The Colloid Chemistry of Silica - American Chemical Society

12 Surface Chemistry and Surface Energy of Silicas Downloaded by UNIV LAVAL on October 29, 2015 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0234.ch012

Alain M. Vidal and Eugène Papirer Centre de Recherches sur la Physico-Chimie des Surfaces Solides, Centre National de la Recherche Scientifique, Mulhouse, France

The establishment of relationships between the surface chemistry and the surface free energy of silicas is important for practical applications of these materials. Inverse gas chromatography, either at infinite dilution or finite concentration, appears to be an effective method for the detection of changes of surface properties induced by chemical or thermal treatments. Silicas of various origins (amorphous or crystalline) with surface chemistries modified by chemical (esterification) or heat treatment were compared. The consequences of these modifications on surface energetic heterogeneities were assessed.dd.

SILICA EXISTS IN A BROAD VARIETY OF FORMS, in spite of its simple chemical formula. This diversity is particularly true for divided silicas, each form of which is characterized by a particular structure (crystalline or amorphous) and specific physicochemical surface properties. The variety results i n a broad set of applications, such as chromatography, dehydration, polymer reinforcement, gelification of liquids, thermal isolation, liquid-crystal posting, fluidification of powders, and catalysts. The properties of these materials can of course be expected to be related to their surface chemistry and hence to their surface free energy and energetic homogeneity as well. This chapter examines the evolution of these different characteristics as a function not only of the nature of the silica (i.e., amorphous or crystalline), but also as a function of its mode of synthesis; their evolution upon modification of the surface chemistry of the solids by chemical or heat treatment is also followed. 0065-2393/94/0234-0245$08.00/0 © 1994 American Chemical Society

In The Colloid Chemistry of Silica; Bergna, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

Downloaded by UNIV LAVAL on October 29, 2015 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0234.ch012

246

T H E C O L L O I D CHEMISTRY O F SILICA

Only two kinds of functional groups can be found on silica surfaces: siloxane bridges and hydroxyls (silanols). However, among the hydroxyls, different types can be identified, namely single free silanols, geminal hydroxyls, hydroxyl pairs associated through hydrogen bonding (either vicinal or brought together, for example, at points of contact between particles or in micropores), inner hydroxyl groups, and adsorbed water (1-3). Various methods, either chemical (4-8) or physical (9-25), can be used for the determination of these surface groups, and their number and type can be easily modified by chemical (e.g., esterification upon reaction with alcohols) or heat treatment. However, for heat treatment, as shown by Fripiat (16), the modification of the surface chemical properties is much more complex than would be expected when only considering the curves relating weight loss to temperature. Thus it should be of interest to relate the evolution of surface silanol groups to the surface free energy of silica samples. The surface free energy of a solid (75) can be expressed as a sum of two components: 7 (the dispersive component), describing London-type interactions, and 7S P (the specific component), including all other interac tions (Η-bonding, polar, and so forth). s

d

S

7s

=

7s

d

+

7s

sp

Two methods can be used for the assessment of the 7s of divided solids: contact-angle measurements and adsorption processes. The drawbacks of the contact-angle measurements are associated with surface roughness of the samples. As for the adsorption process, determination of the compo nents of the surface free energy of the solid is based on interpretation of adsorption isotherms, either complete (calculation from spreading pres sures) or only from the first linear part of the isotherm. In this respect, inverse gas chromatography (IGC), which appears to be the technique of choice (17), was extensively used in this study.

Experimental Details Materials.

Eight silicas from different synthesis processes were studied:

1. Five amorphous silicas: • A, a • P, a m /g) • G, a • C, a • F, a

fumed silica, Aerosil 200 (Degussa, 200 m /g) precipitated silica, Zeosil 175 M P (Rhône-Poulenc, 175 2

2

gel of silica, RP1 (Rhône-Poulenc, 230 m /g) colloidal silica, F D R (Rhône-Poulenc, 10 m /g) fibrillar silica (18), F A T M 220/1 (180 m /g) 2

2

2


12.

V I D A L A N D PAPIRER

Surface Chemistry and Surface Energy of Silicas 247

2. Three crystalline silicas: • L i , synthetic (19) H2S1O5 (84 m /g) • L.2» synthetic H-Magadiite (48 m /g) • L3, synthetic H-Kenyaite (18 m /g) A and F were high-purity silicas. A l l samples but A were slightly porous. 2

2

2

Alcohol-Modified Silicas. The silicas were modified by reaction with alco hols, either methanol (Ci) or hexadecanol (Cie), according to a method previously described (8). Degree of esterification was determined b y elemental analysis, microgravimetry (weight loss associated with the pyrolysis of modified silicas), or radiochemistry (use of C-labeled alcohols). T h e corresponding modified silicas were identified as X C i and X C i e .


14

Inverse Gas Chromatography and Heat Treatment of Silicas. Silica particles of adequate size (0.25-0.5 mm i n diameter) obtained by compression and sieving were used to fill chromatographic columns (stainless steel, 30 cm long, 3 mm i n diameter) connected to a gas chromatograph fitted with a flame ionization detector. Helium was used as carrier gas at a flow rate of 20 cm /min [the flow rate conditions were selected so as to have the best efficiency for the chromatographic columns (obtained at the minimum of the Van Deemter curve)]. Measurements were made at a column temperature of 60 °C. Thermal treatment of silica was done under helium flow by heating the column to the desired temperature. After being heated for 30 min, the column was cooled to the analysis temperature. W i t h alkane probes, symmetrical retention peaks were observed. F o r polar probes, skewed peaks were usually recorded; for such peaks an integrator was used to determine the peak firstorder moment. Calculation of 7 and specific interaction parameters from chromatographic data were described elsewhere (20) and are only briefly described here. W h e n minute amounts of solute are adsorbed, the adsorption process can be described by the initial part of the adsorption isotherm, which is practically linear. Under these conditions Henry's law applies. It is then possible to relate the thermodynamic parameters of adsorption, such as the variation i n free energy upon adsorption of the solute at zero coverage (AG°), to the retention volume of the probe (VN): 3

s

AG°

d

=

In ( C ' V )

-RT

N

where R is the gas constant, Τ is the temperature, and C is a constant depending on the reference state for the adsorbed molecule. It is known from earlier studies that AG° varies linearly with the number of carbon atoms of a homologous alkane series. It is thus possible from the preceding equation to calculate the adsorption free energy increment associated with one C H 2 group: AG(CH ) 2

=

In [V (n)/ V (

-RT

N

N

n +

i)]

where VN(n> and VN(n + i) are the retention volumes of n-alkanes with η and η + 1 carbon atoms, respectively. The quantity AG(CH ) can provide an estimation of the dispersive interactions between one C H 2 group and an adsorbent and is related to the V 2

rf

t

h

e

s o l i d

b

y

A

m

e

r

f

c

a

n

C

h

e

m

f

c

a

|

S

o

c

J

e

t

Library 1155 16th St.. N.W. In The Colloid Chemistry of Silica; Bergna, H.; Washington, O.C. Advances in Chemistry; American Chemical Society: Washington, DC,20036 1994.

y

248


AG CH ) (

2

=

6.023

χ

ΙΟ

χ

23

a H ) ( C

2

x

2 [7s 7(CH )] d

2

1/2

where a(cH ) is the surface area of a C H 2 group (0.06 nm ) and 7(CH ) is the surface tension of a surface made of C H 2 groups only, for example, polyethylene (35.6 mj/m at 20 °C). For interactions of polar probes with polar surfaces, the free energy of adsorption can be expressed by 2

2

2

2

AG

=

A G

D

AG

+

sp

where AG and AG*v are the contributions to the free energy of adsorption of the dispersive and specific interactions, respectively. The generally accepted way of separating London dispersion effects from specific effects, in infinite dilution chromatography, is to compare the chosen solute probe with an n-alkane of approximately the same geometry and polarizability. As a consequence, the specific interaction parameter i is obtained by subtracting âG , corresponding to nonspecific interactions, from A G , measured by inverse gas chromatography:


d

d

s p

I

SP

=

AG

S

P

=

AG

-

AG

d

The accuracy of the measurements was equal to ±0.001 min for the retention time, ±0.1 °C for the column temperature, ±20 Pa for the atmospheric pressure, and ±100 Pa for the pressure drop. Therefore, net retention volumes were known with a precision of about 5%. Consequently, the absolute error for free energy of adsorption and for the specific interaction parameter are estimated to be ±0.1 and ±0.2 kl/mol, respectively.

Results and

Discussion

London Component of the Surface Free Energy of HeatTreated Silicas. Figure 1 shows the evolution of 7s for the different types of silicas versus heat-treatment temperature. The origin of the sample as well as the thermal treatments applied are important in determining 7 A Amorphous silicas A, P, and G exhibit similar behaviors, corresponding to an increase of 7 Λ from about 6 0 - 7 0 mj/m at 60 °C to 100 mj/m at 500 °C, followed by a decrease at temperatures up to 700 °C. These complex variations must be associated with complex chemical changes occurring on the surface of silicas upon heat treatment. Colloidal (C) and fibrous (F) silicas have similar general trends characterized by a continuous increase of > up to 600 °C (e.g., sample C; at 600 °C 7s is 160 mj/m ). The evolution of 7s for crystalline silicas is completely different from that of amorphous silicas. Starting from a very high value at low temperatures, it levels off between 200 and 400 °C, then increases again for higher heat-treatment temperatures. The extremely high values obtained, particularly for the L 2 sample (7 is in the 120-470-mJ/m range), raise a fundamental question as they do not have a physical d

2

2

d

d

2

d

s

d


2




12.

Figure 1. Evolution of the dispersive component of the surface free energy of silicas versus heat treatment temperature.

meaning. These results can be associated with the intercalation of the alkane probes between the sheetlike structure of crystalline silicas, a process that was demonstrated by use of branched alkanes. The behavior of sample L i is a mix of that of crystalline and amorphous silicas, because it behaves as a crystalline silica at low temperature (60-200 °C) and as an amorphous silica above 200 °C (this behavior


250


parallels the evolution of its crystalline organization, which is unstable above 200 °C). These curves point not only to the differences but also to the similarities exhibited by amorphous and crystalline samples. The main differences appear essentially at low treatment temperatures, at which the surface chemical properties of both types of silica are very much different. Interpretation of the evolution of 7 versus temperature in amorphous silicas would suggest that the probes interact with siloxane bridges preferentially through dispersive interactions. This explanation was supported by Brinker et al. (12, 13), who identified by S i N M R and Raman spectroscopies two types of silicon-oxygen species on the surface of silicas: tetra (unstrained) and trisiloxane (strained) rings. The trisiloxane rings, nonexistent at low temperature, form at intermediate temperatures and become prevalent in the 3 5 0 - 6 5 0 °C range. At higher temperatures trisiloxane cycles rearrange to yield less-strained cyclotetrasiloxane units. The evolution of the physicochemical characteristics of the surface of amorphous silicas upon thermal treatment can be envisioned as follows. A t room temperature the surface of the solids is covered by a multilayer of water, the external layers of which are eliminated at 30 °C under vacuum. At higher temperatures, only a monomolecular layer of water interacts with the surface (strongly with silanol groups, weakly with siloxane bridges). At about 100 °C, part of the water is evacuated, and thus the exposed surface will be available for interactions with alkane probes, a process associated with an increase of 7s . At 250 °C all of the physically adsorbed water, but not molecules trapped in pores, has been eliminated. The phenomena observed between 250 and 500 °C can probably be attributed to the condensation of vicinal silanols (8), which yield trisiloxane cyclic compounds. At 500 °C, geminal silanols begin to disappear (21) and trisiloxane rings rearrange to yield tetrasiloxane cycles. Thus, above 500 °C the 7 plot may be taken to represent the condensation of geminal and isolated silanols, a process that may be the reason, or the consequence, of a surface rearrangement that occurs at temperatures up to 800 °C (22). s

d


29

d

s

d

Specific Component of the Surface Free Energy of HeatTreated Silicas. Specific interaction capacities of heat-treated silicas, that is, their ability to interact with polar molecules, were examined with chloroform (Lewis acid probe) and toluene and benzene (amphoteric molecules). Figure 2 provides examples of the evolution of the specific interaction parameter i of the different silicas with chloroform as a probe. A l l amorphous silicas except C showed an Isp that decreased with temperature of treatment, with a more or less pronounced step i n the 2 0 0 - 4 0 0 °C temperature range. This evolution, which parallels the silanol content of the solids (23), suggests that I reflects the interaction of the probe with silanol groups. The I of the silicas treated at the highest s p

sp

sp


12.



l (kJ/mole) sp

1

τ

τ

\

•

-

•

-


—•—ψ

- Thermal treatment temperature ,( C) 200

300

,— A

I

;

e

100

—A. 1

400

500

600

700

Figure 2. Evolution of the specific interaction parameter of silicas with chloroform versus heat treatment temperature.

temperature also appear to be quite different; thus, a nonequivalent surface chemical state of the various amorphous samples is implied. In crystalline silicas the variation of I versus temperature is much more complex. L i behaves as an amorphous sample, in agreement with the evolution of its crystalline structure with temperature, whereas L 2 and L 3 show maxima at 450 and 350 °C, respectively. Such a result means that interactions with chloroform are increasing while the total number of surface hydroxyls, which are theoretically responsible for these interac tions, is decreasing. Thus, two antagonistic mechanisms have to be envisioned, one involving intercalation of the probe within the lamellar layers of the crystalline silicas (up to 350 °C; the interlamellar distance is decreased, and walls covered with silanol groups are thus closer—as a consequence their influence on inserted chloroform molecules will i n crease, yielding higher interaction parameters), the other (above 350 °C) associated with the condensation of hydroxyls, resulting in a loss of active sites and thus in a decrease of chloroform interactions [a process confirmed by S i cross-polarization-magic-angle spinning ( C P - M A S ) N M R spectros copy (23)]. Deactivation of silica surfaces by grafting of alkyl chains [esterification with short-chain (Ci) or long-chain (Cie) alcohols] was reported (24) to be associated with strong decreases in 7 as well as I , which for Ci6-modified samples are very close to those exhibited by polyethylene (known to be a sp

29

s

d

sp


252


surface of very low energy). Thus, the behavior of esterified silicas versus temperature was of interest. Evolution of the Surface Free Energy of Esterified Silicas with Heat-Treatment Temperature. Figure 3 shows the evolution versus temperature of 7 of initial and esterified G. The modifications are associated with a decrease of the dispersive component of the surface free energy, a process very much dependent on the number of carbon atoms of the grafted alkyl chain. Thus, the silica reacted with hexadecanol can be considered completely coated by a hydrocarbon layer, whereas the part of the surface that reacted with methanol, which is unhindered by the methyl grafts, remains available for further interactions. The curves also show the thermal stability of the grafted alkyl chains. Starting at about 250 °C, a steep increase of 7s (particularly with Cie-modified silicas) is evident and is probably related to the pyrolysis of the grafted chains. This process seems to be completed by 500 °C, because the curves corresponding to the esterified samples merge with that of initial silica at this temperature. The increase in 7 of the modified silicas can thus be considered to result from a combination of phenomena corresponding to the behavior of ungrafted silicas and to the degradation of the grafted alkyl chains.


s

d

d

s

d

ι

1

1

ιι

ι

1

I

(mJ/m ) 2

110

90

/f

70

\*

.

ι

50 30«

, 100

Thermal treatment temperature ( C ) e

200

300

400

500

600

700

Figure 3. Evolution of the dispersive component of the surface free energy of esterified silicas versus heat treatment temperature. (Reproduced with permis sion from reference 26. Copyright 1990.) Specific interaction parameters are strongly reduced upon surface modification (hexadecylated silicas have I values close to zero). In methanol-reacted silicas, I values decrease when heat-treatment tempera ture increases. This result suggests that after grafting with methanol, some free hydroxyls are still available for further interactions. This scenario is sp

sp


12.




also likely for Ci6-grafted silicas, but for these silicas the residual silanol groups are completely shielded by the grafted chains and are thus inaccessible to the probes. The results obtained with the different silicas point to the importance of the mode of preparation on their surface characteristics and thus on their surface heterogeneity. Understanding of the surface heterogeneity can be attained by calculation of the distribution function of the energy of adsorption of alkane molecules on the surface of the solids. Distribution Function of Adsorption Energy. F r o m chromato graphic data it is possible to relate the amount of solute adsorbed on a solid to the equilibrium pressure and thus to plot its adsorption isotherm. For a heterogeneous surface, the experimentally measured adsorption isotherm can be described as a sum of local isotherms corresponding to different surface-active sites. The isotherm can then be represented by the following integral equation:

where 0(p,e) is the local adsorption isotherm on sites corresponding to an adsorption energy e, 0(e) is the distribution function of adsorption energy, and ρ is pressure (25, 26). Thus, by knowing the complete isotherm and using an approximation of local isotherm, it is possible to calculate 0(e). Because these are physical adsorption processes, only alkane probes were considered. Figure 4 shows the distributions of energy measured on silicas A, G , and L3 with hexane as a solute. The distributions are bimodal and point to the existence of two different types of adsorption site on the surface. Moreover, the distributions appear to be dependent on the nature of the silica. The distribution calculated for silica L3 is much narrower than those of silicas A and G This result is in agreement with the surface topology of hydroxyls, which are known to be homogeneously distributed on the surface of L3, whereas A exhibits a flat but chemically more heterogeneous surface, and G , which has a higher silanol content, is very heterogeneous (23). However, it is difficult to assign the peak correspond ing to a given energy of interaction to a particular type of surface functional group. Nevertheless, the distribution energy curves yielded by initial silica G were compared to those yielded by the same silica grafted with methanol (GCi) and hexadecanol ( G C 1 6 ) . Esterification would be expected to be associated with a complex modification of the silica surface. It appears (Figure 5) that grafting is indeed followed both by a shifting of the distribution curve toward smaller energies (the longer the chains, the larger the shift) and by a decrease of the height of the peak corresponding to the smaller energy. These shifts can be related to a decrease of the



254

T H E C O L L O I D CHEMISTRY O F

SILICA

Figure 4. Distribution of the energy of adsorption of hexane for different silicas. (Reproduced with permission from reference 27. Copyright 1990.) accessibility of the surface due to the steric hindrance of the grafted alkyl chains. Moreover, because the area under the curves is proportional to the number of adsorption sites, the decrease of peak height could be related to the decrease of the number of accessible silanols. As a consequence, the low energy peak could tentatively be linked with the energy of interaction between the alkane probe and surface hydroxyls.

Figure 5. Distribution of the energy of adsorption of hexane for initial and esterified silicas. (Reproduced with permission from reference 27. Copyright 1990.) In The Colloid Chemistry of Silica; Bergna, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

12.



Conclusions The evolution of the surface free energy components of the different samples showed that the physicochemical surface characteristics of silicas and their surface heterogeneity are dependent on the mode of prepara tion. A n approximation of surface heterogeneity was attained by calcula tion of the distribution function of the energy of adsorption of alkane probes on the solid surfaces.

References Downloaded by UNIV LAVAL on October 29, 2015 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0234.ch012

1. Iler, R. K. The Chemistry of Silica; Wiley Interscience: N e w York, 1979.

2. Kondo, S. Nippon Kagaku Kaishi 1985, 6, 1106. 3. Wagner, M . P. Rubber Chem. Technol. 1976, 49, 703. 4. Hertl, W . J . Phys. Chem. 1968, 72, 1248. 5. Armistead, C . G.; Tyler, A . J.; Hambleton, F. H.; Mitchell, S. Α.; Hockey, J. A .

J. Phys. Chem. 1969, 73, 3947. 6. Evans, B ; White, T. E . J. Catal. 1968, 11, 336. 7. Vidal, Α.; Papirer, Ε.; Donnet, J. B. J. Chim. Phys. 1974, 71, 445. 8. Zaborski, M . ; Vidal, Α.; Ligner, G . ; Balard, H . ; Papirer, E . ; Burneau, A .

Langmuir 1989, 5, 447.

9. Morrow, Β. Α.; Cody, I. Α. J. Phys. Chem. 1972, 77, 1465. 10. Hair, M . L . J. Non-Cryst. Solids 1975, 19, 199. 11. Sander, L.; Callis, J. B.; F i e l d , L . R. Anal. Chem. 1983, 55, 1068. 12. Brinker, C . J.; Kirkpatrick, R. J.; Tallant, D . R.; Bunker, B. C . ; Montez, Β. J.

Non-Cryst. Solids 1988, 99, 418.

13. Brinker, C . J.; Tallant, D . R.; Roth, E . P.; Ashley, C . S. J. Non-Cryst. Solids 1986, 82, 117. 14. Maciel, G . E . ; Sindorf, D . W . J. Am. Chem. Soc. 1980, 102, 7606. 15. Miller, M . L.; Linton, R. W . ; Maciel, G . E . ; Hawkins, B. L . J. Chromatogr. 1985, 319, 9. 16. Fripiat, J. In Soluble Silicates; Falcone, J. S., E d . ; ACS Symposium Series 194, p 165.

17. Conder, J. R.; Young, C . L . Physicochemical Measurements by Gas Chromatog raphy; Wiley Interscience: N e w York, 1979. 18. Aulich, Η. Α.; Eisenrith, K. H . ; Urbach, H. P. J. Mater. Sci. 1984, 19, 1710.

19. L e Bihan, M . T.; Kalt, Α.; Wey, R. Bull. Soc. Fr. Minéral. Cristallogr. 1971, 94, 15. 20. Papirer, E . ; Balard, H . ; Vidal, A . Eur. Polym. J. 1988, 24, 783. 21. Sindorf, D . W . ; Maciel, G. E. J. Am. Chem. Soc. 1983, 105, 1487. 22. F e l t l , L.; Lutovsky, P.; Sosnova, L.; Smolkova, E . J. Chromatogr. 1974, 91, 321. 23. Ligner, G.; Vidal, Α.; Balard, Η.; Papirer, Ε.J.Colloid Interface Sci. 1990, 134, 486. 24. Vidal, Α.; Papirer, E . ; Wang, M . J.; Donnet, J. B . Chromatographia 1986, 23, 227. 25. Rudzinski, W . ; Jagiello, J.; Grillet, Y. J. Colloid Interface Sci. 1982, 48, 478. 26. Jagiello, J.; Ligner, G . ; Papirer, E . J. Colloid Interface Sci. 1990, 137, 128.

27. Legrand, A . P. et al. Adv. Colloid Interface Sci. 1990, 33, 9 1 . RECEIVED 1991.

for review October 3, 1990. ACCEPTED revised manuscript December 30,


The Colloid Chemistry of Silica - American Chemical Society

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