, P.; Shen, Y. Surface Crystallization of Liquid n-Alkanes and Alcohol Monolayers Studied by Surface Vibrational Spectroscopy. Chem. Phys. Lett. 1995, 235, 347−354. (21) Bell, G. R.; Bain, C. D.; Ward, R. N. Sum-Frequency Vibrational Spectroscopy of Soluble Surfactants at the Air/Water Interface. J. Chem. Soc., Faraday Trans. 1996, 92, 515−523. (22) Roke, S.; Schins, J.; Müller, M.; Bonn, M. Vibrational Spectroscopic Investigation of the Phase Diagram of a Biomimetic Lipid Monolayer. Phys. Rev. Lett. 2003, 90, 128101. (23) Liu, J.; Conboy, J. C. Phase Transition of a Single Lipid Bilayer Measured by Sum-Frequency Vibrational Spectroscopy. J. Am. Chem. Soc. 2004, 126, 8894−8895. (24) 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. (25) Yurdumakan, B.; Harp, G. P.; Tsige, M.; Dhinojwala, A. Template-Induced Enhanced Ordering under Confinement. Langmuir 2005, 21, 10316−10319. (26) Guyot-Sionnest, P.; Superfine, R.; Hunt, J.; Shen, Y. Vibrational Spectroscopy of a Silane Monolayer at Air/Solid and Liquid/Solid Interfaces Using Sum-Frequency Generation. Chem. Phys. Lett. 1988, 144, 1−5. (27) Tyrode, E.; Rutland, M. W.; Bain, C. D. Adsorption of CTAB on Hydrophilic Silica Studied by Linear and Nonlinear Optical Spectroscopy. J. Am. Chem. Soc. 2008, 130, 17434−17445. (28) Messmer, M. C.; Conboy, J. C.; Richmond, G. L. Observation of Molecular Ordering at the Liquid-Liquid Interface by Resonant Sum Frequency Generation. J. Am. Chem. Soc. 1995, 117, 8039−8040. (29) Conboy, J. C.; Messmer, M. C.; Richmond, G. L. Investigation of Surfactant Conformation and Order at the Liquid-Liquid Interface by Total Internal Reflection Sum-Frequency Vibrational Spectroscopy. J. Phys. Chem. 1996, 100, 7617−7622. (30) Kurian, A.; Prasad, S.; Dhinojwala, A. Direct Measurement of Acid-Base Interaction Energy at Solid Interfaces. Langmuir 2010, 26, 17804−17807. (31) Prasad, S.; Zhu, H.; Kurian, A.; Badge, I.; Dhinojwala, A. Interfacial Segregation in Polymer Blends Driven by Acid-Base Interactions. Langmuir 2013, 29, 15727−15731. (32) Sirota, E. B.; Wu, X. Z. The Rotator Phases of Neat and Hydrated 1-Alcohols. J. Chem. Phys. 1996, 105, 7763−7773. (33) Tasumi, M.; Shimanouchi, T.; Watanabe, A.; Goto, R. Infrared Spectra of Normal Higher Alcohols-I. Spectrochim. Acta 1964, 20, 629−666. (34) Tanaka, K.; Seto, T.; Hayashida, T. Phase Transformation of nHigher Alcohols (I). Bull. Inst. Chem. Res. Kyoto Univ. 1958, 35, 123− 139. (35) Mosselman, C.; Mourik, J.; Dekker, H. Enthalpies of Phase Change and Heat Capacities of Some Long-Chain Alcohols. Adiabatic 6313

DOI: 10.1021/acs.langmuir.5b01330 Langmuir 2015, 31, 6306−6313