Combined Nitrogen, Hexane, and Benzene Adsorption

Jul 18, 2014 - Boynton Beach, FL, USA). The isotherms were measured over a relative pressure p/p0 range from 0.01 to 0.995 and, on average, consisted ...
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Combined Nitrogen, Hexane, and Benzene Adsorption Characterization of Pores and Surfaces of Lyophobic Mesoporous Silicas T. M. Roshchina,† N. K. Shonija,† F. Bernardoni,‡ and A. Y. Fadeev*,‡ †

Department of Chemistry, M.V. Lomonosov Moscow State University, Moscow 119991, Russian Federation Department of Chemistry and Biochemistry, Seton Hall University, South Orange, New Jersey 07079, United States

‡

S Supporting Information *

ABSTRACT: For lyophobic porous surfaces, structural analysis by vapor adsorption is complicated due to weak adsorbate−adsorbent interactions and limited wetting of the pores (nonzero contact angles). To investigate further, adsorption isotherms of three distinct adsorbates (nitrogen - 77 K, n-hexane and benzene - 298 K) were studied for SBA-15 ordered mesoporous silica where the surface was functionalized with lyophobic perfluoroalkyl groups (C6F13 termini). The results demonstrated a clear advantage of the combined use of the adsorption isotherms of less surface sensitive (nitrogen) and more surface sensitive (hydrocarbons) adsorbates. The adsorption of nitrogen provided basic structural characteristics like surface area, pore volume, and pore size distribution, while the isotherms of benzene and n-hexane were used to characterize wetting (contact angles) and surface energy of the C6F13 surfaces within the pores. For the first time, the statistical film thickness for nitrogen, benzene, and n-hexane are being reported for the adsorption on fluorinated surfaces, thereby providing critical data for the pore size and the contact angle determination of the lyophobic materials.

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adsorption characterization of fluorinated surfaces challenging, as polymolecular adsorption and capillary condensation, especially for organic vapors, can be complicated due to incomplete wetting of the pores by the liquid adsorbate. Prior to this work, quantitative evaluation of pore wetting (nonwetting) and the analysis of pore size distribution for fluorinated materials was not possible due to the unknown values for statistical film thickness for common adsorbates (including nitrogen) on these surfaces. In this work, we report the results of a combined adsorption study that includes nitrogen (77 K), n-hexane, and benzene (298 K) for the characterization of SBA-15 silica grafted with perfluorohexyl (C6F13) groups. The use of different adsorbates allowed for the simultaneous characterization of pore structure and the surface properties of the pores (contact angles and surface energy). For the first time, the statistical film thickness for nitrogen, benzene, and n-hexane are being reported for the adsorption on model wide-pore silica grafted with C6F13 groups, thereby providing critical data for the pore size determination of the lyophobic materials.

INTRODUCTION Low-energy lyophobic materials with developed surfaces that are supported on dispersed and porous substrates are traditionally of interest for water purification, waterproofing, and separations.1,2 In the recent years, there have been a number of emerging applications of functionalized porous and textured materials for superhydrophobic and antifrost coatings,3 heat transfer,4 and energy storage.5 Last but not least, lyophobic materials with well-defined pore structure and surface chemistry are suited extremely well for testing physical models in adsorption,6,7 capillarity, and wetting.8,9 The adsorption characterization of hydrophobic surfaces has been the focus of researchers for a long time; for a review of early works, see ref 10. Over the past decade, the adsorption of nitrogen and organic vapors on monolayers of alkyl,11,12 dimethylsiloxane,13,14 and fluorinated alkyl15 groups chemically grafted to silicas and metal oxides has been reported. The reference adsorption isotherm for nitrogen on hydrophobic (C18H37) silicas has been made available,16 and the use of nitrogen adsorption for the characterization of ordered functionalized silicas has been reviewed.17 The published data show that lyophobization of mineral surfaces (the replacement and/or screening of polar groups with nonpolar organic groups) results in a notable reduction in the energy of adsorption interactions as compared to bare substrates. Among the variety of lyophobic surfaces, surfaces of fluoroalkyl groups occupy a special place due to their low values of surface energy,18 and the fact that these surfaces generally have the least amount of interactions with the adsorbate. This makes the © 2014 American Chemical Society

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EXPERIMENT

Silicas. The preparation of silicas (SBA-15 and S-120), their functionalization through the reaction with ClSi(CH3)2(CH2)2C6F13, and the chemical analysis data for the C6F13-grafted silicas are detailed in the Supporting Information. The structural characteristics of the Received: February 20, 2014 Revised: July 17, 2014 Published: July 18, 2014 9355

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hydroxylated S-120 and SBA-15 were determined by nitrogen adsorption. S-120: surface area - 100 m2/g, total pore volume - 1.27 cm3/g, average pore diameter - ranged from 35 to 50 nm. SBA-15: surface area - 389 m2/g, total pore volume - 0.89 cm3/g, average pore diameter - 8.3 nm. Prior to hydroxylation, S-120 was free from the micropores yet SBA-15 showed some microporosity (Vμ ≈ 0.05 cm3/ g). After the hydroxylation (see the Supporting Information for details), both silicas were essentially free from the micropores (Vμ < 0.01 cm3/g), as was verified by the t-plots and alpha-s methods. No micropores were detected in the C6F13 silicas. Adsorption Isotherms. Table 1 presents physicochemical properties of the adsorbates used in this study. The adsorption−desorption

Table 1. Properties of the Adsorbates

adsorbate

T (K)

p0 (Torr)

crosssectional area (nm2)

nitrogen benzene n-hexane

77 298 298

760 95 150

0.16224 0.49 0.52

a

a

molar volume (cm3/mol)

surface tension (mJ/m2)

γVm/RT (nm)

34.7 89.3 131.3

8.88 28.18 17.91

0.48 1.015 0.949

At the temperature of the adsorption measurements.

isotherms of benzene and n-hexane (298 K) were obtained by the static weight method using a custom-built vapor adsorption system equipped with a McBain−Bakr quartz spring microbalance. The isotherms were measured over a relative pressure p/p0 from 0.01 to 0.999 and, on average, consisted of 60−80 adsorption−desorption points. The adsorption−desorption isotherms of nitrogen (77 K) were obtained with an Autosorb-1 analyzer (Quantachrome Instruments, Boynton Beach, FL, USA). The isotherms were measured over a relative pressure p/p0 range from 0.01 to 0.995 and, on average, consisted of 50−60 adsorption−desorption points. Prior to all the adsorption measurements, the silicas were degassed at 423 K overnight. Specific surface areas were calculated via the Brunauer− Emmett−Teller (BET) method using the range of relative pressure from 0.06 to 0.20. The total pore volume of the sample was obtained as V∑ = amaxVm, where amax is the amount adsorbed at saturation (p/p0 → 1) and Vm is the molar volume of liquid adsorbate at the temperature of the measurements. The pore size distribution and average pore diameter were calculated with the Barrett−Joyner− Halenda (BJH) algorithm using the desorption branch of the adsorption−desorption hysteresis. The diameters of the menisci dm were calculated from the desorption branch of the isotherms in the capillary condensation region using the Kelvin equation (eq 3, R&D). For the adsorption of nitrogen on bare silica and C6F13 silica as well as for the adsorption of benzene and n-hexane on bare silicas, full wetting was assumed (cos θ = 1) and the diameter of the core dc was equal to dm.

Figure 1. Adsorption−desorption isotherms of nitrogen (77 K), nhexane (298 K), and benzene (298 K) for bare SBA-15 (squares) and SBA-15-C6F13 (triangles). Solid points - desorption. Dashed horizontal lines - monolayer capacity for bare SBA-15 by a given adsorbate. The isotherms for n-hexane and nitrogen are offset by 10 and 17 mmol/g, respectively.

adsorbates. The reduction of the intensity of the adsorption interactions was also demonstrated through the decrease of the constant C of the BET equation, which characterized the enthalpy of adsorption20 (Table 2). Another indication of extremely weak adsorption interactions of the fluoroalkyl surfaces was obtained by comparing the relative pressures required to reach the adsorption equal to the monolayer coverage. As shown in Figure 1, the adsorption of the organic adsorbates on SBA-15-C6F13 stayed below the monolayer capacity am (calculated from the cross-sectional area of that particular adsorbate) up until p/p0 ∼ 0.6−0.7. This pressure range was unusually high for the monolayer adsorption, e.g., in comparison with p/p0 ∼ 0.2 which was observed for the monolayer coverage of these adsorbates on bare silica. However, perhaps the most notable and unusual feature of the adsorption on the SBA-15-C6F13 was observed for n-hexane and benzene in the region of the capillary−condensation hysteresis. For SBA-15-C6F13 as compared to SBA-15, the average pore size must be reduced due to the surface grafting, and thus, a shift of the hysteresis loop to a lower pressure was anticipated. On the contrary, the hysteresis loop shifted not to lower but to higher relative pressures (Figure 1). The formal

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RESULTS AND DISCUSSION Adsorption Isotherms of Nitrogen, n-Hexane, and Benzene. The adsorption−desorption isotherms for bare SBA15 and SBA-15-C6F13 are shown in Figure 1. For all of the adsorbates studied, the adsorption isotherms on fluorinated silica laid well below the isotherms of bare silica, indicating a significant reduction of the adsorption interactions due to the presence of perfluoroalkyl groups. Quantitative analysis of this point is given in the Supporting Information, where the reduction in the adsorption interactions is demonstrated upon the comparison of the standard isotherms, i.e., presented as μmol/m2. Since the reduction of the adsorption interactions was observed for all the adsorbates, we concluded that grafting of the perfluoroalkyl groups to silica reduced its dispersive and electrostatic interactions19 (probed by n-hexane and N2) as well as hydrogen bonding19 (probed by benzene) with the 9356

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Table 2. Structural Characteristics of the SBA-15 and SBA-15-C6F13 as Determined from the Adsorption Isotherms adsorbate N2 n-C6H14 C6H6

silica

SBETa (m2/g)

CBET

Vpa (cm3/g)

dc or dm (nm)

dp (nm)

τ0b (nm)

σc (nm)

SBA-15 SBA-15-C6F13 SBA-15 SBA-15-C6F13 SBA-15 SBA-15-C6F13

389 294 (352) 271 120 (144) 294 117 (140)

167 35 5 3 16 11

0.89 0.64 (0.77) 0.90 0.56 (0.67) 0.90 0.55 (0.66)

6.7 5.7 7.7 9.0 8.0 11.2

8.3 7.2 8.8

0.79 0.76 0.55 0.25 0.50 0.23

0.354

9.0

0.4 0.37

For SBET and Vp, values in parentheses are normalized per 1 g of silica. bτ0 - statistical thickness of adsorbed film at the average pore diameter. cσ thickness of a single adsorbed layer.

a

Alternatively, τ can be obtained from the standard adsorption isotherms on the nonporous hydroxylated silica; these data are summarized for nitrogen20−22 and benzene,21,23 respectively. At the beginning of this work, the applicability of the equations shown above as well as the validity of the standard isotherms of nitrogen and benzene for the adsorption on fluorinated surfaces was unknown. Also, to the best of our knowledge, no reliable data on the standard isotherms were available for n-hexane. To obtain this data directly from the experiment, the adsorption isotherms for all of the adsorbates used in this work were measured for two model silicas: bare Silokhrom S-120 and S-120 grafted with C6F13 groups. The thickness of the adsorbed films was calculated by the known equation22 τ = aVm/SBET, where a is the amount adsorbed (mol/g), Vm is the molar volume of adsorbate, and SBET is the BET surface area of silica24 determined by N2. The S-120 silica has been studied in our previous works;13,15 it was a high purity material with a uniform surface and no micropores. The difference in the adsorption of hydrocarbons (per unit area) on bare S-120 and SBA-15 as well as on S-120-C6F13 and SBAC6F13 did not exceed 20% (p/p0 < 0.5), indicating a similar chemical nature of these surfaces. Due to the relatively large pores (dpore ∼ 35−50 nm), the capillary condensation for S-120 and S-120-C6F13 did not begin until p/p0 ∼ 0.8, thus providing a wide range of pressures for the determination of the adsorbed film thickness. Figure 3 summarizes the τ data for the adsorption of nitrogen, benzene, and n-hexane on bare silica (compiled from refs 20−22 along with data obtained in this work) and C6F13 silicas (this work). The thickness of the adsorbed film for nitrogen was not very sensitive to the surface chemistry: values of τ for bare and C6F13 silicas were almost identical. Similar observations about the poor sensitivity of nitrogen adsorption to the chemical nature of the adsorbent have been made earlier by several groups.16,21,22 Analyzing the data in Figure 3, we concluded that, for the adsorption of nitrogen on SBA-C6F13, τ could be determined either through eq 1 or using the standard adsorption isotherm.20−22 For the adsorption of hydrocarbons, however, the picture was quite different: the thickness of the adsorbed films varied significantly depending on the chemical nature of the surface. For both benzene and n-hexane, the values of τ on C6F13 silica were much lower than those on bare silica. At p/p0 ∼ 0.7, the point corresponding to the maximal change of the pore volume during the pore emptying for the SBA-C6F13, τ for benzene and n-hexane were less than 0.3 nm. This was less than 0.37 and 0.4 nmthe van der Waals thickness of the benzene ring and the diameter of the CH3 group in alkanes, respectively.23 Such low values of the adsorbed film thickness suggested that, during the pore emptying process in the C6F13 silicas, the menisci of the condensed adsorbates were in contact with partially uncovered

use of the Kelvin equation with these isotherms resulted in a gross overestimation of the pore size, showing ∼20−40% increase in the pore diameter for SBA-15-C6F13 vs parental SBA-15. This was clearly incorrect and merely indicated failure of the commonly made assumption about complete wetting (cos θ = 1) of the pore walls by the condensing adsorbate. We believe that, for the capillary condensation of hydrocarbons in the C6F13 pores, wetting was incomplete (cos θ < 1) and the diameter of the meniscus dm was greater than the core diameter dc (Figure 2). The magnitude of the shift of the hysteresis loop

Figure 2. PSD curves obtained from the benzene isotherms: (1) PSD by the core diameter dc for bare SBA-15; (2) PSD by the pore diameter dp for bare SBA-15; (3) PSD by the diameter of meniscus dm for SBA-15-C6F13.

was dependent on the nature of the adsorbates: the largest shift was observed for benzene, then n-hexane, while for nitrogen the shift was virtually absent. This trend was consistent with the range of surface tension of the liquid adsorbates (mJ/m2), 8.88 (77 K, N2) < 17.91 (n-C6H14, 298 K) < 28.18 (298 K, C6H6), and consequently with the gradual decrease of the pore wetting in that range. Statistical Thickness of the Adsorbed Films on C6F13 Surfaces. In the analysis of the adsorption isotherms, and especially in the determination of the pore sizes, the knowledge of the statistical thickness of the adsorbed film τ (nm) over the range of relative pressures is of prime importance. The following equations are used for nitrogen20,21 τ = 10−1

and benzene τ = 10−1

13.99 0.034 − log p/p0

(1)

5.924 −0.0156 − log p /p0

(2)

22

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h = (VSBA − VSBA − C6F13)/SSBA

(5)

We noted that both values of thickness were notably smaller than the length of the fully stretched grafted groups (∼1.2 nm)26 and were comparable to the van der Waals diameter of the CF3 group,23 0.54 nm, suggesting a horizontal orientation of grafted groups on the surface. By nitrogen, the total pore volume (V) and the specific surface area (S) of the SBA-C6F13 reduced, respectively, by 15 and 11% as compared to bare SBA15. Using the model of cylindrical pores with the uniformly grafted layer, the structural parameters of bare and modified silicas were related as follows:27 2 ⎛ d C F ⎞2 ⎛ 2h ⎞ VC6F13 = VSBA ⎜ 6 13 ⎟ = VSBA ⎜1 − ⎟ dSBA ⎠ ⎝ ⎝ dSBA ⎠

SC6F13 = SSBA

surfaces of pores. Conceivably, the absence of the continuous films of the adsorbate impeded wetting (increased the contact angle and dm), thereby resulting in a shift of the capillary condensation loop to higher p/p0 as compared to bare silica. Characterization of Pore Structure and Surfaces Properties of C6F13 Silica. Further, we used the combined data obtained from the nitrogen, benzene, and n-hexane adsorption isotherms to characterize pore structure and wetting in lyophobic pores. As a starting point of this analysis, we assumed that, unlike hydrocarbons, nitrogen fully wetted the C6F13 surfaces (cos θ = 1). The validity of this assumption was based on the adsorbed film thickness data for the C6F13 surfaces. In the range of the relative pressures corresponding to the capillary condensation in the SBA-C6F13 (p/p0 ∼ 0.7− 0.8), the thickness of the adsorbed film of nitrogen was approximately two monolayersthe amount generally considered sufficient for complete wetting by liquid adsorbate.21 Using the capillary condensation data for nitrogen (desorption branch), the core diameters (dc) of the SBA-15-C6F13 were determined directly from the Kelvin equation: 4γVm RT ln(p0 /p)

(3)

dNp 2

dNc 2

Pore diameters were obtained as = + 2τN2. Average pore diameter, pore volume, and surface area data for bare and C6F13 grafted SBA-15 are given in Table 2. From the reduction of the average pore diameter due to grafting h = 0.5(dSBA − dSBA − C6F13)

dSBA

⎛ 2h ⎞ = SSBA ⎜1 − ⎟ dSBA ⎠ ⎝

(7)

These equations used VC6F13 and SC6F13 that were normalized to 1 g of silica (Table 2, values in parentheses). Using two different values for the thickness of the grafted layer (0.31 and 0.6 nm), eqs 6 and 7 predicted 16 and 32% reduction in the pore volume and 8 and 15% reduction in the surface area for SBA-C6F13 as compared to bare SBA-15, i.e., in satisfactory agreement with the values observed experimentally. The isotherms of benzene and n-hexane, along with the structural data obtained by nitrogen, were used to assess wetting and surface energy of the C6F13 pores. The contact angles of hydrocarbons in the C6F13 pores were evaluated using dc = dm cos θ. The dm values were determined from the isotherms by the Kelvin equation. The dc values were evaluated by different methods listed below (all used the most probable values of dc): (1) In the simplest and crudest approach, dc of SBA-C6F13 was assumed equal to dc of bare SBA-15 determined by n-hexane and benzene (all the raw data used for the calculations are given in Table 1). (2) dc of SBA-C6F13 was determined as dp-2τ, where dp was the BJH pore diameter of SBA-C6F13 by nitrogen and τ was the adsorbed film thickness for benzene or n-hexane on model C6F13 silica. (3) dc of SBA-C6F13 was determined as dp*-2τ, where dp* was the pore diameter of SBA-C6F13 calculated by eq 6 using the pore volume of SBA-C6F13 and the pore volume and the BJH pore diameter of bare SBA-15 by benzene and n-hexane, respectively. (4) Similar to approach 3 except the pore diameter of SBAC6F13 was determined as dp** = 4V/S, where V was the pore volume of SBA-C6F13 by benzene and n-hexane and S was the surface area of SBA-C6F13 by nitrogen. The results of the contact angle calculations are summarized in Table 3. With the exception of method 1, which gave a notably smaller value, the contact angles by all other methods were close to each other. The contact angles increased with an increase of the surface tension of the liquid, and for both benzene and n-hexane, the values obtained were in a reasonable agreement with the contact angles determined by the sessile drop technique for the planar silica surface (Si wafer) grafted with the C6F13 groups (Supporting Information). Using the average values (methods 2−4) for the contact angles, the free surface energy of the C6F13 surfaces was

Figure 3. Statistical thickness of the adsorbed films vs p/p0 for different adsorbates on bare silica and C6F13 silica (this work unless otherwise stated): (1) N2 on S-120, (2) N2 on S-120-C6F13, (3) N2 ref 20, (4) N2 by eq 1, (5) N2 ref 21, (6) benzene on S-120, (7) benzene ref 23, (8) benzene by eq 2, (9) n-hexane on S-120, (10) and (11) benzene and n-hexane on S-120-C6F13.

dcN2 = dmN2 =

dC6F13

(6)

(4)

the thickness of the grafted layer (h) was determined to be 0.6 nm. Alternatively, the thickness of the C6F13 layer was determined to be 0.31 nm from the reduction of the pore volume due to grafting (normalized to 1 g of silica) and the surface area of bare silica:11 9358

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Langmuir Table 3. Values of cos θ and θ for Benzene and n-Hexane on the C6F13 Surfaces

(2)

cos θ θ (deg)

0.71 44

0.60 53

cos θ θ (deg)

0.85 31

0.74 42

(3)

(4)

benzene 0.64 0.63 50 51 n-hexane 0.79 0.79 38 38

sessile drop data for the C6F13/Si wafers, adv/rec

0.87/0.91 30/24

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CONCLUSION The results of this work demonstrated the advantage of the combined use of adsorption isotherms of less surface sensitive (nitrogen) and more surface sensitive (hydrocarbons) adsorbates for characterization of the lyophobic mesopores. The adsorption isotherms of nitrogen provided basic structural characteristics like surface area, pore volume, and pore size distribution, while the isotherms of benzene and n-hexane provided valuable insight in wetting and surface properties of the C6F13 silicas. ASSOCIATED CONTENT

S Supporting Information *

Standard adsorption isotherms (μmol/m2), the details on the preparation of the C6F13 silicas, and the contact angle measurements for the C6F13 monolayers supported on Si wafers. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Plueddemann, E. P. Silane coupling agents, 2nd ed.; Plenum: New York, 1991. (2) Ali, I.; Al-Othman, Z. A.; Nagae, N.; Gaitonde, V. D.; Dutta, K. K. Recent Trends in Ultra-fast HPLC: New Generation Superficially Porous Silica Columns. J. Sep. Sci. 2012, 35 (23), 3235. (3) Varanasi, K. K.; Deng, T.; Smith, J. D.; Hsu, M.; Bhate, N. Frost Formation and Ice Adhesion on Superhydrophobic Surfaces. Appl. Phys. Lett. 2010, 97 (23), 234102/1. (4) Jing, T.; Kim, Y.; Lee, S.; Kim, D.; Kim, J.; Hwang, W. Frosting and Defrosting On Rigid Superhydrophobic Surface. Appl. Surf. Sci. 2013, 276, 37. (5) Gokulakrishnan, N.; Parmentier, J.; Trzpit, M.; Vonna, L.; Paillaud, J. L.; Soulard, M. Intrusion/Extrusion of Water into Organic Grafted SBA-15 Silica Materials for Energy Storage. J. Nanosci. Nanotechnol. 2013, 13 (4), 2847. (6) Trens, P.; Denoyel, R.; Glez, J. C. Comparative Adsorption of Argon and Nitrogen for the Characterization of Hydrophobic Surfaces. Colloids Surf., A 2004, 245, 93. (7) Thommes, M.; Morell, J.; Cychosz, K. A.; Fröba, M. Combining Nitrogen, Argon, and Water Adsorption for Advanced Characterization of Ordered Mesoporous Carbons and Periodic Mesoporous Organosilicas. Langmuir 2013, 29, 14893. (8) Tarasevich, Yu. I. State and Structure of Water in Vicinity of Hydrophobic Surfaces. Colloid J. 2011, 73 (2), 257. (9) Helmy, R.; Kazakevich, Y.; Ni, C.; Fadeev, A. Y. Wetting in Hydrophobic Nanochannels: a Challenge of Classical Capillarity. J. Am. Chem. Soc. 2005, 127 (36), 12446. (10) Kiselev, A. V. Adsorption Properties of Hydrophobic Surfaces. J. Colloid Interface Sci. 1968, 28 (3/4), 430. (11) Bernardoni, F.; Fadeev, A. Y. Adsorption and Wetting Characterization of Hydrophobic SBA-15 Silicas. J. Colloid Interface Sci. 2011, 356, 690. (12) Marcinko, S.; Helmy, R.; Fadeev, A. Y. Adsorption Properties of SAMs Supported on TiO2 and ZrO2. Langmuir 2003, 19, 2752. (13) Roshchina, T. M.; Shonia, N. K.; Zubareva, N. A.; Fadeev, A. Y. Adsorption and Wettability of Lyophobic Organo-silicon Monolayers Supported on Silica. Russ. J. Phys. Chem. A 2003, 77 (9), 1482. (14) Kazakevich, Y. V.; Fadeev, A. Y. Adsorption Characterization of Oligo(dimethylsiloxane)-Modified Silicas: An Example of Highly Hydrophobic Surfaces with Non-Aliphatic Architecture. Langmuir 2002, 18 (8), 3117. (15) Gurevich, K. B.; Roshchina, T. M.; Shonia, N. K.; Kustov, L. M.; Ivanov, A. V. Peculiarities of Adsorption of Organic Compounds and Water on Silicas with Bonded Perfluoroalkyl Groups. J. Colloid Interface Sci. 2002, 254 (1), 39. (16) Kruk, M.; Antochshuk, V.; Jaroniec, M.; Sayari, A. New Approach to Evaluate Pore Size Distributions and Surface Areas for Hydrophobic Mesoporous Materials. J. Phys. Chem. B 1999, 103 (48), 10670. (17) Kruk, M.; Jaroniec, M. Gas Adsorption Characterization of Ordered Organic-Inorganic Nanocomposite Materials. Chem. Mater. 2001, 13, 3169. (18) Nishino, T.; Meguro, M.; Nakamae, K.; Matsushita, M.; Ueda, Y. The lowest surface free energy based on −CF3 alignment. Langmuir 1999, 15, 4321. (19) Kiselev, A. V. Intermolecular Interactions in Adsorption and Chromatography; Vysshaya Shkola: Moscow, 1986. (20) Gregg, S. J., Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (21) Karnaukhov, A. P. Adsorption. Texture of Dispersed and Porous Materials; Nauka: Novosibirsk, Russia, 1999. (22) Dubinin, M. M. Capillary Phenomena and Information on the Pore Structure of Adsorbents. In The Modern Theory of Capillarity. The

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calculated by the methods of surface components28 and the equation of state.29 The two methods were in close agreement, giving ∼14 (by n-hexane) and ∼18 mJ/m2 (by benzene) for the free surface energy, which was well within the range of the values reported for Teflon and other fluorinated surfaces.30 The use of n-hexane and benzene isotherms alone for the determination of the surface area and the pore volume of SBAC6F13 was problematic. The BET surface area of SBA-C6F13 by n-hexane and benzene demonstrated ∼2-fold decrease as compared to bare SBA-15. An even larger discrepancy was observed in the surface area of SBA-C6F13 assessed by hydrocarbons vs nitrogen (Table 2). Although the pore volumes of bare SBA-15 assessed by different adsorbates agreed well with each other (Table 2), the pore volumes of SBA-C6F13 demonstrated a notable (∼15−17%) reduction from nitrogen to hydrocarbons. It should be noted here that the pore volume calculations assumed that the liquid density of the adsorbate was not affected by the pore surfaces. This seemed true for wetting systems, but for the nonwetting systems, the density of the adsorbate might be lower than in the bulk due to the loose packing of the molecules at the lyophobic surfaces, thereby leading to the underestimated pore volumes of the SBA-15-C6F13 by the hydrocarbons as compared to those by nitrogen.

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ACKNOWLEDGMENTS

The support from the Seton Hall Research Council and from Merck Inc. is acknowledged.

dc calc. method, see text (1)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 9359

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Centennial Gibbs’ Theory of Capillarity; Rusanov, A. I., Goodrich, F. C., Eds.; Khimia publishers: Leningrad, Russia, 1980; p 100. (23) Zefirov, Yu. V.; Zorky, P. M. New Applications of van der Waals Radii in Chemistry. Russ. Chem. Rev. 1995, 64 (5), 415. (24) As it came up during the review of this manuscript, the statistical film thickness value was tight to the BET surface area. In this regard, it is worth noting that, although the BET method has been adopted as a principal method for the surface area determination of dispersed and porous materials, there is no general consensus on the standard value for the cross-sectional area of nitrogen. The value recommended in the famous monograph by Greg and Sing20 of 0.162 nm2 was challenged by Jelinek and Kovats25 who provided convincing evidence for the new value 0.135 nm2. Although the use of a different value of nitrogen cross section (0.135 vs 0.162 nm2) would result in 20% difference in the BET surface area and the statistical film thickness, respectively, we do not think that the choice of one or the other value would impact the main point of our work, which was the combined use of nitrogen and hydrocarbon adsorption isotherms for the characterization of the pores and contact angles of the lyophobic mesoporous silicas. For the ease of direct comparison with the literature, we used 0.162 nm2the value used unexceptionally in the classical works (ref 20 and citations therein). (25) Jelinek, L.; Kováts, E. sz. True Surface Areas from Nitrogen Adsorption Experiments. Langmuir 1994, 10, 4225. (26) Determined by the ACD Lab software. (27) Unger, K. K. Porous Silica, its Properties and Use as Support in Column Liquid Chromatography; Journal of Chromatography Library, Vol. 16; Elsevier: Amsterdam, The Netherlands, 1979. (28) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Interfacial Lifshitzvan der Waals and Polar Interactions in Macroscopic Systems. Chem. Rev. 1988, 88, 927. (29) Li, D.; Neumann, A. W. Equation of state for interfacial tensions of solid-liquid systems. Adv. Colloid Interface Sci. 1992, 39, 299. (30) Fadeev, A. Y. Hydrophobic Monolayer Surfaces: Synthesis and Wettability. In Encyclopedia for Surface and Colloid Science, 2nd ed.; Somasundaran, P., Ed.; Taylor & Francis: New York, 2006; Vol. 4, p 2854.

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dx.doi.org/10.1021/la500660s | Langmuir 2014, 30, 9355−9360