On the Origin of Mesoscale Structures in Aqueous Solutions of Tertiary

Article pubs.acs.org/JPCB

On the Origin of Mesoscale Structures in Aqueous Solutions of Tertiary Butyl Alcohol: The Mystery Resolved Marián Sedlák* and Dmytro Rak Institute of Experimental Physics, Slovak Academy of Sciences, Watsonova 47, 040 01 Košice, Slovakia S Supporting Information *

ABSTRACT: We have performed a detailed experimental study on aqueous solutions of tertiary butyl alcohol which were a subject of long-standing controversies regarding the puzzling presence of virtually infinitely stable large-scale structures in such solutions occurring at length scales exceeding appreciably dimensions of individual molecules, referred to also as mesoscale structures. A combination of static and dynamic light scattering yielding information on solution structure and dynamics and gas chromatography coupled with mass spectrometry yielding information on chemical composition was used. We show that tertiary butyl alcohol clearly exhibiting such structures upon mixing with water does not contain any propylene oxide, which was previously considered as a source of these structures (an impurity expected to be present in all commercial samples of TBA). More importantly, we show that no mesoscale structures are generated upon addition of propylene oxide to aqueous solutions of TBA. The ternary system TBA/water/propylene oxide exhibits homogeneous mixing of the components on mesoscales. We show that the source of the mesoscale structures is a mesophase separation of appreciably more hydrophobic compounds than propylene oxide. These substances are explicitly analytically identified as well as their disappearance upon filtering out the mesoscale structures by nanopore filtration. We clearly show which substances are disappearing upon filtration and which are not. This enables us to estimate with rather high probability the chemical composition of the mesoscale structures. Visualization of large-scale structures via nanoparticle tracking analysis is also presented. Video capturing the mesoscale particles as well as their Brownian motion can be found in the Supporting Information.

1. INTRODUCTION

report inhomogeneities in aqueous solutions of 2-butoxyethanol.5 Independent of the forgotten TBA story, years later, a short communication by Georgalis et al.6 appeared reporting the observation of a slow dynamic mode in dynamic light scattering (DLS) experiments on three different types of electrolytes in aqueous solutions. This mode was interpreted as being due to sub-micrometer size clusters of ions. In 2006, a series of three extensive works by Sedlak7−9 showed that mesoscale structures appreciably exceeding dimensions of individual molecules are not a privilege of one or a few special system(s) but rather present in whole classes of systems. It was shown quite surprisingly that the majority of solutions and mixtures as used in everyday life and research practice do possess long-lived

Aqueous solutions of tertiary butyl alcohol (TBA), the last from a series of alcohols still freely miscible with water, have been a subject of interest, discussion, as well as controversies regarding the observation of anomalously high light scattering, indicating the presence of some sort of large structures present in such solutions. The first observation of the anomalously high light scattering was reported in 1972 by Vuks and Shurupova1 and interpreted as a “phase transition” between clathrate-like more ordered structure and less ordered molecular structure. Beer and Jolly2 reported that observations similar to those found in ref 1 were dependent on the sample history as well as purity. Euliss and Sorensen3 noted enhanced inhomogeneities upon cooling and attributed them possibly to clathrates formed by TBA, water, and some help gas. Bender and Pecora4 did not report any inhomogeneities in aqueous TBA solutions but did © 2014 American Chemical Society

Received: January 27, 2014 Published: February 21, 2014 2726

dx.doi.org/10.1021/jp500953m | J. Phys. Chem. B 2014, 118, 2726−2737

The Journal of Physical Chemistry B

Article

THF contained hydrophobic butylhydroxytoluene antioxidant as an additive in various concentrations.15 Coming back to the TBA story, Subramanian and Anisimov16 reported recently on resolving the mystery of aqueous solutions of tertiary butyl alcohol. The mesoscale structures were reported to originate as a consequence of the presence of propylene oxide in solution (“an impurity expected to be present in all commercial samples of TBA”16,17). This conclusion was made on the basis of the observation that the slow dynamic mode in DLS disappeared irreversibly after intensive filtration through nanoporous filters and was then regenerated upon addition of trace amounts of propylene oxide. No effect was reported when other substances were added, such as 10−4 mole fraction isopropanol, 10−4 mole fraction isobutanol, and 4 × 10−5 mole fraction cyclohexane.16 Similarly, no effect was seen upon adding PO to isopropanol aqueous solution. It was concluded that the formation of mesoscale structures requires specific interactions of the major components (TBA, water in this case) with minor components (as PO in this case). As authors correctly note in their work,16 many open questions arise, such as what is the internal structure of such structures, what controls their size, are they thermodynamically stable or kinetically arrested, and can results from this system be generalized to other systems? The aim of this work was the following: (i) to perform a much more detailed light scattering investigation in a much wider variety of experimental conditions and mixture compositions including static light scattering not performed so far on these systems, (ii) to perform high precision analytical measurements via GC-MS (gas chromatography coupled with mass spectrometry) and to correlate them with structural and dynamical data from static and dynamic light scattering, and (iii) according to results from GCMS to try to purify the components and to repeat measurements on pure (or significantly purer) major components. We wanted among others to answer the question whether the structures present prior to filtration are the same as structures generated by PO addition. In principle, these might be two different types of structures. Our approach proved to be useful. We show that no mesoscale structures are generated upon addition of propylene oxide to aqueous solutions of TBA. The ternary system TBA/ water/propylene oxide exhibits homogeneous mixing of the components on mesoscales. We show that the source of the mesoscale structures is a mesophase separation of appreciably more hydrophobic compounds than propylene oxide. These substances are explicitly analytically identified as well as their disappearance upon filtering out the mesoscale structures by nanopore filtration. We clearly show which substances are disappearing upon filtration and which are not. This enables us to estimate with rather high probability the chemical composition of the mesoscale structures. We also explain the reason why addition of PO misleadingly mimics the generation of mesoscale structures.

large-scale inhomogeneities. Around 100 different solute− solvent pairs were investigated in a detailed study7−9 with the aim to classify systems with respect to the capability of formation of the large-scale structures as well as to try to shed some light on the mechanism of their formation. The presence and intensity of the large-scale structures was correlated with properties of constituent molecules and ions such as their charge, dipole moment, protic vs aprotic character, etc. It was found9 that electrolytes of both inorganic and organic origin exhibit large-scale structures in aqueous solutions (in all cases) and in organic solvents (selectively). Solutions of nonelectrolytes including mixtures of liquids exhibit large-scale structures in aqueous solutions (in all cases) and in organic solvents (selectively). Nonaqueous mixtures did not exhibit large-scale organization in the case of nonpolar and weakly polar components. Since anomalously high light scattering and a slow dynamic mode in dynamic light scattering can have various origins, a detailed light scattering analysis was performed7 with a conclusion that the source of the slow mode scattering is real objects (not fluctuations) and that these are discrete objects (not bicontinuous phases with large correlation lengths). They possess smaller or larger polydispersities, in certain cases with sizes spanning up to a decade (radii from ∼30 to ∼300 nm). A nice agreement between size distributions obtained from static light scattering and nanoparticle tracking analysis (analysis of particle dynamics from microscopic video images) was published recently.10 Average sizes as well as size distributions are usually concentrationdependent.7 A detailed study was also devoted to the kinetics of the structure formation as well as to its long-time stability.8 The large-scale structures develop upon mixing the components of the solution or mixture on time scales varying from minutes to weeks, depending on the concrete system. Importantly, a truly homogeneous mixture is obtained at the beginning, and only af terward, a buildup of large structures begins. The long-time stability of large-scale structures was studied in detail over time intervals ranging up to 15 months. While in some systems the structures appeared virtually infinitely stable, a very slow ceasing was observed in others (weeks to months). While the characteristics are, at least qualitatively, similar in various systems, it is not clear whether the mechanism of the formation of mesoscale structures is the same in all systems or whether we deal with more than one mechanism. The presence of large structures was confirmed in works from several groups, namely, in aqueous solutions of several common well-soluble compounds (urea, α-cyclodextrin, ethanol),11,12 in aqueous solutions of amino acids,13 in mixtures of water with several organic liquids,14 and in THF/water mixtures.11,12,15 Several interpretations of the origin of these structures were hypothesized in the literature. It was reported9 that mesoscale structures appear in solvents capable of 3D hydrogen-bond networking and therefore hypothesized that the effect may be related to solute−solute and solute−solvent hydrogen bonding. They were interpreted also as nanobubbles,11,12 while subsequently the nanobubble nature of these structures was dismissed.10,14 While the work of Häbich et al.14 was focused mainly on verification of the eventual nanobubble nature of such structures, it was noted that the scattering signal is sensitive to the purity of the substances and thus, in principle, may originate from some kind of mesophase separation of minority components in the mixtures. A similar conclusion was drawn from LS and SANS data on THF/water mixtures where

2. EXPERIMENT Materials. TBA (tert-butyl alcohol) was from Sigma-Aldrich (99.7%). PO (propylene oxide) was also from Sigma-Aldrich (99.5%). Both compounds were additionally purified with emphasis on removing mainly hydrophobic impurities using the following procedure. 30% water solution of TBA was stirred with activated charcoal (DARCO, Sigma-Aldrich) overnight at 70 °C. Then, TBA was distilled off, dried using a water absorbing molecular sieve (Merck, Darmstadt), and distilled 2727



Article

GC-MS (Gas Chromatography Coupled with Mass Spectrometry). Samples were analyzed on an Agilent 7890A GC system with an Agilent 5975 C Mass Selective Detector (Agilent, Palo Alto, CA) and nonpolar HP-5 ms column (30 m × 0.25 mm × 0.25 μm). Two methods were used: direct injection and the HS-SPME method (headspace solid-phase microextraction). Direct injection of 1 μL of sample was realized with a pulse of 140 kPa, 24 s, at 280 °C and followed by analysis with the following temperature program: 50 °C − 2 min − 2 °C/min − 300 °C − 10 min. Helium was used as a carrier gas (99.998%; flow rate: 0.9 mL/min; SIAD, Bergamo, Italy). In the case of the HS-SPME method, 3 mL aliquots of solutions in 20 mL glass vials with a silicone septum were sampled using a DVB/CAR/PDMS (divinylbenzene/carboxen/ polydimethylsiloxane) extraction fiber (SUPELCO, Bellefonte, USA) for 1 h at ambient temperature. Extraction fiber was then placed in the headspace. Desorption occurred in the injection port at 280 °C for 5 min; the flow rate was 2 mL/min for 12 s and then 0.9 mL/min for the rest of the analysis. Separated compounds were identified on the basis of their mass spectra in the range 29−520 m/z (m being the mass and z the charge), comparison with the NIST08 spectra library, and taking into account other facts and factors such as expected elution order and literature data on boiling points. Nanoparticle Tracking Analysis (NTA). NTA was carried out with an LM10B Nanoparticle characterization system from Nano Sight (Amesbury, U.K.) with a trinocular microscope and an LM12 viewing unit with a 60mW laser working at λ = 405 nm. Video sequences were recorded via CCD camera operating at 30 frames per second (fps) and evaluated via the NANOSIGHT NTA 2.3 Analytical Software Suite. Blank NTA experiments with pure water or pure TBA were performed to exclude a possible contamination of water or viewing unit with parasitic scatterers. Perfectly clean dark images (without any traces of particles) were obtained on pure liquids.

once again. The same method was used to purify the propylene oxide (PO). In this case, due to the low boiling point of PO, the aqueous solution with activated charcoal was stirred at room temperature. Water was purified by reversed osmosis and activated carbon TOC reduction, freshly double-distilled in a quartz apparatus, and subsequently deionized by analytical grade mixed-bed ion exchange resins (Bio-Rad, Richmond, CA). We have used also water from commercial apparatus ELGA Purelab Ultra Analytic (Elga, United Kingdom). No dependence of results on water source was observed. The resistivity of water was always above 15 MΩ cm. Static Light Scattering (SLS). SLS measurements were made using a 40 mW HeNe laser, model 25LHP928 (CVI Melles Griot, Albuquerque, NM) with 632.8 nm vertically polarized beam. No change of data with laser power was observed in the range 1−40 mW. A laboratory made goniometer with an angular range from 30 to 135° was used to collect data for both static and dynamic light scattering experiments. Scattering intensities were measured by photon counting. Scattering intensities were normalized using doubly distilled and filtered benzene as a standard and expressed as I/IB ratios, where IB is benzene total scattering. Temperature was set with a precision of ±0.03 °C. Great attention was paid to the purity of samples. Scattering cells were thoroughly cleaned from dust. All solvents were filtered through nanoporous filters because of the same reason. Experiments on blank samples (nonstructured mixtures or pure liquids) showed a complete dust removal. Angular dependencies of scattering intensity were usually measured several times on one sample and subsequently averaged. Dynamic Light Scattering (DLS). An ALV5000E correlator with a fast correlation board option (ALV, Langen, Germany) was used for photon correlation measurements. Characteristic decay times of dynamic modes τi and their relative amplitudes Ai(τi) were evaluated through the moments of distribution functions of decay times A(τ) obtained by fitting correlation curves using CONTIN18 and GENDIST19,20 programs as g(1)(t ) =

∫0

∞

A(τ )e−t / τ dτ

3. RESULTS AND DISCUSSION Figure 1 shows typical examples of light scattering data on aqueous solutions of TBA. Autocorrelation functions from dynamic light scattering clearly exhibit two modes. Both modes are diffusive in nature, since the characteristic frequencies of both exhibit q2 dependences, where q is the scattering vector (not shown). The fast mode corresponds to the “ordinary molecular scattering”, more exactly to the concentration (mixture composition) fluctuations relaxing by diffusion, and the slower mode is due to the presence of large structures appreciably exceeding molecular dimensions (also referred to as mesoscale structures). While both fast and slow diffusive modes are clearly seen at 330 g/kg due to comparable amplitudes, the slow diffusive mode is dominating at 150 g/kg, since its amplitude is incomparably higher than the fast mode amplitude. The calculated apparent hydrodynamic radii are Rhapp = 66 nm (cTBA = 150 g/kg) and 53 nm (cTBA = 330 g/kg), respectively. These values were obtained via the Stokes−Einstein formula using viscosities of particular TBA/H2O mixtures. Corresponding angular dependencies of the slow mode amplitude from static light scattering are shown in Figure 1B. Radii of gyration obtained from Guinier approximation are Rg = 60 nm (cTBA = 150 g/kg) and 45 nm (cTBA = 330 g/kg), respectively. The whole concentration diagram is shown in Figure 2. Evidently, the ability to form mesoscale structures is concentrationdependent with a maximum somewhere between 100 and 200

(1)

Diffusion coefficients were calculated as Di = (1/τi)q−2, where q is the scattering vector defined as q = (4πn/λ0) sin(θ/2), with n being the solution refractive index, λ0 the laser wavelength, and θ the scattering angle. Two diffusive modes were detected. They were characterized by diffusion coefficients Df, Ds and amplitudes Af, As (subscripts f and s refer to faster and slower, respectively). Correlation curves at various angles were recorded concurrently with integral scattering intensities I(θ) (solution scattering) and IB(θ) (scattering of a benzene standard). Normalized scattering amplitudes of the two dynamic modes Af(θ) and As(θ) were calculated as A s (θ ) =

I(θ )/IB(θ ) 1 + A f (θ )/A s(θ )

(2)

A f (θ ) =

I(θ )/IB(θ ) 1 + A s(θ )/A f (θ )

(3)

assuming that I(θ)/IB(θ) = Af(θ) + As(θ). Dimensionless ratios As(θ)/Af(θ) and Af(θ)/As(θ) were taken from DLS spectra of relaxation times. 2728



Article

strongly present, the concentration-normalized intensity is still relatively high, but the scattering signal in terms of its absolute value is becoming weak and hence measurements are becoming more and more difficult and data less reliable. Long-term stability of mesoscale structures was tested by repeated measurements on selected samples over intervals of several weeks or even months with a conclusion that these structures were stable. This is in agreement with the previous work on aqueous TBA solutions16 as well as on aqueous solutions of other organic compounds.8 A distinct feature of aqueous TBA solutions (not found in some other types of solutes investigated in our laboratory in the past) was the reported influence of temperature: the effect of mesoscale structures was more pronounced at lower temperatures than at higher temperatures.16 In order to obtain a closer insight into this, we have performed detailed static and dynamic light scattering measurements upon temperature cycling (Figure 3).

Figure 1. (A) Typical example of bimodal autocorrelation functions from dynamic light scattering experiments on aqueous TBA solutions. Red curve: concentration 150 g/kg (molar fraction 0.041). Green curve: concentration 330 g/kg (molar fraction 0.107). While both fast and slow diffusive modes are clearly seen at 330 g/kg, the slow diffusive mode is dominating at 150 g/kg, since its amplitude is incomparably higher than the fast mode amplitude. Scattering angle θ = 90°. (B) Static light scattering angular dependencies of the slow mode amplitude. Corresponding radii of gyration Rg = 60 nm (cTBA = 150 g/kg) and Rg = 45 nm (cTBA = 330 g/kg), respectively. T = 25 °C.

Figure 3. (A) Temperature dependence of scattering intensity from mesoscale structures. Scattering angle θ = 45°. Blue symbols correspond to gradual cooling and red symbols to gradual heating, respectively. (B) Temperature dependence of the ratio Dsp/Ds, where Dsp is the slow diffusion coefficient predicted for temperature T provided that no change occurs in the sample upon changing temperature except for changes of solution viscosity η. T = 25 °C is taken as a reference temperature so that Dsp/Ds(25 °C) = 1. The decrease of Dsp/Ds with T is due to the decrease of the apparent hydrodynamic radius Rh,app of mesoscale structures. Scattering angle θ = 45°. Figure 2. Concentration-normalized scattering intensity from mesoscale structures in aqueous solutions of TBA. Scattering angle θ = 45°. T = 7 °C.

Temperature changes appear to be fully reversible without any hysteresis. Scattering signal from mesoscale structures becomes stronger upon cooling and weakens upon heating (Figure 3A). This can be due to an increase in the number of mesoscale structures and/or their size, and/or their scattering contrast with respect to the rest of the solution (“matrix”). Figure 3B clearly shows that the size does change reversibly with

g/kg that corresponds to TBA molar fractions of 0.026−0.057. The effect is ceasing at high concentrations and is practically undetectable for TBA concentrations of ∼700−1000 g/kg. Upon going to low concentrations, the effect is still relatively 2729



Article

afterward solely due to “molecular scattering” (fast mode). Then, propylene oxide was added to the sample via a syringe filter such that the resulting concentration of added propylene oxide in the sample (in solution) was cPO = 0.9 mass %. Added PO indeed generated scattering signal similar to the signal prior to filtration. What is, however, evident in this experiment is that the generated intensity of the slow mode after addition of PO is still approximately 3 times smaller than the former intensity in the TBA/water mixture prior to filtration in spite of the fact that the concentration of added PO is incomparably higher than what can be expected in TBA of p.a. quality with determined 99.7% purity. The 0.9% concentration of PO in TBA solution corresponds to the 6% concentration of PO in pure TBA. It is of course absolutely unrealistic that the TBA would contain such a concentration of PO, since all impurities altogether constitute 0.3%. This was the first observation leading to questioning the origin of mesoscale structures in aqueous solutions of TBA as due to the action of PO. Subsequently, the influence of PO was investigated in more detail in a whole variety of concentrations and mixture compositions. Figure 5 shows a summary of results from experiment where aqueous TBA solution (c = 330g/kg) was first cold-filtered at 7

temperature, namely, that the size increases upon cooling and decreases upon heating. The ratio Dsp/Ds is shown in Figure 3B, where Dsp is the slow diffusion coefficient expected at temperature T provided that no change occurs in the sample upon changing temperature except for changes of solution viscosity η. Ds is the actually measured slow diffusion coefficient at temperature T. T = 25 °C is taken as a reference temperature so that Dsp/Ds(25 °C) = 1. The ratio Dsp/Ds can be thus interpreted as the ratio of apparent hydrodynamic radii of mesoscale structures Rhapp,T/Rhapp,25°C. A similar result was obtained at a TBA concentration of 330 g/kg. However, since the scattering from mesoscale structures is much weaker here, almost a complete disappearance of signal was observed above 40 °C. Its reappearance was observed upon cooling back. The disappearance of the slow mode signal does not necessarily mean a complete disintegration of structures but can mean that their size and/or scattering contrast get so small that the signal drops below the detectability limit. The next focus in this work was on the verification of the reported claim16 that the source of the mesoscale structures in aqueous TBA solutions is propylene oxide (PO), an impurity assumed16 to be present in all commercial TBA products, including those of p.a. quality. This claim was based on experiments which showed that mesoscale structures could be irreversibly eliminated by intensive filtration through membrane filters with nanopores and subsequently regenerated by PO addition.16 Our expected goal was not only to repeat and verify these findings in our laboratory but also to try to answer the question of whether the structures present in solution prior to filtration and those generated by PO addition are the structures of the same origin, simply because they do not have to be. Figure 4 shows results of an experiment where 150 g/kg TBA solution was first measured without filtration (just the components of the mixture, i.e., water and TBA, were separately filtered prior to mixingadded via nanoporous filters directly to the scattering cell and afterward gently mixed). Subsequently, the mesoscale structures were completely eliminated via filtration of the mixture and the scattering was

Figure 5. Intensity of scattering from mesoscale structures in aqueous solutions of TBA (c = 330 g/kg) as a function of the concentration of added propylene oxide. TBA solution was first cold-filtered at 7 °C such that As = 0, and subsequently, cold PO (T = 7 °C) was added. After each PO addition, the sample was maintained and monitored at 7 °C for a couple of days (typically 2−6) while the intensity was still changing (vertical arrows). Sample compositions are specified in Table 1. Scattering angle θ = 45°.

°C such that the mesoscale structures were eliminated (As = 0), and subsequently cold PO (T = 7 °C) was gradually added. Work at lower temperature was chosen, since the effect is more pronounced at lower temperatures (Figure 3). After each PO addition, the sample was maintained and monitored at 7 °C for a couple of days (typically 2−6) while the intensity was still changing (vertical arrows). Sample compositions are specified in Table 1. Figure 6 shows angular dependencies of scattering from large-scale structures generated by PO addition. The scenario is usually such that intensity goes down after addition of PO and the angular dependence becomes less steep which means that the structures get smaller after addition of PO. Then, structures grow over several days and consequently angular dependencies become steeper. The inset in Figure 5 shows in detail data at very low PO concentrations and documents that no effect is

Figure 4. Angular dependencies of the overall light scattering intensity (fast mode “molecular scattering” plus slow mode scattering originated from mesoscale structures): (blue ○) TBA solution 150 g/kg prior to filtration; (blue ●) the same sample after filtration; (red □) the filtered sample after addition of filtered dust-free propylene oxide. The resulting concentration of added propylene oxide in the sample c = 0.9 mass %. The whole experiment was done at 7 °C, including cold PO added. Scattering intensity is normalized to benzene scattering IB. 2730



Article

Table 1. Mixture Compositionsa 1 2 3 4 5 6 7 8 9 10 11 12 13 a

cTBA (%)

cH2O (%)

cPO (%)

32.87 32.74 32.61 31.47 30.19 27.90 25.62 22.75 17.65 11.11 3.92 1.95 0.47

67.13 66.85 66.59 64.26 61.64 59.98 52.30 46.44 36.04 22.67 7.99 3.99 0.97

0.00 0.40 0.80 4.26 8.18 15.12 22.08 30.82 46.32 66.22 88.09 94.06 98.56

Figure 7. Angular dependencies of scattering from TBA/PO mixture upon gradual addition of water. The initial composition of the mixture was 57.5 mass % TBA and 42.5 mass % PO. Then, water was gradually added to this mixture. Concentration of added water in the mixture (mass %): (black ○) 0, (red ○) 0.012, (green ○) 2.37, (blue ○) 3.97, (teal ○) 5.52, (olive ○) 9.87, and (magenta ○) 19.72. The whole experiment was performed at 7 °C. Scattering intensity is normalized to benzene scattering IB.

Data refer to the experiment from Figure 5.

Table 2 shows radii of gyration of mesoscale structures calculated in Guinier approximation as a function of the Table 2. Radii of Gyration of Mesoscale Structures in TBA/ PO Mixtures as a Function of the Concentration of Added Watera

Figure 6. Angular dependencies of scattering from mesoscale structures in aqueous solutions of TBA (c = 330 g/kg) as a function of the concentration of added propylene oxide. Data refer to the experiment described in Figure 5. The same color of the symbols means measurements almost at the same time. An open symbol means measurement shortly before addition of PO, and a closed symbol means measurement shortly after addition of PO. The whole experiment was done at 7 °C, including cold PO added.

a

cH2O (%)

cPO (%)

cTBA (%)

Rg (nm)

0.00 0.012 2.37 3.97 5.52 9.87 19.72

42.53 42.53 41.52 40.84 40.18 38.33 34.14

57.47 57.46 56.10 55.19 54.30 51.80 46.13

70.3 85.1 139 144 173 185

Data refer to experiment from Figure 7.

concentration of added water. It is evident from Figure 7 that also the polydispersity increases. Hence, the radii of gyration listed in Table 2 are rather some upper limit estimates of the particular size distributions. Figure 8 shows the kinetics of the mesoscale structure formationreal time monitoring via light scattering. The TBA/PO mixture was measured first without addition of water where only one mode (“ordinary molecular scattering”) is seen in DLS data. Then, one drop of filtered cold water (7 °C) was added to the mixture in the scattering cell, the cell was shortly gently shaken to homogenize the mixture, and measurement continued. A gradual increase of intensity due to the growth of large scattering objects (mesoscale structures) is evident. An accompanied gradual increase of intensity fluctuations is also evident, as expected. Figure 9 shows a ternary diagram of the TBA/water/PO system summarizing mixture compositions that have been investigated in this work. The height of the vertical bars corresponds to scattering As from mesoscale structures measured at an angle of 45°, normalized to benzene scattering. The overall dependence of As on the mixture composition over the whole ternary diagram will be discussed later in this paper. We would like to stress at this point that, in all cases (all

seen up to cPO = 1% and that it is necessary to increase the PO concentration to cPO ≈ 4% to reproduce the intensity found in unfiltered TBA solution. Concentrations of cPO = 1 and 4% in solution correspond to cPO = 3 and 12% in pure TBA. This is of course in strong contrast to the 99.7% purity of the TBA used. Results from Figure 5 are thus in agreement with conclusions drawn from Figure 4. The effect of the formation of mesoscale structures reaches a maximum for cPO = 60−70% and gradually ceases at high cPO. Figure 7 shows results of an experiment where the TBA/PO mixture with the composition 42.5/57.5 was investigated. This mixture shows no evidence of any mesoscale structuring. Only the fast mode (“ordinary molecular scattering”) is present. Then, small amounts of filtered water were gradually added directly to the scattering cell and scattering was monitored. As small addition of water as that leading to the concentration of water in the mixture cH2O = 0.012% resulted in the formation of mesoscale structures. Mesoscale structures then grew as a function of added water content (overall increase of scattering accompanied with gradually steeper and steeper angular dependencies). 2731



Article

This finding together with other above-mentioned facts lead us to the next step, namely, to correlating light scattering data with analytical measurements by GC-MS. Figure 10 shows a

Figure 8. Real-time monitoring of the origination of mesoscale structures. TBA/PO mixture with mass composition 57.5% TBA and 42.5% PO was measured at a temperature of 7 °C (scattering signal from t = 0 to t = 5 min). Only one mode (“ordinary molecular scattering”) was seen in DLS data. Then, one drop of filtered cold water (7 °C) was added to the mixture directly to the scattering cell, the mixture was shortly gently shaken to homogenize it, and measurement continued immediately. A gradual increase of intensity as well as increase of fluctuations due to the growth of large scattering objects (mesoscale structures) is evident. Scattering angle θ = 45°. Scattering intensity is normalized to benzene scattering IB.

Figure 10. GC-MS (gas chromatography coupled with mass spectrometry) elugram. Direct injection of TBA. Injection temperature 280 °C. (2) 2-Methoxy-2-methylpropane; (3) 1-(1,1-dimethylethoxy)2,2-dimethylpropane; (4) methylhexanol; (5) dodecane; (6) tetradecane; (7) hexadecane; (8) octadecane; (9) eicosane.

GC-MS elugram of pure TBA. First of all, it should be mentioned that no propylene oxide was detected in our TBA. Main components contained in our TBA are listed in the figure caption together with numbers of corresponding peaks in the elugram. Because a nonpolar column was used in the GC-MS experiment, more hydrophilic compounds eluted at shorter times while more hydrophobic compounds eluted at longer times. It should be taken into account that sizes (areas) of peaks corresponding to particular compounds are not directly proportional to their concentrations, since these areas depend (aside from compound concentrations) also on other parameters. Figure 11 shows results of GC-MS analysis on aqueous TBA solution (c = 150 g/kg) prior to filtration and after filtration, respectively. It is clearly evident which compounds are eliminated by filtration and which are not. More detailed data are listed in Table 3, but it is clear that dodecane, tetradecane, hexadecane, and octadecane, i.e., extremely hydrophobic compounds, are practically completely eliminated (corresponding peaks decreased 1000 times or more). On the other hand, compounds less hydrophobic like 2-methoxy-2-methylpropane, 1-(1,1dimethylethoxy)-2,2-dimethylpropane, or methylhexanol are not eliminated from solution by filtration. The third group of compounds numbered as 10−14, i.e., silans and siloxans, originate from the leaking of the GC column and extraction fiber when exposed to water vapors and are not coming from the sample itself. They are as expected practically the same whether filtered or unfiltered sample is analyzed. Light scattering data for the filtered and unfiltered samples from Figure 11 are in the Supporting Information. The filtered sample shows no presence of mesoscale structures. It is possible to conclude with very high confidence that mesoscale structures are composed mainly of the highly hydrophobic, relatively long hydrocarbons dodecane, tetradecane, hexadecane, and octadecane, while less hydrophobic compounds are not involved (or very marginally) in mesoscale structures. Propylene oxide belongs to weakly hydrophobic compounds; it is miscible with

Figure 9. Ternary diagram of the TBA/water/PO system at T = 7 °C. The height of the vertical bars corresponds to scattering As from mesoscale structures measured at an angle of 45°. Commercial TBA and PO both of p.a. grade were used without special purification (used “as is”).

compositions) where mesoscale structures were found, the scattering signal from mesoscale structures could be finally irreversibly eliminated by filtration via fine nanoporous filters, including mixtures with huge concentrations of PO on the order of tens of percent. This finding is in disagreement with the interpretation that mesoscale structures in aqueous TBA solutions are due to the presence of PO as an impurity contained in TBA16 and that the irreversible disappearance of these structures is due to the removal of PO segregated in these structures.16 It can be easily verified by weighing the filter before and after filtration that only a very small amount of filtered material is retained on the filter and hence practically all PO is passing through the filter to the filtered solution when filtering mixtures with high PO content. Evidently the mesoscale structures disappear not due to the removal of PO. 2732



Article

Figure 12. GC-MS (gas chromatography coupled with mass spectrometry) elugram. Direct injection of propylene oxide. Injection temperature 280 °C. (1) 2,4-Dimethylfuran; (2) 2-ethyl-3-methyl-1,3dioxolane; (3) 1,1′-oxybis(2-propanol); (4) 2,2′-oxybis(1-propanol); (5) tripropylene glycol; (6) hexadecane; (7) octadecane; (8) eicosane; (9) pentacosane; (10) squalene; (11) dodecyl acrylate; (12) diisobutyl phthalate; (13) bis(2-ethylhexyl) adipate; (14) bis(2-ethylhexyl) phthalate.

propylene oxide itself. It is clear that our PO contains exactly such long hydrophobic hydrocarbons which were identified as mesoscale structure makers in the case of the TBA/water mixture (Figure 11). These are the compounds numbered in Figure 12 as (6) hexadecane, (7) octadecane, (8) eicosane, (9) pentacosane, and (10) squalene. By adding propylene oxide to the TBA/water mixture, the concentration of these “mesoscale structure makers” increases, but since their concentration in PO is low, one needs to add extremely high concentrations of PO to regenerate the mesoscale structures after filtration. That is what was exactly observed in the light scattering experiments described above. It is actually the action of “mesoscale structure makers”, not PO itself, that leads to the regeneration of the effect after filtration. Other compounds that can be identified in the GC-MS elugram of PO in Figure 12 are hydrophilic (1−5) and partly hydrophilic/partly hydrophobic compounds (11− 14). These are expected to not be principally involved.

Figure 11. GC-MS (gas chromatography coupled with mass spectrometry) elugrams. (A) TBA aqueous solution, c = 150 g/kg, before filtration (red) and after filtration (green). (B) Zoomed plot showing peaks with small amplitudes in detail. Elugrams were obtained by the HS-SPME method (headspace solid-phase microextraction). Desorption occurred in the injection port at 280 °C for 5 min. (1) TBA; (2) 2-methoxy-2-methylpropane; (3) 1-(1,1-dimethylethoxy)2,2-dimethylpropane; (4) methylhexanol; (5) dodecane; (6) tetradecane; (7) hexadecane; (8) octadecane; (10) dimethylsilandiol; (11) hexa-methylcyclotetrasiloxane; (12) octamethylcyclopentasiloxane; (13) decamethylcyclopenta-siloxane; (14) dodecamethylcyclohexasiloxane. Further parameters are given in Table 3.

water up to 40%. This supports our conclusions from light scattering experiments that PO is not a source of mesoscale structures in TBA/water mixtures. Another explanatory argument comes from Figure 12 which shows a GC-MS elugram of

Table 3. Summary of HS-SPME GC-MS Data (Headspace Solid-Phase Microextraction Gas Chromatography-Mass Spectrometry)a abundance (a.u.) 1 2 3 4 5 6 7 8 10 11 12 13 14

substance

tr (min)

unfiltered

filtered

tert-butyl alcohol 2-methoxy-2-methylpropane 1-(1,1-dimethylethoxy)-2,2-dimethylpropane methylhexanol dodecane tetradecane hexadecane octadecane dimethylsilandiol hexamethylcyclotrisiloxan octamethylcyclotetrasiloxane decamethylcyclopentasiloxane dodecamethylcyclohexasiloxane

1.571 2.651 5.071 5.510 15.309 20.719 25.559 29.922 2.651 4.680 9.416 14.057 18.868

1167463229 109786601 7163730 25969673 44585954 53964376 25226268 7438535 33449397 12575989 4615266 6176944 2811742

1247473634 109113720 7046643 28566900 50471 29291 10567 4137 28902715 10438659 4194355 4885203 2563775

content decrease, times 1 1.08 1.09 0.97 943 1969 2551 1921 1.24 1.29 1.18 1.35 1.53

a

Filtered and unfiltered aqueous solution of TBA, cTBA = 150 g/kg. tr is the retention time. Content decrease is calculated from the ratio of abundances from unfiltered and filtered solutions, respectively. 2733



Article

To test the conclusions given above, we have decided to purify TBA and propylene oxide with special emphasis on removing highly hydrophobic compounds such as long hydrocarbons. Details of purification are in the experimental part. Figure 13 shows a summary of experiments similar to

Figure 14. Scattering from mesoscale structures in aqueous solutions of TBA (c = 150 g/kg). TBA was purified with emphasis on removal of hydrophobic compounds, as described in the Experiment section, and afterward, a known amount of octadecane was added to TBA such that the resulting concentration of octadecane in TBA was (blue ○) 0.00053%, (green ○) 0.0053%, and (red ○) 0.053%. Afterward, the octadecane-doped TBA was mixed with water. Temperature T = 7 °C.

mesoscale structure; nevertheless, the 0.0053% concentration is quite a realistic number that can be expected in TBA of p.a. quality with 99.7% purity and the relative composition of minority compounds is given by Figure 10. This is in contrast to unrealistic concentrations of propylene oxide necessary to generate the effect of mesoscale structures. Figure 15 shows direct visualization of the long-lived mesoscale structures by a NTA (nanoparticle tracking analysis) apparatus. Structures are clearly visualized by optical microscope as individual bright spots directly in the approximately 80 μm wide laser beam (volume approximately 80 × 100 × 20 μm3). It should be stressed that these structures/particles are not being directly “imaged”. The particles act as point scatterers whose dimensions are below the Abbé limit, only above which can structural information and shape be resolved by optical microscopy. Some spots are larger and some smaller, which can be due to either different sizes of particles or different positions in the beam. Video sequence capturing of the Brownian motion of the mesoscale particles can be found in the Supporting Information. This method21−26 permits one to further track the Brownian pathways of individual particles over a suitable period of time (typically several seconds), compute their diffusion coefficients based on their mean square displacements, subsequently compute their hydrodynamic radii via the Stokes−Einstein formula, and finally yield size distributions. Determination of unknown particle concentrations is also possible. A more detailed NTA analysis will be the subject of an upcoming separate experimental work. Current NTA data confirm our conclusions from the light scattering work, namely, (i) that these are real objects, not fluctuations, and (ii) that these are discrete objects (not bicontinuous phases with large correlation lengths).

Figure 13. (A) Intensity of scattering from mesoscale structures in aqueous solutions of TBA (c = 330 g/kg) as a function of the concentration of added propylene oxide. TBA solution was first coldfiltered at 7 °C such that As = 0, and subsequently, cold PO (T = 7 °C) was added. After each PO addition, the sample was maintained and monitored at 7 °C for a couple of days (typically 2−6) while the intensity was still changing (vertical arrows). Red symbols correspond to mixtures prepared with untreated compounds (adapted from Figure 5). Green symbols correspond to mixtures prepared with purified compounds, as described in the Experiment section. (B) Zoomed plot showing small values in detail. Scattering angle θ = 45°.

those presented in Figure 5 which were now repeated with purified components (both TBA and PO purified). As can be seen in plot A, the effect is almost eliminated. A careful inspection of very small values of scattering from mesoscale structures As in the zoomed plot B indicates that the shape of the dependence is practically identical to that in plot A; the kinetics is very similar, and just the values are very small (2 orders of magnitude smaller). We can therefore conclude that we deal here with some residual effect which is the same in nature, just dramatically eliminated. Figure 14 shows the results of an experiment where purified TBA was doped with known amounts of octadecane and afterward such TBA was mixed with water at a concentration of cTBA = 150 g/kg. The presence of mesoscale structures is evident, and light scattering data are semiquantitatively similar to data on nonpurified TBA mixed with water. In terms of scattering intensity from mesoscale structures, the closest match to data on unpurified TBA mixed with water is obtained at 0.0053% octadecane concentration. It is well possible that various hydrocarbons may have various abilities to form

4. CONCLUSIONS It can be concluded that binary mixtures of TBA/water and ternary mixtures of TBA/PO/water are homogeneous on large length scales appreciably exceeding molecular dimensions and do not possess mesoscale structures. Such structures form from substantially more hydrophobic compounds that were exactly analytically identified by GC-MS and were found to be mostly 2734



Article

Figure 15. Microscopic images from the NTA (nanoparticle tracking analysis) experiment on an aqueous solution of TBA (c = 150 g/kg). TBA was first purified, subsequently doped with a known amount of octadecane (0.0053% octadecane in TBA), and finally mixed with water (see Figure 14). Two still images from a video sequence are shown where the captured area is 80 × 100 μm2 while the focal depth is approximately 20 μm. Below are individual Brownian motion tracks of several selected particles as monitored and evaluated by the NTA software. The experiment was performed at room temperature. The whole video is available in the Supporting Information.

long hydrocarbons such as dodecane (C12H26) and longer ones. These hydrophobes present in both TBA and PO as minority components segregate upon mixing with water into relatively well-defined and long-term stable mesoscale structures with sizes on the order of ∼100 nm. Less hydrophobic minority components (typically containing oxygen or hydroxyl groups) were found to be also present in both TBA and PO but are not involved in the mesoscale structures. Propylene oxide itself belongs to this category. The observation of the generation of mesoscale structures by addition of PO into TBA/water mixtures led to a conclusion that PO is a source of mesoscale structures in TBA/water mixtures.16,17 In fact, the hydrophobes contained in PO are the source of mesoscale structures developing upon PO addition. Another experiment confirming the nature of observed mesoscale structuring is shown in Figure 16. Following the diagram in Figure 9, we have chosen the mixture composition where the effect reaches its maximum (cTBA = 18.3%, cPO = 15.5%, cH2O = 66.2%). Scattering intensity is as high as AS ∼ 900 at this composition. Then, the mixture was filtered via fine filters to completely eliminate AS and the sample was immediately monitored via light scattering. At the beginning, the overall intensity was equal to Af (monomodal correlation curve, just “ordinary molecular scattering”), and then, a gradual reappearing of the slow mode (mesoscale structures) was monitored online. The intensity grew relatively quickly within the first 30 min, and then, a slow increase (another order of magnitude) was observed over the next 2 days. No increase was observed afterward. The final intensity was As = 12, i.e., 75 times lower than the initial intensity As ∼ 900. It can be easily verified by

Figure 16. Online monitoring of a gradual reappearing of the slow mode (mesoscale structures) after filtration by a fine filter. The TBA/ PO/H2O mixture composition corresponds to that with the maximum effect of formation of mesoscale structures (cTBA = 18.3%, cPO = 15.5%, cH2O = 66.2%). I/IB is the benzene-normalized overall scattering intensity. Fast mode (“ordinary molecular scattering”) intensity Af = 0.22. Scattering angle θ = 45°.

weighing the filter before and after filtration that only a very small amount of filtered material is retained on the filter and hence practically all PO is passing through the filter to the filtered solution in this experiment. We have thus demonstrated that PO is not the source of mesoscale structures even in that part of the diagram which represents the maximum effect of mesoscale structuring. Regarding the explanation of the partial reappearance of the structures upon filtration, two scenarios can be envisaged. The first would be based on an assumption that 2735



Article

(impurities) could also be involved in the formation of these structures. In summary, on the basis of the results of this and our previous work, we can conclude that aqueous mixtures of organic compounds as used in everyday life and research practice do possess long-lived large-scale inhomogeneities in a quite substantial manner, even research-grade compounds of p.a. quality and even after special in-lab purification procedures. Under conditions strongly favoring mesoscale segregation, very special and sophisticated procedures would be needed to completely eliminate the effect by producing compounds of virtually absolute purity. Under conditions strongly favoring mesoscale segregation, a partial reappearance of mesoscale structures can be seen even after complete elimination of the effect by nanopore filtration (Figure 16). Many questions remain open, especially those related to the stabilization mechanism of these mesoscale structures preventing macroscopic phase separation. Another related issue is the presence of similar structures in solutions of ionic species (organic and inorganic salts)6−10 and polyions27−31 which may or may not have similar origin.

not all hydrophobes are initially incorporated in mesoscale structures and some portion is still molecularly dissolved. Upon removal of the mesoscale structures, the quasi-equilibrium is distorted and molecularly dissolved hydrophobes segregate into new mesoscale structures. The second scenario would be based on an assumption that the structures present in the mixture prior to filtration are partially retained on the filter and partially broken into small fragments that pass the filter and that could coalesce later into larger structures. However, we do not see any indication of broken fragments very shortly after filtration and therefore the first scenario seems to be more probable. Regarding the dependence of the degree of large-scale structure formation on mixture composition, it can be concluded as follows. In binary mixtures of TBA and water (Figure 2), a maximum can be observed around 150 g/kg, while the effect gradually ceases for TBA concentrations larger than 500 g/kg. When TBA concentration is high, hydrophobic compounds can be sufficiently solubilized such that they can be molecularly dissolved. Upon decreasing TBA content (increasing water content) in the mixture, hydrophobes become more and more segregated into large-scale structures. Going with TBA concentration further down (below 150 g/kg), the scattering from these structures become eventually weaker, since the decrease of TBA concentration means also a decrease of the absolute value of the concentration of hydrophobes in the mixture. Gradual addition of water into the TBA/PO mixture (Figure 7) decreases the ability of the mixture to solubilize hydrophobes molecularly, and therefore, these are more and more segregated. When the water content approaches 70%, the effect reaches its maximum (Figure 9). Further increases of water content go at the expense of the concentrations of TBA and PO and hence also at the expense of the concentration of hydrophobes as a “building material” for mesoscale structures. A more detailed discussion on the whole ternary diagram shown in Figure 9 is not much possible, since this diagram implicitly reflects not only different mixture compositions in terms of TBA/PO/water but also different types and quantities of hydrophobes contained in TBA and PO, respectively. The role of TBA and/or PO in the stabilization of mesoscale structures cannot be therefore assessed from Figure 9. A separate detailed study is necessary for this purpose. The effect investigated in this work can be characterized as a mesoscopic solubility. Hydrophobic compounds are neither macroscopically phase separated nor molecularly dissolved in TBA/water mixtures. While it is clear that water is the source of the segregation and hydrophobes are “the building material”, the role of the main solute (TBA in our case) in the stabilization of discrete mesoscale structures will be investigated in detail in the near future. Results obtained on the given system have much broader implications, since, as described in the Introduction, practically all aqueous mixtures and solutions of organic molecules do exhibit large-scale structures with similar characteristics.7,9,11−17 It is well possible that in other aqueous systems (aqueous solutions or mixtures of other organic molecules), less hydrophobic compounds may also be involved, since this may depend on the ability of the main solute component (as TBA in this work) to solubilize minority components present as impurities. Since TBA is a good solvent for hydrophobes, only very hydrophobic compounds (long hydrocarbons C12H26−C30H62) segregate upon mixing TBA with water. Compounds with a lower ability to solubilize hydrophobes may yield mesoscale structures upon mixing with water, while far less hydrophobic minority compounds

■

ASSOCIATED CONTENT

S Supporting Information *

Figure showing the light scattering characterization of the unfiltered sample from Figure 11 and video sequence capturing the Brownian motion of mesoscale particles in aqueous solution of TBA. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Support of the Slovak Research and Development Agency (Grant No. 048610) and Scientific Grant Agency VEGA (Grant No. 2/0182/14) is acknowledged. This work was realized within the frame of the project “Build up of the infrastructure of the Centre of excellence for progressive materials with nanoand sub-micrometer structure, Project No. 26220120035”, which is supported by the Operational Program “Research and Development” of the Slovak republic financed through European Regional Development Fund. The authors are also thankful to P. Bartak from Palacky University Olomouc for acquiring and interpreting GCMS elugrams.

■

REFERENCES

(1) Vuks, M. F.; Shurupova, L. V. The Scattering of Light and Phase Transition in Solutions of Tertiary Butyl Alcohol in Water. Opt. Commun. 1972, 5, 277−278. (2) Beer, C. W., Jr.; Jolly, D. J. Comments on “The Scattering of Light and Phase Transition in Solutions of Tertiary Butyl Alcohol in Water. Opt. Commun. 1974, 11, 150−151. (3) Euliss, G. W.; Sorensen, C. M. Dynamic Light Scattering Studies of Concentration Fluctuations in Aqueous t-butyl Alcohol Solutions. J. Chem. Phys. 1984, 80, 4767−4773. (4) Bender, T. M.; Pecora, R. A Dynamic Light Scattering Study of the tert-Butyl Alcohol-Water System. J. Phys. Chem. 1986, 90, 1700− 1706.

2736



Article

(5) Bender, M. T.; Pecora, R. Dynamic Light Scattering Measurements of Mutual Diffusion Coefficients of Water-Rich 2-butoxyethanol Water Systems. J. Phys. Chem. 1988, 92, 1675−1677. (6) Georgalis, Y.; Kierzek, A. M.; Saenger, W. Cluster Formation in Aqueous Electrolyte Solutions Observed by Dynamic Light Scattering. J. Phys. Chem. B 2000, 104, 3405−3406. (7) Sedlák, M. Large-Scale Supramolecular Structure in Solutions of Low Molar Mass Compounds and Mixtures of Liquids: I. Light Scattering Characterization. J. Phys. Chem. B 2006, 110, 4329−4338. (8) Sedlák, M. Large-Scale Supramolecular Structure in Solutions of Low Molar Mass Compounds and Mixtures of Liquids: II. Kinetics of the Formation and Long-Time Stability. J. Phys. Chem. B 2006, 110, 4339−4345. (9) Sedlák, M. Large-Scale Supramolecular Structure in Solutions of Low Molar Mass Compounds and Mixtures of Liquids. III. Correlation with Molecular Properties and Interactions. J. Phys. Chem. B 2006, 110, 13976−13984. (10) Sedlák, M.; Rak, D. Large-Scale Inhomogeneities in Solutions of Low Molar Mass Compounds and Mixtures of Liquids: Supramolecular Structures or Nanobubbles? J. Phys. Chem. B 2013, 117, 2495−2504. (11) Jin, F.; Ye, J.; Hong, L.; Lam, H.; Wu, C. Slow Relaxation Mode in Mixtures of Water and Organic Molecules: Supramolecular Structures or Nanobubbles? J. Phys. Chem. B 2007, 111, 2255−2261. (12) Jin, F.; Li, J.; Ye, X.; Wu, C. Effects of pH and Ionic Strength on the Stability of Nanobubbles in Aqueous Solutions of AlphaCyclodextrin. J. Phys. Chem. B 2007, 111, 11745−11749. (13) Hagmeyer, D.; Ruesing, J.; Fenske, T.; Klein, H. W.; Schmuck, C.; Schrader, W.; da Piedade, M. E. M.; Epple, M. Direct Experimental Observation of the Aggregation of Alpha-Amino Acids into 100−200 nm Clusters in Aqueous Solution. RSC Adv. 2012, 2, 4690−4696. (14) Häbich, A.; Ducker, W.; Dunstan, D. E.; Zhang, X. Do Stable Nanobubbles Exist in Mixtures of Organic Solvents and Water? J. Phys. Chem. B 2010, 114, 6962−6967. (15) Li, Z. Y.; Cheng, H.; Li, J. Y.; Hao, J. K.; Zhang, L.; Hammouda, B.; Han, C. C. Large-Scale Structures in Tetrahydrofuran - Water Mixture with a Trace Amount of Antioxidant Butylhydroxytoluene (BHT). J. Phys. Chem. B 2011, 115, 7887−7895. (16) Subramanian, D.; Anisimov, M. A. Resolving the Mystery of Aqueous Solutions of Tertiary Butyl Alcohol. J. Phys. Chem. B 2011, 115, 9179−9183. (17) Subramanian, D.; Ivanov, D. A.; Yudin, I. K.; Anisimov, M. A.; Sengers, J. V. Mesoscale Inhomogeneities in Aqueous Solutions of 3methylpyridne and Tertiary Butyl Alcohol. J. Chem. Eng. Data 2011, 56, 1238−1248. (18) Provencher, S. W. A Constrained Regularization Method for Inverting Data Represented by Linear Algebraic or Differential Equations. Comput. Phys. Commun. 1982, 27, 213−219. (19) Jakeš, J. Testing of the Constrained Regularization Method of Inverting Laplace Transform on Simulated Very Wide Quasielastic Light-Scattering Auto-Correlation Functions. Czech. J. Phys. 1988, 38, 1305−1316. (20) Štěpánek, P. Data Analysis in Dynamic Light Scattering. Dynamic Light Scattering. The Method and Some Applications; Clarendon: Oxford, U.K., 1993; Chapter 4, pp 177−240. (21) Carr, B.; Hole, P.; Malloy, A.; Nelson, P.; Wright, M.; Smith, J. Applications of Nanoparticle Tracking Analysis in Nanoparticle Research − a Mini Review. Eur. J. Parenter. Pharm. Sci. 2009, 14, 45−50. (22) Syvitski, J. P. M. Principles, Methods, and Application of Particle Size Analysis; Cambridge University Press: Cambridge, U.K., 1991. (23) Walker, J. G. Improved Nano-Particle Tracking Analysis. Meas. Sci. Technol. 2012, 23, 065605. (24) Filipe, V.; Hawe, A.; Jiskoot, W. Critical Evaluation of Nanoparticle Tracking Analysis (NTA) by NanoSight for the Measurement of Nanoparticles and Protein Aggregates. Pharm. Res. 2010, 27, 796−810. (25) Malloy, A.; Carr, B. Nanoparticle Tracking Analysis - The Halo (TM) system. Part. Part. Syst. Charact. 2006, 23, 197−204.

(26) Saveyn, H.; De Baets, B.; Thas, O.; Hole, P.; Smith, J.; Van der Meeren, P. Accurate Particle Size Distribution Determination by Nanoparticle Tracking Analysis Based on 2-D Brownian Dynamics Simulation. J. Colloid Interface Sci. 2010, 352, 593−600. (27) Lin, S. C.; Lee, W. I.; Schurr, J. M. Brownian Motion of Higly Charged Poly(L-Lysine). Effects of Salt and Polyion Concentration. Biopolymers 1978, 17, 1041−1064. (28) Schmitz, K. S. Macroions in Solution and Colloidal Suspension; VCH Publishers: New York, 1993. (29) Sedlák, M. The Ionic Strength Dependence of the Structure and Dynamics of Polyelectrolyte Solutions as Seen by Light Scattering: The Slow Mode Dilemma. J. Chem. Phys. 1996, 105, 10123−10133. (30) Sedlák, M. What Can Be Seen by Static and Dynamic Light Scattering in Polyelectrolyte Solutions and Mixtures? Langmuir 1999, 15, 4045−4051. (31) Sedlák, M. Structure and Dynamics of Polyelectrolyte Solutions by Light Scattering. Physical Chemistry of Polyelectrolytes; Marcel Dekker: New York, 2001; Chapter 1, pp 1−58.

2737


On the Origin of Mesoscale Structures in Aqueous Solutions of Tertiary

Recommend Documents