Is There a LiquidâLiquid Phase Transition in Confined Triphenyl

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Is There a Liquid-Liquid Phase Transition in Confined Triphenyl Phosphite? Magdalena Tarnacka, Olga Madejczyk, Mateusz Dulski, Paulina Maksym, Kamil Kaminski, and M. Paluch J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05336 • Publication Date (Web): 10 Aug 2017 Downloaded from http://pubs.acs.org on August 11, 2017

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The Journal of Physical Chemistry

Is There a Liquid-Liquid Phase Transition in Confined Triphenyl Phosphite? Magdalena Tarnacka†‡*, Olga Madejczyk†‡,Mateusz Dulski$‡, Paulina Maksym†‡, Kamil Kaminski†‡*, Marian Paluch†‡ † Institute of Physics, University of Silesia, Uniwersytecka 4, 40-007 Katowice, Poland ‡ Silesian Center of Education and Interdisciplinary Research, University of Silesia, 75 Pulku Piechoty 1A, 41500 Chorzow, Poland $ Institute of Materials Science, University of Silesia, 75 Pulku Piechoty 1A, 41-500 Chorzow, Poland * Corresponding author: (MT) [email protected] (KK) [email protected]

Abstract The effect of nanoscale confinement on the formation of glacial phase in triphenyl phosphite (TPP), usually assigned to liquid-liquid transition (LLT), was investigated by a combination of differential scanning calorimetry (DSC), broadband dielectric (BDS) and Raman spectroscopy. Although the application of micronscale confinement revealed the presence of prominent LLT [Kurita, R. and Tanaka, H. Phys. Rev. Lett. 2007, 98, 235701]; surprisingly under the nanolevel geometrical restriction, there is no evidence of its existence. Interestingly at temperature corresponding to the LLT phenomena, the nanoconfined TPP undergoes crystallization of suppressed nucleation step. Our data indicates that most likely the formation of so-called glacial phase is a result of the crystallites formed within liquid matrix, where the non-crystallized molecules adjacent to their surface forming the interfacial layer of slowed down molecular dynamics.

1 ACS Paragon Plus Environment


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I.

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Introduction Liquid-liquid phase transition (LLT) is considered to be a transformation between

different liquid states of single component and remains one of the most intriguing and controversial physical phenomenon. Although the occurrence of LLT has been reported for various types of compounds 1,2,3,4,5 and confirmed by numerous molecular dynamics (MD) simulation6,7,8,9, the nature of observed changes is still a puzzle. Thus, the existence of LLT itself is a matter of hot debate. One of the most illustrative example of ongoing discussion on LLT is the unique behavior observed in triphenyl phosphite (TPP, see Scheme 1). This simple van der Waals glass former transforms at certain conditions into an unknown “apparently amorphous” state (so-called glacial phase10 ), which, as verified experimentally, differs by density, fragility, glass transition temperature, sound velocity, spin-lattice relaxation times and the structure factor S(q) from the supercooled liquid, (quenched) glass and crystal11,12,13. Although initially assigned as defect-ordered phase10,11,14, this unique behavior was also discussed to be a result of, i.e., the formation of liquid-crystal, plastic-crystal phase12,15, or “arrested” crystallization resulting in the mixture of nanocrystallites within viscose liquid16. However, further studies devoted to TPP indicated that the formation of glacial phase occurs due to the prominent firstorder LLT17,18,19,20,21,22,23,24, where this new phase is an amorphous state of the second liquid phase of TPP (liquid 2). This liquid 2 was reported to be characterized by different polarity and molecular ordering than a mixture of liquid 1 and crystal 1, indicating that this unusual behavior cannot be simply an “arrested” crystallization but rather a result of LLT transition25. On the other hand, the most recent work devoted to LLT in TPP indicates that the ultimately formed glacial phase contains nanocrystallities, which nucleation and growth were postulated to be preceded by LLT, or perhaps occur concurrently 26 . Therefore, it is argued that the formation of glacial phase is a two-step process. First, LLT takes place leading to the liquid 2.


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Second, liquid 2 crystallizes to different (second) polymorphic form (crystal 2), which molecular ordering is similar to that of liquid 2 and is assumed to frustrates the nucleation of crystal 124,25. This leads to a mixture of crystal phase within amorphous-like solid24,25,26. Unfortunately due to difficulties in avoiding crystallization of TPP and its competition with LLT, the nature of occurring changes remains a complex and difficult matter. An answer to the question about the origin of such remarkable behavior of TPP might be the application of spatially restricted conditions, where a finite-size effect might allow for deeper insight into the LLT transition. Numerous works devoted to the confinement effects display its strong influence on, i.e., phase transition temperatures (as melting, crystallization or liquid-glass), molecular mobility, crystal growth and surface free energy27. Thus, one can expect that the applied geometrical restriction would suppress crystallization and all ongoing changes would be a result of LLT. In this context, it is worthwhile to mention confinement studies on TPP at micronscale by R. Kurita and H. Tanaka28, where authors reported the prominent LLT occurring under applied geometrical restriction. Nevertheless, we believe that an increase of confinement conditions down to nanolevel scale might reveal some crucial factors allowing in final confirmation/or denial of the existence of LLT within TPP. For that purpose, we incorporated TPP within native aluminum oxide (AAO) membranes of two different pore diameters (d = 35 nm and d = 150 nm). As first, we studied the behavior of bulk TPP by using differential scanning calorimetry (DSC), broadband dielectric (BDS) and Raman spectroscopy. Then, the same experimental protocols were employed in the case of confined samples. To our knowledge, this is the first study on the LLT transition under nanoscale confinement reported to date.

II. Experimental section 2.1. Materials



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Triphenyl phosphite (TPP) with purity higher than 98% was supplied by Sigma-Aldrich and used as received. The chemical structure is presented in Scheme 1(a). The nanoporous aluminum oxide membranes used in this study were supplied from Synkera Co (see Scheme 1(b)). Details concerning porosity, pore distribution, etc. can be found at the Web page of producer29. 2.2. Sample preparation/Infiltration procedure Prior to filling, AAO membranes were dried in an oven at 423 K under vacuum to remove any volatile impurities from the nanochannels. After cooling, they were placed in TPP. Then, the whole system was maintained at T=293 K in vacuum (10-2 bar) for 24 h to let the compound flow into the nanocavities. After completing the infiltration process, the surface of AAO membrane was dried and the excess sample on the AAO surface was removed by use of paper towel. 2.3. BDS measurements Isobaric measurements of the complex dielectric permittivity ε*(ω) = ε’(ω)–iε”(ω) were carried out using a Novocontrol Alpha dielectric spectrometer over the frequency range from 10-2 to 106 Hz at ambient pressure. The temperature was maintained with a Quatro Cryosystem using a nitrogen gas cryostat; control was better than 0.1 K. Dielectric measurements of bulk TPP were performed in a parallel-plate cell (diameter: 10 mm, gap: 0.1 mm) immediately after preparation of the amorphous sample. AAO membranes filled with TPP were also placed in a similar capacitor (diameter: 10 mm, membrane: 0.005 mm) 30 , 31 . Nevertheless, the confined samples are a heterogeneous dielectric consisting of a matrix and an investigated compound. Because the applied electric field is parallel to the long pore axes, the equivalent circuit consists of two capacitors in parallel composed of ε*TPP and ε*AAO. Thus, the measured total impedance is related to the individual values through 1/Z*c=1/Z*TPP +1/Z*AAO. Time


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dependent dielectric measurements were made in the temperature range 213 – 223 K and 219 – 227 K for bulk and confined systems, respectively. 2.4. DSC measurements Calorimetric measurements were carried out by Mettler-Toledo DSC apparatus equipped with a liquid nitrogen cooling accessory and an HSS8 ceramic sensor (heat flux sensor with 120 thermocouples). Temperature and enthalpy calibrations were performed by using indium and zinc standards. The sample was prepared in an open aluminum crucible (40µL) outside the DSC apparatus. Samples were scanned at various temperatures at constant heating rate 10 K/min. Each measurement at a given temperature was repeated 3 times. For each experiment a new sample was prepared. 2.5. Raman measurements Raman spectra were recorded using WITec alpha300 R system equipped with Nd:YAG laser (532 nm at 40 mW of power) and a high sensitivity back-illuminated Newton CCD camera. The excitation laser radiation was coupled into a microscope through a single-mode optical fiber with a 50 mm diameter. An air Olympus MPLAN (50x/0.76NA) objective was used. Raman scattered light was focused onto a multi-mode fiber (50 mm diameter) and monochromator with a 600 line/mm grating. Raman spectra were accumulated in the 500– 3800 cm-1 range by 20 scans with integration time of 15 s and a resolution of 3 cm-1. The measurements were performed at room temperature, 228 K and 218 K using THMS600 Linkam stage with temperature stabilization ±0.5 K and collected every 60 min. All data were manipulated by performing a baseline correction and cosmic ray removal. The spectrometer’s monochromator was calibrated using the Raman scattering line of a silicon plate (520.7 cm-1).

III.

Results and Discussion



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According to the literature, TPP might transform into glacial phase by two ways: (i) slow heating of quenched sample and (ii) annealing of supercooled material in temperature range 210 to 223 K. For the purpose of this paper, we applied the second protocol. Nevertheless, we extended the examined range of temperatures so that we would be able to explore the behavior of TPP also beyond the “glacial” range and compare the observed changes. The DSC curves of TPP annealed at T = 219 – 229 K are presented in Figure 1(a). As it can be seen, all collected data reveal the presence of significant exothermic peak, which moves towards shorter times with increasing temperature typically reported for LLT17,32. One can recall that the such scenario is also characteristic for the first order transition, and can be observed also in the case of, i.e., crystallization. In order to characterize the progress of recorded event, the kinetic curves were constructed accordingly to the following equation: t

dH

∫ dt

dt

t0

α DSC = t

∞

dH ∫t dt dt 0

=

At , A∞

(1)

where dH/dt is the rate of heat evolution, t0 and t∞ are the time when transition starts and ends, At and A∞ are areas under normalized DSC curves for partially and fully crystallized material, respectively. The obtained curves are presented as an inset in Figure 1(a). As it can be seen, kinetics curves shift to shorter times with increasing temperature and all have pronounced sigmoidal shapes, usually reported for crystallization. Note that Hedoux et al.32 suggested that the observed sigmoidal curves indicate the ongoing crystallization instead of LLT. Nevertheless, there is no consensus in this matter, since Senker and Rossler, who observed the comparable scenario, strictly assigned it to the prominent LLT17. To calculate the constant rates of this process, the obtained data were fitted by the Avrami model33, commonly used in the case of the kinetics studies, i.e., crystallization: 6 ACS Paragon Plus Environment

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α =1− exp(− ktn ) ,

(2)

where α is the conversion, k is a constant rate and n is the Avrami exponent that can be related to the mechanism of crystallization (or chemical reaction). As illustrated, the Avrami fits describe the constructed curves in an accurate way. Determined values of k are presented in Figure 1(b). As shown, there is a pronounced change in the slop of k(T)-dependence occurring at T = 223 K, indicating a possible change in the mechanism of the studied process. To quantify the observed effect, we estimated the activation energies of both distinguished regimes by fitting the data with Arrhenius function:

k = k0 exp(Ea / RT) ,

(3)

where k0 is a pre-exponential factor, Ea is the activation barrier and R is the gas constant. As presented, the calculated values of Ea differ significantly for both regimes and are equal to 53 kJ/mol and 128 kJ/mol above and below 223 K, respectively. One can recall that the change in k(T)-dependence at T = 223 K corresponds perfectly to the upper limit of “glacial” range. Thus, it can be concluded that the two regimes characterized by different Ea correspond to two various transitions: LLT followed by crystallization. It is also worthwhile to mention that the estimated value of Ea of the formation of glacial phase agrees with literature17. As observed, despite the difference in Ea indicating different origin of occurring changes, both transitions (LLT and crystallization) revealed similar DSC signal and shape of constructed kinetic curves, what make them impossible to distinguish by DSC measurements. Therefore in the next step of the analysis, we explored the change in the TPP behavior (below T =223 K) by BDS technique, which might allow to differentiate those two competitive transitions due to their dynamical properties. Dielectric spectra collected upon the annealing of bulk TPP at T = 217 and T = 213 K (below the determined threshold value) are shown in Figure 2(a) and 2(b). As illustrated, the collected spectra evolve in unusual manner.



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The transition, assigned to the formation of glacial phase, is manifested by (i) shift to the lower frequencies and (ii) broadening of the structural relaxation peak of reduced intensity (see Figure 2 (b)). Interestingly, similar pattern of behavior is reported also at elevated pressure 34. As demonstrated in the literature, molecular dynamics of glacial phase is much different, including higher glass transition temperature, Tg = 212 K, and lower fragility in comparison to the supercooled TPP (Tg = 205 K)18. Basing on above discussion, one can conclude that the combination of DSC and BDS techniques enabled us to assign TPP behavior above and below T = 223 K to two different overlapping transitions. For deeper insight and better understanding of LLT, TPP was incorporated within nanoporous native aluminum oxide (AAO) membranes of pore size d = 35 nm and d = 150 nm (for details see Experimental Section) and then annealed at indicated temperature conditions. The examined nanosystems are measured at thermodynamic conditions corresponding to the regime, where no confinement effect is observed and the confined samples revealed the bulk-like dynamics (see Figure 2(c)). Note that there are two fractions (core and interfacial) of molecules in uniaxial AAO pores, which interact with each other. As an effect, a clear change in dynamics of core molecules can be observed due to vitrification of the adsorbed molecules35,36,37 (see the relaxation map presented in Figure 2(c)). Nevertheless herein, we investigated the behavior of confined TPP characterized by the same dynamics as bulk sample at temperatures corresponding to the “glacial” transition, where no additional factors (i.e., negative pressure 38 ,39 or annealing of surface molecules 40 ) contribute to the obtained data. Dielectric spectra obtained upon the annealing of confined samples are shown in Figure 3(a) and 3(b) for TPP incorporated within AAO temples of d = 35 nm and d = 150 nm, respectively. Surprisingly, the continuous decrease of intensity of the structural relaxation with time was detected (see Figure 3 (a) and 3(b)); while, no change within confined TPP at T


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= 217 K can be seen (see the inset of Figure 3(a)). Obtained results are quite unexpected for at least two reasons. First, the observed changes correspond perfectly to the scenario typically reported in the case of crystallization both bulk (see Figure 2(a)) and confined samples27,35. One might expect that under spatial restriction TPP crystallization would be suppressed due to confinement effect. Instead, the obtained results strictly indicate that crystallization, no LLT, occurs in pores. Second, changes occurring within nanoporous materials are seen only above T = 217 K. Below this temperature, only slight changes in dielectric strength of the structural process were recorded upon annealing of the sample for few days. One can argue that, accordingly to Kurita and Tanaka predictions28, an increase of confinement from microns to nanoscale should shift the LLT range to lower temperatures; thus, we might be well above the conditions of the formation of glacial phase. Nevertheless, it should be stressed that, as mentioned above, we examined TPP behavior characterized by the same molecular dynamics as bulk sample. Therefore, we expected that, due to similar dynamics of both bulk and confined material, LLT should be detected at more less the same temperatures. Note that for TPP incorporated in pores of diameter d = 150 nm measurements were carried out down to 213 K and only subtle changes were observed at these conditions. However for TPP infiltrated in pores of d = 35 nm, we did not perform further measurements below T = 217 K; since the additional/confinement effects (i.e., equilibration of the interfacial molecules) are dominating loss spectra. Just to add that at those conditions, the nanosystem reaches isochoric condition, where different effects control the behavior of soft matter under confinement38,39. It is worthwhile to mention that the behavior of confined TPP below those conditions was analyzed elsewhere (for details see Ref. [37]). To get insight into the kinetics of changes observed in dielectric loss spectra of confined TPP (mainly amplitude of the structural relaxation process), isothermal data were analyzed with the use Havriliak-Negami (HN) function41. To follow the ongoing changes, we



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monitored the variation of ∆ε, which can be used as a very good indicator of phase transition, crystallization process etc. To present all kinetic curves at one graph, data were renormalized according to the equation:

αBDS =

∆ε − ∆ε (t ) , ∆ε − ∆ε (∞)

(4)

where ∆ε is the dielectric strength of the α-process at the beginning of the crystallization, ∆ε(∞) is the long-time limiting value, while ∆ε(t) is the value at a given time of the process. Obtained kinetics curves of bulk and confined TPP are presented in Figure 4(a) and 4(b), respectively. As illustrated in Figure 4(a), the kinetics curves obtained for the bulk sample are characterized by the sigmoidal shapes, similarly as previously reported in the case of DSC data (see the inset in Figure 1(a)). However, different scenario can be seen for confined samples, where kinetic curves revealed that ongoing changes follow more exponential character. That simply indicates that the nucleation step of crystallization seems to be suppressed due to the applied confinement. One can recall that comparable results were obtained also for crystallization of phenyl salicylate (salol) incorporated within AAO templates27. The collected data were further fitted by Avrami equation to calculate rate constants, k, which were furthermore plotted versus inverse temperature in Figure 4(c). As it can be seen, the crystallization process under confinement is significantly slower in comparison to the bulk TPP (see line in Figure 4(c), which denotes T = 221 K). Nevertheless, the examined process slows down with a decreasing pore size; while, the activation energy, determined by Arrhenius equation, is the same for both applied pore diameter, Ea ~ 90 kJ/mol (see Figure 4(c)). Note that this value is lower or higher in comparison to the formation of “glacial” phase (Ea,T223K = 53 kJ/mol) observed in the bulk TPP, respectively. One can recall that similar variation of the activation energy of crystallization of


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nanomaterials was also reported is the case of abovementioned crystallization of salol, where Ea of confined samples was lower or higher when compared to the bulk, due to different polymorphic forms crystallized under confinement depending on the pore diameter, d27. As illustrated, the applied herein nanoscale confinement revealed the ongoing crystallization of suppressed nucleation step and modified activation energy, suggesting some interaction between the host (nanocavities) and guest (TPP). At the same time, the obtained data strictly indicates that no LLT occurs within confined sample, in contrast to the prediction given in literature28.. One can recall that accordingly to the literature the glacial phase is a heterogeneous mixture of nanocrystallities within amorphous-like solid as a result of crystallization immediately preceded by LLT25,26. Nevertheless herein, we observed only one process, indicating that the reported changes are merely due to crystallization. We believe that the occurring process might be illustrated as follow. The annealing of supercooled TPP at T = 210 – 223 K leads to a partial crystallization of TPP and the formation of crystallites within liquid matrix16,32, which is manifested by the systematic decrease of intensity of the structural relaxation process. At the same time, the non-crystallized molecules adsorbed at surface of crystallites and formed an interfacial layer of reduced mobility, suppressing further crystallization, what is demonstrated by the emergence of a new broad relaxation peak in a lower frequency range (see Figure 2(b)). In this context, it is worthwhile to mention recent studies on polymers nanocomposites (PNCs) 42 , 43 , where slowing down of the molecular dynamics was assigned to the formation of interfacial layer, i.e., due to the adding silica nanoparticles to polymer matrix. Note that an increase of the silica nanoparticles within the polymer is also manifested by shift, widening and decrease in amplitude of segmental relaxation43, comparable to the bulk TPP behavior (see Figure 2(b)). Thus by the analogy, it can be concluded that the changes in the distribution of relaxation times resulted from partially immobilized molecules adjacent to the surface of crystallites forming interfacial



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layer44,45. We believe that this picture explains the formation of glacial phase in appropriate way. It is worthwhile to mention that the comparable results were also obtained for other “LLT compound” n-butanol, where the partially crystallization of sample leading to the coexisting of two phases (liquid and crystal) was observed, instead of putative LLT46. In order to verify the above conclusion, we performed additional Raman measurements, which for the bulk material have been performed at two temperatures (T = 218 K and T = 228 K) to follow TPP behavior below and above the abovementioned threshold temperature (T = 223 K). The time-dependent Raman spectra of bulk sample with respect to the reference spectrum measured at room temperature are presented in a full spectra range and CH region in Figure 5(a) and 5(c). Note that the recorded Raman spectra of bulk TPP are in good agreement with data previously reported by Hedoux et al.47. Upon the annealing at T = 218 K, the intensities of bands in range 3070 cm-1, 1244-1234 cm-1, 1050-950 cm-1, 950-700 cm-1 and 400-120 cm-1 increase and their full width at half maximum (FWHM) narrows over time with respect to the reference. However, the most significant alterations were found for CH band centered around 3070 cm-1, where a linear increase of its integral intensity with time was seen, as illustrated in Figure 5(c) and Figure 5(f). Surprisingly, the same behavior (increase of integral intensities and narrowing of FWHM) of abovementioned bands was also observed in the case of time-dependent measurements at T = 228 K. One can recall that no significant differences within bulk TPP occurring at T = 218 K and T = 228 K were detected, indicating that both above and below T = 223 K bulk TPP undergoes crystallization, not LLT. Additionally independently on the temperature condition (below and above the glacial phase), final Raman spectra were similar, indicating that TPP crystallized to the same polymorphic form. Note that TPP was also reported to crystallize to other polymorphic crystal phase (crystal 2)48,49,50; nevertheless herein, we observed only one form independently to the applied annealing temperature.


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As a next step, we examined the confined TPP. The time-dependent Raman spectra obtained for TPP within AAO membranes of d = 150 nm are presented in full spectra range in Figure 5(b). Note that the recorded Raman spectra of confined TPP are in relatively good agreement with bulk data and those reported previously by Hedoux et al.47. However by comparing spectra measured at T = 298 K, small differences in the position of main bands and intensities with respect to the bulk can be seen. One can observe the blue shift of the most intense Raman band from 1015 cm-1 (bulk) to 1009 cm-1 (d = 150 nm) and 1005 cm-1 (d = 35 nm) and/or C-C benzene ring band from 1600 cm-1 (bulk) to 1597 cm-1 (d = 150 nm) and 1594 cm-1 (d = 35 nm) with reducing pore diameter as an effect of applied confinement. Such changes allow to suppose that the examined molecular system, restricted by the pore walls may occur in different structural configuration. Thus, the most pronounced changes detected in the range 1015 - 1005 cm-1, associated with anti-symmetric stretching P-O-Ph (where Ph denotes phenyl) modes, might result from the inter- and intramolecular reorganization of aromatic rings. More interesting results provide analysis of less intense bands from 1244 1234 cm-1, 950 - 700 cm-1 and 400 - 120 cm-1 ranges. As presented, Raman spectra of confined systems clearly show a bands shifting towards higher and/or lower wavenumbers, respectively for d = 150 nm and d = 35 nm, as an effect of modification of bond length within P–O–Ph units due to conformational changes, molecular subunits rearrangement and/or whole molecule reorganization. Interestingly for d = 150 nm, P-O-Ph band is shifted towards higher wavenumber when compared to bulk system. It might suggest that TPP molecules have a certain freedom of movement favoring conformational changes. It also seems that hydroxyl units of pore walls may constitute some kind of catalyst for relatively quick molecular rearrangement which enforce molecular interaction, random at a given moment of time. As a result, such behavior leads to rotational movement of benzene rings and lengthening of P-O binding (Figure 5(a)). In turns, completely opposite trend was found in case of TPP within d =



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35 nm. One can assume that an increase in geometry constraints induce stronger interaction of TPP molecules with hydroxyl groups of pore walls which provide lower dynamics of molecules and might have an impact on shortening P-O bond. As a result, one can expect, that in contrast to other described systems, an increase in the vibrational strength within phosphite units. Moreover, it can added that the band shift towards lower wavenumber indicates lower molecular movement of TPP molecules due to higher geometry restrictions. Thus, high geometry constraints induce the variation in molecular packing and imply stronger interaction between TPP molecules as well as TPP molecules and hydroxyl groups of pore walls. As a result, lower molecular movement hinders conformational changes and the whole system seems to be more ordered, already at the time TPP incorporates into the AAO channels. Further analysis revealed that upon the annealing at T = 218 K, changes arose within the examined nanosystems are the same as recorded in the case of bulk sample, where an increase of intensities and narrowing of FWHM of, i.e., CH bands located around 3070 cm-1 is noted (see in Figures 5(c), 5(d) and 5(e)). Nevertheless, the intensity of stretching CH band of confined samples is significantly weaker in comparison to the bulk material, which might be assigned to the interaction of CH group with hydroxyl ones bind to AAO templates walls (see Figure 5(f)). As indicated, the confined TPP undergoes crystallization to the same polymorphic form as observed in the case of bulk sample. However, the changes detected for pore systems are much slower than in the case of bulk material implying that crystallization becomes slower with the reduction of pore diameter (Figure 5(f)). Nevertheless, it should be mentioned that the lack of LLT transition under confinement might be caused by the conformational rearrangements of TPP within pores, as captured by Raman measurements. In this context, one can recall that Kurita and Tanaka discussed the affected liquids structuring as one of the possible reason, which may suppress formation of glacial phase in TPP28. Additionally, it is also worthwhile to add that beside the conformational changes forced by the


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applied geometrical constrain also dynamical heterogeneity of the sample might be another important factor which influences the behavior of TPP. One can add that although molecular dynamics (average structural relaxation times) of bulk and confined TPP agreed for the investigated range of temperatures, there was quite significant discrepancy in the shape of the α-process (for details see Ref. [37]). Dielectric studies clearly indicated that confined TPP is more dynamically heterogeneous with respect to the bulk material. Therefore, conformational changes along with the significant enhanced dynamical heterogeneity of confined TPP can be responsible for a different behavior of bulky and confined TPP. Finally, we decided to characterize the ongoing crystallization under confinement. As first, we calculated the critical nucleus size, rc, which accordingly to the classical crystallization theory can be estimated by the following equation51:

rc =

2σ sl T * , ∆H f ρ s ∆T

(5)

where σsl is the solid-liquid interfacial tension, T* is the solid-liquid equilibrium (melting) temperature, ∆Hf is the bulk enthalpy of fusion, and ∆T= (T* - T) is the degree of supercooling. The determined values of rc plotted versus the relaxation time, τα, are presented in Figure 6(a)52. Values of τα were taken from Ref. [37]. As illustrated, rc decrease with an increasing τα, i.e., for d = 35 nm rc changes in the range 2-1.65 nm. Surprisingly, the highest value of rc was estimated for TPP confined within d = 35 nm, while the smallest for the bulk sample. Note that rc calculated for o-terphenyl and benzyl alcohol confined within controlled pore glass (CPG) reaches value of 3.1 and 2.5 nm, respectively for ∆T= 0.85T*53, while rc of TPP for ∆T= 0.85T* reaches 2.9 nm. One can observe that the calculated critical nucleus size is significantly lower with respect to the pore diameter of the applied AAO membranes, which explains the incomplete suppression of crystallization process. Next, we plotted constant rates of crystallization (estimated from Avrami equation, Eq. (2)) versus τα, as presented in Figure 6(b). As shown, the decoupling between both variable is seen(slope of all linear fitting 15 ACS Paragon Plus Environment


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function was s ~ 0.5). Interestingly, similar decoupling between rates of crystallization and structural relaxation process is observed for the bulk and confined sample.

IV. Conclusion The comprehensive studies on the formation of glacial phase in TPP (usually discussed as LTT) under spatially restriction condition were carried out. Combined DSC, BDS and Raman techniques revealed that the confined TPP undergoes crystallization of suppressed nucleation step and affected activation energy, due to interaction between the host (nanocavities) and guest (TPP) molecules. Surprisingly, we did not observe any changes indicating polyamorphism or polymorphism within TPP at both bulk and confined conditions. These insights suggest that the formation of glacial phase in bulk TPP is not a result of the (almost) concurrent LLT and crystallization, but is merely due to the latter process and arose from the crystallites produced within liquid matrix, where the non-crystallized molecules adjacent to the surface form the interfacial layer of slowed down molecular dynamics.

AUTHOR INFORMATION Corresponding Author * (MT) e-mail: [email protected] (KK) e-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT K.K., P.M. and O.M. are thankful for a financial support from the Polish National Science Centre within OPUS project (Dec. no 2015/17/B/ST3/01195).

Schemes and Figures 16 ACS Paragon Plus Environment

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O O

P O

(a)

(b)

Scheme 1. The chemical structure of examined triphenyl phosphite (a) and the characteristic of applied AAO membranes (b), taken from the producer web page [29].

(a)

T = 219 K T = 221 K T = 223 K T = 225 K T = 227 K T = 229 K

TPP bulk

increasing temperature

conversion, αDSC

1.0

0.8

Ea,T>223K = 53 kJ/mol Ea,T

Is There a LiquidâLiquid Phase Transition in Confined Triphenyl

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Is There a LiquidâLiquid Phase Transition in Confined Triphenyl