- Email: [email protected]
Adsorption behavior of a silane coupling agent on colloidal silica studied by gel permeation chromatography
Adsorption Behavior of a Silane Coupling Agent on Colloidal Silica Studied by Gel Permeation Chromatography NORIHIRO NISHIYAMA, 1 ROBERT SHICK, AND HATSUO ISHIDA Department o f Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44106-1712 Received January 31, 1990; accepted October 4, 1990 The adsorption behavior of a silane coupling agent deposited onto silica is examined as a function of both drying time and silane concentration using gel permeation chromatography (GPC). Information concerning the bonding of the silane to the surface, the amount adsorbed, and the molecular species present is obtained from GPC by measuring the extractable, physisorbed silane. Chemisorption occurs rapidly within 1 day, and continues to gradually increase until 14 days. Fourier transform-infrared diffuse reflectance spectroscopy (DRIFT) is used to obtain information concerning the adsorption behavior of the chemisorbed silane. The effect of surface coverage on the orientation of silane molecules has been studied. The effect of acid on the adsorption behavior is investigated as a function of concentration of acid. Addition of hydrochloric acid increases the molecular weight of the adsorbed silane species. The molecular species chemisorbed on the silica surface is different with different concentrations of hydrochloric acid, and strongly affects the adsorption behavior. Higher-molecular-weight species of silane molecules reduce the maximum amount of chemisorbed silane. In addition, the non-hydrogen-bonded carbonyl component increases with increasing concentration of hydrochloric acid. © 1991AcademicPress,Inc.
gel permeation chromatography (GPC). They reported that the silane coupling agent forms It is well known that the mechanical prop- a multilayer on the surface consisting of a erties of polymeric composite materials are chemisorbed layer and a physisorbed layer. controlled by the fiber/matrix interface. The The former is covalently bonded to the surface, properties of an interphase composed of silane and the latter is associated by the hydrogen coupling agents are strongly influenced by the bonding and weaker van der Waals attractions. amount of silane adsorbed and the adsorption The adsorption behavior, molecular weight, behavior of the silane. Therefore, the deter- and molecular structure of these species is mination of these parameters is essential for controlled by the isoelectric point of the suba fundamental understanding of the rein- strate and the pH of the solution. They also forcement mechanism in polymer composites. investigated molecular structure changes asIshida et al. (1-4) have investigated the ad- sociated with pH changes. sorption behavior of silane coupling agents Nishiyama et al. have investigated the adonto various substrates. They coated silane on sorption behavior of silane coupling agent onto various surfaces by evaporating the solvent out colloidal silica in the solution system using of the silane solution, thereby depositing the electron spin resonance (5) and 29Si nuclear silane on the surface. They characterized the magnetic resonance spectroscopy (6). The resulting molecular species using Fourier species ofsilane adsorbed onto a colloidal silica transform-infrared spectroscopy (FT-IR) and surface were determined while the system remained in solution. Nishiyama et al. reported that little chemisorption occurs from solution Current address: Department of Dental Materials, Nihon University, School of Dentistry at Matsudo, 870-1 until the dimer species are produced; therefore, they concluded that the major constituent of Sakaecho 2-Nishi Matsudo Chiba 271, Japan. INTRODUCTION
146 0021-9797/91 $3.00 Copyright© 1991by AcademicPress,Inc. All rightsofreproductionin any formreserved.
Journal of Colloidand InterfaceScience, Vol. 143,No. 1,April1991
ADSORPTION
the adsorbed species was the dimer species of silane. In addition, the adsorption behavior varies with the methylene chain length in wmethacryloxyalkyltriethoxysilane. A silane with a shorter chain is adsorbed on the silica surface in a r a n d o m fashion; however, a silane with a longer chain is adsorbed onto the silica surface in a more ordered fashion. The present work attempts to correlate the behavior between two types of silica treatment systems, namely, those which are dried after removal from the treating solution and those which are left in the solution. To obtain information concerning the a m o u n t of chemisorbed silane and the corresponding molecular species on the silica surface in the deposited system, the physisorbed silane was removed from the silica surface using tetrahydrofuran ( T H F ) , and the a m o u n t of physisorbed silane was determined using GPC. The chemisorbed silane on the silica surface was measured by Fourier transforminfrared diffuse reflectance spectoscopy ( D R I F T ) after the physisorbed silane had been removed. Examination of the carbonyl region gave insight into the probable structure of the adsorbed species. In addition, the influence of hydrochloric acid on the chemisorption and physisorption behavior of the silane on the silica surface was also investigated using GPC and D R I F T as a function of the concentration of hydrochloric acid. The relationship between the adsorption behavior and the molecular species of the silane was discussed. On the basis of these results, an adsorption model of silane in the deposited system was proposed. EXPERIMENTAL
3,-Methacryloxypropyltrimethoxysilane, (3,MPS) from Aldrich was used as received. The colloidal silica from Degussa was also used as received and has a reported surface area of 130 mZ/g, BET (7).
Silane Treatment The silane coupling agent was dissolved in a 70% by volume ethanol-30% by volume
ON
SILICA
147
H 2 0 solution. (The water was de-ionized and distilled. The p H of the water was 5.5.) Concentrations of silane were 2.7, 5.5, 10.9, 14.0, 22.0, 33.0, and 43.5 m g / m l . Silane was allowed to hydrolyze in these solutions for 1 h. Following hydrolysis, 10 g of the colloidal silica was added to 150 ml of the silane solution. After 1 h, the suspension was centrifuged, and the supernatant solution was discarded. The colloidal silica with the deposited silane was dried in a desiccator at r o o m temperature (23 + I ° C ) for varying times: 1, 4, 7, 10, 14, and 21 days. Deionized and distilled water of p H 5.5 was adjusted with hydrochloric acid (EM Science). The concentrations of hydrochloric acid were 10 -4, 10 -2, and 10 -l mole/liter. The acid solution was then dissolved into ethanol (H20: ethanol = 30:70, v / v ) , and q/-MPS was added at a concentration of 14.0 m g / m l . The treatm e n t was then performed in the same m a n n e r as described above.
Determination of Chemisorbed Amount of Silane onto Colloidal Silica Surface The following experimental procedure was used to determine the initial, deposited amount ofsilane. Colloidal silica was dispersed in the silane solution, 1 g for 15 ml solution. After 1 h, this suspension was centrifuged, and the supernatant solution was discarded. The treated silica was then allowed to dry at 40°C until the weight of silica became constant. The initial a m o u n t of silane deposited onto the silica was determined by the change in weight of the colloidal silica before and after deposition of 3,-MPS onto the silica surface. To determine the a m o u n t of chemisorbed silane on the silica surface, the a m o u n t of physisorbed silane was measured as a function of drying time of the silica using GPC. The chemisorbed amount of silane was determined by subtracting the measured physisorbed a m o u n t from the initial a m o u n t of silane deposited on the silica surface. The silica was treated by varying the concentration of the silane solution. The silica was then dried in a Journal of Colloid and Interface Scienca, Vol. 143, No. 1, April 1991
148
NISHIYAMA, SHICK, AND ISHIDA
desiccator at r o o m temperature and sampled as a function of time. Silica was dispersed into THF, 0.5 g for 10 ml solvent, and this suspension was stirred for 2 h to completely remove the physisorbed silane. After stirring, the suspension was centrifuged, and a controlled volume of the supernatant solution containing the previously physisorbed silane was injected into the G P C (Waters 510, Styragel 50 and 100 nm) for analysis. The U V detector (Waters 484) response was calibrated by integrating the chromatograms based on known concentrations of 7-MPS in THF. The cutoff value for T H F is 210 nm, and therefore it is ineffective to use the kmaxfor 3,-MPS at 211 n m attributed to the conjugation of the methacrylate group. To avoid the problem caused by the cutoff of the mobile solvent, THF, the absorbance at 225 n m was used for determination of the a m o u n t of physisorbed silane. At this wavelength, 3,-MPS still shows 12% of its m a x i m u m intensity while the effect of the T H F is greatly reduced. To correlate the molar response of Beer's law to weight, it was assumed that the predominant molecular species was the silane triol.
Adsorption Behavior of Silane Coupling Agent onto Colloidal Silica Surface After the treated colloidal silica was dried for 14 days, the colloidal silica was repeatedly rinsed with T H F to completely remove the physisorbed silane, until no silane was detected in the supernatant solution for G P C analysis. A Fourier transform-infrared spectrometer ( B o m e m DA3) equipped with a narrow bandpass m e r c u r y - c a d m i u m - t e l l u r i d e ( M C T ) detector, with a specific detectivity D* of 2.1 X 10 ,o cm Hz ~/2/W, was used to obtain a spectrum of the silane coupling agent chemisorbed on the silica surface. A diffuse reflectance ( D R I F T ) attachment (Barnes / Spectra Tech No. 0030-081 ) was employed. Prior to the measurement, 100 mg of the silica powder was dispersed in 1 g of KBr powder. The FTIR spectrum was measured with a 4-cm -~ resolution under a nitrogen purge. A blocker was Journal of Colloid and Interface Science, Vol. 143,No. 1, April 1991
used to reduce the specularly reflected light as m u c h as possible. RESULTS AND DISCUSSION
Effect of Concentration of Silane Typical G P C chromatograms showing molecular species and a m o u n t of silane extracted from the treated silica are shown in Fig. 1 as a function of drying time. The assignment of G P C peaks is summarized in Table I. The concentration of 3,-MPS in silane solution for this figure is 22.0 m g / m l . These chromatograms are due entirely to the physisorbed silane, which is held to the silica surface only by hydrogen bonding and van der Waals forces and can therefore be easily removed with THF. It can also be seen that as the drying time progresses, the molecular species ofsilane change. Initially, the m o n o m e r fraction decreases as the dimer and trimer are produced, as shown in Fig. 1. However, it is important to remember for this system that both covalent and hydrogen-bonded silane contributes to the measured peaks assigned to the dimer and trimer
[3 Amountofphysisorbed silane(rag/g)
C
Dryingtime(day)
A
1day
4days
7days
41.0rng/g
~
34.9mg/g
10days
28.8mg/g
14days
21days
48.5mg/g
~10.8
/
~ . ~
mg/g
9.0mg/g
40002000 8(30 Molecularweight
FIG. 1. GPC chromatograms of physisorbed silane as a function of drying time for 22.0 mg/ml treatment. Molecular weight is calibrated by polystyrenestandard.
149
A D S O R P T I O N ON SILICA TABLE I GPC Peak Assignments
A B C D E
Silane species
Molecular weight~
M o n o m e r speciesb D i m e r species Trimer species Tetramer species Octamer species
432 715 917 1447 3020
a Molecular weight is calibrated by polystyrene standard. b M o n o m e r species include four kinds of silane molecular species: OCH 3
I --Si--OCH3,
I OCH 3
OH
OH
I
I
--Si--OCHs,
I OCH3
--Si--OH,
I OCHs
OH
I --Si
OH.
/ OH
species. For greater drying times, species of higher molecular weight are produced. It is also evident in Fig. 1 that there is a decrease in the a m o u n t of removable, physisorbed silane as drying time is increased. The relationship between amount of silane which is physisorbed onto the silica surface and drying time can be seen in Fig. 2. The a m o u n t of physisorbed silane decreases with increasing drying time. Specifically, the decrease in amount of removable silane in the supernatant solution indicates the formation of a covalent bond, the - S i - O - S i - bond, between the Si-OH group of the silane coupling agent and the Si-OH group of the silica surface, thereby indicating an increase in the chemisorbed fraction of silane. The amount of chemisorbed silane on the silica surface increases dramatically within 1 day, continues to increase more gradually for 14 days, and then plateaus, indicating the maximum adsorption of chemisorbed silane for each concentration. For treatments under 22.0 mg/ml, most of the deposited silane is chemisorbed onto the silica surface. However, with increasing concentrations of silane solution, one begins to see the presence of the physisorbed silane due to saturation of the chemisorbed layer. By changing the concentration of the treating solution, the amount of silane cova-
lently bonded to the silica surface within I day varies. The initial amount of chemisorption within 1 day increases with increasing concentration of silane in a silane solution. This is understandable because the probability of silane coming into contact with the silica surface increases with increasing concentration of silane. Figure 3 shows GPC chromatograms as a function of the concentration of treating solution after drying for 14 days. These chromatograms indicate that the molecular species of physisorbed silane are oligomers, and the fact that these molecular weights are different indicates that the oligomer species are dependent on the deposited amount of silane. Presumably this is because the probability of condensation increases as the amount of silane deposited on the silica surface increases. For example, above 33.0 mg/ml, octamer is produced. Therefore, as the amount of deposited silane increases, the molecular weight of the species produced increases. Accordingly the molecular species which would be chemisorbed on the silica surface are dependent on drying time and also on the concentration of the silane in the treating solution. Initially the species of lower molecular weight are chemisorbed, and then with in150
~
I O0
E 50
6
12
18
24
Drying~trte (day)
FIG. 2. A m o u n t of silane coupling agent physisorbed on silica surface as a function of drying time. Concentrations of silane in silane solution: ©, 5.5; A, 10.9; D, 22.0; 0, 33.0 m g / m l . Journal of Colloid and Interfuce Science, Vol. 143, No. 1, April 1991
150
NISHIYAMA, SHICK, A N D ISHIDA
B
I0.9mg/ml / ~
/~
40002000 800 Molecularweight FIG. 3. GPC chromatograms o f physisorbed silane as a function of concentration of silane coupling agent in silane solution. Molecular weight is calibrated by polystyrene standard.
creasing drying time, the oligomers are chemisorbed. Figure 4 demonstrates the a m o u n t of chemisorbed silane, after drying for 21 days, as a function of the concentration of the treating solution. The a m o u n t of chemisorbed silane increases monotonically with the concentration of treating solution until the concentration reaches 33.0 m g / m l . At this point, the chemisorbed layer has saturated, and the a m o u n t of silane chemisorbed on the silica surface is about I00 m g / g , which is the maxi m u m a m o u n t of chemisorbed silane on the silica surface. Saturation of the chemisorbed layer probably occurs because of the steric hindrance of the organic group in the silane molecule. Saturated chemisorption, which has 100 mg silane for every gram of silica, corresponds to approximately 2.3 silane molecules chemisorbed on 1 n m 2 of silica surface. This is in good agreement with a previous study (5). The colloidal silica has 3 or 4 S i - O H groups for 1 n m 2 of silica surface (7); thereJournal of Colloid and Interface Science,
Vol. 143,No. 1, April1991
fore, unreacted S i - O H groups on the silica surface still remain (8). Shindorf and Maciel (9) determined the n u m b e r of chemisorbed silanes on silica surface using solid-state 29Si NMR. They reported chemisorption of 1.5 molecules per 1 n m 2 of silica whose surface has 4.5 S i - O H groups on 1 n m 2. Their report is in reasonable agreement with the results presented here. The D R I F T - I R spectra of the silane chemisorbed onto the silica surface is shown in Fig. 5 as a function of concentration of silane in solution. It should be noted that for these spectra the physisorbed silane has been completely removed. For treatments under 10.9 m g / m l , the C ~ O band at 1706 cm -1 , which is assigned to the hydrogen-bonded C z group with the S i - O H group on the silica surface (10), is primarily observed. However, when the concentration of silane in solution increases to 14.0 m g / m l (when the n u m b e r of chemisorbed silane molecules per 1 n m 2 silica surface is 1.2 molecules) a new C z O band at 1726 cm -1 appears, reflecting the different environment of the non-hydrogen-bonded C ~ O group (10). This band at 1726 cm -~ is attributed to the free C ~ O stretch, and it clearly increases with increasing amounts of
150 3
0
{ g N lOO
24
/o ~ so
fi
o/O
/
o/
1
o
o/ 1'0
2~0
3'0
4'0
500
Concentrationofsilaneinsolution(rng/mll FIG. 4. A m o u n t of saturated chemisorbed silane coupling agent on silica surface and n u m b e r of chemisorbed silane molecules per 1 n m 2 silica surface as a function of the concentration of silane in solution.
ADSORPTION ON SILICA
Number of chemisorbed silane molecules per I nm2 2.20
2.26 ~
1,16
~
_
_
/
_
~
~
_
0.94
0.52
0.30 I
t8oo
t7'50
i
tToo
IB'50
iB'oo
Wavenumber (cm-1)
FIG. 5. DRIFT FT-IR spectra of chemisorbed silane molecules on silica surface as a function of concentration of silane in solution. chemisorbed silane (6). Fry and Maciel ( 11 ) showed similar behavior for adsorbing species using deuterium NMR: that is, as species were first adsorbed, they would lie on the surface. As the chemisorbed fraction increased, at some coverage less than the saturated value, adsorbed species would begin to stand away from the surface within the chemisorbed layer. By using curve fitting, it is possible to isolate the two fractions of the carbonyl, as shown in Fig. 6 where O is the ratio of chemisorbed stlane to the m a x i m u m saturated a m o u n t ) . It has been reported that the specific absorptivity increases with surface interaction, Tensmeyer et al. (12) and Hoffman and Brindley (13) found the increase to be almost a factor of 2 for a ketone adsorbed on clay. Therefore, our standing c o m p o n e n t is probably even higher than is indicated by the free C ~ O. The behavior of the carbonyl band indicates that the adsorption behavior of silane is dependent upon the n u m b e r of chemisorbed silane molecules per 1 n m 2 silica surface. As the n u m b e r of chemisorbed molecules increases, they begin to stand up from the surface (free C = O) instead of lying parallel to the surface (hydrogen-bonded C = O ) . This transition from
151
lying to standing occurs for treatment above 14.0 m g / m l , which corresponds to the point where the chemisorbed silane is slightly more than half of the saturated value. As the molecular weight of species chemisorbed on the silica surface increases, there is insufficient space for the chemisorbed molecules to continue lying down; therefore, silane molecules are chemisorbed standing away from the silica surface. The adsorption mechanism depends on the initial a m o u n t of deposited silane because the total a m o u n t of deposited silane controls the a m o u n t of chemisorbed silane as well as the molecular weight of the adsorbing species. It should be mentioned that the trend of having standing orientation is higher for the thermodynamically equilibrated, adsorbed silane than for the kineticatly deposited silane. There is also infrared evidence concerning the adsorption of silane species in the free - O H region of the spectra. A sharp peak is observed at 3744 cm -I which is due to the non-hydrogen-bonded silanols on the untreated silica surface. This frequency is shifted to slightly lower frequency than for the transmission spectrum due to the optical perturbation of the D R I F T experiment. Silane mol-
100 a--a
75
w
,~
5o
, 0
, 05
,
0
FIG. 6. Fraction of each C = O peak in FT-IR as a function of surface coverage. O = fraction of maximum possiblechemisorbedsilane./1 Hydrogenbonded; ©, nonhydrogen bonded. Journal of Colloid and Interface Science, Vol. 143, No. 1, April 1991
152
NISHIYAMA, SHICK, AND ISHIDA
ecules bond to the silica surface by reacting carbonyls are free of hydrogen bonding, and with these free silanols forming a siloxane that the free silanol band is nearly diminished bond. In addition, the free silanol may dis- around 2.3 molecules/nm 2, it can be conappear due to hydrogen bonding with the car- . cluded that the silica surface is completely, cobonyl of the silane. Therefore, as the surface valently covered with silane molecules. reaction progresses, one would expect a deIn a solution system (5, 6), the molecular crease in the free silanol to result. This decrease species produced for a concentration of 30 mg/ is quite apparent in Fig. 7. Here the intensity ml are mainly the dimer species and it is of the free silanol, in Kubelka Munk units, is mainly physisorbed on the silica surface with plotted as a function of surface coverage. A hydrogen bonding. However, in a deposited continuous reduction in intensity with cov- system, the concentration of silane would inerage is observed, which would tend to suggest crease and the adsorption aspect changes from that the molecules are bonding directly to the hydrogen bonding to covalent bonding as the surface rather than following a two-layer ad- ethanol and water are evaporated during the sorption mechanism. Large errors are intro- drying. As a result of evaporation, the conduces as the free silanol is consumed and be- densation ofsilanes to each other becomes accomes obscured by the other silanols. Despite celerated due to increased concentration of the large errors at high coverage, it also appears reacting species. Therefore, the molecular that the slope for low coverage is steeper than species chemisorbed onto the silica surface in for higher coverages. Indeed, it was observed a system where the treated silica has been dried in Figs. 5 and 6 that at low coverages the car- are different than in those found in a solution bonyl is hydrogen bonded to a silanol, and at system. higher coverages, the silane molecule stands away from the surface exhibiting a free carEffect of Concentration of HCl bonyl. Using this information, one would exThe GPC chromatograms of physisorbed pect a steeper slope at low coverages because a silane may consume a surface free sitanol silane are shown in Fig. 8 after drying for 1 both by chemically bonding and by hydrogen day as a function of the concentration of hybonding to its carbonyl. However, at higher drochloric acid in the silane treating solution. coverage, the silane would consume a surface Recall that the assignment of GPC peaks is in free silanol only by reaction. It is interesting Table I. The concentration of silane in the to note that this change in slope appears at treating solution is constant at 14.0 mg/ml. approximately 1 m o l e c u l e / n m 2, nearly the For the nonacidic condition, the dimer species same coverage as where the free carbonyl is are the main component of the physisorbed first observed. Considering the fact that some silane. However, in the presence of acid, the molecular weight increases with increasing concentrations of hydrochloric acid. For ex~" 0.040 i c 0.035 ample, the molecular species produced on the 0.030 \ silica surface are mainly trimer species in the 0.025 case of 10 -2 mole/liter and tetramer species 0.020 \ in the case of 10 -~ mole/liter. This shows that \ 0.015 the molecular weight has a clear dependence 0.010 on the concentration of hydrochloric acid. 0 . 0 0 5 E ° - ' - - ~ -i r i 0.000 Figure 9 shows the GPC chromatograms as 2.5 0.5 1.0 1.5 2.0 0.0 a function of concentration of HC1; in this Molecules per nm 2 case, however, the drying time is 14 days. For FIG. 7. Intensityof 3744-cm-~band ( freesurfacesilanol) a nonacidic condition, there is a large distriin Kubelka Munk units as a function of surface coverage. bution in molecular weight of the silane species
+i"
Journal of Colloid and Interface Science, Vol. 143, No. I, April 1991
ADSORPTION ON SILICA
153
lution system, however, the acid stays in the solution and stabilizes the hydrolyzed silanes to form lower-molecular-weight species. Figure 10 demonstrates the relationship between amount of physisorbed silane and drying time as a function of the concentration of hydrochloric acid. Here, the initial amount ofsilane deposited in the nonacidic condition is different from that in the acidic condition, but recall that the concentration of silane in the treating solution is the same. In the nonacidic condition, the amount of silane depos40002000 800 Molecularweight ited is 52 mg/g, and in the acidic condition the amount is about 60 mg/g. This is probably FIG. 8. GPC chromatogramsof physisorbedsilane as a caused by the looser packing of the silica due function of concentration of hydrochloricacid after 1 day of drying. Molecularweight is calibrated by polystyrene to increased ionic repulsion from the presence of hydrochloric acid. Worse packing allows a standard. greater interstitial volume and hence more silane solution, which results in deposition of produced at the silica surface, although the di- more silane. For the nonacidic condition, the mer and trimer species are present as the main amount of chemisorbed silane on the silica component. However, in the presence of acid, surface increases dramatically within 1 day mainly the tetramer and octamer are pro- and gradually increases with increasing the duced. The dependence of molecular weight drying time until the chemisorbed layer saton acid is seen to continue from short to long urates, as mentioned previously. By then, most drying times. of the silane is chemisorbed on the silica surIn previous papers (6, 14~ 15), the hydro- face. However, for the acidic condition, the lysis and condensation behavior of a silane behavior in the chemisorption of silane molcoupling agent were investigated in a solution ecules to the silica surface is different from system, using 29Si NMR. It was found that acid accelerates the hydrolysis of the methoxy groups, -Si-OCH3, in silane molecules ( 16 ). B Concentrationof hydrochloric However, the acid also decelerates the interacid(tool/l) molecular condensation of the silane by sta0 mol/l bilizing the Si-OH moiety. In this case, mainly oligomers are produced. For a nonacidic con10-4mol/l~ , ' ~ dition, higher-molecular-weight species were observed in the solution system. Thus the 102~ovl /E /~/ condensation behavior in the solution system is quite different from that obtained in the de0,mo posited system, whose results are shown here. This difference could be caused by the pH 40002000 800 change due to the evaporation of hydrochloric Molecularweight acid which occurs when the ethanol solution evaporates. Therefore, it seems that the acid PIG. 9. GPC chromatogramsof physisorbedsilane as a accelerates hydrolysis and then evaporates function of concentration of hydrochloric acid after 14 from the silica surface, allowing the production days of drying. Molecularweight is calibratedby polystyof higher-molecular-weight species. In a so- rene standard. B
Conce
Journal of Colloid and Interface Science, VoL 143,No. l, April 1991
154
NISHIYAMA, SHICK, AND ISHIDA 80
%
.~
60@ o
•~ 40
i ~° ; °'"°
<
)
g 0
6
12
18
24
Drying time (day)
FIG. 10. Amount of silane coupling agent physisorbed on silica surface as a function of drying time. Concentrations ofhydrochrolic acid: O, 0, A, 10-4; [3, 10-2; 0, 10-~ mole/liter. that in the nonacidic condition because one can see a substantial increase in the a m o u n t of physisorbed silane as compared with the nonacidic condition. Addition of hydrochloric acid decreases the a m o u n t of chemisorbed silane within 1 day until the concentration of hydrochloric acid is 10 -2 mole/liter. The effects of acid are perhaps explained by changes in charge distribution. For the case of 10 -~ mole/liter, as more acid is added, the a m o u n t of chemisorbed silane is seen to go through a m i n i m u m and then increase. The hydrochloric acid first decelerates the condensation reaction between the S i - O H group of the silane and the S i - O H group on the silica surface, and then begins to increase with additional hydrochloric acid. The amount of physisorbed silane increases with increasing concentration of hydrochloric acid. Therefore, the chemisorption behavior strongly depends on the concentration of hydrochloric acid as catalyst for hydrolysis. This behavior is in good agreement with the stability of silanol which shows the most stable p H to be approximately 2-3 (17). Figure 11 shows the relationship between the a m o u n t of silane and how it is attached to Journal of Colloid and Interface Science, Vol. 143, No. 1, April 1991
the silica as a function of the concentration of silane in solution. Under an acidic condition, it is possible to see a slightly different tendency in the a m o u n t of physisorbed silane as compared with the data for the nonacidic condition. These differences in the physisorbed a m o u n t are probably caused by differences in the silane molecular species which are chemisorbed onto the silica surface. For an acidic condition equal to or above 10 2 mole/liter and for 1 day drying, the trimer or tetramer species are chemisorbed on the silica surface. The experimental conditions are such that the physisorbed silanes are the only source of chemisorbed silane produced after prolonged drying periods. Thus, the G P C chromatograms of the physisorbed silane will give us insight into the species leading to chemisorption, especially after the initial rapid chemisorption period. Those oligomeric species have a larger occupied area and steric hindrance caused by the organic group compared to the m o n o m e r or dimer species. Therefore, the chemisorbed
200
[3
.~ 150
loo
9/°/ 0
10
20
30
40
50
Concentration of silane in solution (rag/ml)
FIG. I I. Amount of silane on silica surfaceas function of concentration of silane in silane solution. O, Chemisorbcd amount; A physisorbed amount; D, deposited amount; *, initial deposited amount under acidic conditions. Concentration of hydrochrolic acid: ~7 (chemisorbcd), • (physisorbed), 10 4; © (chemisorbed), • (physisorbcd), I0-2; * (chemisorbed), ~ (physisorbed), I 0 - i mole/liter.
ADSORPTION
a m o u n t of silane decreases with increasing molecular weight of silane molecules. FT-IR spectra of silane chemisorbed on silica surface are shown in Fig. 12 as a function of the concentration of hydrochloric acid. Recall that the C z O peak is sprit into two peaks, reflecting two different environments. Here, the peak at 1706 cm -~ is assigned to the hydrogen-bonded C z O group and the peak at 1726 cm -1 is attributed to the non-hydrogenbonded C z O group. The peak intensity of the non-hydrogen-bonded C ~ O increases with increasing concentration of hydrochloric acid as shown in Fig. 13. As before, each fraction is obtained by curve fitting. However, in this case, the number of silane molecules chemisorbed per 1 nm 2 are very close as shown in Fig. 10, even though the fraction of the nonhydrogen-bonded component increases with increasing concentration of hydrochloric acid. This is probably caused by the difference in the molecular species chemisorbed on the silica surface. Thus, as mentioned above, the molecular species chemisorbed on the silica surface varies with the concentration of hy-
Numberof chemisorbedsilane molecules per 1 nm2 1.21
~ R~ = 0.2
155
~oo I
75
~ o
5O
25
i
0
i
i
I0 "4
i
I
10-2
Concentration of hydrochloricacid (rnol/l) FIG. 13. C o m p o n e n t fraction o f C ~ O p e a k in F T - I R as a f u n c t i o n o f surface coverage. A, H y d r o g e n b o n d e d ; ©, n o n - h y d r o g e n b o n d e d .
drochloric acid. In 10 -1 mole/liter, mainly the tetramer species are chemisorbed (Fig. 8 ). The higher the molecular weight of silane molecules, the larger is the free volume and steric hindrance of molecular species. As the molecular weight increases it would be expected that it would be more difficult to hydrogen bond to the surface. Therefore, one would expect a higher fraction of the adsorbed species not to be hydrogen bonded to the surface, which is the behavior observed. CONCLUSIONS
1.24
I.I6
18()0
ON SILICA
17150
17bO
16'50
16'00
Wavenumber(cm-i)
FIG. 12. FT-IR spectra ofchemisorbed silane molecules of silica surface as a function of concentration of hydrochloric acid. Concentrations of hydrochloric acid: (A) 10-', (B) 10 -2, (C) 10 -4, (D) 0 mole/liter.
The molecular species chemisorbed on the silica surface are dependent on both the drying time and the concentration of silane in the treating solution. The amount of chemisorbed silane becomes saturated for approximately two molecules for 1 nm z. The specific adsorption behavior depends on the number ofsilane molecules deposited on the silica surface. At low concentration, silane molecules are chemisorbed lying parallel to the silica surface. With increasing amounts of chemisorption, above roughly half the saturated chemisorbed amount, additional chemisorbed molecules are forced to stand away from the silica surface. Journal of Colloid and Interface Science, Vol. 143, No. 1, April 1991
156
NISHIYAMA, SHICK, AND ISHIDA
The molecular species ofsilane chemisorbed on the silica surface also depend on the hydrolysis condition of the silane. Addition of acid followed by drying accelerates the intermolecular condensation of the silane molecules. Increasing acid content increases the molecular weight of the physisorbed silane, which in turn indicates that higher-molecularweight species have been chemisorbed. In addition, the adsorption behavior of silane molecules is dependent on the molecular species chemisorbed on the surface. The higher the molecular weight of the silane molecule, the greater is the free volume of the molecular species, therefore the fraction of the standing component increases. ACKNOWLEDGMENTS One of the authors, N. Nishiyama, acknowledges the financial support of the Nihon University School of Dentistry at Matsudo, Japan. REFERENCES 1. Ishida, H., and Miller, J. D., Macromolecules 17, 1659 (1984).
Journalof Colloidand InterfaceScience,Vol. 143,No. 1,April1991
2. Miller, J. D., Hoh, K., and Ishida, H., Polym. Eng. Sci. Polym. Comp. 5, 18 (1984). 3. Ishida, H., and Miller, J. D., J. Polym. Sci. Polym. Phys. Ed. 23, 2227 (1985). 4. Ishida, H., Polym. Sci. Technol. 27, 25 (1985). 5. Nishiyama, N., Katsuki, H., Horie, K., and Asakura, T., J. Biomed. Mater. Res. 21, 1029 (1987). 6. Nishiyama, N., Horie, K., and Asakura, T., J. Colloid InterfaceSci. 129, 113 (1989). 7. Young, G. J., J. ColloidSci. 13, 67 (1958). 8. Shindorf, D. W., and Maciel, G. E., J. Amer. Chem. Soc. 105, 3767 (1983). 9. Shindrof, D. W., and Maciel, G. E., J. Phys. Chem. 86, 5208 (1982). 10. Miller, J. D., and Ishida, H., Surf. Sci. 65, 443 (1984). 11. Fry, J., and Maciel, G. E., unpublished work, Chemically Modified Oxide Surface Symposium, Midland, MI, June 1989. 12. Tensmeyer, L. G., Hoffmann, R. W., and Brindley, G. W., J. Phys. Chem. 64, 1655 (1960). 13. Hoffmann, R. W., and Brindley, G. W., J. Phys. Chem. 65, 443 ( 1961 ). 14. Nishiyama, N., Asakura, T., and Horie, K., Rep. Prog. Polym. Phys. Japan 30, 601 (1987). 15. Nishiyama, N., Asakura, T., and Horie, K., J. Colloid Interface Sci. 124, 14 ( 1988 ). 16. Savard, S., Blanchard, L. P., Leonard, J., and Prud'Homme, R. E., Polym. Composite 5, 242 (1984). 17. Plueddemann, E. P., "Silane Coupling Agents," p. 52. Plenum, New York, 1982.
gel permeation chromatography (GPC). They reported that the silane coupling agent forms It is well known that the mechanical prop- a multilayer on the surface consisting of a erties of polymeric composite materials are chemisorbed layer and a physisorbed layer. controlled by the fiber/matrix interface. The The former is covalently bonded to the surface, properties of an interphase composed of silane and the latter is associated by the hydrogen coupling agents are strongly influenced by the bonding and weaker van der Waals attractions. amount of silane adsorbed and the adsorption The adsorption behavior, molecular weight, behavior of the silane. Therefore, the deter- and molecular structure of these species is mination of these parameters is essential for controlled by the isoelectric point of the suba fundamental understanding of the rein- strate and the pH of the solution. They also forcement mechanism in polymer composites. investigated molecular structure changes asIshida et al. (1-4) have investigated the ad- sociated with pH changes. sorption behavior of silane coupling agents Nishiyama et al. have investigated the adonto various substrates. They coated silane on sorption behavior of silane coupling agent onto various surfaces by evaporating the solvent out colloidal silica in the solution system using of the silane solution, thereby depositing the electron spin resonance (5) and 29Si nuclear silane on the surface. They characterized the magnetic resonance spectroscopy (6). The resulting molecular species using Fourier species ofsilane adsorbed onto a colloidal silica transform-infrared spectroscopy (FT-IR) and surface were determined while the system remained in solution. Nishiyama et al. reported that little chemisorption occurs from solution Current address: Department of Dental Materials, Nihon University, School of Dentistry at Matsudo, 870-1 until the dimer species are produced; therefore, they concluded that the major constituent of Sakaecho 2-Nishi Matsudo Chiba 271, Japan. INTRODUCTION
146 0021-9797/91 $3.00 Copyright© 1991by AcademicPress,Inc. All rightsofreproductionin any formreserved.
Journal of Colloidand InterfaceScience, Vol. 143,No. 1,April1991
ADSORPTION
the adsorbed species was the dimer species of silane. In addition, the adsorption behavior varies with the methylene chain length in wmethacryloxyalkyltriethoxysilane. A silane with a shorter chain is adsorbed on the silica surface in a r a n d o m fashion; however, a silane with a longer chain is adsorbed onto the silica surface in a more ordered fashion. The present work attempts to correlate the behavior between two types of silica treatment systems, namely, those which are dried after removal from the treating solution and those which are left in the solution. To obtain information concerning the a m o u n t of chemisorbed silane and the corresponding molecular species on the silica surface in the deposited system, the physisorbed silane was removed from the silica surface using tetrahydrofuran ( T H F ) , and the a m o u n t of physisorbed silane was determined using GPC. The chemisorbed silane on the silica surface was measured by Fourier transforminfrared diffuse reflectance spectoscopy ( D R I F T ) after the physisorbed silane had been removed. Examination of the carbonyl region gave insight into the probable structure of the adsorbed species. In addition, the influence of hydrochloric acid on the chemisorption and physisorption behavior of the silane on the silica surface was also investigated using GPC and D R I F T as a function of the concentration of hydrochloric acid. The relationship between the adsorption behavior and the molecular species of the silane was discussed. On the basis of these results, an adsorption model of silane in the deposited system was proposed. EXPERIMENTAL
3,-Methacryloxypropyltrimethoxysilane, (3,MPS) from Aldrich was used as received. The colloidal silica from Degussa was also used as received and has a reported surface area of 130 mZ/g, BET (7).
Silane Treatment The silane coupling agent was dissolved in a 70% by volume ethanol-30% by volume
ON
SILICA
147
H 2 0 solution. (The water was de-ionized and distilled. The p H of the water was 5.5.) Concentrations of silane were 2.7, 5.5, 10.9, 14.0, 22.0, 33.0, and 43.5 m g / m l . Silane was allowed to hydrolyze in these solutions for 1 h. Following hydrolysis, 10 g of the colloidal silica was added to 150 ml of the silane solution. After 1 h, the suspension was centrifuged, and the supernatant solution was discarded. The colloidal silica with the deposited silane was dried in a desiccator at r o o m temperature (23 + I ° C ) for varying times: 1, 4, 7, 10, 14, and 21 days. Deionized and distilled water of p H 5.5 was adjusted with hydrochloric acid (EM Science). The concentrations of hydrochloric acid were 10 -4, 10 -2, and 10 -l mole/liter. The acid solution was then dissolved into ethanol (H20: ethanol = 30:70, v / v ) , and q/-MPS was added at a concentration of 14.0 m g / m l . The treatm e n t was then performed in the same m a n n e r as described above.
Determination of Chemisorbed Amount of Silane onto Colloidal Silica Surface The following experimental procedure was used to determine the initial, deposited amount ofsilane. Colloidal silica was dispersed in the silane solution, 1 g for 15 ml solution. After 1 h, this suspension was centrifuged, and the supernatant solution was discarded. The treated silica was then allowed to dry at 40°C until the weight of silica became constant. The initial a m o u n t of silane deposited onto the silica was determined by the change in weight of the colloidal silica before and after deposition of 3,-MPS onto the silica surface. To determine the a m o u n t of chemisorbed silane on the silica surface, the a m o u n t of physisorbed silane was measured as a function of drying time of the silica using GPC. The chemisorbed amount of silane was determined by subtracting the measured physisorbed a m o u n t from the initial a m o u n t of silane deposited on the silica surface. The silica was treated by varying the concentration of the silane solution. The silica was then dried in a Journal of Colloid and Interface Scienca, Vol. 143, No. 1, April 1991
148
NISHIYAMA, SHICK, AND ISHIDA
desiccator at r o o m temperature and sampled as a function of time. Silica was dispersed into THF, 0.5 g for 10 ml solvent, and this suspension was stirred for 2 h to completely remove the physisorbed silane. After stirring, the suspension was centrifuged, and a controlled volume of the supernatant solution containing the previously physisorbed silane was injected into the G P C (Waters 510, Styragel 50 and 100 nm) for analysis. The U V detector (Waters 484) response was calibrated by integrating the chromatograms based on known concentrations of 7-MPS in THF. The cutoff value for T H F is 210 nm, and therefore it is ineffective to use the kmaxfor 3,-MPS at 211 n m attributed to the conjugation of the methacrylate group. To avoid the problem caused by the cutoff of the mobile solvent, THF, the absorbance at 225 n m was used for determination of the a m o u n t of physisorbed silane. At this wavelength, 3,-MPS still shows 12% of its m a x i m u m intensity while the effect of the T H F is greatly reduced. To correlate the molar response of Beer's law to weight, it was assumed that the predominant molecular species was the silane triol.
Adsorption Behavior of Silane Coupling Agent onto Colloidal Silica Surface After the treated colloidal silica was dried for 14 days, the colloidal silica was repeatedly rinsed with T H F to completely remove the physisorbed silane, until no silane was detected in the supernatant solution for G P C analysis. A Fourier transform-infrared spectrometer ( B o m e m DA3) equipped with a narrow bandpass m e r c u r y - c a d m i u m - t e l l u r i d e ( M C T ) detector, with a specific detectivity D* of 2.1 X 10 ,o cm Hz ~/2/W, was used to obtain a spectrum of the silane coupling agent chemisorbed on the silica surface. A diffuse reflectance ( D R I F T ) attachment (Barnes / Spectra Tech No. 0030-081 ) was employed. Prior to the measurement, 100 mg of the silica powder was dispersed in 1 g of KBr powder. The FTIR spectrum was measured with a 4-cm -~ resolution under a nitrogen purge. A blocker was Journal of Colloid and Interface Science, Vol. 143,No. 1, April 1991
used to reduce the specularly reflected light as m u c h as possible. RESULTS AND DISCUSSION
Effect of Concentration of Silane Typical G P C chromatograms showing molecular species and a m o u n t of silane extracted from the treated silica are shown in Fig. 1 as a function of drying time. The assignment of G P C peaks is summarized in Table I. The concentration of 3,-MPS in silane solution for this figure is 22.0 m g / m l . These chromatograms are due entirely to the physisorbed silane, which is held to the silica surface only by hydrogen bonding and van der Waals forces and can therefore be easily removed with THF. It can also be seen that as the drying time progresses, the molecular species ofsilane change. Initially, the m o n o m e r fraction decreases as the dimer and trimer are produced, as shown in Fig. 1. However, it is important to remember for this system that both covalent and hydrogen-bonded silane contributes to the measured peaks assigned to the dimer and trimer
[3 Amountofphysisorbed silane(rag/g)
C
Dryingtime(day)
A
1day
4days
7days
41.0rng/g
~
34.9mg/g
10days
28.8mg/g
14days
21days
48.5mg/g
~10.8
/
~ . ~
mg/g
9.0mg/g
40002000 8(30 Molecularweight
FIG. 1. GPC chromatograms of physisorbed silane as a function of drying time for 22.0 mg/ml treatment. Molecular weight is calibrated by polystyrenestandard.
149
A D S O R P T I O N ON SILICA TABLE I GPC Peak Assignments
A B C D E
Silane species
Molecular weight~
M o n o m e r speciesb D i m e r species Trimer species Tetramer species Octamer species
432 715 917 1447 3020
a Molecular weight is calibrated by polystyrene standard. b M o n o m e r species include four kinds of silane molecular species: OCH 3
I --Si--OCH3,
I OCH 3
OH
OH
I
I
--Si--OCHs,
I OCH3
--Si--OH,
I OCHs
OH
I --Si
OH.
/ OH
species. For greater drying times, species of higher molecular weight are produced. It is also evident in Fig. 1 that there is a decrease in the a m o u n t of removable, physisorbed silane as drying time is increased. The relationship between amount of silane which is physisorbed onto the silica surface and drying time can be seen in Fig. 2. The a m o u n t of physisorbed silane decreases with increasing drying time. Specifically, the decrease in amount of removable silane in the supernatant solution indicates the formation of a covalent bond, the - S i - O - S i - bond, between the Si-OH group of the silane coupling agent and the Si-OH group of the silica surface, thereby indicating an increase in the chemisorbed fraction of silane. The amount of chemisorbed silane on the silica surface increases dramatically within 1 day, continues to increase more gradually for 14 days, and then plateaus, indicating the maximum adsorption of chemisorbed silane for each concentration. For treatments under 22.0 mg/ml, most of the deposited silane is chemisorbed onto the silica surface. However, with increasing concentrations of silane solution, one begins to see the presence of the physisorbed silane due to saturation of the chemisorbed layer. By changing the concentration of the treating solution, the amount of silane cova-
lently bonded to the silica surface within I day varies. The initial amount of chemisorption within 1 day increases with increasing concentration of silane in a silane solution. This is understandable because the probability of silane coming into contact with the silica surface increases with increasing concentration of silane. Figure 3 shows GPC chromatograms as a function of the concentration of treating solution after drying for 14 days. These chromatograms indicate that the molecular species of physisorbed silane are oligomers, and the fact that these molecular weights are different indicates that the oligomer species are dependent on the deposited amount of silane. Presumably this is because the probability of condensation increases as the amount of silane deposited on the silica surface increases. For example, above 33.0 mg/ml, octamer is produced. Therefore, as the amount of deposited silane increases, the molecular weight of the species produced increases. Accordingly the molecular species which would be chemisorbed on the silica surface are dependent on drying time and also on the concentration of the silane in the treating solution. Initially the species of lower molecular weight are chemisorbed, and then with in150
~
I O0
E 50
6
12
18
24
Drying~trte (day)
FIG. 2. A m o u n t of silane coupling agent physisorbed on silica surface as a function of drying time. Concentrations of silane in silane solution: ©, 5.5; A, 10.9; D, 22.0; 0, 33.0 m g / m l . Journal of Colloid and Interfuce Science, Vol. 143, No. 1, April 1991
150
NISHIYAMA, SHICK, A N D ISHIDA
B
I0.9mg/ml / ~
/~
40002000 800 Molecularweight FIG. 3. GPC chromatograms o f physisorbed silane as a function of concentration of silane coupling agent in silane solution. Molecular weight is calibrated by polystyrene standard.
creasing drying time, the oligomers are chemisorbed. Figure 4 demonstrates the a m o u n t of chemisorbed silane, after drying for 21 days, as a function of the concentration of the treating solution. The a m o u n t of chemisorbed silane increases monotonically with the concentration of treating solution until the concentration reaches 33.0 m g / m l . At this point, the chemisorbed layer has saturated, and the a m o u n t of silane chemisorbed on the silica surface is about I00 m g / g , which is the maxi m u m a m o u n t of chemisorbed silane on the silica surface. Saturation of the chemisorbed layer probably occurs because of the steric hindrance of the organic group in the silane molecule. Saturated chemisorption, which has 100 mg silane for every gram of silica, corresponds to approximately 2.3 silane molecules chemisorbed on 1 n m 2 of silica surface. This is in good agreement with a previous study (5). The colloidal silica has 3 or 4 S i - O H groups for 1 n m 2 of silica surface (7); thereJournal of Colloid and Interface Science,
Vol. 143,No. 1, April1991
fore, unreacted S i - O H groups on the silica surface still remain (8). Shindorf and Maciel (9) determined the n u m b e r of chemisorbed silanes on silica surface using solid-state 29Si NMR. They reported chemisorption of 1.5 molecules per 1 n m 2 of silica whose surface has 4.5 S i - O H groups on 1 n m 2. Their report is in reasonable agreement with the results presented here. The D R I F T - I R spectra of the silane chemisorbed onto the silica surface is shown in Fig. 5 as a function of concentration of silane in solution. It should be noted that for these spectra the physisorbed silane has been completely removed. For treatments under 10.9 m g / m l , the C ~ O band at 1706 cm -1 , which is assigned to the hydrogen-bonded C z group with the S i - O H group on the silica surface (10), is primarily observed. However, when the concentration of silane in solution increases to 14.0 m g / m l (when the n u m b e r of chemisorbed silane molecules per 1 n m 2 silica surface is 1.2 molecules) a new C z O band at 1726 cm -1 appears, reflecting the different environment of the non-hydrogen-bonded C ~ O group (10). This band at 1726 cm -~ is attributed to the free C ~ O stretch, and it clearly increases with increasing amounts of
150 3
0
{ g N lOO
24
/o ~ so
fi
o/O
/
o/
1
o
o/ 1'0
2~0
3'0
4'0
500
Concentrationofsilaneinsolution(rng/mll FIG. 4. A m o u n t of saturated chemisorbed silane coupling agent on silica surface and n u m b e r of chemisorbed silane molecules per 1 n m 2 silica surface as a function of the concentration of silane in solution.
ADSORPTION ON SILICA
Number of chemisorbed silane molecules per I nm2 2.20
2.26 ~
1,16
~
_
_
/
_
~
~
_
0.94
0.52
0.30 I
t8oo
t7'50
i
tToo
IB'50
iB'oo
Wavenumber (cm-1)
FIG. 5. DRIFT FT-IR spectra of chemisorbed silane molecules on silica surface as a function of concentration of silane in solution. chemisorbed silane (6). Fry and Maciel ( 11 ) showed similar behavior for adsorbing species using deuterium NMR: that is, as species were first adsorbed, they would lie on the surface. As the chemisorbed fraction increased, at some coverage less than the saturated value, adsorbed species would begin to stand away from the surface within the chemisorbed layer. By using curve fitting, it is possible to isolate the two fractions of the carbonyl, as shown in Fig. 6 where O is the ratio of chemisorbed stlane to the m a x i m u m saturated a m o u n t ) . It has been reported that the specific absorptivity increases with surface interaction, Tensmeyer et al. (12) and Hoffman and Brindley (13) found the increase to be almost a factor of 2 for a ketone adsorbed on clay. Therefore, our standing c o m p o n e n t is probably even higher than is indicated by the free C ~ O. The behavior of the carbonyl band indicates that the adsorption behavior of silane is dependent upon the n u m b e r of chemisorbed silane molecules per 1 n m 2 silica surface. As the n u m b e r of chemisorbed molecules increases, they begin to stand up from the surface (free C = O) instead of lying parallel to the surface (hydrogen-bonded C = O ) . This transition from
151
lying to standing occurs for treatment above 14.0 m g / m l , which corresponds to the point where the chemisorbed silane is slightly more than half of the saturated value. As the molecular weight of species chemisorbed on the silica surface increases, there is insufficient space for the chemisorbed molecules to continue lying down; therefore, silane molecules are chemisorbed standing away from the silica surface. The adsorption mechanism depends on the initial a m o u n t of deposited silane because the total a m o u n t of deposited silane controls the a m o u n t of chemisorbed silane as well as the molecular weight of the adsorbing species. It should be mentioned that the trend of having standing orientation is higher for the thermodynamically equilibrated, adsorbed silane than for the kineticatly deposited silane. There is also infrared evidence concerning the adsorption of silane species in the free - O H region of the spectra. A sharp peak is observed at 3744 cm -I which is due to the non-hydrogen-bonded silanols on the untreated silica surface. This frequency is shifted to slightly lower frequency than for the transmission spectrum due to the optical perturbation of the D R I F T experiment. Silane mol-
100 a--a
75
w
,~
5o
, 0
, 05
,
0
FIG. 6. Fraction of each C = O peak in FT-IR as a function of surface coverage. O = fraction of maximum possiblechemisorbedsilane./1 Hydrogenbonded; ©, nonhydrogen bonded. Journal of Colloid and Interface Science, Vol. 143, No. 1, April 1991
152
NISHIYAMA, SHICK, AND ISHIDA
ecules bond to the silica surface by reacting carbonyls are free of hydrogen bonding, and with these free silanols forming a siloxane that the free silanol band is nearly diminished bond. In addition, the free silanol may dis- around 2.3 molecules/nm 2, it can be conappear due to hydrogen bonding with the car- . cluded that the silica surface is completely, cobonyl of the silane. Therefore, as the surface valently covered with silane molecules. reaction progresses, one would expect a deIn a solution system (5, 6), the molecular crease in the free silanol to result. This decrease species produced for a concentration of 30 mg/ is quite apparent in Fig. 7. Here the intensity ml are mainly the dimer species and it is of the free silanol, in Kubelka Munk units, is mainly physisorbed on the silica surface with plotted as a function of surface coverage. A hydrogen bonding. However, in a deposited continuous reduction in intensity with cov- system, the concentration of silane would inerage is observed, which would tend to suggest crease and the adsorption aspect changes from that the molecules are bonding directly to the hydrogen bonding to covalent bonding as the surface rather than following a two-layer ad- ethanol and water are evaporated during the sorption mechanism. Large errors are intro- drying. As a result of evaporation, the conduces as the free silanol is consumed and be- densation ofsilanes to each other becomes accomes obscured by the other silanols. Despite celerated due to increased concentration of the large errors at high coverage, it also appears reacting species. Therefore, the molecular that the slope for low coverage is steeper than species chemisorbed onto the silica surface in for higher coverages. Indeed, it was observed a system where the treated silica has been dried in Figs. 5 and 6 that at low coverages the car- are different than in those found in a solution bonyl is hydrogen bonded to a silanol, and at system. higher coverages, the silane molecule stands away from the surface exhibiting a free carEffect of Concentration of HCl bonyl. Using this information, one would exThe GPC chromatograms of physisorbed pect a steeper slope at low coverages because a silane may consume a surface free sitanol silane are shown in Fig. 8 after drying for 1 both by chemically bonding and by hydrogen day as a function of the concentration of hybonding to its carbonyl. However, at higher drochloric acid in the silane treating solution. coverage, the silane would consume a surface Recall that the assignment of GPC peaks is in free silanol only by reaction. It is interesting Table I. The concentration of silane in the to note that this change in slope appears at treating solution is constant at 14.0 mg/ml. approximately 1 m o l e c u l e / n m 2, nearly the For the nonacidic condition, the dimer species same coverage as where the free carbonyl is are the main component of the physisorbed first observed. Considering the fact that some silane. However, in the presence of acid, the molecular weight increases with increasing concentrations of hydrochloric acid. For ex~" 0.040 i c 0.035 ample, the molecular species produced on the 0.030 \ silica surface are mainly trimer species in the 0.025 case of 10 -2 mole/liter and tetramer species 0.020 \ in the case of 10 -~ mole/liter. This shows that \ 0.015 the molecular weight has a clear dependence 0.010 on the concentration of hydrochloric acid. 0 . 0 0 5 E ° - ' - - ~ -i r i 0.000 Figure 9 shows the GPC chromatograms as 2.5 0.5 1.0 1.5 2.0 0.0 a function of concentration of HC1; in this Molecules per nm 2 case, however, the drying time is 14 days. For FIG. 7. Intensityof 3744-cm-~band ( freesurfacesilanol) a nonacidic condition, there is a large distriin Kubelka Munk units as a function of surface coverage. bution in molecular weight of the silane species
+i"
Journal of Colloid and Interface Science, Vol. 143, No. I, April 1991
ADSORPTION ON SILICA
153
lution system, however, the acid stays in the solution and stabilizes the hydrolyzed silanes to form lower-molecular-weight species. Figure 10 demonstrates the relationship between amount of physisorbed silane and drying time as a function of the concentration of hydrochloric acid. Here, the initial amount ofsilane deposited in the nonacidic condition is different from that in the acidic condition, but recall that the concentration of silane in the treating solution is the same. In the nonacidic condition, the amount of silane depos40002000 800 Molecularweight ited is 52 mg/g, and in the acidic condition the amount is about 60 mg/g. This is probably FIG. 8. GPC chromatogramsof physisorbedsilane as a caused by the looser packing of the silica due function of concentration of hydrochloricacid after 1 day of drying. Molecularweight is calibrated by polystyrene to increased ionic repulsion from the presence of hydrochloric acid. Worse packing allows a standard. greater interstitial volume and hence more silane solution, which results in deposition of produced at the silica surface, although the di- more silane. For the nonacidic condition, the mer and trimer species are present as the main amount of chemisorbed silane on the silica component. However, in the presence of acid, surface increases dramatically within 1 day mainly the tetramer and octamer are pro- and gradually increases with increasing the duced. The dependence of molecular weight drying time until the chemisorbed layer saton acid is seen to continue from short to long urates, as mentioned previously. By then, most drying times. of the silane is chemisorbed on the silica surIn previous papers (6, 14~ 15), the hydro- face. However, for the acidic condition, the lysis and condensation behavior of a silane behavior in the chemisorption of silane molcoupling agent were investigated in a solution ecules to the silica surface is different from system, using 29Si NMR. It was found that acid accelerates the hydrolysis of the methoxy groups, -Si-OCH3, in silane molecules ( 16 ). B Concentrationof hydrochloric However, the acid also decelerates the interacid(tool/l) molecular condensation of the silane by sta0 mol/l bilizing the Si-OH moiety. In this case, mainly oligomers are produced. For a nonacidic con10-4mol/l~ , ' ~ dition, higher-molecular-weight species were observed in the solution system. Thus the 102~ovl /E /~/ condensation behavior in the solution system is quite different from that obtained in the de0,mo posited system, whose results are shown here. This difference could be caused by the pH 40002000 800 change due to the evaporation of hydrochloric Molecularweight acid which occurs when the ethanol solution evaporates. Therefore, it seems that the acid PIG. 9. GPC chromatogramsof physisorbedsilane as a accelerates hydrolysis and then evaporates function of concentration of hydrochloric acid after 14 from the silica surface, allowing the production days of drying. Molecularweight is calibratedby polystyof higher-molecular-weight species. In a so- rene standard. B
Conce
Journal of Colloid and Interface Science, VoL 143,No. l, April 1991
154
NISHIYAMA, SHICK, AND ISHIDA 80
%
.~
60@ o
•~ 40
i ~° ; °'"°
<
)
g 0
6
12
18
24
Drying time (day)
FIG. 10. Amount of silane coupling agent physisorbed on silica surface as a function of drying time. Concentrations ofhydrochrolic acid: O, 0, A, 10-4; [3, 10-2; 0, 10-~ mole/liter. that in the nonacidic condition because one can see a substantial increase in the a m o u n t of physisorbed silane as compared with the nonacidic condition. Addition of hydrochloric acid decreases the a m o u n t of chemisorbed silane within 1 day until the concentration of hydrochloric acid is 10 -2 mole/liter. The effects of acid are perhaps explained by changes in charge distribution. For the case of 10 -~ mole/liter, as more acid is added, the a m o u n t of chemisorbed silane is seen to go through a m i n i m u m and then increase. The hydrochloric acid first decelerates the condensation reaction between the S i - O H group of the silane and the S i - O H group on the silica surface, and then begins to increase with additional hydrochloric acid. The amount of physisorbed silane increases with increasing concentration of hydrochloric acid. Therefore, the chemisorption behavior strongly depends on the concentration of hydrochloric acid as catalyst for hydrolysis. This behavior is in good agreement with the stability of silanol which shows the most stable p H to be approximately 2-3 (17). Figure 11 shows the relationship between the a m o u n t of silane and how it is attached to Journal of Colloid and Interface Science, Vol. 143, No. 1, April 1991
the silica as a function of the concentration of silane in solution. Under an acidic condition, it is possible to see a slightly different tendency in the a m o u n t of physisorbed silane as compared with the data for the nonacidic condition. These differences in the physisorbed a m o u n t are probably caused by differences in the silane molecular species which are chemisorbed onto the silica surface. For an acidic condition equal to or above 10 2 mole/liter and for 1 day drying, the trimer or tetramer species are chemisorbed on the silica surface. The experimental conditions are such that the physisorbed silanes are the only source of chemisorbed silane produced after prolonged drying periods. Thus, the G P C chromatograms of the physisorbed silane will give us insight into the species leading to chemisorption, especially after the initial rapid chemisorption period. Those oligomeric species have a larger occupied area and steric hindrance caused by the organic group compared to the m o n o m e r or dimer species. Therefore, the chemisorbed
200
[3
.~ 150
loo
9/°/ 0
10
20
30
40
50
Concentration of silane in solution (rag/ml)
FIG. I I. Amount of silane on silica surfaceas function of concentration of silane in silane solution. O, Chemisorbcd amount; A physisorbed amount; D, deposited amount; *, initial deposited amount under acidic conditions. Concentration of hydrochrolic acid: ~7 (chemisorbcd), • (physisorbed), 10 4; © (chemisorbed), • (physisorbcd), I0-2; * (chemisorbed), ~ (physisorbed), I 0 - i mole/liter.
ADSORPTION
a m o u n t of silane decreases with increasing molecular weight of silane molecules. FT-IR spectra of silane chemisorbed on silica surface are shown in Fig. 12 as a function of the concentration of hydrochloric acid. Recall that the C z O peak is sprit into two peaks, reflecting two different environments. Here, the peak at 1706 cm -~ is assigned to the hydrogen-bonded C z O group and the peak at 1726 cm -1 is attributed to the non-hydrogenbonded C z O group. The peak intensity of the non-hydrogen-bonded C ~ O increases with increasing concentration of hydrochloric acid as shown in Fig. 13. As before, each fraction is obtained by curve fitting. However, in this case, the number of silane molecules chemisorbed per 1 nm 2 are very close as shown in Fig. 10, even though the fraction of the nonhydrogen-bonded component increases with increasing concentration of hydrochloric acid. This is probably caused by the difference in the molecular species chemisorbed on the silica surface. Thus, as mentioned above, the molecular species chemisorbed on the silica surface varies with the concentration of hy-
Numberof chemisorbedsilane molecules per 1 nm2 1.21
~ R~ = 0.2
155
~oo I
75
~ o
5O
25
i
0
i
i
I0 "4
i
I
10-2
Concentration of hydrochloricacid (rnol/l) FIG. 13. C o m p o n e n t fraction o f C ~ O p e a k in F T - I R as a f u n c t i o n o f surface coverage. A, H y d r o g e n b o n d e d ; ©, n o n - h y d r o g e n b o n d e d .
drochloric acid. In 10 -1 mole/liter, mainly the tetramer species are chemisorbed (Fig. 8 ). The higher the molecular weight of silane molecules, the larger is the free volume and steric hindrance of molecular species. As the molecular weight increases it would be expected that it would be more difficult to hydrogen bond to the surface. Therefore, one would expect a higher fraction of the adsorbed species not to be hydrogen bonded to the surface, which is the behavior observed. CONCLUSIONS
1.24
I.I6
18()0
ON SILICA
17150
17bO
16'50
16'00
Wavenumber(cm-i)
FIG. 12. FT-IR spectra ofchemisorbed silane molecules of silica surface as a function of concentration of hydrochloric acid. Concentrations of hydrochloric acid: (A) 10-', (B) 10 -2, (C) 10 -4, (D) 0 mole/liter.
The molecular species chemisorbed on the silica surface are dependent on both the drying time and the concentration of silane in the treating solution. The amount of chemisorbed silane becomes saturated for approximately two molecules for 1 nm z. The specific adsorption behavior depends on the number ofsilane molecules deposited on the silica surface. At low concentration, silane molecules are chemisorbed lying parallel to the silica surface. With increasing amounts of chemisorption, above roughly half the saturated chemisorbed amount, additional chemisorbed molecules are forced to stand away from the silica surface. Journal of Colloid and Interface Science, Vol. 143, No. 1, April 1991
156
NISHIYAMA, SHICK, AND ISHIDA
The molecular species ofsilane chemisorbed on the silica surface also depend on the hydrolysis condition of the silane. Addition of acid followed by drying accelerates the intermolecular condensation of the silane molecules. Increasing acid content increases the molecular weight of the physisorbed silane, which in turn indicates that higher-molecularweight species have been chemisorbed. In addition, the adsorption behavior of silane molecules is dependent on the molecular species chemisorbed on the surface. The higher the molecular weight of the silane molecule, the greater is the free volume of the molecular species, therefore the fraction of the standing component increases. ACKNOWLEDGMENTS One of the authors, N. Nishiyama, acknowledges the financial support of the Nihon University School of Dentistry at Matsudo, Japan. REFERENCES 1. Ishida, H., and Miller, J. D., Macromolecules 17, 1659 (1984).
Journalof Colloidand InterfaceScience,Vol. 143,No. 1,April1991
2. Miller, J. D., Hoh, K., and Ishida, H., Polym. Eng. Sci. Polym. Comp. 5, 18 (1984). 3. Ishida, H., and Miller, J. D., J. Polym. Sci. Polym. Phys. Ed. 23, 2227 (1985). 4. Ishida, H., Polym. Sci. Technol. 27, 25 (1985). 5. Nishiyama, N., Katsuki, H., Horie, K., and Asakura, T., J. Biomed. Mater. Res. 21, 1029 (1987). 6. Nishiyama, N., Horie, K., and Asakura, T., J. Colloid InterfaceSci. 129, 113 (1989). 7. Young, G. J., J. ColloidSci. 13, 67 (1958). 8. Shindorf, D. W., and Maciel, G. E., J. Amer. Chem. Soc. 105, 3767 (1983). 9. Shindrof, D. W., and Maciel, G. E., J. Phys. Chem. 86, 5208 (1982). 10. Miller, J. D., and Ishida, H., Surf. Sci. 65, 443 (1984). 11. Fry, J., and Maciel, G. E., unpublished work, Chemically Modified Oxide Surface Symposium, Midland, MI, June 1989. 12. Tensmeyer, L. G., Hoffmann, R. W., and Brindley, G. W., J. Phys. Chem. 64, 1655 (1960). 13. Hoffmann, R. W., and Brindley, G. W., J. Phys. Chem. 65, 443 ( 1961 ). 14. Nishiyama, N., Asakura, T., and Horie, K., Rep. Prog. Polym. Phys. Japan 30, 601 (1987). 15. Nishiyama, N., Asakura, T., and Horie, K., J. Colloid Interface Sci. 124, 14 ( 1988 ). 16. Savard, S., Blanchard, L. P., Leonard, J., and Prud'Homme, R. E., Polym. Composite 5, 242 (1984). 17. Plueddemann, E. P., "Silane Coupling Agents," p. 52. Plenum, New York, 1982.