water interactions with hydrophobic versus hydrophilic nanosilica

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

WATER INTERACTIONS WITH HYDROPHOBIC VERSUS HYDROPHILIC NANOSILICA Vladimir M. Gun'ko, Volodymyr V Turov, Evgeniy M. Pakhlov, Tatyana V. Krupskaya, Mykola Vasylyovych Borysenko, Mykola T. Kartel, and Barbara Charmas Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03110 • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 19, 2018

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WATER INTERACTIONS WITH HYDROPHOBIC VERSUS HYDROPHILIC NANOSILICA Volodymyr M. Gun’ko, †,* Volodymyr V. Turov, † Evgeniy M. Pakhlov, † Tetyana V. Krupska, † Mykola V. Borysenko, † Mykola T. Kartel, † Barbara Charmas§ †

Chuiko Institute of Surface Chemistry, 17 General Naumov Street, Kyiv 03164, Ukraine § Faculty of Chemistry, Maria Curie-Skłodowska University, 20031 Lublin, Poland

ABSTRACT It is well known that interaction of hydrophobic powders with water is weak, and upon mixing, they typically form separated phases. Preparation of hydrophobic nanosilica AM1 with a relatively large content of bound water with no the formation of the separated phases was the aim of this study. Unmodified nanosilica A-300 and initial AM1 (A-300 completely hydrophobized by dimethyldichlorosilane), compacted A-300 (cA-300), and compacted AM1 (cAM1) containing 5058 wt.% of bound water were studied using low-temperature 1H NMR spectroscopy, thermogravimetry, infrared spectroscopy, microscopy, small angle X-ray scattering, nitrogen adsorption, and theoretical modeling. After mechanical activation (~20 atm) upon stirring of AM1/water mixture at the degree of hydration h = 1.0 or 1.4 g of distilled water per gram of dry silica, all water is bound and the blend has the bulk density of 0.7 g/cm3. The temperature and interfacial behaviors of bound water depend strongly on a dispersion media type (air, chloroform, and chloroform with trifluoroacetic acid (4:1)) because the boundary area between immiscible water and chloroform should be minimal. Water and chloroform molecules are of different sizes affecting their distribution in pores (voids between silica nanoparticles in their aggregates) of different sizes. Structural, morphological, and textural characteristics of silicas, and environmental features affect not only the distribution of bound water, but also the amounts of strongly (frozen at T < 260 K) and weakly (frozen at 260 K < T < 273 K) bound and strongly (chemical shift δH = 4-6 ppm) and weakly (δH = 1-2 ppm) associated waters. Despite the changes in the characteristics of cAM1, it demonstrates a flotation effect. The developed system with cAM1/bound water could be of interest from a practical point of view due to controlled interactions with aqueous surroundings. KEYWORDS: Hydrophobic nanosilica AM1; Hydro-compacted AM1; Structural characteristics; Strongly and weakly bound water; Strongly and weakly associated water; Surroundings effects

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INTRODUCTION Various hydrophobic silicas are of importance from a practical point of view1-6 and produced by many companies for a variety of applications.7-13 Many problems appearing upon the use of hydrophobic materials are caused by features of their interactions with water, the degree of hydrophobicity, durability of a hydrophobic functional layer, etc.14-20 Interaction of water with a solid surface depends not only on the type of functionalization (hydrophobization), but also on the surface topology and confined space effects, as well on the presence of co-adsorbates and a dispersion medium type.18,21-24 On the other hand, water can strongly affect the organization of fumed metal or metalloid oxides (FMO) with respect to the secondary particles such as aggregates of nonporous nanoparticles (NPNP) and agglomerates of aggregates.18,25,26 Additionally, in the systems with hydrophobic particles and water, air bubbles can play an important role in flotation and decrease in the wettability of a particle surface and whole secondary structures with hydrophobized NPNP.18,27 Features of interactions of water with hydrophobic and hydrophilic particles allow one to create “dry” water28 or the reverse system with hydrophobic cores and hydrophilic shells containing bound water.29-31 Note that water bound to hydrophilic components of complex materials (such as polymers, rubbers, and paints filled by pigment particles, etc.) can lead to some negative effects. For example, upon freezing, water (ice) having a larger volume than liquid one can destroy the materials. Water as an active solvent and/or a promotor of acid-base reactions can affect a degradation rate of the materials located in wet air under sun light. Therefore, a deep insight into features of structure and location of bound water and its interaction with surroundings (e.g., hydrophilic and hydrophobic nanosilicas used as fillers) is of importance from a practical point of view.14-31 Interaction between water molecules by the hydrogen bonds is several times stronger than that between water molecules and such hydrophobic functionalities as Si(CH3)n (n = 1-3) interacting with water by weak van-der-Waals forces.18,32 However, the interactions between these functionalities per se can be weaker than that with water that causes the hydrophobic hydration 2 ACS Paragon Plus Environment

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effects.33,34

As

a

whole,

water

molecules

location

in

hydrophobic

environment

is

thermodynamically unfavorable due to reduction of entropy and interaction energy (i.e. Gibbs free energy increases) and an increase in the heat capacity.35,36 These interface features, as well as the confined space effects, can influence the properties of water bound in complex systems containing hydrophobic and hydrophilic components.18,26

At a mosaic surface with disordered hydrophilic and hydrophobic patches (e.g. at partially hydrophobized silica surface), the clusterization of water can be enhanced.18,37 If the clusterization of water occurs under certain external actions (e.g. mechanical loading) that small water clusters can be more densely incorporated between hydrophobic functionalities. This can result in removal of air bubbles bound to hydrophobic structures located in water. Depending on conditions, it is possible preparation of structures with hydrophilic or hydrophobic cores and hydrophobic or hydrophilic shells, e.g., with hydrophilic and hydrophobic nanosilicas. The first pair corresponds to, e.g., “dry” water,28 the second one corresponds to such materials as Flotosorb (working as a surfactant),27 Enterosgel (a sorbent for medical applications),18 etc. However, there is a problem of durability of hydrophobic functional layer, especially under mechanical loading.39 Note that ≡Si-OSiR3 structures are more strongly stable in comparison to those formed at a surface of non-silica materials.1-6 For completely hydrophobized nanosilica, a certain amount of adsorbed water remains and it is well observed in the infrared (IR) spectra.18 This effect can be explained by the presence of water in the form of residual silanols (inaccessible for modifier molecules) and intact water molecules bound to residual silanols or located inside nanoparticles.1-4,18 The water molecules can penetrate into the particle volume during a relatively long period (e.g., several days for preheated hydrophilic nanosilica).1-4,18 At a surface of nanosilica, there are some silanols, which are poorly accessible or inaccessible for such reactants as silanes. They are characterized by an IR band at 3680-3660 cm−1.1,2 These groups are relatively stable upon heating.2 Thus, fumed silicas completely hydrophobized by alkylsilanes forming such surface functionalities as –Si(CH3)3, (CH3)2Si-O3 ACS Paragon Plus Environment

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Si(CH3)2, etc. can contain small amounts of residual silanols and bound water. A question erases on the possibility of a strong increase in the amounts of water bound to hydrophobic nanosilica upon certain treatment with no decomposition of surface functionalities. A similar effect was studied for complex systems with hydrophilic and hydrophobic FMO.26 However, it is of interest to study the interaction of a pure hydrophobic nanosilica with water under mechanical loading. Fumed silica forms very loose powder with a large empty volume (Vem) up to 25 cm3/g.1-4,18 Mechanical treatment of dry or wetted hydrophilic nanosilica results in increased bulk density (ρb), which depends strongly on treatment conditions. Simple wetting-drying of hydrophilic nanosilica can strongly increase the value of ρb. However, it is smaller (up to 0.25 g/cm3 for A-300) than that after mechanical compaction of the wetted powder (0.4-0.6 g/cm3).18,26 For hydrophobic nanosilica mixed with n-hexane and dried or unmodified nanosilica mixed with alcohol, the compaction effects are smaller than that for unmodified nanosilica treated with water.18,40 The aim of this work was to study hydrophobic nanosilica compacted due to stirring with water under certain mechanical loading in comparison to hydrophilic nanosilica prepared by a similar way. This study is of interest not only from a theoretical point of view, but also from a practical one. Compacted hydrophobic nanosilica containing a certain amount of bound water and remaining hydrophobic at a macro-level (e.g. demonstrating supernatant macroparticles after mixing with water) can play a role of a surfactant stronger interacting with the aqueous media (despite remained hydrophobic properties) than the initial hydrophobic nanosilica.

EXPERIMENTAL Materials. Such commercial fumed silicas as initial hydrophilic A-300 (specific surface area SBET = 295 m2/g, bulk density ρb ≈ 0.04 g/cm3) and completely hydrophobized A-300 (AM1, SBET = 285 m2/g, ρb ≈ 0.04 g/cm3) (Pilot plant of Chuiko Institute of Surface Chemistry, Kalush, Ukraine) were used as initial samples. AM1 was prepared by hydrophobization of A-300 by dimethyldichlorosilane giving pair-crosslinked dimethylsilyl (DMS) groups. AM1 was selected as a 4 ACS Paragon Plus Environment

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hydrophobic nanosilica because the volume of the pair-crosslinked DMS groups is smaller than that of trimethylsilyl (TMS) of longer functionalities. This is favorable for preparation of target hydrocompacted AM1 (cAM1). Nanosilica A-300 is composed of nonporous nanoparticles (NPNP) of dav = 9.2 nm in average diameter, and dav = 9.8 nm for AM1. Both unmodified and hydrophobized NPNP form aggregates (< 1 µm in size) and loose agglomerates of aggregates (> 1 µm in size) forming visible structures at a low value of ρb ≈ 0.04 g/cm3 (see Figures 1 and S1-S5 in Electronic Supporting Information (ESI) file). Hydrophilic nanosilica was wetted and dried that result in increased bulk density of the powder to ρb ≈ 0.2 g/cm3. This silica with added water (1:1.125) was carefully stirred that results in ρb ≈ 0.6 g/cm3 for compacted hydrophilic nanosilica (cA-300). Compacted hydrophobic AM1 (cAM1) with added water (1:1 or 1:1.4) was prepared by careful grinding in a porcelain mortar (with strong hand-loading giving ~20 atm, estimated from the geometry of the mortar and a pestle used, and a loading weight) for 30 min. This treatment results in uniform dense powder at ρb ≈ 0.7 g/cm3 (at h = 1 g/g) with no a separated water phase and empty volume Vem ≈ 1/ρb – 0.5/ρ0,SiO2 – 0.5/ρ0,H2O ≈ 0.701 cm3/g. Note that fresh cAM1 sinks in water and does not float to a bulk water surface during a long period (Figure S1). This occurs due to removal of the air bubbles from AM1 upon hydro-compaction. A similar effect was observed for hydrocompacted blends of hydrophobic AM1 with hydrophilic fumed silica A-300 or alumina Al-100. After drying of cAM1 in air, it can float at the bulk water surface (Figure S1). To avoid the effects of bulk water upon 1H NMR spectroscopy measurements, relatively small amounts of water (1.0-1.4 g per gram of dry solids) and acid (trifluoroacetic acid F3COOD, TFAA) added to chloroform medium (1:4) were used in the NMR experiments. Microphotographs (Primo Star optical microscope, Carl Zeiss) of used samples show some structural features of them (see Figure S1 in ESI file). 1

H NMR spectroscopy. 1H NMR spectra of static samples (volume of 0.5 cm3 placed into a 5

mm ampoule) of initial A-300 and AM1 and compacted cA-300 and cAM1 with bound water located in air or chloroform media (alone or with added TFAA, w/w 4:1) were recorded using a 5 ACS Paragon Plus Environment

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Varian 400 Mercury spectrometer (magnetic field 9.4 T) utilizing 60o pulses of 1 µs duration. An ampoule with a silica sample (50-200 mg depending on ρb) was filled by chloroform alone (the ampoule was shaken for several minutes) and then a certain amount of TFAA was added (shaking again). A portion of TFAA can easily penetrate to a sample during shaking of the ampoule. Each spectrum was recorded by co-addition of eight scans with a 2 s delay between each scan. Relative mean errors were less than ±10% for 1H NMR signal intensity for overlapped signals, and ±5% for single signals. Temperature control was accurate and precise to within ±1 K. The accuracy of integral intensities was improved by compensating for phase distortion and zero line nonlinearity with the same intensity scale at different temperatures. To prevent supercooling, the spectra were recorded starting at T = 200-210 K for samples precooled to this temperature at a cooling rate of 5 K/min and maintained at the lowest temperature for 10 min. Then the samples were heated to 280285 K at a heating rate of 5 K/min with steps ∆T = 10 K or 5 K and maintained at a fixed temperature for 8 min for data acquisition at each temperature. The used conditions provided a practically equilibrium state of samples at each temperature. Applications of this method and NMR cryoporometry to nanooxides were described in detail elsewhere.18,41,42 Note that signals of immobile (frozen) molecules and functionalities of particles were not registered in the 1H NMR spectra due to a narrow bandwidth (20 kHz), a large difference in the transverse relaxation time of mobile and immobile phases, and the use of static samples.18 Changes in the Gibbs free energy (∆G) of bound water were determined from the temperature dependences of the amounts of unfrozen water (Cuw in mg of water per gram of dry sample) at T = 200−273 K18 and tabulated ∆G data for ice.43 The area under the ∆G(Cuw) curve determines interfacial Gibbs free energy, γS (eq. S7 in ESI file) as the modulus of overall changes in ∆G(Cuw) due to interaction of water with a solid surface.18 Water can be frozen in narrower pores (or voids between nanoparticles) at lower temperatures as described by the Gibbs−Thomson relation for the freezing/melting point depression for liquids confined in cylindrical pores at radius R18,41,42 (see some calculation details in ESI file). 6 ACS Paragon Plus Environment

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Infrared spectroscopy. The infrared (IR) spectra were recorded in the range of 4000-300 cm−1 using a Specord M80 (Carl Zeiss). Samples with A-300 or AM1 were pressed (~1300-1500 atm) into thin pellets (weight 15-20 mg). The transmittance IR spectra recorded with 4 cm−1 steps were converted into the absorbance spectra. Thermogravimetry (TG) with differential thermal analysis (DTA). Thermograms (TG, differential TG (DTG), and DTA) were recorded using a Derivatograph Q-1500 D apparatus (MOM, Hungary) upon heating of samples (~0.2 g) in air at a heating rate of 10 oC/min. Microscopy. The particulate morphology of nanooxides was analyzed using field-emission scanning electron microscopy (SEM) employing a QuantaTM 3D FEG (FEI, USA) apparatus operating at the voltage of 15 or 30 kV. Unmodified silica (A-300), modified A-300 (AM1) and compacted AM1 (cAM1) samples were mounted on circular aluminum stubs with a double sticky carbon tape and then coated with Pd/Au (Quorum Techn. Polaron SC7640/CA7625). Optical microphotographs were prepared using a Primo Star optical microscope (Carl Zeiss). Textural characteristics. To analyze the textural characteristics of samples degassed at 373 K for several hours, low-temperature (77.4 K) nitrogen adsorption–desorption isotherms were recorded using a Micromeritics ASAP 2420 adsorption analyzer. The specific surface area (SBET) was calculated according to the standard BET method.44 The total pore volume Vp was evaluated from the nitrogen adsorption at p/p0 ≈ 0.98-0.99, where p and p0 denote the equilibrium and saturation pressure of nitrogen at 77.4 K, respectively.45 Calculations of the pore size distributions (PSD)46 are described in detail in ESI file. SAXS. Small-angle X-ray scattering (SAXS) analysis of initial and compacted silicas was carried out using an Empyrean (PANalytical, Netherlands) diffractometer with CuKα radiation (with parallel beam X-ray mirror with a W/Si crystal) using a transmission mode with scans over the 0.115 - 5o range at a step size of 0.01o using a continuous scan mode at 293 K. Before the SAXS measurements, the samples were poured onto a mylar film (6 µm in thickness), leveled and gently kneaded by hand. The beam weakness after passing through the sample was measured, then it was 7 ACS Paragon Plus Environment

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corrected taking into account the background observed calculating the absorption factor of each sample. SAXS patterns were analyzed using PANanalytical EasySAXS V. 2.0.0.405 program (to calculate the particle size distributions, PaSD) and some approaches described in details in ESI file using homemade software to calculate the pore size distributions (PSD)47 and other textural characteristics.

RESULTS AND DISCUSSION Morphological, textural and structural characterization. The particulate morphology of initial AM1 (Figures 1 and 2) is similar to that of unmodified nanosilica A-300 having the same value of ρb = 0.04 g/cm3 (see Figures S1-S5 in ESI file). Pair-crosslinked surface functionalities (CH3)2Si-O-Si(CH3)2 (formed upon hydrolysis of the Si-Cl bonds and condensation of the residual silanols from neighboring functionalities attached to a silica surface, see cluster models in ESI file) form a relatively thin surface layer on a surface of each nonporous nanoparticle (NPNP) practically with no cross-linking of adjacent NPNP.

Figure 1. SEM images of (a) AM1 and (b) cAM1 (scale bar 1 µm).

Typically, all fumed oxides form aggregates of NPNP (< 1 µm in size) and agglomerates of aggregates (> 1 µm)1-6 (Figures 1 and S1-S5). Contribution of small aggregates of NPNP of 30-80 8 ACS Paragon Plus Environment

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nm in diameter is smaller for initial AM1 than for cAM1 due to the compaction effects (Figure 2a). However, the pore size distribution (PSD) intensity is lower for cAM1 than for AM1 or A-300 at R = 0.2-10 nm (Figure 2b) because contribution of narrow voids between NPNP in compacted aggregates decreases. The hydrophobization of A-300 results in certain reorganization of aggregates of NPNP and agglomerates occurs in AM1 (Figure 3); however, the PSD shape is the same.

Figure 2. (a) Particle size distributions and (b) incremental pore size distributions calculated using SAXS data.

Figure 3. Incremental pore (voids between nonporous nanoparticles) size distributions of unmodified A-300 (curve 1) and initial AM1 (2) calculated using nitrogen adsorption data.

The PSD (voids between NPNP in the secondary structures18,48) of AM1 is similar to that of unmodified A-300 (Figure 3); however, a small narrowing of the PSD peaks of AM1 is observed. For voids between adjacent NPNP (a peak at R = 2 nm), narrow voids at R < 1 nm practically 9 ACS Paragon Plus Environment

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disappear in AM1 (Figures 2b and 3). For voids between NPNP from the second or third coordination spheres in aggregates and agglomerates (a peak at R = 10 – 20 nm), contribution of macropores decreases for AM1. The first effect can be caused both by increased sizes of modified NPNP due to the attached DMS layer and changes in the organization of aggregates of NPNP. Note that the PSD (Figure 3) do not describe the total range of pores because nitrogen is poorly adsorbed in large macropores,44,45 and SAXS PSD (Figure 2b) describe broader pores. The empty volume Vem = 1/ρb – 1/ρ0 in the powders of non-treated A-300 or AM1 having the bulk density ρb ≈ 0.04 g/cm3 and true density ρ0 ≈ 2.2 and 2.15 g/cm3 for A-300 and AM1 NPNP, respectively, is much larger (≈24.5 cm3/g) than the pore volume Vp (typically Vp < 1 cm3/g) measured from the nitrogen adsorption at p/p0 = 0.98-0.99. The mechanical compaction of nanosilica results in stronger changes in the PSD (see Figures S6 and S7 in ESI file) than that due to hydrophobization by DMS (Figure 3) or hexamethyldisilazane (giving trimethylsilyl (TMS) groups attached to a silica surface with no crosslinking) (Figure S7). Thus, the hydro-compaction of AM1 or A-300 or mechanochemical activation of A-300 in a ball-mill results in stronger changes in the PSD (Figures 2b, S6 and S7) than the surface hydrophobization (Figure 3), and these changes affect the SAXS patterns (Figure S8) and the corresponding PaSD (Figures 2a, S9a), PSD (Figure 2b) and chord size distributions, CSD (Figure S9b). Note that the hydro-compaction of AM1 leads to appearance of a small number of very small nanoparticles (Figure S9a, d < 3 nm), which can represent the fragments of partially destroyed NPNP of AM1. The CSD (Figure S9b), corresponding to lines located in the silica phase with no voids, show the compaction effect because the CSD of cAM1 is located below that of AM1. Thus, fumed silicas, both initial and modified, could be assigned to “soft” powder materials characterized by easy changes in such textural characteristics as PaSD, PSD, Vp, and Vem; however, relative changes in the specific surface area are smaller upon the treatments used (according to the SAXS data, this change is about 1.5% only).


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Figure 4. Thermogravimetry (TG curves 1, 2, and 3 and DTG curves 4, 5, and 6) and DTA (curves 7, 8, and 9) data for AM1 (curves 1, 4, and 7), AM1 compacted with water (2, 5, and 8), and unmodified nanosilica A-300 (3, 6, and 9).

Nanosilicas located in air can adsorb certain water amounts, which depend on the textural characteristics and chemical structure of a surface. The amount of water bound in the initial AM1 powder located in air (Figure 4, curve 1) is two times smaller than that in unmodified A−300 (curve 3). Thus, despite surface hydrophobization in AM1, it can adsorb a certain amount of water from air with no mechanical treatment and hydro-compaction of the powder. The latter leads to a significant increase in the content of bound water (Figure 4, curves 2, 5, and 8). According to the IR spectra, despite the absence of free silanols in initial or treated AM1 (Figures 5 and S10, a band at 3748 cm-1)1,2 that shows complete hydrophobization of the silica surface in AM1, water can be adsorbed onto the surface (Figures 5 and S10, a band at 3500-3000 cm−1).1-6 This result can be explained by several reasons. First, there are poorly accessible silanols (e.g., located in narrow and shallow pores), which are observed in the IR spectra of initial and treated AM1, as well of A-300 (Figures 5 and S10, a band at 3680-3660 cm-1).1,2 Second, there are intact water molecules and silanols inside NPNP, which can be completely removed only at high temperatures (up to 1000 oC).1-6 Heating of nanosilica at high temperatures affects the textural characteristics due to removal of silanols and bound water (see Table S1 in ESI file).1-4 The heating results in increased SBET (NPNP decrease in size) and then in decreased SBET due to sintering of 11 ACS Paragon Plus Environment

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NPNP at higher temperatures and removal of the surface hydroxyls that leads to changes in the orientation of adsorbed N2 molecules with increased occupied surface area per a molecule. Third, for AM1 prepared by modification of A-300 by dimethyldichlorosilane with cross-linked pairs of neighboring functionalities, the accessibility of surface siloxane bonds for water molecules is greater than that for attached TMS groups. The latter form larger surface umbrellas than DMS (see models of AM1 particle in Figures 5b, S11, S14a, and S15, and silica modified by trimethylsilyl groups, Figure S14b).18,38 Therefore, water molecules can easily penetrate to polar structures at AM1 surface in comparison to A-300 completely hydrophobized by the TMS functionalities.

a

b

Figure 5. (a) Infrared spectra of thin pellets (pressed at 1300 or 1500 atm) with AM1 initial (curve 1), preheated at 125oC for 2.5 hours (2), compacted with water and pressed into a thin pellet at 1300 atm (3), and nanosilica A-300 unmodified (4) and preheated at 450 oC for 1 hour (5); (b) model of dry AM1 nanoparticle (see models hydrated AM1 particles in ESI file).

However, during sample pressing at 1300-1500 atm prior to the IR spectra recording, a portion of weakly bound water (WBW) forming large domains in nanosilicas can be removed. Therefore, intensity of a broad band of water at 3500-3200 cm−1 is maximal for initial nanosilica (Figures 5a, curve 4, and S10) because WBW can be more easily removed from cAM1. However, this band (Figure 5a, curve 3) is more intensive for cAM1 (initially containing 50 wt.% of water and 45 wt.% observed in the TG curve, Figure 4, curve 2) than that of A-300 preheated at 450 oC (Figure 5a, 12 ACS Paragon Plus Environment

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curve 5). Minimal amounts of water is observed for AM1 initial (curve 1) or preheated (curve 2) that is in agreement with the TG data (Figure 4). Thus, the amounts and the behavior of water bound in initial and compacted hydrophilic and hydrophobic silica powders significantly differ due to several factors. Certain features of the interfacial behavior of water bound to nanosilicas studied can be analyzed using 1H NMR spectra recorded at different temperatures for samples with relatively small amounts of bound water (h = 1.0-1.4 g/g) and located in various dispersion media (vide infra). Interfacial phenomena. Water bound to cAM1 (h = 1 or 1.4 g/g) (Figures 6a and S12a,b) and cA-300 (h = 1.125 g/g) (Figure 6a, dotted-dashed lines) located in air is characterized by 1H NMR signals of similar shapes at the chemical shift of proton resonance at δH ≈ 5 ppm. This value is similar to that of bulk water, i.e., this water can be attributed to strongly associated water (SAW), but for cAM1, signals are broader. The latter can be explained by reduced mobility of clustered water bound to cAM1 due to the presence of surface barriers with cross-linked DMS groups (see models in ESI file). If the systems are located in the chloroform media instead of air, the behavior of water strongly changes vs. temperature (Figures 6b,c and S12c,d). A major portion of water bound to cAM1 represents weakly bound water (WBW) frozen at 260 K < T < 273 K (Figures 6b, S12c,d, and S13). Note that theoretical calculations of the 1H NMR spectra of free water clusters and water clusters bound to AM1 particles (Figures S14-S21) show that there is 1.0-1.5 ppm upfield shift for water bound to hydrophobic AM1 particles in comparison to free water clusters.


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Figure 6. 1H NMR spectra of hydro-compacted cAM1 at h = 1 g/g (a) (solid lines) in air and (b) in CDCl3, hydrocompacted cA-300 at h = 1.125 g/g (a) (dotted-dashed lines) in air and (c) in CDCl3; inserts (b, c) show the spectra at high temperatures.

Additionally, a decrease in the numbers of water molecules in the free clusters (Figure S10, curves 4 and 6, and models in Figures S15-S21) or a certain disorder of the cluster structure (curve 5) leads to a small upfield shift (< 0.5 ppm). At T < 270 K, a major contribution of unfrozen water 14 ACS Paragon Plus Environment

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corresponds to weakly associated water (WAW) appearing in the spectra at δH =1-2 ppm. WAW corresponds to small 2D and 3D clusters (see Figures S18 and S19)18 contacted with the siloxane bridges and hydrophobic DMS barriers. For cA-300, contribution of WAW is small (Figures 6c, and S13). However, a significant portion of SAW corresponds to WBW, since it is frozen at 260 K < T < 273 K. If TFAA is added to the system that a relative contribution of WAW decreases and SAW represents a solution of TFAA since a significant downfield shift (characteristic for acidic solution) is observed (Figure S12e,f). The temperature and interfacial behaviors of bound water well reflected in the temperature dependences of the amounts of unfrozen water (Figure S13) and related structural and thermodynamic characteristics (Figure 7 and Table 1). The amounts of strongly bound water (SBW) frozen at T < 260 K are greater for cAM1 located in air than that for cA-300 (Table 1, Cuws and PSD/IPSD at R < 5 nm, Figure 7a,b) that is rather unexpected result. However, the amount of WBW is greater for cA-300, because water tends to form larger structures to reduce the contact area with hydrophobic surface of cAM1, and a portion of water becomes unbound (UBW) (frozen at 273.15 K) and is not fixed in the NMR cryoporometry PSD. This water and a fraction of WBW was removed on pressing of pellets for the IR spectra recording as described above. The effect of the chloroform medium is stronger for water bound to cAM1 (i.e., there is the displacement of water into larger structures frozen at T close to 273 K; therefore, non-fixed by the NMR cryoporometry PSD) than that for cA-300 (Figures 7a,b and S13 and Table 1). Similar effects of the chloroform medium are observed for cAM1 at h = 1.4 g/g (Figures 7c,d, S13, and Table 1). Addition of TFAA leads to enhancement of bonding of the acidic solution (Figures 7c,d and S13 and Table 1, Cuws and γS values increase but decreases) due to both the colligative properties of the acidic solution and confined space effects in voids between NPNP in their aggregates. The values of γS* (Table 1) can characterize the wettability of samples. It is maximal for cAM1 located in air because chloroform can displace a portion of water from a surface of nanosilica.


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Figure 7. NMR cryoporometry calculations of (a, c) differential and (b, d) incremental size distributions of pores filled by unfrozen water bound to hydro-compacted cAM1 at h = (a, b) 1 g/g and (c, d) 1.4 g/g and hydro-compacted cA-300 (a, b) at h =1.125 g/g located in air, CDCl3 or CDCl3/TFAA (4:1).

The amount of SBW is relatively small for all systems (with one exception of the system with TFAA) because both cA-300 and cAM1 are composed of NPNP and contribution of narrow voids between them is small. This contribution decreases with compaction (Figures S12 and S13), despite contribution of some mesopores could increase. WBW is located in broad mesopores and macropores, but SBW is located in narrow mesopores and nanopores. A small amount of SBW and a large amount of WBW interacting with cAM1 can provide the preservation of the flotation effects for wetted-dried cAM1 then located in bulk water (Figure S1).


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Table 1 Characteristics of water bound to compacted cA-300 and cAM1 located in air or chloroform medium Sample h Сuww γS* Snano,uw γS Medium Сuws Smeso,uw Smacro,uw Vnano,uw Vmeso,uw Vmacro,uw −∆Gs (mg/g) (mg/g) (J/g) (K) (g/g) (mJ/m2) (m2/g) (m2/g) (m2/g) (cm3/g) (cm3/g) (cm3/g) (kJ/mol) cA-300 1.125 Air 40 1085 2.41 7.22 118.3 268.1 4 48 9 0.002 0.710 0.413 cA-300 1.125 CDCl3 30 1095 2.53 6.10 96.8 268.4 10 44 9 0.005 0.626 0.494 cAM1 1.0 Air 50 950 2.08 8.37 167.4 267.4 0.3 44 5 0 0.719 0.406 cAM1 1.0 CDCl3 10 990 3.09 3.11 74.0 253.8 32 10 0 0.012 0.132 0.856 cAM1 1.4 Air 1.97 12.89 207.9 266.9 0 55 7 0 0.998 0.127 70 1330 cAM1 1.4 CDCl3 2.93 8.05 87.5 266.5 50 42 0 0.019 0.441 0.940 15 1385 cAM1 1.4 CDCl3/TFAA 2.94 35.74 148.9 260.6 19 211 0 0.008 1.371 0.021 350 1050 Note. Cuws and Cuww are the amounts of weakly and strongly bound waters; ∆Gs is the changes in the Gibbs free energy of water layer closely located to a surface; γS is the modulus of the total changes in the Gibbs energy of bound water unfrozen at T < 273.15 K; γS* = γS/Suw, Suw = Snano,uw + Smeso,uw + Smacro,uw, Vuw = Vnano,uw + Vmeso,uw + Vmacro,uw, is the average melting temperature; Snano,uw and Vnano,uw, Smeso,uw and Vmeso,uw, and Smacro,uw and Vmacro,uw are the specific surface area and pore volume of nanopores at R < 1 nm, mesopores at 1 nm < R < 25 nm and macropores at R > 25 nm, respectively, in contact with unfrozen water (see some details in ESM file).

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UBW and WBW can be easily evaporated in comparison to SBW. The air bubbles (located in places free of WBW/UBW contacted with hydrophobic functionalities) can remain in dried cAM1 after wetting in bulk water that provide the flotation effect (Figure S1). This effect was observed for both cAM1 maintained in air or closed glass ampoules for a month in contrast to fresh cAM1 sinking in the bulk water. Thus, WBW can be easily evaporated from cAM1 due to the organization of secondary particles without contribution of narrow nanopores.18,48

Figure 8. Chemical shifts δH of unfrozen water in the 1H NMR spectra of (a) cA-300 (h = 1.125 g/g) and cAM1 at h = (a) 1 g/g and (c) 1.4 g/g located in air and chloroform or CDCl3/TFAA media vs. T, and (b, d) corresponding functions s(T) = −T(∂(lnδ(T))/∂T)P vs. T.

The system nonuniformity affects the temperature dependence of δH of water bound to nanosilicas (Figure 8a,c). This results in complex changes in the entropy of bound unfrozen (melted ice) water vs. temperature (Figure 8b,d) calculated as described in detail elsewhere (see also ESI file).49 The maximal peak of the function of s(T) (Figure 8b,d) corresponds to increased entropy of WBW, which appears due to melting of ice at 265-273 K. However, for WAW bound to cAM1 (h = 1.4 g/g) located in chloroform, this peak becomes broader and shifts toward lower temperatures (Figure 8d). This is due to larger amounts of bound water (1.4 g/g) stirred with 18 ACS Paragon Plus Environment

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cAM1 that changes the organization of the secondary particles in comparison to the system at h = 1 g/g (Figure 8b). This assumption is confirmed by the behavior of SAW (Figure 8b,d). The behavior of water (mainly SAW) bound to cAM1 at h = 1.0 g/g (Figure 8b) and 1.4 g/g (Figure 8d) located in air differs only slightly; however, the curve is broader for the former. The change of the dispersion medium from air to chloroform results in the appearance of a certain amount of WAW, which is more strongly bound due to the influence of the hydrophobic environment (both DMS functionalities and CDCl3). This result is rather unexpected. CONCLUSIONS To change the interaction of a hydrophobic nanosilica powder with water, a simple method of stirring under mechanical loading of ~20 atm was used. This treatment results in the formation of compacted hydrophobic nanosilica cAM1 with a relatively large content of bound water (1.01.4 g/g) without the formation of separated phases. In this system, the air bubbles are removed. Therefore, fresh cAM1 sinks and does not float at the bulk water surface. Unmodified nanosilica A−300 (SBET = 295 m2/g, bulk density ρb ≈ 0.04 g/cm3) and initial AM1 (A-300 completely hydrophobized by dimethyldichlorosilane, SBET = 285 m2/g, ρb ≈ 0.04 g/cm3), compacted A-300 (cA-300, with bound water ρb ≈ 0.6 g/cm3) and compacted AM1 (cAM1, with bound water ρb ≈ 0.7 g/cm3) containing 50-58 w.% of bound water were studied using low-temperature 1H NMR spectroscopy, thermogravimetry, infrared spectroscopy, microscopy, SAXS, nitrogen adsorption, and theoretical modeling. After stirring (~20 atm) of AM1/water mixture (h = 1.0 or 1.4 g/g), all water is bound. Contributions of SAW, WAW, SBW, of WBW in the systems studied depend on several factors: (i) a type of silica, i.e., hydrophilic A-300 or hydrophobic AM1 initial or pretreated; (ii) dispersion media (air, chloroform, and chloroform with TFAA); (iii) temperature; and (iv) interaction with TFAA depending on the sizes of water clusters and domains having different activity as a solvent. As a whole, the boundary area between immiscible liquids (water and chloroform) should be minimal to obtain minimal Gibbs free energy of the system. The water


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and chloroform molecules are of different sizes that affect their distributions in pores (voids between nanoparticles in their aggregates) of different sizes. Therefore, chloroform can displace water into large voids or narrow voids inaccessible for the CDCl3 molecules. The structural, morphological, textural, and environmental features affect not only the distribution of bound water in voids between NPNP, but also the amounts of SBW (frozen at T < 260 K) and WBW (frozen at 260 K < T < 273 K), and SAW (δH = 4-6 ppm) and WAW (δH = 1-2 ppm). Despite the changes in many characteristics of cAM1, it demonstrates the flotation effect after drying in air. Therefore, the developed system with cAM1/bound water could be of interest from a practical point of view due to stronger interactions with aqueous surroundings depending on maintain conditions of wetted or wetted-dried cAM1.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +38044 4229627. ORCID Volodymyr M. Gun’ko: 0000-0001-6333-3441 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

(1) (2) (3) (4) (5) (6) (7)

(8) (9)

REFERENCES Iler, R.K. The Chemistry of Silica. Wiley: Chichester, 1979. Legrand, A.P. (Ed.) The Surface Properties of Silicas. Wiley: New York, 1998. Auner, N.; Weis, J. (Eds) Organosilicon Chemistry VI. From Molecules to Materials. Wiley: Weinheim, 2005. Bergna, H.E. (Ed.) Colloidal Silica: Fundamentals and Applications. Taylor & Francis LLC, Salisbury, 2005. Chuiko, A.A. (Ed.) Chemistry of Silica Surface. UkrINTEI: Kiev, 2001 (in Russian). Blitz, J.P.; Gun'ko, V.M. (Eds.) Surface chemistry in biomedical and environmental science. NATO Science Series II: Mathematics, Physics and Chemistry. Springer: Dordrecht, Vol. 228, 2006. Hongwu International Group Ltd, HWNano. The Difference Between Hydrophilic Silica And Hydrophobic Silica Nanoparticles. http://www.hwnanomaterial.com/the-difference-betweenhydrophilic-silica-and-hydrophobic-silica-nanoparticles_p255.html AEROXIDE® – Fumed Metal Oxides, Technical Overview 13, Evonik Resource Efficiency GmbH, 2017. AEROSIL® and SIPERNAT® Products in Powder Detergents, TECHNICAL INFORMATION 1417, Evonik, 2017.


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(10) Improving Performance of Milled Powder with AEROSIL® Fumed Silica and SIPERNAT® Specialty Silica. TECHNICAL INFORMATION 1410, Evonik Resource Efficiency GmbH, 2017. (11) SIPERNAT® specialty silica and AEROSIL® fumed silica as flow aid and anticaking agent, Technical Information 1351, Evonik Resource Efficiency GmbH, 2015. (12) Successful Use of AEROSIL Fumed Silica in Liquid Systems, Technical Information 1279. Evonik Resource Efficiency GmbH, 2009. (13) Guo, Z.; Huang, Z.; Wang, Y.; Wang, D.; Han, M.-Y.; Yang, W. Phase Engineering of Hydrophobic Meso-Environments in Silica Particles for Technical Performance Enrichment. Langmuir 2018, 34, 7428-7435 DOI: 10.1021/acs.langmuir.8b01040. (14) Fukada, K.; Kawamura, N.; Shiratori, S. Trace Material Capture by Controlled Liquid Droplets on a Superhydrophobic/Hydrophilic Surface. Anal. Chem. 2017, 89(19), 10391–10396 DOI: 10.1021/acs.analchem.7b02369. (15) Ishida, N.; Matsuo, K.; Imamura, K.; Craig, V. S. J. Hydrophobic Attraction Measured between Asymmetric Hydrophobic Surfaces. Langmuir 2018, 34(12), 3588–3596 DOI: 10.1021/acs.langmuir.7b04246. (16) Martínez-Gómez, A.; López, S.; García, T.; de Francisco, R.; Tiemblo, P.; García, N. Long-Term Underwater Hydrophobicity: Exploring Topographic and Chemical Requirements. ACS Omega 2017, 2 (12), 8928–8939 DOI: 10.1021/acsomega.7b01717. (17) Min, J.; Baek, S.; Somasundaran, P.; Lee, J. W. Anti-Adhesive Behaviors between Solid Hydrate and Liquid Aqueous Phase Induced by Hydrophobic Silica Nanoparticles. Langmuir 2016, 32(37), 9513– 9522 DOI: 10.1021/acs.langmuir.6b02729. (18) Gun’ko, V.M.; Turov, V.V. Nuclear Magnetic Resonance Studies of Interfacial Phenomena. CRC Press: Boca Raton, 2013. (19) Henderson, M.A. Interaction of water with solid surfaces: fundamental aspects revisited. Surf. Sci. Report. 2002, 46, 1-308. (20) Birdi, K.S. (Ed.) Handbook of Surface and Colloid Chemistry. Third edition. CRC Press: Boca Raton, 2009. (21) Brovchenko, I.; Oleinikova, A. Interfacial and Confined Water. Elsevier: Amsterdam, 2008. (22) Martínez Casillas, D. C.; Longinotti, M. P.; Bruno, M. M.; Vaca Chávez, F.; Acosta, R. H.; Corti, H. R. Diffusion of Water and Electrolytes in Mesoporous Silica with a Wide Range of Pore Sizes. J. Phys. Chem. C 2018, 122, 3638-3647 DOI: 10.1021/acs.jpcc.7b11555. (23) Al-Abadleh, H.A.; Grassian, V.H. Oxide surfaces as environmental interfaces. Surf. Sci. Report. 2003, 52, 63–161. (24) Forny, L.; Saleh, K.; Pezron, I.; Komunjer, L.; Guigon, P. Influence of mixing characteristics for water encapsulation by self-assembling hydrophobic silica nanoparticles. Powder Technol. 2009, 189, 263269. (25) Kelesidis, G. A.; Furrer, F. M.; Wegner, K.; Pratsinis, S. E. Impact of Humidity on Silica Nanoparticle Agglomerate Morphology and Size Distribution. Langmuir 2018, in press, DOI: 10.1021/acs.langmuir.8b00576. (26) Turov, V.V.; Gun’ko, V.M.; Pakhlov, E.M.; Krupska, T.V.; Tsapko, M.D.; Charmas, B.; Kartel, M.T. Influence of hydrophobic nanosilica and hydrophobic medium on water bound in hydrophilic components of complex systems. Colloids and Surfaces A: Physicochem. Eng. Aspects 2018, 552, 39– 47. (27) Fujii, S.; Yokoyama, Y.; Nakayama, S.; Ito, M.; Yusa, S. I.; Nakamura, Y. Gas Bubbles Stabilized by Janus Particles with Varying Hydrophilic-Hydrophobic Surface Characteristics. Langmuir 2018, 34 (3), 933–942 DOI: 10.1021/acs.langmuir.7b02670. (28) Binks, B.P.; Murakami, R. Phase inversion of particle-stabilized materials from foams to dry water. Nature Materials 2006, 5, 865–869. 21 ACS Paragon Plus Environment

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(29) Mironyuk, I.F.; Gun'ko, V.M.; Zarko, V.I. System with hydrophilic and hydrophobic silicas - water. Report. NAS Ukr. 1999, N3, 149-154 (in Russian). (30) Simovic, S.; Prestidge, C. A. Hydrophilic Silica Nanoparticles at the PDMS Droplet - Water Interface. Langmuir, 2003, 19 (9), 3785–3792. DOI: 10.1021/la026803c (31) Diab, R.; Canilho, N.; Pavel, I. A.; Haffner, F. B.; Girardon, M.; Pasc, A. Silica-Based Systems for Oral Delivery of Drugs, Macromolecules and Cells. Adv. Colloid Interface Sci. 2017, 249, 346–362 DOI: 10.1016/j.cis.2017.04.005. (32) Butler J. A.V. The energy and entropy of hydration of organic compounds. Trans. Faraday Soc. 1937, 33, 229-238. (33) Yaminsky, V.V.; Vogler, E.A. Hydrophobic hydration. Curr. Opin. Colloid Interface Sci. 2001, 6, 342-349. (34) Chandler, D. Interfaces and the driving force of hydrophobic assembly. Nature 2005, 437, 640-647. (35) Imai, T.; Hirata, F. Hydrophobic effects on partial molar volume. J. Chem. Phys. 2005, 122, 094509. (36) Imai, T.; Hirata, F. Reply to "Comment on 'Hydrophobic effects on partial molar volume'". J. Chem. Phys. 2005, 123, 167104. (37) Snyder, P.W.; Lockett, M.R.; Moustakas, D.T.; Whitesides, G.M. Is it the shape of the cavity, or the shape of the water in the cavity? Europ. Phys. J. - Special Topics 2014, 223, 853-889. (38) Gun'ko, V.M.; Turov, V.V.; Bogatyrev, V.M.; Charmas, B.; Skubiszewska-Zięba, J.; Leboda, R.; Pakhovchishin, S.V.; Zarko, V.I.; Petrus, L.V.; Stebelska, O.V.; Tsapko, M.D. Influence of partial hydrophobization of fumed silica by hexamethyldisilazane on interaction with water. Langmuir 2003, 19, 10816-10828. (39) Milionis, A.; Loth, E.; Bayer, I. S. Recent Advances in the Mechanical Durability of Superhydrophobic Materials. Adv. Colloid Interface Sci. 2016, 229, 57–79 DOI: 10.1016/j.cis.2015.12.007. (40) Gun'ko, V.M.; Voronin, E.F.; Mironyuk, I. F.; Leboda, R.; Skubiszewska-Zięba, J.; Pakhlov, E.M.; Guzenko, N.V.; Chuiko, A.A. The effect of heat, adsorption and mechanochemical treatments on stuck structure and adsorption properties of fumed silicas. Colloids Surf. A: Physicochem. Eng. Aspects 2003, 218, 125–135. (41) Strange, J.H.; Rahman, M.; Smith, E.G. Characterization of porous solids by NMR. Phys. Rev. Lett. 1993, 71, 3589–3591. (42) Mitchell, J.; Webber, J.B.W.; Strange, J.H. Nuclear magnetic resonance cryoporometry. Physics Reports 2008, 461, 1–36. (43) Glushko, V.P. (Ed.) Handbook of thermodynamic properties of individual substances. Nauka: Moscow, 1978 (in Russian). (44) Gregg, S.J.; Sing, K.S.W. Adsorption, Surface Area and Porosity. 2nd ed. Academic Press: London, 1982. (45) Adamson, A.W.; Gast, A.P. Physical Chemistry of Surface. 6th edition, Wiley: New York, 1997. (46) Gun'ko, V.M. Composite materials: textural characteristics. Appl. Surf. Sci. 2014, 307, 444–454. (47) Pujari, P. K.; Sen, D.; Amarendra, G.; Abhaya, S.; Pandey, A. K.; Dutta, D.; Mazumder, S. Study of pore structure in grafted polymer membranes using slow positron beam and small-angle X-ray scattering techniques. Nuclear. Instr. Method Phys. Res. B 2007, 254, 278–282. (48) Gun’ko, V. M.; Turov, V. V.; Zarko, V. I.; Goncharuk, O. V.; Pahklov, E. M.; Skubiszewska-Zięba, J.; Blitz, J. P. Interfacial phenomena at a surface of individual and complex fumed nanooxides. Adv. Colloid Interface Sci. 2016, 235, 108–189. doi:10.1016/j.cis.2016.06.003. (49) Mallamace, F.; Corsaro, C.; Broccio, M.; Branca, C.; González-Segredo, N.; Spooren, J.; Chen, S.-H.; Stanley, H.E. NMR evidence of a sharp change in a measure of local order in deeply supercooled confined water. Proc. Natl. Acad. Sci. USA 2008, 105, 12725-12729.


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Figure 1. SEM images of (a) AM1 and (b) cAM1 (scale bar 1 µm). 230x107mm (300 x 300 DPI)


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Figure 2. (a) Particle size distributions and (b) incremental pore size distributions calculated using SAXS data. 77x33mm (300 x 300 DPI)


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Figure 3. Incremental pore (voids between nonporous nanoparticles) size distributions of unmodified A-300 (curve 1) and initial AM1 (2) calculated using nitrogen adsorption data. 72x62mm (300 x 300 DPI)


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Figure 4. Thermogravimetry (TG curves 1, 2, and 3 and DTG curves 4, 5, and 6) and DTA (curves 7, 8, and 9) data for AM1 (curves 1, 4, and 7), AM1 compacted with water (2, 5, and 8), and unmodified nanosilica A300 (3, 6, and 9). 75x59mm (300 x 300 DPI)


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Figure 5. (a) Infrared spectra of thin pellets (pressed at 1300 or 1500 atm) with AM1 initial (curve 1), preheated at 125oC for 2.5 hours (2), compacted with water and pressed into a thin pellet at 1300 atm (3), and nanosilica A-300 unmodified (4) and preheated at 450 oC for 1 hour (5); (b) model of dry AM1 nanoparticle (see models hydrated AM1 particles in ESI file). 70x33mm (300 x 300 DPI)


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Figure 6. 1H NMR spectra of hydro-compacted cAM1 at h = 1 g/g (a) (solid lines) in air and (b) in CDCl3, hydro-compacted cA-300 at h = 1.125 g/g (a) (dotted-dashed lines) in air and (c) in CDCl3; inserts (b, c) show the spectra at high temperatures. 222x515mm (300 x 300 DPI)


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Figure 7. NMR cryoporometry calculations of (a, c) differential and (b, d) incremental size distributions of pores filled by unfrozen water bound to hydro-compacted cAM1 at h = (a, b) 1 g/g and (c, d) 1.4 g/g and hydro-compacted cA-300 (a, b) at h =1.125 g/g located in air, CDCl3 or CDCl3/TFAA (4:1). 147x121mm (300 x 300 DPI)


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Figure 8. Chemical shifts δH of unfrozen water in the 1H NMR spectra of (a) cA-300 (h = 1.125 g/g) and cAM1 at h = (a) 1 g/g and (c) 1.4 g/g located in air and chloroform or CDCl3/TFAA media vs. T, and (b, d) corresponding functions s(T) = -T(d(lnδ(T))/dT)P vs. T. 149x126mm (300 x 300 DPI)


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TOC A simple way to prepare strongly hydrated hydrophobic nanosilica powder. WATER INTERACTIONS WITH HYDROPHOBIC VERSUS HYDROPHILIC NANOSILICA Volodymyr M. Gun’ko, Volodymyr V. Turov, Evgeniy M. Pakhlov, Tetyana V. Krupska, Mykola V. Borysenko, Mykola T. Kartel, Barbara Charmas

221x235mm (72 x 72 DPI)


water interactions with hydrophobic versus hydrophilic nanosilica

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