Behavior of Liquid Crystals Confined to Mesoporous Materials as

764

J. Phys. Chem. B 2008, 112, 764-775

Behavior of Liquid Crystals Confined to Mesoporous Materials as Studied by Spectroscopy of Methyl Iodide and Methane as Probe Molecules

13C

NMR

Pekka Tallavaara and Jukka Jokisaari* Department of Physical Sciences, NMR Research Group, UniVersity of Oulu, P. O. Box 3000, FIN-90014 UniVersity of Oulu, Finland ReceiVed: August 27, 2007; In Final Form: NoVember 2, 2007

The behavior of thermotropic nematic liquid crystals (LCs) Merck Phase 4 and ZLI 1115 confined to mesoporous controlled pore glass materials was investigated using 13C nuclear magnetic resonance spectroscopy of probe molecules methyl iodide and methane. The average pore diameters of the materials varied from 81 to 375 Å, and the temperature series measurements were performed on solid, nematic, and isotropic phases of bulk LCs. Chemical shift, intensity, and line shape of the resonance signals in the spectra contain lots of information about the effect of confinement on the state of the LCs. The line shape of the 13C resonances of the CH3I molecules in LCs confined into the pores was observed to be even more sensitive to the LC orientation distribution than, for example, that of 2H spectra of deuterated LCs or 129Xe spectra of dissolved xenon gas. The effect of the magnetic field on the orientation of LC molecules inside the pores was examined in four different magnetic fields varying from 4.70 to 11.74 T. The magnetic field was found to have significant effect on the orientation of LC molecules in the largest pores and close to the nematic-isotropic phase transition temperature. The theoretical model of shielding of noble gases dissolved in LCs based on pairwise additivity approximation was utilized in the analysis of CH4 spectra. For the first time, a first-order nematic-isotropic phase transition was detected to take place inside such restrictive hosts. In the larger pores a few degrees below the nematic-isotropic phase transition of bulk LC the 13C quartet of CH3I changes as a powder pattern. Results are compared to those derived from 129Xe NMR measurements of xenon gas in similar environments.

Introduction Liquid crystals (LCs) play an important role in modern technology. Different kind of hands-on applications based on LCs can be found from almost every technical instrument. Perhaps the most familiar ones are various LC applications in display technology such as TVs, monitors, polymer-dispersed liquid crystals (PDLCs), watches, calculators, etc. Thermotropic properties of LCs have resulted in the widespread use of them in thermography. Flow visualizations, heat transfer measurements, shear stress observations, and extensive medical applications are only a few examples in which LC thermography is nowadays used. Thermal mapping, radiation detection, and the use of LCs as anisotropic solvents for the study of various physicochemical properties of molecules are things that also speak for importance of LCs these days.1 Porous materials can be found almost everywhere in everyday life. Practically all solid and semisolid materials are porous to varying degrees. Only metals, some plastics, and some dense rocks are exceptions. Hydrology, petroleum, chemical, biochemical, and electrochemical engineering, as well as medicine are all interested in physical and chemical properties of porous media in numerous applications.2 Nuclear waste storage, oil recovery, and passive solar energy collection devices are some examples of technological applications where porous materials exhibit an increasingly bigger role.3 In addition, they offer a good pattern setup for the study of various basic physical problems. * To whom correspondence should be addressed. Email: Jukka. [email protected]. Phone: +35885531308 or +358405956146 (mobile). Fax: +35885531287.

Behavior of isotropic fluids and especially anisotropic LCs in various kinds of porous materials have been studied by using different techniques such as alternating current (AC) and relaxation-mode calorimetry,4-6 Raman spectroscopy,7 dipolarcorrelation effect,8,9 spin relaxation time T1 and T2 measurements,9,10 light scattering,11 X-ray diffraction,12 and nuclear magnetic resonance (NMR) spectroscopy.13-16 In NMR spectroscopy, LC molecules are usually labeled with deuterium in one or several positions, and 2H NMR spectrum is measured.17 Line-shape analysis of deuterium spectrum has proven to give versatile information about LC orientation and phase behavior of LC molecules in the pores both in smectic, nematic, and isotropic phases. This is, however, quite an insensitive method due to very small chemical shift change of 2H signal in different temperatures and chemical environments. An alternative to isotope labeling of LC molecules is to utilize 129Xe gas as a probe to study phase behavior and orientation order of LC molecules in porous media.18-20 129Xe isotope (spin I ) 1/2) of xenon is an extremely suitable candidate for LC NMR studies because its chemical shift is extremely sensitive to the changes taking place in its local environment.21,22 In the present study, Merck thermotropic nematic LC Phase 4 (eutectic mixture of p-methoxy-p′-butylazoxy-benzenes) was confined to four different controlled pore glass (CPG) materials with nominal pore diameters varying from 81 to 375 Å. In addition, Merck thermotropic nematic LC ZLI 1115 (trans-4(4-heptyl-cyclohexyl)-benzonitrile) was confined to a CPG material with nominal pore diameter 81 Å. CPG compounds are nearly pure silica, and therefore they are chemically very inert. They have an extremely rigid structure that makes them incompressible and furthermore spectroscopically transparent.

10.1021/jp076840i CCC: $40.75 © 2008 American Chemical Society Published on Web 01/01/2008

Liquid Crystals Confined to Mesoporous Materials Narrow pore size distribution and large internal surface area enable NMR studies of effects of pore surface on the behavior of LCs as a function of pore size. The behavior of Phase 4 and ZLI 1115 in the pores was studied at different temperatures ranging from solid to isotropic phases by measuring 13C NMR spectra from carbon-13 enriched methyl iodide and methane dissolved in the LC. Results are compared to those obtained earlier from 129Xe NMR studies in which xenon gas was dissolved into Phase 4 and the mixture was confined to CPG materials with nominal pore diameters 81 and 156 Å. 129Xe NMR was also used to study the behavior of ZLI 1115 in pores with nominal diameter 81 Å. According to our knowledge this is the first study of this kind in which the heteronuclear spin system, a small molecule, is used to probe the behavior of LC molecules in confined geometries. Experimental Section CPG porous materials used in this study were obtained from CPG Inc. (Lincoln Park, New Jersey). According to the manufacturer, the nominal pore diameters of the materials are 81, 156, 237, and 375 Å, and the size of particles is 125-177 µm in each material. The acronym CPG followed by the pore diameter in angstroms is used to refer to the substances (e.g., CPG 81). Thermotropic LCs Phase 4 (also known as N4) and ZLI 1115 (S1115, PCH-7) were purchased from Merck (Darmstadt, Germany), and they were used without further purification. According to the manufacturer, the nematic range of Phase 4 is 293-347 K and that of ZLI 1115 303-331 K. Both LCs are nematic and uniaxial. 13C isotope enriched methyl iodide 13CH3I (99%) and methane gas 13CH4 (99%) were obtained from Euriso-top (Gif-Sur-Yvette, France) and Isotec Inc. (Miamisburg, USA), respectively. 129Xe isotope enriched (85%) xenon gas was delivered by Spectra Gases Inc. (New Jersey). The samples were prepared as follows. First, the CPG porous material was dried in a 10 mm medium wall NMR tube in a vacuum line overnight at a temperature of ∼470 K. Then, the tube was unfastened from the line and sealed with a plug and Parafilm. Next, the medium (Phase 4 or ZLI 1115) was degassed by several freeze-pump-thaw cycles, and after bubbles were no longer seen to build up, the CPG powder was immersed in excess of medium. System was mixed completely, and gaseous impurities were removed in the same way as from bulk LC. ZLI 1115 was mixed with CPG 81 and Phase 4 with CPG 81, 156, 237, and 375 materials. After that, the sample was frozen by snow or cold ice water and unfastened from the vacuum line, and about 10 mol % of 13C isotope enriched methyl iodide was added. After connecting the sample tube to the vacuum line again, about 2 atm of 13C isotope enriched methane gas was added by immersing a sample tube in a liquid nitrogen bath, and the glass tube was sealed with a flame. The preparation method for samples containing 129Xe gas, LC, and CPG 81 or 156 is described elsewhere.18 One-dimensional proton coupled and decoupled 13C NMR spectra were recorded mainly on Bruker Avance DRX 500 and DPX 200 spectrometers (Larmor frequencies 125.8 and 50.3 MHz for 13C, respectively) equipped with 10 mm high-resolution BBO probe heads and BVT2000 and BVT3200 variable temperature units, respectively. The effect of the magnetic field on the orientation of LC molecules inside the pores was also tested at different magnetic fields on Bruker Avance DSX 300 and DPX 400 spectrometers (Larmor frequencies 75.5 and 100.6 MHz for 13C, respectively) with 10 mm high-resolution BBOprobe heads and BVT2000 and BVT3200 variable temperature

J. Phys. Chem. B, Vol. 112, No. 3, 2008 765 units, respectively. 1D 129Xe measurements were performed on DRX 500 with the same probe as 13C measurements. The experiments were performed mainly within the temperature range ∼272-349 K and from low to high temperature to avoid the super cooling effect of the LC. The samples were frozen tens of degrees below the melting point of bulk LC in the NMR spectrometers before actual measurements were initiated. The spectra were measured at 1 K intervals, and the temperature was allowed to stabilize for 10-15 min after each temperature change. During the measurements the temperature did not fluctuate more than (0.1 K. The lowest and the highest temperatures were calibrated with the aid of the 1H NMR spectra from the standard temperature calibration samples (4% methanol in deuterated methanol and 80% ethylene glycol in DMSOd6).23 Between these points, the correct temperature was assumed to change linearly as a function of the temperature shown by the VT unit of the spectrometer. The xenon-129 spectra were referenced to 6.16 atm pressure xenon gas but corrected with well-known shielding equation of bulk xenon gas24 to correspond to zero pressure at each temperature. Theory Thermotropic LCs are commonly used as solvents in NMR spectroscopy to partially orientate dissolved molecules. LC molecules have only a short-range positional order in the nematic phase, and if the LC molecules are elongated in shape, as they are in this study, they tend to align with their long axes parallel to a common axis, known as n, of the LC. This direction is often also called the optic axis. When a liquid crystalline sample is placed in an external magnetic field B, to the NMR spectrometer for example, the sample is magnetized. The minimum energy density due to the magnetization is reached when n is oriented parallel with the external magnetic field in the case where the anisotropy of diamagnetic susceptibility, ∆χ, is positive. If ∆χ is negative, the minimum of the energy density is reached when n is oriented perpendicular to the external field.25 Consequently, because the ∆χ of Phase 4 and ZLI 1115 is positive their director orients along the external magnetic field. In the present system, LCs were confined to randomly interconnected, roughly cylindrical pores with various sizes. Then several factors contribute to the orientation of molecules in nematic phase. Director configuration results from interplay between the external magnetic field, pore wall surface interactions, size of cavities, elastic forces of LCs, and space geometry of used porous material. These factors will be examined below. In the present case we used CPG as a porous material. It is a mesoporous silica material manufactured by acid leaching of a phase-separated borosilicate base material Na2O-B2O3-SiO2, by which the sodium borate phase (NaBO2) is removed, leaving behind a silicon oxide (SiO2) framework with a highly interconnected pore system with uniform and controlled pore sizes. The porosity of the CPG materials is determined by the chosen composition ratio of Na2O-B2O3 to SiO2, and the mean pore size can be controlled by an annealing process, i.e., reheating for variable periods of time after the initial temperature quench.26 This process creates a very narrow pore size distribution with at least 80% of the pores within (10% of the mean pore diameter. The atomic force microscopy (AFM) measurements have revealed that the CPG grain surface is smooth on the nanometer scale, with no preferred direction within the surface plane. Because no direct information can be obtained from the surfaces of the pore walls, it is expected to be similar with the surface

766 J. Phys. Chem. B, Vol. 112, No. 3, 2008

Tallavaara and Jokisaari

of CPG grain.27 Smooth, nontreated CPG pore surfaces are expected to enforce axial orientational anchoring of the LC molecules,28 i.e., molecules are oriented parallel with the long axis of pore. Silane (SiH4) treatment of pore surfaces is known to enforce homeotropic orientational anchoring of the molecules29 in which molecules are oriented perpendicular to the pore surface. In this work no silane treatment was used, and consequently bare silica surface tends to orientate LC molecules along the surfaces. The external magnetic field, however, tends to orientate the molecules parallel with the field, and therefore these two factors are competing with each other. For example, near a smooth surface of pore wall, the orientation of LC molecules is completely determined by the surface interactions between the pore wall and molecule. This surface interaction, however, diminishes with increasing distance from the surface, and sufficiently far from the wall the nematic LCs behave like bulk LC and orientate parallel with the external field. This distance is called magnetic coherence length, ξM, and it is determined through the equation30

ξM )

( )

1 µ0K B ∆χ

1/2

(1)

where B is the external magnetic flux density, µ0 is permeability of vacuum, K is elastic constant of the medium, and ∆χ is anisotropy of diamagnetic susceptibility. In the magnetic field used in this work (4.70-11.74 T), the magnetic coherence length is usually approximated to be 2.5-1 µm. It is thus much larger than the average pore diameter (0.0081-0.0375 µm), and thus we can conclude that the magnetic field has a negligible influence on the orientation of LC molecules in the pores. On the other hand, because the size of the spherical CPG grains is on average 150 µm in diameter, the spaces between the grains are so large that the external magnetic field dominates as an orientation force, and consequently LC molecules behave like in bulk in between the particles. The surface induced ordering is usually described in terms of wetting31 or to be precise as orientational wetting32 in the case of a nematic LC solid interface. In the isotropic phase, there is a surface-induced residual nematic order due to a strongly anchored nematic layer at the surface of the pore. The value of the orientational order parameter at the surface, S0, can be significantly different from that of the bulk, and it is a direct measure of the strength of the anisotropic substrate-fluid interaction, which is usually called the surface coupling constant G. The difference in measured order parameters can be attributed to the different surface treatments of pores.33 The Sheng theory,34 which is based on the Landau-de Gennes formalism and which describes orientational ordering of LCs near a solid surface, predicts that S0 decreases as temperature increases. In some cases,17,33,35 however, the orientational order of the first molecular layer at the pore surface has been observed to be temperature independent with the remaining order penetrating into pores following the Sheng theory. The interfacial thickness of the surface layer, σ0, is approximately on the order of molecular dimensions36 (between 10 and 20 Å, depending on the surface energy). The surface induced nematic order can consequently be expressed in the form17

S(r) )

{

S0 -[(R-σ0)-r]/ξN

S 0e

R e r e R - σ0 R - σ0 e r e 0

(2)

where r is the distance from the middle of pore, R is the radius of the pore, and ξN describes the nematic correlation length, which is a measure of the distance over which the local

fluctuations are correlated.37 Equation 2 is valid only when ξN , R. ξN is determined through equation

ξN ) ξ0

[T -T*T*]

1/2

(3)

where ξ0 is the material dependent zero temperature coherence length, T is temperature, and T* is the second-order transition temperature, i.e., it is the temperature when the nematic correlation length ξN becomes infinite, and it is typically ∼1 K below the nematic-isotropic phase transition temperature. It is also the lowest temperature down to which one can supercool the isotropic phase. The nematic correlation length represents the size of nematic-like domains in the isotropic phase. In small domains, LC molecules are parallel to each other, but the orientations of successive domains do not correlate. The nematic correlation length is expected to be relatively large when T < TNI.38 In nanometer scale cavities ξN is thus much longer than the radius of pore R, and therefore in such circumstances surface effects dominate and the order parameter S(r) is equal to the surface-order parameter S0 in the pores. In the highly confined spaces, i.e., when the size of the cavity is on the order of correlation length, the orientational order parameter S(r) is presented in the form39

S(r) ) S0

cosh(r/ξN) cosh(R/ξN)

(4)

Crawford et al.33 have in addition observed from deuterium NMR measurements that the diffusion of LCs in the isotropic phase in the surface layer of cylindrical polymer cavities of Nuclepore is about three orders slower than in the rest of cylinder, confirming that the interaction between the surface layer and pore wall is local in nature. The pore structure of CPG porous material is fundamentally the same as in Vycor glass. They both are prepared in the same way; only in the final treatment of the CPG material, there are some minor differences compared to the Vycor glass, which may make surface irregularities smaller in CPG.40 Iannacchione et al.41 have modeled Vycor glass as a 3D network of randomly connected pore segments with the ratio of segment length to the pore diameter about four. Because of uniform manufacturing methods of both porous materials, this can be considered to be true also for CPG. Consequently the lengths of pore segments used in this study are about 300 Å in the smallest pore diameter sample (CPG 81) and 1500 Å in the largest one (CPG 375). Typical self-diffusion coefficients of methane42 and xenon gas43 in thermotropic LCs are about 10-10 m2/s in the direction of the LC director. Unfortunately, the self-diffusion coefficient of methyl iodide in LC is not known to us, but presumably it is of the same order as that of xenon and methane. The self-diffusion coefficients of LC molecules are usually about one order smaller, i.e., ∼10-11 m2/s, along the nematic director. As was shown earlier,18 the chemical shift difference of xenon-129 dissolved in the nematic phase of bulk Phase 4 oriented parallel with the external magnetic field and xenon in Phase 4 confined to pores of CPG material with diameter 81156 Å is about 10 ppm (∼1400 Hz, in the magnetic field of 11.74 T). To observe different signals from these two sites, xenon must stay in one site longer than the NMR time scale τ ) 1/(2π∆ν) ≈ 0.1 ms. Therefore, the distance traveled by a xenon atom during τ in one dimension is about (Dτ)1/2 ≈ 1000 Å. The same diffusion length is valid also when medium Phase 4 is replaced by ZLI 1115. As will be shown later, the 13C chemical shift difference of methane and methyl iodide, in a

Liquid Crystals Confined to Mesoporous Materials

J. Phys. Chem. B, Vol. 112, No. 3, 2008 767

similar environment as xenon described above, is about 0.7 and 1.4 ppm, respectively. In the magnetic field of 11.74 T, these are about 90 and 180 Hz, and the NMR time scales are consequently 1.8 and 0.9 ms, respectively. Because of translational diffusion, a methane atom changes its local position in one dimension during τ by an average distance of about 4200 Å. For methyl iodide, an average diffusion length in the time scale of τ is about 3000 Å. Roughly the same values are valid for methane and methyl iodide if ZLI 1115 is used as a medium instead of Phase 4. From these diffusion lengths, it can be concluded that molecules map the average orientation of LC in a pore because pore diameters are much smaller than translational diffusion lengths. Randomly connected pore structure, however, slows down the diffusion of molecules in some degree. In fact, Vilfan et al.9 have pointed out by using deuterium NMR spectroscopy that the effective diffusion coefficient of LC 5CB in the nematic phase is about one order larger in the bulk phase than in the CPG 75 matrix. Consequently it can be assumed that diffusion is faster in larger pores than it is in smaller ones and that the above-mentioned diffusion distances 4200 and 3000 Å diminish to 1300 and 900 Å, respectively, if diffusion is assumed to be ten times slower in the pores than it is in the bulk. If an average translational diffusion distance of a molecule is estimated from the time duration of FID signal, i.e., how long it takes when the intensity of FID signal is attenuated to zero mainly due to the loss of phase coherence (about 16 ms in the nematic phase and 30 ms in the isotropic phase in samples containing methane and methyl iodide), the length scale is much larger than that obtained from the NMR time scale calculations. During the presence of transverse magnetization component the molecules diffuse on average about 13 000 Å in the nematic phase and 17 000 Å in the isotropic phase in one dimension. If diffusion is assumed to be about one order smaller in pores than it is in bulk, then the corresponding values are 4000 and 5500 Å, respectively. From these or previously estimated diffusion length values, we can conclude that the chemical shift of the signal of xenon, methane, or methyl iodide observed from the porous materials containing smallest pores is an average of the chemical shifts characteristic of many different pores (and different pore orientations), whereas the chemical shift of methyl iodide reflects the orientation in the pores in the case of largest pores (CPG 375). According to previous discussion, pore walls of CPG material tend to orient LC molecules parallel with the pore axis. Because of translational diffusion, molecules map the average orientation of LC in a pore, not a local orientation in some part of the pore. The nuclear magnetic shielding of noble gas atoms as well as of methane dissolved in LC can be explained by using pairwise additivity approximation developed by Ylihautala et al.44 According to the model, the average of the shielding tensor element in the direction of the applied magnetic field (Z) can be presented in the form44

{

〈σZZ〉 ) F(T) σ0[1 - β1(T - T0)] +

}

2 ∆σ0[1 - β2(T - T0)]P2(cos θ)S(T) (5) 3 where F(T) describes the temperature dependence of the density of the used LC, σ0 is the shielding constant divided by density at reference temperature T0 (chosen to be bulk nematic-isotropic phase transition temperature TNI), and ∆σ0 is its anisotropy. Coefficients β1 and β2 express the temperature dependence of shielding and shielding anisotropy, respectively. P2(cos θ) )

(1/2)(3 cos2 θ - 1) is the well-known second-order Legendre polynomial, where θ is the angle between the LC director n and the external magnetic field B0. Term S(T) ) (1/2)〈3 cos2 β - 1〉 is the second-rank orientational order parameter of LC, where β is the angle between the director and the long molecular axis. Angle brackets denote that the orientational order parameter is a time average. As can be seen from the anisotropy term of eq 5 (from the latter part of the equation), shielding is directly proportional to the product of P2(cos θ)S(T). In the isotropic phase, this product is zero because the second rank orientational order parameter S(T) vanishes due to randomly and isotropically oriented LC molecules. In the nematic phase, P2(cos θ) should in principle produce a perfect chemical shift anisotropy (CSA) powder pattern line shape in isotropically oriented pore systems. However, if a probe atom or molecule dissolved in LC travels through several differently oriented pores during the time scale NMR measurement is performed, S(T) is effectively reduced. This diffusion averaging could in an extreme case lead to the observation of isotropic signal from the system in which LC molecules form perfect nematic phases inside the pores. In addition, in the interconnection region, where several pore segments join together, the director distribution cannot be unambiguously determined.27 These joining points also effectively decrease the P2(cos θ) term. The macroscopic bulk susceptibility correction σb has to be taken into account because eq 5 represents only the microscopic part of the effects of solvent on the shielding of solute. For a long cylindrical sample parallel with the applied magnetic field, the susceptibility effect is

σb ) -

F(T) 1 2 χ + ∆χS(T) 3 3 M

[

]

(6)

where χ is diamagnetic susceptibility of LC, ∆χ is its anisotropy, and M is the molar mass of LC. Temperature dependence of density F(T) of used LCs is not known, but a good approximation is that the density changes linearly as a function of temperature both in isotropic and nematic phases.1 In the isotropic phase, the density behavior can be approximated by the function F(T) ) F0[1 - R(T - T0)] and in the anisotropic phase by the function F(T) ) F0{[1 - R(T - T0)] + ∆F/F0}, where F0 is the density at the reference temperature T0 ) TNI, R is the isobaric thermal expansion coefficient, and ∆F represents the density jump in the isotropic-nematic phase transition. Thermal expansion coefficient R is usually different in the isotropic and nematic phases. Because ∆χ is positive both for Phase 4 and ZLI 1115, the director n orients parallel with the applied magnetic field B0 in nematic phase, and consequently P2(cos θ) ) 1. The temperature dependence of the second-rank orientational order parameter S(T) in the nematic phase can be modeled by the Haller function S(T) ) (1 - yT/T0)z, where y and z are adjustable parameters. Therefore when density functions, Haller function, and macroscopic bulk susceptibility correction are added to eq 5, the average of shielding tensor element in anisotropic phase becomes

{

〈σZZ〉aniso ) F0 [1 - R(T - T0)] +

{

}

∆F × F0

2 σ0[1 - β1(T - T0)] + ∆σ0[1 - β2(T - T0)] × 3 T z 1 2 T z 1-y χ + ∆χ 1 - y T0 3M 3 T0

(

)

[

(

) ]}

(7)


Tallavaara and Jokisaari

The equation for the isotropic phase is obtained by setting the anisotropic parameters S, ∆σ0, ∆χ, as well as density jump ∆F equal to zero. The experimental 13C chemical shift, δexp, of methyl iodide or any molecule possessing at least a 3-fold symmetry axis partially oriented in LC solvent is45

2 δexp ) δiso - ∆σSzzP2(cos θ) 3

(8)

In the equation, δiso is the isotropic chemical shift, ∆σ ≡ σzz (1/2)(σxx + σyy) is the anisotropy of the shielding tensor in the molecule fixed coordinate system (x, y, z), where the z axis coincides with the molecular symmetry axis, and Szz is the order parameter of the molecule symmetry axis. P2(cos θ) term has the same meaning as in eq 5, and it is equal to 1 in the present cases. For methyl iodide, the order parameter Szz is obtained from the experimental 1H-13C dipole-dipole coupling

DCH ) -

µ0pγCγH 8π2rCH3

SCH

(9)

where SCH ) P2(cos R)Szz is the orientational order parameter of the C-H bond, with R being the angle between the C-H bond and the z axis, rCH is the C-H bond length, and the other symbols have their usual meaning. The contribution from the respective J tensor to the experimental dipole coupling should be taken into account, but for C-H coupling it is negligible.46 The melting-point depression ∆T of liquids and LCs confined in a porous material can be used to characterize the pore-size distribution. The depressed melting point of a liquid in a pore is generally attributed to the reduced crystal size in the pore and the larger surface-to-volume ratio. The melting point depression of a liquid in a porous material is given by GibbsThompson equation47

∆T ) T0 - T )

2σslT0 kp ≡ ∆HfFsRp Rp

(10)

where T0 and T are the melting points of the bulk and confined substances, respectively, σsl is the surface energy of the solidliquid interface, ∆Hf is the specific bulk enthalpy of fusion, Fs is the density of the solid, and Rp is the pore radius. The constant kp is characteristic to each substance. In cryoporometry, the fact that the melting-point depression of a confined fluid in pores is inversely proportional to the pore radius is used to determine the pore-size distributions. In our earlier work,18 it was observed that the majority of xenon atoms squeeze out from the bulk LC during slow freezing of substance, whereas if the sample is cooled very rapidly by immersing it in liquid nitrogen, xenon atoms do not squeeze out from the solid, but the atoms are occluded by the solid. Xenon was also observed to squeeze out from the organic mediums during slow freezing.48,49 When the bulk medium freezes slowly between the CPG or silica particles, a part of the xenon that squeeze out from the frozen fluid ends up into the unfrozen liquid inside the pores. Results and Discussion LC in CPG 81 and 156. 13C NMR spectra measured on a DPX 200 spectrometer from the sample containing porous material CPG 81, Phase 4, 13C-enriched methyl iodide, and methane gas are shown in Figure 1. The temperature range is 278-334 K, and calibrated temperatures are shown between the spectra. This and all other spectra presented below were

Figure 1. 13C NMR spectra measured from the sample containing porous material CPG 81, Phase 4, methyl iodide, and methane gas in the magnetic field of 4.70 T. The measurement temperatures are shown between the spectra. The measurements were carried out from low to high temperature.

measured from low to high temperatures. Because CPG is composed of nearly pure silica, it is chemically very inert and has an extremely rigid structure. Therefore solvent or pressure does not affect the structure of matrices. Also structural characteristics of the CPG matrix are nearly temperature independent in the used temperature range, and consequently all the observed temperature effects can be attributed to the change in the physical properties of the LC or solute molecules. Different signals in the spectra are labeled as follows: capital letters I and N are abbreviations from words “isotropic” and “nematic”, and they refer to the phases sampled by methane or methyl iodide molecules or xenon atoms. Subscripts B and C after capital letters are abbreviations from the words “bulk” and “confined”, and they express whether signal arises from solute molecules dissolved in bulk LC between the CPG particles or from molecules confined inside the pores, respectively. For example symbol IC means that the signal originates from solute molecules confined inside the pores, and that environment sampled by molecule is on average isotropic. Signal IC. At the lowest temperatures, the spectrum arising from the methyl iodide confined inside the pores is denoted by IC. The spectrum IC is a quartet, each peak having relative intensities of 1:3:3:1. The spacing between the adjacent peaks is equal and is about 150 Hz. Heterogeneous environment broadens line widths of signals; full width at half-maximum is between 30 and 40 Hz, whereas in bulk it is only a couple of hertz. The above-mentioned peak spacing value is, however, within error limits the same as the 13C-1H spin-spin coupling constant JCH of methyl iodide.50,51 Therefore, we can conclude that the phase sampled by methyl iodide molecule below the bulk solid-nematic phase transition temperature in the smallest pores is on average isotropic. In fact, the same quartet is observed over the whole temperature range studied. The spinspin coupling constant seems to stay constant, but a small chemical shift change is observed at solid-nematic and nematic-isotropic phase transition temperatures. The 13C chemical shifts of methyl iodide and methane gas as a function of temperature are presented in Figures 5, 6, and 7. Temperature evolution of the line shape of the quintet from methane is not considered here because of the low signal-to-noise ratio. The spectra measured from the CPG 156 sample resemble very much those shown for CPG 81 in Figure 1, including also the isotropic quartet over the whole temperature range studied.


13

Figure 2. C NMR spectra from the sample containing CPG 237, Phase 4, methyl iodide, and methane gas measured in the magnetic field of 4.70 T.


Figure 5. 13C chemical shift of CH3I dissolved in Phase 4 confined to four different sizes CPG materials as a function of reduced temperature T* ) T/TNI measured in the magnetic field of 11.74 T. Note that the CPG 375 sample was also measured in the magnetic field of 4.70 T. All the measurements were performed from low to high temperature. Symbols in the figure represent the chemical shifts of the signals in Figures 1-4. Open symbols indicate the chemical shifts of CH3I in the bulk phase, and closed symbols are associated to the chemical shifts from the intraporous phases. The shape of the symbols in the insert indicates from what porous material sample chemical shift of the signals were obtained. Solid, nematic, and isotropic refer to the phases of bulk LC.

Figure 3. 13C NMR spectra from the sample containing CPG 375, Phase 4, methyl iodide, and methane gas measured in the magnetic field of 4.70 T.

Figure 6. 13C chemical shift of 13CH3I dissolved in Phase 4 confined to CPG 375 material as a function of reduced temperature T* measured in the magnetic fields of 4.70, 7.05, 9.39, and 11.74 T. All the measurements were performed from low to high temperature. Symbols in the figure represent the chemical shifts of the signals in Figures 3-4. Open symbols indicate the chemical shifts of CH3I in the bulk phase, and closed symbols are associated to the chemical shifts from the intraporous phases. The shape of the symbols in the insert indicates from what porous material sample chemical shift of the signals were obtained. Solid, nematic, and isotropic refer to the phases of bulk LC.

Figure 4. 13C NMR spectra from the sample containing CPG 375, Phase 4, methyl iodide, and methane gas measured in the magnetic field of 11.74 T.

Signal NB. At the temperature 287 K, bulk LC between the CPG particles melts and methyl iodide as well as methane gas squeeze out from the pores to melted liquid between the particles. This is seen as diminishing intensity of signal IC and

a birth of signal NB in the spectra. The observed solid-nematic phase transition temperature is close to the value announced by manufacturer (293 K) for bulk LC. Small change in phase transition temperatures stems from methyl iodide and methane dissolved in the LC and possible uncertainty in temperature calibration. Signal NB is a quartet, and it originates from methyl iodide dissolved in the bulk LC between the CPG particles. It was shown earlier that Phase 4 molecules outside the pores orientate parallel with the external magnetic field18 and behave like bulk LC. The spacing between the peaks of quartet is now |2DCH + JCH| where DCH is dipolar coupling and JCH the spinspin coupling between carbon and proton in methyl iodide. The nonzero DCH confirms that there is a preferential molecular


Figure 7. 13C chemical shift of CH4 dissolved in Phase 4 confined to four different sizes CPG materials as a function of reduced temperature T* measured in the magnetic field of 11.74 T. Note that the CPG 375 sample was also measured in the magnetic field of 4.70 T. All the measurements were performed from low to high temperature. Open symbols indicate the chemical shifts of CH4 in the bulk phase, and closed symbols are associated to the chemical shifts from the intraporous phases. In the isotropic phase, signals from these two phases coalesce, and consequently only one signal is seen. The shape of the symbols in the insert indicates from what porous material sample chemical shift of the signals were obtained. Solid, nematic, and isotropic refer to the phases of bulk LC.

orientation, and that environment is anisotropic. According to the experiments JCH stays practically constant but DCH decreases as temperature increases because it is directly proportional to the second-rank orientational order parameter of methyl iodide. The examination of the sum of methane and methyl iodide signal intensities just below and above solid-nematic phase transition temperature reveals that in slow freezing almost all solute molecules squeeze out from freezing LC to the pores. Melting point of bulk LC in CPG 156 sample was about 287 K. Signal IB. Nematic-isotropic phase transition takes place at 324 K, which is over 20° lower than the value announced by manufacturer, 347 K, for bulk LC. Again this is mainly due to high concentrations of methyl iodide in CPG 81 sample that shifts the phase transition temperature so much. In general, with a few exceptions, when a non-nematic solute is added to a nematic, the nematic-isotropic phase transition temperature, TNI, decreases with increase in non-nematic concentration.52-54 In other samples, concentrations were slightly smaller, and consequently phase transition temperatures were closer to those announced by the manufacturer for bulk LC. For example in CPG 156 sample, TNI was about 332 K. As the nematic phase changes to isotropic, signal NB disappears and signal IB appears. Signal IB coalesces with signal IC, and signals cannot be resolved in Figure 1. On the contrary, separate signals from these two different phases can easily be seen in proton-decoupled spectra (see Figure 5 which displays chemical shifts as a function of temperature). The chemical shift of signal IB is about 0.2 ppm larger than that of signal IC. In CPG 156, the difference is smaller, being about 0.1 ppm. Chemical shift difference occurs most likely from susceptibility effects but possibly also the direct or indirect interaction of methyl iodide molecules with the surface of the pores or from pore wall induced orientational order close to surface has to be considered as will be discussed in more details below. LC in CPG 237. Figure 2 shows 13C NMR spectra from the sample that is otherwise identical with that described in Figure 1, but the average pore diameter of CPG is 237 Å. There are lots of similarities between the series of spectra in Figures 1

Tallavaara and Jokisaari and 2. The quartet marked by the symbol NB and observed over the nematic temperature range 288-334 K originates from methyl iodide in between the CPG particles. It behaves equally as a function of temperature as shown in Figure 1, which is quite impending because particle sizes are equal, and consequently empty spaces in between the particles are also of the same size. In the isotropic phase, quartets from the two different sites cannot be resolved, but from the proton-decoupled spectra they can be resolved, and the chemical shift difference between these sites is about 0.1 ppm. The line shape of the signal originating from CH3I confined inside the pores below the nematic-isotropic phase transition temperature, marked by NC, is quite different from the corresponding one, marked by IC, in Figure 1. The spectrum is not anymore a quartet but resembles a 13C powder pattern of CH3I. This indicates that the average length of the pore segments is so long that the environment, sampled by methyl iodide molecules during the NMR time scale due to translation thermal diffusion, is not isotropic but anisotropic. Consequently, solute molecules do not diffuse through several differently oriented pores during the time FID signal is collected and map several different LC distributions therein, which would average out anisotropic shielding tensor elements of CH3I. Instead they visit only one or a couple of differently oriented pores during the time scale of NMR measurement. The director of the nematic phase built up inside the pores is mainly parallel with the pore axis. As there is an isotropic distribution of the direction of the pores in the sample, the resonance frequencies of the 13C atoms of CH3I in different pores form powder pattern line shape spectra. About 2 K below the nematic-isotropic phase transition temperature, signal NC changes to isotropic quartet marked by IC observed also in isotropic phase. LC in CPG 375. In Figure 3 are presented 13C NMR spectra from the sample that was identical with those described in Figures 1 and 2, but in this case the average pore diameter was 375 Å. Again the same signals are seen from CH3I between the pore particles and in the isotropic phase as was seen previously from other CPG samples described above. In this case, however, chemical shift difference between two sites in isotropic phase was so small that separate signals could not be seen even from the proton-decoupled spectra. Line shape of the signal NC originating from methyl iodide confined inside the pores below the nematic-isotropic phase transition temperature is now even more complicated than it was in Figure 2. The only difference between the CPG 237 and 375 samples is that in the latter, the average pore diameter is slightly larger, and also pore segments are a little bit longer. Line shape of the signal NC in Figure 3 may be affected by an increased orientation distribution of LC molecules toward the external magnetic field in the pores. Figure 4 seems to support this hypothesis because in an even stronger magnetic field (B0 ) 11.74 T) the line shape appears to evolve quite drastically. In Figure 4, 13C NMR spectra from the same sample as in Figure 3 are presented, but in this case, spectra were measured in the magnetic field of 11.74 T. As can be seen, the line shape of the signal NC originating from the methyl iodide confined inside the pores is quite different from that in Figures 1, 2, or 3. Magnetic field thus has a significant contribution to the orientation of LC molecules inside the pores. Complete understanding of the line shape would require development of a theoretical model that takes into account asymmetric LC distribution inside the pores and diffusion averaging. It is out of this research project but would definitely be worth investigating because line shape of the 13C spectra from CH3I seems to

Liquid Crystals Confined to Mesoporous Materials be even more sensitive to the LC orientation, and orientation changes in the pores than are respective xenon or deuterium spectra. Consequently we do not try to explain in more detail line shapes of the signals. Line shape of the signal NC, however, evolves quite intensively as temperature increases, and about 12 K below nematic-isotropic phase transition temperature (331 K), a smaller quartet comes up in the spectra marked as dashed lines in Figure 4. Emergence of the quartet indicates that all LC molecules are not oriented parallel with the pore axis, but there is a significant portion of LC molecules that are oriented toward the applied magnetic field. Magnetic coherence length decreases as temperature increases, as was described in the Theory chapter, and Figure 4 proves that close to phase transition temperature magnetic field starts to dominate as an orienting force and surface interactions of pore walls play only a minor role. Separation between quartet peaks is smaller than it is in the quartet observed outside of pores, and this indicates that the order parameter Szz of methyl iodide is smaller (by about 23-32%) in the pores than it is in the bulk. At lower magnetic fields, in which magnetic coherence length is longer, anisotropic quartet from methyl iodide emerges at higher temperature closer to the phase transition temperature; at 9.40 T the quartet arises about 6 K, and at 4.70 T about 3 K below nematic-isotropic phase transition temperature. Line shape of the signal NC evolves progressively from that presented in Figure 3 to that presented in Figure 4 as magnetic flux density is increased. 13C Chemical Shift of CH I. The 13C chemical shifts of CH I 3 3 obtained from the proton-decoupled spectra from the samples examined in Figures 1-4 are presented as a function of reduced temperature T* ) T/TNI in Figures 5 and 6. The effects of different magnetic flux densities on the chemical shift of the signals are also considered. In all cases, line shapes of the spectra were almost perfectly symmetric, and therefore only symmetric Lorentzian/Gaussian line shape functions were used in fitting process to pick peak position more reliably. As can be seen from the previous figures and also from the proton-decoupled spectra (not shown), almost all methyl iodide molecules squeeze out from the LC medium as it slowly freezes. The majority of CH3I molecules squeeze to the pores, and consequently only one signal is observed below the bulk solidnematic phase transition temperature in Figures 5 and 6. The chemical shift of the signal originating from the smallest pores (CPG 81) changes linearly as a function of temperature in solid, nematic, and isotropic phases of bulk LC. The slope of the chemical shift vs temperature curve changes, however, at the solid-nematic phase transition, and at the nematic-isotropic phase transition clear chemical shift jump is observed. It is seen from Figure 5 that in larger pores an abrupt chemical shift change is observed also as bulk LC melts. Discontinuities are about 0.03 ppm in CPG 156, 0.06 ppm in CPG 237, and 0.07 ppm in CPG 375 in the magnetic field of 11.74 T. In the smaller fields, small chemical shift alterations are also observed. Below we will consider some possible reasons that might cause the observed phenomena. From the intensity examination of the signals as a function of temperature, we can conclude that the concentration of methyl iodide molecules decreases in the pores as bulk LC melts between the particles. Part of the molecules trapped inside the pores during slow freezing of medium squeezes out as LC melts. Concentration change affects the chemical shift of the signal at varying degrees. The relative concentration change of methyl iodide inside the pores at melting point can be calculated because the intensity of the NMR signal intensity is directly proportional to the amount of material in a given volume. Consequently, by

J. Phys. Chem. B, Vol. 112, No. 3, 2008 771 measuring the intensity of the signal from methyl iodide confined inside the pores just above and below the melting point and taking their ratio we get that the concentration of solutes in pores decreases about 30% during the phase transition in each sample. When bulk LC between the pore particles melts also magnetic susceptibility changes to some extent. However, because the particle sizes and consequently also spaces between the particles are roughly equal susceptibility alteration and changes in chemical shift should be roughly equal in each sample. It is unlikely that the change of the magnetic coherence length when the bulk LC melts would substantially affect the chemical shift of the signal from the methyl iodide inside the pores. Namely as can be seen from eq 1, the magnetic coherence length is directly proportional to the square root of the quotient of the elastic constant and the anisotropy of diamagnetic susceptibility of LC. The elastic constant K is directly proportional to the square of the orientational order parameter S of LC,55 whereas ∆χ is directly proportional to S.56 Their ratio K/∆χ is consequently proportional to S. As the bulk LC melts, methyl iodide concentration inside the pores decreases and the LC orientational order parameter increases due to smaller amount of solute molecules to disturb the nematic state.54 As the orientational order inside the pores increases, also the magnetic coherence length increases, and consequently the absolute value of the chemical shift of the signal should also increase. This was not observed in the present case. Linear chemical shift vs temperature behavior of signals from CPG 81-156 samples in nematic phase indicates that the effect of magnetic field on the orientation of LC molecules inside the pores is relatively small. Thus pore wall surface interactions dominate, and LC molecules as well as methyl iodide molecules are oriented parallel with the pore axis. Isotropic quartet and linear chemical shift vs temperature behavior of methyl iodide signal confirms that the solute molecules experience on average isotropic environment inside the smallest pores. Chemical shift of the signal from CPG 237 sample changes also linearly as a function of temperature in the nematic phase, indicating the negligible effect of magnetic field on the orientation of LC molecules inside the pores. Asymmetric line shape of signal NC (see Figure 2) indicates, however, that the average orientational order parameter SZZ of the methyl iodide is not zero. Pore segments are consequently sufficiently long that the SZZ does not completely average out due to fast diffusion averaging in NMR time scale as explained before or irregularities, caused by the surface deformations to the director distribution, are smaller in larger pores. Because magnetic coherence length decreases as temperature increases, the chemical shift of the methyl iodide signal from the largest pores (CPG 375) approaches that observed from the bulk nematogen, in which director is parallel with the external magnetic field. Curving of the chemical shift of the signal from the CPG 375 sample slightly upward when approaching the nematic-isotropic phase transition also in the lower magnetic field (B0 ) 4.70 T) indicates that the torque of the magnetic field is strong enough to slightly orientate the director toward the field in the pores. A slightly larger concentration of methyl iodide in CPG 81 sample than in other samples is clearly seen as a smaller dipolar coupling, chemical shift of signal NB and the nematic mesophase temperature region in Figures 1 and 5. In general chemical shifts of the signals outside of the pores, NB, are almost identical in each sample. This is reasonable because the particle size of used porous material is relatively large, and therefore the chemical

772 J. Phys. Chem. B, Vol. 112, No. 3, 2008 shift of signal observed outside of the pores is close to that observed from bulk LC. Small deviations occur mainly from inaccuracy in temperature determination (∼2 K) (Figures 5 and 6) and differences in solute concentration (Figure 5). Phase transition from nematic to isotropic takes place at temperature TNI (T* ) 1.00). At phase transition, signal NB disappears. Surprisingly a clear discontinuity point in the chemical shift curve is observed in all samples. This is interesting because in respective 129Xe NMR studies in the same environment (same LC and porous material) no phase transitions were seen to take place inside the smallest pores within the whole temperature range studied. In general, the xenon atom is considered to be very sensitive probe atom to local environment changes because of its exceptionally large NMR chemical shift range due to the large and polarizable electron cloud of the atom (see below). The discontinuity in chemical shift is the bigger the pore size in diameter being about 0.09 ppm in CPG 81, 0.12 ppm in CPG 156, and 0.20 ppm in CPG 237. In CPG 375 transition is greatest, about 0.82 ppm in the magnetic field of 11.74 T and 0.29 ppm in the field of 4.70 T. The same discontinuation point was also observed in the chemical shift curve when samples were slowly cooled in the NMR spectrometer and measurements were performed from the isotropic phase to nematic phase (data not shown). This means that a bulk like first-order nematic-isotropic phase transition takes place inside the pores, phenomenon that to our knowledge has not been observed earlier in such restrictive hosts. As can be seen from Figure 5 the phase transition from the nematic to the isotropic phase occurs almost at the same temperature inside and outside of the pores. This means that the concentration of the solute molecules in the pores and in the bulk phase is almost the same. Because of inaccuracy in the temperature (∼2 K) and because spectra were measured at one Kelvin intervals, we could not see (at least very clearly) pore-size-dependent phase transition temperature, typical to first-order phase transitions. Usually the nematic-isotropic transition is shifted only about 1-3 K to lower temperature in systems in which surface effects are limited to the boundary layer and pore diameters are larger than the nematic correlation length.14 For example the nematic-isotropic phase transition temperature of 7CB liquid crystal in a Vycor porous material of average pore diameter ∼70 Å was shifted about 4 K.41,57 In the isotropic phase, two signals are observed from CPG 81-237 samples, one from the pores and another outside of pores. The chemical shift of the signal originating from the pores is larger than that observed between the pore particles, and it is about 0.19 ppm larger in CPG 81 and 0.11-0.12 ppm in CPG 156 and 237 samples. Differences in the chemical shift seem to increase slightly in CPG 81 and stay constant in error limits in CPG 156 and 237 as a function of temperature. In all samples, chemical shifts of both signals change linearly as a function of temperature. A little bit larger chemical shift from the pores may be a consequence of direct or indirect interactions of methyl iodide molecules with the surface of the pores. Another possible explanation is that in the isotropic phase, there is a surfaceinduced residual nematic order due to a strongly anchored nematic layer at the surface of the pore. The value of the orientational order parameter of LC at the surface is S0, and order parameter is expected to decrease as the distance from the pore surface increases (see eqs 2 and 4). In the middle of the pore, orientational order parameter may be zero, and in this case, the phase is truly isotropic. Because of diffusion averaging, methyl iodide molecules map the whole pore segment on the NMR time scale and experience consequently an average

Tallavaara and Jokisaari director distribution of LCs inside the pores which is in this case isotropic. In the spectra this is seen as an isotropic quartet. It is good to bear in mind that isotropic spectra were observed from CPG 81 and 156 samples also when bulk LC is in nematic phase. One difference between these two spectra observed in nematic and isotropic phases is that in the nematic phase LC director is parallel with the pore axis also in the middle of pore whereas in isotropic phase, LC orientation distribution may be arbitrary. Thus in the isotropic phase, the effective order parameter of methyl iodide Szz is smaller, and according to eq 8 this decreases the average chemical shift of the signal. The product SZZP2(cos θ) should consequently be nonzero. Because the chemical shift of the signal from the pores changes linearly as a function of temperature, the effective order parameter stays constant above nematic isotropic phase transition temperature. In bigger pores the effect of the surface walls on the orientation of LCs is smaller than in smaller pores. Thus the chemical shift difference of the signals is smaller, and the discontinuity jump of the chemical shift of the signal in phase transition is larger. That is why signals coalesce in the largest pores (CPG 375) and why separate signals were not observed in the isotropic phase. In Figure 6, the 13C chemical shift of CH3I dissolved in Phase 4 confined to CPG 375 porous material is presented as a function of reduced temperature T*. The spectra were measured in the fields of 4.70, 7.05, 9.39, and 11.74 T from low to high temperature, and as can be seen from the figure, magnetic field has a clear influence on the chemical shift of methyl iodide confined inside the pores in the nematic phase. In the lowest field chemical shift of the signal changes almost linearly as temperature increases whereas in higher field strengths clear curving of the chemical shift curve toward the values observed from methyl iodide dissolved in bulk LC is seen. Curving starts at lower temperature in higher fields because magnetic coherence length is inversely proportional to the field strength. 13C Chemical Shift of CH . In Figure 7 the 13C chemical 4 shifts of CH4 obtained from the proton-decoupled spectra are presented as a function of reduced temperature. Spectra were mainly measured in the magnetic field of 11.74 T, but the largest pore size sample was measured also at a lower field (4.70 T). Data are displayed so that the isotropic values overlap. There are a lot of similarities in chemical shift and intensity behavior of methane gas signal compared to that observed in methyl iodide described in the previous chapter. Below the bulk solid-nematic phase transition temperature, only one signal is seen in the spectra. Methane gas behaves in the same way as methyl iodide and squeezes out from freezing LC medium to the pores where, according to Gibbs-Thompson equation, freezing temperature of fluid is lower. This is seen as increase in intensity of the signal originating from methane trapped inside the pores. Again almost all methane molecules squeeze out as bulk LC between particles freezes. As bulk LC melts the intensity of signal from the pores decreases suddenly. At the same temperature also the chemical shift of the signal increases about 0.15 ppm in each sample. There may be several reasons for an abrupt chemical shift change. One possibility is that the susceptibility of the environment changes as bulk LC melts, and this changes the average chemical shift. The effect of the magnetic field may also be one reason for the chemical shift behavior. 13C chemical shifts of methane confined inside the largest pores (CPG 375) are slightly larger than those observed from other samples (CPG 81-237) in which chemical shift behavior seems to be identical as a function temperature.



TABLE 1: Values of the Parameters Obtained by Fitting of Eq 7 to the Experimental 13C Shielding Data for CH4 Dissolved in Bulk Phase 4 and ZLI 1115 Liquid Crystals between the Controlled Pore Glass Particlea Phase 4

ZLI 1115

parameter

CPG 81

CPG 156

CPG 237

CPG 375

CPG 81

T0 ) TNI [K] F0 [g/cm3] ∆F/F0 [%] R [10-4/K] σ0 [ppm cm3/g] β1 [10-4/K] β2 [10-4/K] ∆σ0 [ppm cm3/g] y z χ [10-9 m3/mol] ∆χ [10-10 m3/mol] M [g/mol]

324.0 0.964b (0.42)b 5.39b, (12.16)b 2.249c -41.0 (25.2) (-1.77) (0.998)b (0.209)b -2.40 (3.93) 284.15

330.5 c d c,d 2.266c -31.7 (24.9) (-1.99) d d -2.40 (3.91) 284.15

333.1 c d c,d 2.228c -50.5 (0) (-1.92) d d -2.40 (3.89) 284.15

329.1 c d c,d 2.244c -40.3 (0) (-1.89) d d -2.41 (3.89) 284.15

322.0 0.982 0.25 26.8, (21.7) 2.259c -97.2 (-92.0) (-2.00) (0.999) (0.198) -2.40 (3.89) 283.45

a

Parentheses indicate that values of the parameters are obtained from the nematic phase; values without parenthesis are obtained from the isotropic phase. b Value obtained from earlier 129Xe shielding data for xenon in bulk Phase 4 LC.18 c The value is fixed to value obtained from isotropic phase and in the table it refers to CPG 81 column when there is * symbol in the row. d The value is fixed to value obtained from nematic phase and it refers to the values in the CPG 81 column. All measurements were performed in the magnetic field of 11.74 T.

Methane molecule has a tetrahedral symmetry, and thus it does not orient in LC environments. Dipolar coupling between proton and carbon should consequently be zero. In practice, however, nonzero dipolar couplings of a few hertz have been observed in bulk LCs and their mixtures, also in Phase 4.58-60 These residual dipolar couplings are due to the fact that the various vibrational modes of methane do exhibit orientation resulting in a correlation between vibrational and reorientational motions.61-63 The anisotropic forces acting in LC environment do not deform the molecular structure of the methane; instead they affect the electron distribution of molecule leading to an anisotropic 13C shielding tensor. The anisotropy is from -1.77 ppm to -2.00 ppm at the nematic-isotropic phase transition temperature as given in Table 1. The values of the parameters presented in Table 1 were obtained from the two-stage fitting procedure of eq 7 to the experimental 13C shielding data for CH4 dissolved in bulk Phase 4 and ZLI 1115. First, the isotropic part of eq 7 was fitted to the data obtained from the isotropic phase. Part of the parameters were fixed to the values obtained earlier from 129Xe measurements of xenon gas in bulk Phase 4.18 Second, eq 7 was fitted to the data collected in the nematic phase. σ0 and β1 were fixed to the values obtained in the isotropic phase before fitting procedure in nematic phase was done. These are listed in Table 1. 13C chemical shift behavior of the signal N , originating from B the methane dissolved in the bulk LC between the pore particles, as a function of temperature in the nematic phase originates mainly from the anisotropy of shielding at the reference temperature ∆σ0 and to a lesser degree from the anisotropy of diamagnetic susceptibility ∆χ and the density jump ∆F/F0 in the phase transition. Relative contribution of ∆χ to the chemical shift is, however, much larger than it is for 129Xe in various LCs. Larger chemical shift of the signal NC from the CPG 375 pores may consequently originate from anisotropic forces caused by LC molecules to the electron distribution of methane molecule. LC molecules orient toward the external magnetic field in the pores as was already explained in the previous chapter, and anisotropic orientation distribution deforms the electronic system of methane, increasing the average chemical shift closer to the value observed from bulk LC. The effect of the magnetic field on the orientation of LC molecules is smaller in a smaller magnetic field, and therefore the chemical shift of the signal from the CPG 375 pores is smaller in the field of 4.70 T than it is in the field of 11.74 T especially close to the

phase transition temperature. In the isotropic phase separate signals that were observed in the nematic phase from the two different sites now coalesce and only one signal is seen. ZLI 1115 in CPG 81. 13C NMR temperature series measurements from the sample that contained LC ZLI 1115, porous material CPG 81, 13C-enriched methyl iodide, and methane gas were also performed with and without proton decoupling. The results were similar to those obtained earlier from 129Xe NMR studies in which xenon gas was used as a probe atom to study the behavior of Phase 4 in CPG 81.18 Only a minor change, about 0.05 ppm, in the chemical shift curve of the signal from methyl iodide confined inside the pores was observed at melting temperature of bulk LC, whereas no first-order nematicisotropic phase transition was detected to take place inside the pores. Chemical shift of the signal changed linearly as a function of temperature over the whole temperature range studied and no discontinuous points were seen. Signal from the pores was symmetric isotropic quartet confirming the interpretation that solute molecules experienced isotropic environment on average. In ZLI 1115, the orientational order parameter of the symmetry axis of methyl iodide SZZ was larger than in Phase 4, which was seen as a larger dipolar coupling between proton and carbon nucleus, DCH. In the isotropic phase, two quartets were observed; the chemical shift of the quartet from the pores was about 0.8 ppm larger than that originating between the pore particles just above the nematic-isotropic phase transition temperature TNI. The chemical shift difference, however, decreased linearly as temperature increased being about 0.7 ppm 25 K above TNI. Either direct or indirect pore wall interactions with the methyl iodide molecules decreased progressively as temperature increased or surface-induced residual nematic order S0 decreased as a function of temperature, which for one decreased average order parameter SZZ of CH3I. A larger chemical shift difference of two sites in isotropic phase compared to that observed in Phase 4 medium may be a consequence of stronger interaction of CH3I with pore walls or surface-induced order is larger for ZLI 1115 than it is for Phase 4. In general, S0 has been observed to be different for different LCs,64,65 and in addition it has been discovered to depend from the surface treatments of the pore walls.33,35 Temperature dependence of chemical shift of signal from methane gas inside the pores was also linear on the whole temperature range examined, and only one signal was observed in proton decoupled spectra from methane in solid and isotropic phases.

774 J. Phys. Chem. B, Vol. 112, No. 3, 2008 129Xe

Results. Behavior of 129Xe atoms of xenon gas dissolved in Phase 4 confined to CPG 81 and 156 porous materials are already examined elsewhere.18 In summary, xenon gas experiences isotropic environment inside the pores in nematic and isotropic phases. In the spectra this is seen as a linear chemical shift vs temperature behavior of the 129Xe signal and symmetric Lorentzian/Gaussian line shape function. In addition, no first-order nematic-isotropic phase transition was detected to take place inside the CPG 81-156 materials. The behavior of 129Xe atoms in ZLI 1115 medium confined to CPG 81 porous material did not deviate from that observed in Phase 4. The only difference between 13C spectra of CH3I and 129Xe spectra of xenon gas in ZLI 1115 and CPG 81 mixtures was that in the former chemical shift difference between two states in the isotropic phase decreased along the second-order polynomial curve from 0.80 to 0.71 ppm as temperature increased from 323 to 347 K, whereas in the latter according to leastsquare fit chemical shift difference increased linearly from 0.77 to 0.85 ppm as temperature increases from 324 to 343 K. Decrease of chemical shift difference of two signals as a function of temperature may originate from the decrease of surfaceinduced orientational order parameter. In many papers, S0 has been observed to stay constant or decrease as a function temperature.17,33,64 When Phase 4 was used as a medium chemical shift difference of 129Xe signals in isotropic phase in CPG 81 matrix increased linearly from 1.39 to 1.53 ppm as temperature increased from 344 to 362 K. For CH3I in the same environment chemical shift difference of 13C resonance signals increased linearly from 0.18 to 0.20 ppm as temperature increased from 325 to 348 K. Conclusions We have studied the behavior of LC Phase 4 and ZLI 1115 molecules inside the pores of CPG material by using 13C NMR spectroscopy of methane and methyl iodide dissolved in the LCs. LCs are oriented parallel to the pore axis, and because CH3I molecules map several differently oriented pores during the NMR time scale due to translational diffusion, they experience an average isotropic environment if the length of the pore segments is sufficiently small. In the present case this was the situation in the smallest CPG pores in which nominal pore diameter was 81-156 Å. In the spectra this was seen as isotropic quartet and linear chemical shift vs temperature behavior of 13C signal in nematic phase. In slightly larger pores with nominal pore diameter 237 Å, 13C powder pattern spectra was observed inside the pores. The length of the pore segments was consequently sufficiently long so that the environment sampled by solute molecules was not averaged to be isotropic, and this was seen as powder pattern line shape spectra. As the average pore diameter increased to 375 Å powder pattern spectra became more complicated in shape because the orientational distribution of solute molecules was not isotropic anymore. We believe that the observed line shape is a consequence of partial orientation of LC molecules toward the external magnetic field in the pores. In the chemical shift curve of the signal this was seen as curving of the chemical shift value toward those observed from bulk LC between the pore particles. This was seen especially clearly close to the nematic-isotropic phase transition temperature because there the magnetic coherence length was the smallest and the effect of magnetic field on the orientation of LC molecules was the largest. The magnetic coherence length is inversely proportional to the field strength, and this was easily seen in the curving behavior of the chemical shift of the signal. At the nematic-isotropic phase transition

Tallavaara and Jokisaari temperature first-order nematic-isotropic phase transition was observed to take place inside the pores. To our knowledge this is the first time such a phenomenon is observed in such restrictive hosts. In the analysis of the chemical shift of the signals in the isotropic phase, assumption that there is surface induced orientational order was used as an aid to explain observed chemical shift behavior of signals from inside and outside of the pores. 129Xe experiments of xenon gas dissolved in similar environments support part of the results described above, but there are some results that are not completely clear and therefore need further investigation. Acknowledgment. The authors are grateful to the Academy of Finland (Grants 43979 and 203278) and the Graduate School of Computational Chemistry and Molecular Spectroscopy (LASKEMO) for financial support. P.T. acknowledges the grants from the Research Foundation of Orion Corporation, the Tauno To¨nning Foundation, the Oskar O ¨ flund Foundation, the Emil Aaltonen Foundation, the Alfred Kordelin Foundation, and the Magnus Ehrnrooth Foundation. References and Notes (1) Demus, D.; Goodby, J.; Gray, G. W.; Spiess, H.-W.; Vill, V. Handbook of Liquid Crystals; Wiley: Weinheim, Germany, 1998; Vol. 1. (2) Dullien, F. A. L. Porous Media, Fluid Transport and Pore Structure; Academic Press: San Diego, 1992. (3) Kralj, S.; Zidansˇek, A.; Lahajnar, G.; Zˇ umer, S.; Blinc, R. Phys. ReV. E 1998, 57 (3), 3021. (4) Iannacchione, G. S.; Crawford, G. P.; Qianm. S.; Doane, J. W.; Finotello, D.; Zumer, S. Phys. ReV. E 1996, 53 (3), 2402. (5) Kutnjak, Z.; Kralj, S.; Lahajnar, G.; Zˇ umer, S. Phys. ReV. E 2003, 68, 21705. (6) Kutnjak, Z.; Kralj, S.; Lahajnar, G.; Zˇ umer, S. Phys. ReV. E 2004, 70, 51703. (7) Shao, Y.; Zerda, T. W. J. Phys. Chem. B 1998, 102, 3387. (8) Grinberg, F.; Kimmich, R. J. Chem. Phys. 1996, 105 (8), 3301. (9) Vilfan, M.; Apih, T.; Gregorovie`, A.; Zalar, B.; Lahajnar, G.; Zˇ umer, S.; Hinze, G.; Bo¨hmer, R.; Althoff, G. Magn. Res. Im. 2001, 19, 433. (10) Vilfan, M.; Vrbancˇicˇ-Kopacˇ, N. In Liquid Crystals in Complex Geometries Formed by Polymer and Porous Network; Crawford, G. P., Zˇ umer, S., Eds.; Taylor and Francis: London, Great Britain, 1996. (11) Bellini, T.; Clark, N. A.; Muzny, C. D.; Wu, L.; Garland, C. W.; Schaefer, D. W.; Oliver, B. J. Phys. ReV. Lett. 1992, 69 (5), 788. (12) Clegg, P. S.; Stock, C.; Birgeneau, R. J.; Garland, C. W.; Roshi, A.; Iannacchione, G. S. Phys. ReV. E 2003, 67, 21703. (13) Zeng, H.; Finotello, D. Phys. ReV. Lett. 1998, 81 (13), 2703. (14) Iannacchione, G. S.; Crawford, G. P.; Qian, S.; Doane, J. W.; Finotello, D.; Zˇ umer, S. Phys. ReV. E 1996, 53 (3), 2402. (15) Crawford, G. P.; Ondris-Crawford, R.; Zˇ umer, S.; Doane, J. W. Phys. ReV. Lett. 1993, 70 (12), 1838. (16) Jin, T.; Zalar, B.; Lebar, A.; Vilfan, M.; Zumer, S.; Finotello, D. Eur. Phys. J. E 2005, 16, 159. (17) Crawford, G. P.; Stannarius, R.; Doane, J. W. Phys. ReV. A 1991, 44 (4), 2558. (18) Tallavaara, P.; Telkki, V.-V.; Jokisaari, J. J. Phys. Chem. B 2006, 110, 21603. (19) Tallavaara, P.; Jokisaari, J. Phys. Chem. Chem. Phys. 2006, 8, 4902. (20) Long, H. W.; Luzar, M.; Gaede, H. C.; Larsen, R. G.; Kritzenberger, J.; Pines, A.; Crawford, G. P. J. Phys. Chem. 1995, 99, 11989. (21) Jokisaari, J. Prog. NMR Spectrosc. 1994, 26, 1. (22) Jokisaari, J. In NMR of Ordered Liquids; Burnell, E. E., de Lange, C. A., Eds.; Kluwer: 2003. (23) Van Geet, A. L. Anal. Chem. 1968, 40, 2227. (24) Jameson, A. K.; Jameson, C. J.; Gutowsky, H. S. J. Chem. Phys. 1970, 53, 2310. (25) Jokisaari, J. Encyclopedia of NMR Spectroscopy; 1996; Vol. 1. (26) Qiao, Y.; Schoenhoff, M.; Findenegg, G. H. Langmuir 2003, 19 (15), 6160. (27) Kralj, S.; Zidansˇek, A.; Lahajnar, G.; Musˇevicˇ, I.; Zˇ umer, S.; Blinc, R.; Pintar, M. M. Phys. ReV. E 1996, 53 (4), 3629. (28) Zidansˇek, A.; Lahajnar, G.; Kralj, S. Appl. Magn. Reson. 2004, 27, 1. (29) Zidansˇek, A.; Kralj, S.; Repnik, R.; Lahajnar, G.; Rappolt, M.; Amenitsch, H.; Bernstroff, S. J. Phys.: Condens. Matter 2000, 12, 431.

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Behavior of Liquid Crystals Confined to Mesoporous Materials as

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