Structural characterization of (3-aminopropyl)triethoxysilane-modified

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1776

Anal. Chem. 1988, 60, 1776-1786

Structural Characterization of (3-Aminopropyl)triethoxysilane-Modified Silicas by Silicon-29 and Carbon- 13 Nuclear Magnetic Resonance G. Stephen Caravajal,' Donald E. Leyden,* Gregory R. Quinting, and Gary E. Maciel* Department of Chemistry, Colorado State University, Ft. Collins, Colorado 80523

Nuclear magnetic resonance (NMR) studles have been carried out on a series of samples prepared by derlvatlzatlon of silica gels with (3-aminopropyl)triethoxysilane (APTS) under systematically varled sillca pretreatment temperatures, APTS reaction conditions, and APTS-modifled sillca posttreatment temperatures. %iand I3C technlques employlng cross polarization (CP) and magic-angle spinning (MAS) were used. 13C CP/MAS NMR studies of the structures of the APTSmodlfied slllcas reveal that the amino groups in samples prepared In dry toluene can be either hydrogen bonded or protonated by acidic slianols at the silica gel surface; the relative amount of the protonated form Increases wlth the amount of water present at the dllca surface. Silica hydrath also affects the amlno group chemical envlronments In sampies prepared In aqueous solution. Relative Intensities In 13C CP/MAS NMR spectra are used for determlnatlon of the number of residual ethoxy groups per silane moiety. For samples prepared ln dry toluene, ethoxy group reactions were found to increase as the amount of water at the slllca gel surface increases. For the samples prepared In aqueous solution, the 13CNMR spectra show that reaction of the ethoxy groups has occurred completely. Three different silane chemical environments at the silane/slllca Interface are seen in the %iCP/MAS NMR spectrum, carrespondlng to attached silane moletles with one, two, and three siloxane bonds. The number of siloxane bonds formed durlng the preparation of APTS-modlfied sillca is largest for silica gel samples wlth the largest amounts of water at the silica gel surface. Curing produces an Increase in the number of siloxane bonds at the Interface and Is optimized In the presence of surface water and at curing temperatures greater than 150 OC.

Aminoalkane-substituted silanes, such as (3-aminopropy1)triethoxysilane (APTS), are extensively used for the chemical modification of various silica and alumina surfaces employed in bonded phase liquid chromatography ( I ) , affinity chromatography ( 2 ) ,trace metal analysis ( 3 ) ,immobilization of transition-metal catalysts ( 4 ) ,and as coupling agents in the treatment of glass fibers ( 5 ) . The complete characterization of the chemical state of APTS-modified silica requires a detailed knowledge of the nature of the siloxane attachment to the surface, Le., the product of silylation of the silica surface, as well as the nature of the interactions, if any, between the amino group and other acid or base sites in the system, e.g., the nature and extent of hydrogen bonding and/or Bronsted protonation of the amino group. Numerous physical methods, including a variety of spectroscopic experiments (6-IO),have been applied to the study of APTS-modified silica and closely Present address: Procter and Gamble, Ivorydale Technical

Center, 5299 Spring Grove Ave., Cincinnati, OH 45217.

related systems, including APTS polymers. In spite of these many studies, which have included some useful, albeit fragmentary applications of modern solid-state NMR techniques (7-9), a clear and consistent picture of this complex system has not yet emerged. The potential structural complexity of APTS-modified silica is to some degree represented by the variety of possible chemical structures shown in Figure 1, which does not even include the structural possibility of any unreacted >Si-O-CH,CH, groups in the system. The present paper summarizes a systematic study of APTS-modified silica gel by means of %i and 13C NMR, using cross polarization (CP) and magic-angle spinning (MAS) (11). The 29SiCP/MAS results are extremely useful for characterizing the nature of the attachment between the silane atom of the APTS-derived moiety and the surface. The 13C CP/ MAS results provide useful information on the status of the amino group in these systems. Special emphasis is given to the role of water, as manifested by different silica gel pretreatment parameters, reaction conditions, and reaction product posttreatment temperatures.

EXPERIMENTAL SECTION NMR Measurements. Solid-state 13Cand %i NMR spectra were obtained in natural abundance at frequencies of 50.3 and 39.7 MHz, respectively, on a modified Nicolet NT-200 spectrometer equipped with a home-built CP/MAS unit. All cross polarization spectra were obtained with spin-temperature alternation (12). The CP/MAS probes were based on a doubly tuned, single-solenoidarrangement. Magic-angle spinning was routinely carried out at 1.5-2.0 kHz (13C) and 3.5-4.0 kHz (%i) with rotors machined from Kel-F and Delrin, respectively. Modified silica samples prepared under anhydrous conditionswere used to determine if moisture from the laboratory air was altering samples during CP/MAS experiments. In test experiments, no 13Cor =Si spectral differences were found between samples spun with dry nitrogen and compressed air, so the latter was used for all samples on which data are reported in this paper. For 13C spectra, the magic angle was adjusted to within 0.lo using the 79Brspectrum of a sample of KBr placed in the spinner (13). 13C spectra were obtained with a 1-ms contact time and a recycle of 1 s. 29Sispectra were acquired with a 5-ms contact time and a recycle time of 1 s. Solid-state W i CP/MAS NMR spectra were obtained with 1K data tables and a spectrum width of 20 kHz. A total of 10000-40000 accumulations were coadded for each spectrum. 29SiNMR spectra were obtained by using the CP sequence with the flip-backfeature (14). 13Cand %i spectra were externally referenced to liquid tetramethylsilane (TMS), based on substitution of hexamethylbenzene and tetrakis(trimethy1silyl)methane, respectively. All chemical shifts in this paper are reported in parts per million, with lower values corresponding to higher shielding. Solution-state 13C and 29SiNMR spectra were acquired on a Bruker WP-2OOSY spectrometer at 50.3 and 39.7 MHz, respectively. Spectra were obtained in the pulse mode with simultaneous broad-band decoupling. Spectral widths of 20 kHz (W)or 10 kHz (29Si)with 16K data tables were used. NMR spectra were recorded on samples that were 15-25% (v/v) in deuteriated chloroform, used as solvent and for deuterium lock, with TMS as an internal chemical shift reference.

0003-2700/88/0360-1776$01.50/0 0 1988 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988

1

1

/ / / / / / / / / / / / / / / / / /

1777

I

/////

Figure 1. Structural possibilities anticipated for APTSderivatized silica gel.

Spectral decomposition of overlapped peaks in solid-stateNMR spectra was accomplished by utilizing the standard Nicolet 1180 software (NTCCAP). Each spectrum was decomposed by assuming a minimum number of contributing resonances capable of satisfactorily representing the observed spectrum. For each resonance, the chemical shift, line width, and intensity were treated as variable parameters. A 100% Gaussian line shape was used for all spectral lines. Reagents. Reagent grade toluene (Fisher Scientific Co.) was refluxed over sodium metal and distilled under dry nitrogen prior to use. As a comparison, a freshly opened bottle of toluene was dried over molecular sieves (J. T. Baker) without distillation. Solid-state 13Cand 29SiNMR spectra of APTS-modified silica samples prepared by using distilled toluene and toluene dried over molecular sieves showed no spectral differences,so the latter was used for all reactions reported in this paper. The (3-aminopropy1)triethoxysilane(APTS) and n-butyltrimethoxysilane (BTMS) were obtained from Petrarch Systems, Inc. The silanes were vacuum distilled immediately prior to use, with the first 5 mL of distillate being discarded. The purity of the distillate was confirmed by solution 13C and 29SiNMR. Vacuum distillation was found to be essential in order to obtain reproducible results. N-Propylamine (NPA) and 1,3-diaminopropane (DAP), both from Eastman Kodak, were dried over molecular sieves prior to use. A freshly opened bottle of absolute ethanol (AAPER Alcohol and Chemical Co.) was used for adsorption studies. Deuteriated chloroform (Cambridge Isotope Lab.) was used as received. Sample Preparation. A slurry of silica gel (J.T. Baker, 60-200 mesh, surface area 290 m2/g) in water was filtered and dried under vacuum Torr) for 24 h at 25 "C (room temperature, RT), 110 or 200 "C to obtain silica with varying degrees of surface hydration. Silica gel samples pretreated in this manner were reacted with a 5% (v/v) solution of APTS (or BTMS) in toluene for 1 h at 25 "C under dry nitrogen. Each slurry was suction filtered and washed with three 200-mL portions of dry toluene. APTS-modified silicas prepared from the pretreated silica gel samples were split into five portions, producing three sets of samples, each set containing five fractions. Each of the five fractions from each of the three sets of samples was dried under vacuum (lo-* Torr) for 24 h at one of the following five temperatures: 25,65, 110, 150, and 200 "C. This set of 15 samples provided the basis for studying the combined effects of silica hydration and curing on the structure of APTS-modified silica prepared in a toluene medium. For another series of samples, a 5% (v/v) aqueous solution of APTS was reacted with silica at 25 "C for 1h; the fiitered product was washed with water and split into five portions. Each fraction was dried at one of the five temperatures listed above. All samples were stored in a desiccator under dry nitrogen, and their NMR spectra were obtained within 2 weeks of the time the samples were initially prepared. Percent carbon and percent nitrogen were determined (Huffman Laboratories) for all of the modified silica samples indicated above; prior to elemental analysis, each sample was heated at 200 "C under vacuum for 24 h. In experiments designed to examine the NMR characteristics of adsorbed, rather than covalently bound, species, a 5% (v/v) solution of NPA or DAP in dry toluene was mixed under dry nitrogen with three different silica gel samples previously dried at 25, 110, and 200 "C, respectively. Each mixture was stirred

Table I. APTS Loading Levels of Si02-APTS(A,200) Samples Based on Nitrogen Contents (millimoles of APTS per gram of silica gel)a 200 series

110 series

RT series

AQ series

0.58

0.63

1.19b

1.04

'Results of elemental analyses (Huffman Laboratories). bAnalysis for Si02-APTS(RT,RT) sample is 1.27 mmol of APTS/g. for 1 h at 25 "C. After this treatment, each slurry was suction filtered and washed with three 200-mL portions of dry toluene. These treated silica samples were dried at 25 "C for 24 h and stored in a desiccator.

RESULTS AND DISCUSSION 1. Overall Patterns. Table I summarizes the nitrogen contents of all of the AE'TS-derivatized silica samples prepared from a given silica sample (a specific series) and cured (dried at Torr) at a temperature of 200 "C. One sees that the highest APTS loading level is achieved with silica gel that has initially been dried at 25 "C (room temperature or RT series) and Torr, and the smallest loading level is achieved with Torr prior to silylathe 200 Series (dried at 200 "C and tion). The designations shown for each column in Table I (200 series, 110 series, RT series, AQ series) specify the conditions under which the silica gel was prepared prior to the silylation reaction in dry toluene. The 200 series, 110 series, and RT series refer to experiments that began with heating the underivatized silica to 200 "C or 110 "C or 25 "C, respectively, at Torr prior to silylation, and the AQ series consisted of samples resulting from silylation reactions carried out in an aqueous medium. All modified silica samples in this paper will be designated by the notation Si0,-X(A,B), where X represents the reagent used in the surface modification (adsorption or silylation) and A and B indicate the temperature a t which the sample was dried (at Torr) prior to or after, respectively, the treatment of the silica with X. Figures 2 and 3 show the 29Siand 13CCP/MAS spectra, respectively, of the set of five samples obtained for each of four series of APTSderivatized silica gels. The numbers given at the left sides of Figures 2 and 3 specify the temperatures at which each Torr. sample was dried (cured) a t Inspection of the spectra represented in Figures 2 and 3 yields no evidence of spinning sidebands (11). Hence, we conclude that the effective chemical shift anisotropies of the 13Cand 29Siresonances of these systems are small compared with the modest MAS frequencies employed. For both the 29Siand I3C NMR spectra, structural assignments are indicated a t the bottom of each figure. These assignments, and others discussed below, are based on solid-state and liquidsample NMR data taken from the literature (7-9, 15-19). The %i spectra shown in Figure 2 have major peaks in the regions of -49, -58, and -66 ppm, due to the specified types

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ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988

200 Series

RT Series

110 Series

POST REACT 1UN

150

A

RT -100

-50

-50

-150

-100

-150

R’=

I -58 PPM I

0‘

CUHlNG TEMPERATW 1

‘cl

*

c2

1

-150

F

F 81

R

Y

0’6’0

d6

0

IV

v

VI

I

?

1

e’i-0-p-0-ai-

m

111

samples 110 Series

200 Series Cl

‘0

I1

SO,-APTS(A,B)

PUST REACT I ON

-100

-50

/ / / I / / / ////I/

I

spectra of

-150

-100

R , s l , ~R‘o-71‘ 0-91-

Q

Figure 2. 2gSi CP/MAS

-50

I -66 PPM I

b EIO-?-OR’

n o r Et

9

ppn

I -Y9 PPM 1

R= CH2CH2C~2~~2

AQ Series

AQ Series

RT Series

c3

1

-1

60

40

20

60

0

p, \

I

40

20

0

P

kH CH 0 -

/

3\*

I

I’lppm Figure 3. I3C CP/MAS

spectra of

90,-APTS(A,B)

\

58ppm

43

21

IO ppm

samples.

of silane silicons of the attached +SiCH2CH2CH2NH2moiety, and peaks a t -100 and -109 ppm, due to (+Si-O),SiOH and (+Si-O),Si moieties of the silica surface. No peak is observed at -45 ppm, which is the 29Sichemical shift of neat, liquid APTS and is the position expected for the 29Siresonance of APTS that is physisorbed at the silica surface. The I3C spectra shown in Figure 3 have major peaks at about 56 and 17 ppm, due to the CH2and CH3 (aand 0)carbons of unreacted ethoxy groups and marked in Figure 3 with asterisks, and peaks at about 43, 27-21, and 10 ppm, due to the C1, C2, and C3, carbons, respectively, of the H2NCH2(1)CH,(2)CHz(3)Si+ moiety. 2. 29SiNMR Spectral Details. The 93 CP/MAS NMR spectra are expected to provide valuable information on the nature of the attachment of the silane silicon to the surface and on the bonding patterns of that Wit silicon atom (where R = CHzCH2CH2NH2).Trends in the details of such issues

can be gleaned from the spectra by examining patterns of relative intensities. Qualitatively, the relative intensities of the silane resonances shown in Figure 2 for the Si02-APTS(A,RT) samples represented in Table I show the same pattern of surface-silane loading levels as indicated by the microchemical nitrogen analysis data of Table 11, except for the apparent permutation of order for samples of the 200 series and 110 series. However, one should note that the intensities represented in the spectra of Figure 2 were not plotted on the basis of an absolute intensity scale, so they should not be compared from one spectrum to another. The meaningful relative intensity comparisons are within a given spectrum. Relative intensity patterns can be extracted in principle from partially overlapping peaks by deconvolution, followed by integration. Of course, such patterns are quantitatively useful only to the extent that the individual peak intensities can be related quantitatively to the relative concentrations

ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988

Table 11. Relevant 29SiCross Polarization Parameters" peak, ppm

TSiH,

ms

TlpH,

ms

1*/1(5

ms)b

SiO2-APTS(2O0,RT) -49 -58 -100 -109c

1.42 f 0.15 1.25 f 0.32 1.34 f 0.18 2.39 f 0.15

-49 -58 -100 -109

2.06 0.92 0.97 3.61

f f f f

-58 -66 -100 -109c

1.13 0.96 0.91 2.74

f f f f

-58 -66 -100 -109c

0.64 f 0.85 f 0.58 f 1.81 f

-58 -66 -100 -109c

1.72 1.36 2.89 5.70

8.25 f 0.50 8.90 f 1.71 44.4 f 11.5 >500

1.89 1.79 1.15 1.15

Si0,-APTS(ll0,RT) 0.07 0.03 0.05 0.06

4.87 f 0.13 7.75 f 0.12 191 f 7 1 >500

3.06 1.91 1.03 1.35

Si0,-APTS(RT,RT) 0.05 0.02 0.05 0.06

3.74 f 0.11 5.66 i 0.06 46.8 f 3.8 >500

3.85 2.43 1.12 1.20

Si0,-APTS(AQ,RT) 0.02 0.02 0.04 0.03

3.88 f 0.04 4.02 f 0.01 35.7 f 2.0 >500

3.63 3.48 1.15 1.20

SiO,-APTS (RT,200) f 0.08 f 0.02 f 0.10 f 0.09

11.1 f 0.4 15.6 f 0.2 10.2 f 0.2

>500

1.66 1.41 1.98 1.73

Obtained from variable-contact-time experiments. I * / I ( T ) calculated from eq 1 for T = 5 ms. cInterpretation of values derived for the -109 ppm peaks is difficult because of the distribution of structural situations contributing to this peak, as described in the text. of the corresponding individual types of silicon environments displayed in the spectra. This issue can be evaluated, and appropriate corrections made, only if values are known for the pertinent NMR relaxation parameters, namely THsi(the 1H-29Si cross-polarization time constant) and TlpH (the rotating-frame lH spin-lattice relaxation time). Pertinent data on a few representative APTS-derivatized silica samples were obtained from variable contact-time experiments and are summarized in Table 11. This table lists, in addition to TsiH and TlpH,the values of I * / I ( T )calculated from eq 1 (20) for T = 5 ms. The quantity, I*, is the observed magnetization

I ( T ) = I * ( 1 - e-r/THSi)(e-r/TlpH)

(1)

(in this case, 29Si)one would obtain if cross polarization were infinitely fast and rotating-frame spin-lattice relaxation were infinitely slow; I ( T ) is the 29Si magnetization one actually obtains with the CP contact time ( T ) ; the quantity I* is defined by the equation I* = I0(yH/ysi),where I,, represents the equilibrium (29Si)magnetization. The factor, I * / I ( T ) ,evaluated for the contact time actually used in the experiments (5 ms in the present case), is the factor one should apply to the intensity obtained directly from the spectrum in order to have numbers that can be used for analytically meaningful comparisons. Examination of Table I1 reveals that there is substantial variation in each of the parameters, T s ~ HT,l p ~and , I * / I (5 ms) among the various 29Si resonances represented. One should note that the -109 ppm peak represents Si(OSit), silicon atoms near the surface, with no directly bonded OH groups. This broad structural category includes all Si environments of the type, Si-O-(-Si-O-)"H, with n (the number of -0-Si-0- bridges separating the silicon atom of interest (Si) from a hydroxyl proton a t the surface) having, in principle, any integer value greater than 0. Associated with this kind of structural heterogeneity, one expects a dramatic

1779

variation in TsiHand TlpHvalues for individual structural environments. There are many more Si environments corresponding to large n values than for small n values, but the CP characteristics for larger n values are likely to be less favorable than for small n values. Hence, it is difficult to know how to interpret Tsxand TlpHvalues in terms of the structural entities they represent. For each sample the -109 ppm peak has the largest TsiH and TlpHvalues, and the combination in each case yields a I*/I ( 5 ms) value that is intermediate among the values for other peaks in the spectrum. Nevertheless, because of the structural heterogeneity described above for the -109 ppm peak, we will not refer to that peak in any further discussions in this paper. For R-Si< resonances of each individual sample represented in Table I1 (the -49, -58, and -66 ppm peaks), the I*/Z (5 ms) values are within 38% of each other. On this basis we can conclude that, although the relative 29SiNMR intensities of the R-Sit resonances shown in Figure 2 cannot be used directly in a strictly quantitative fashion, it appears that the correct qualitative trends can be deciphered directly from the relative intensities seen in the spectra. If one examines the SiO,-APTS(A,RT) spectra in Figure 2 and corresponding data summarized in Table 11, it is seen that the apparent unusually high degree of silylation for the R T series, Le., sample Si0,-APTS(RT,RT), cannot be "explained away" in terms of the I * / I (5 ms) values. The values for the -109 ppm peak are constant within about 10% for this series and these quantities are bigger for the R-Sit peaks for the SiO,-APTS(RT,RT) sample than for the other members of this series (except for the -66 ppm peak of the SO,-APTS(AQ,RT) sample). This is consistent with the pattern of loading levels shown in Table I. Taking the values of I*/I ( 5 ms) into account also makes it possible to rationalize the apparently larger degree of silylation implied by the uncorrected intensities shown in Figure 2 for the 200 series compared with the 110 series. In Table I1 we see that the I * / I ( 5 ms) value for the -49 ppm peak is about 62% larger for the 110 series than for the 200 series, whereas for the -100 ppm peak the values are about 10% smaller for the 110 series. Because of the consequences of spin dynamics, as described above in terms of I * / I (5 ms) values, most of the 29SiNMR considerations and conclusions of this paper must be considered qualitative. In a few cases, for which the relevant spin dynamics have been characterized quantitatively on the appropriate samples, quantitative conclusions are warranted. For each series represented in Figure 2, we can see qualitatively a shift of intensity from the -49 ppm peak to the -58 ppm peak to the -66 ppm peak as the postreaction curing temperature is increased. Examination of the peak assignments given at the bottom of the figure reveals the qualitative significance of this trend as the curing temperature increases, the average number of Si-0-Si attachments between the silane silicon atoms and the surface (or other silane silicons) increases. For the 200 series (silica samples dried at 200 "C and lo-, Torr prior to silylation), the siloxane region of the spectrum for the sample cured a t room temperature (RT) is dominated by the -49 ppm peak identified with Si-O-Si(R)(OEt)(OR'). A much smaller peak is seen at -58 ppm, identified with silane silicons that have two Si-0-Si attachments to the surface (or to other silane silicons); there is no peak at -66 ppm for R-Si silicons with three Si-0-Si linkages. The ratio of intensities between the -49 and -58 ppm peaks decreases at higher curing temperatures until it becomes less than one at the highest curing temperature of 200 "C. For a 150 "C curing temperature, the product from the 200 series shows a small peak a t -66 ppm due to R-Sit silicons with three Si-0-Si linkages to the surface (or to other RSi silicons); for the 200 series sample cured at 200 "C, the -66 ppm peak

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ANALYTICAL CHEMISTRY, VOL. 60, NO.

17, SEPTEMBER 1. 1988

Table 111. Data Obtained from the %Si NMR Spectra of Si0,-APTS(A,B) Samples, Describing the Types and the Percentages of Molecular Structures Present at the Silane/Silica Interface relative intensities for each Si NMR A,B

-49 ppm

peak position,a % -58 ppm -66 ppm

corrected relative intensities: % -49 ppm -58 ppm -66 ppm

Na,(29Si)b

Nav(13C)b

23 0 1.5 1.6 0 77 200,RT 76 24 19 0 1.6 1.4 0 81 110,RT 73 27 69 31 0.0 0.26 0 59 41 0 RT,RT 0 0 60 0 44 56 RT,200 0 40 0 0 34 0 67 33 AQW 0 66 a Corrected intensity = (uncorrected intensity) X (1*/1(5 ms))(IOO%)/(totalcorrected intensity). bNa,(29Si) and Na,(l3C)have been calculated from the relative intensities corrected according to eq 1. ~~

becomes a still larger fraction of the total silane silicon signal. Similar trends are observed for the 110 series, although the overall intensities of the R-Si signals are sufficiently smaller (vide supra) that these trends are not so apparent, especially for the -66 ppm peak. For the R T series, prepared from silica gel that was initially Torr, evidence for the -49 ppm peak, dried at 25 “C and identified with silane silicons having only one Si-0-Si linkage (presumably to the surface), is found only for the sample cured at room temperature. In this series, the dominant structure for low-temperature curing (RT and 65 “C) is R-Sit moieties with two Si-0-Si linkages (-58 ppm) and for high curing temperatures (150 and 200 “C) is R-Sit moieties with three Si-0-Si linkages (-66 ppm). Similar trends are seen in the spectra of the AQ series, based on silylation reactions carried out in aqueous media, although there is no clear evidence for the -49 ppm peak even for the lowest curing temperature. The spectra shown in Figure 2 provide no evidence of the presence of a pentacoordinate silicon, with a N-Si bond, as proposed by Pleuddeman for the +Si-CH2CH2CH2NH2system (21). Such a structure would be expected to display a 29SiNMR resonance in the region, -120 to -180 ppm (22,23), and its absence in the ?3i spectrum of APTS-derivatized silica has been noted previously (9). Determination of the number of ethoxy groups remaining unreacted after the initial silylation procedure and/or after curing would enable one to deduce the average molecular structure at the silane/silica interface and provide information about the hydrolysis/condensation reactions. Although the 13C/MAS results provide information that is somewhat more direct on this point (vide infra), examination of the 29Si CP/MAS results from this point of view is also worthwhile. As indicated earlier in this section, three main peaks associated with attached silane moieties in different molecular environments are present in the spectra of Figure 2. ?3i peaks at -49, -58, and -66 ppm are assigned to structures I, I1 + 111, and IV V VI, respectively, in Figure 2. These 29Si chemical shift/structure assignments were based on previous solid-state 29Si CP/MAS NMR results (15, 16) and are in agreement with results recently reported by Sudholter and co-workers (9). On this basis, we can see that *?3i CP/MAS NMR cannot distinguish between silicon nuclei in structures I with R’ = H and with R‘ = Et, because of the similarity of their expected chemical shifts (16). However, another structure, -Si(R)(OH),, with a known 29Sichemical shift of about -42 ppm, can be ruled out as a significant contributor for all of the APTS-derivatized silica samples of this study, because such a peak is not observed in any of the spectra shown in Figure 2. The differences among structures I, II/III, and IV/V/VI in Figure 2 are the number and types of siloxane bonds and the number of residual ethoxy or uncondensed hydroxy groups per attached silane moiety. As noted above, the number of attached silane moieties with one, two, and three siloxane bonds can be determined from the relative intensities at -49,

+ +

-58, and -66 ppm, respectively. Hence, the percentages of structures I, II/III, and IV/V/VI in a sample of APTSmodified silica can be determined from the relative intensities in its 29SiNMR spectrum, to the extent that these NMR intensities of the silane silicons can be used directly or have been suitably corrected by the factor, I * / I (5 ms). This approach was applied to the 29SiNMR spectra of Figure 2 and the results are summarized in Table 111, along with other results to be discussed below. 3. ‘3c NMR Spectral Details. The 13C CP/MAS spectra are expected to provide information on the structure/dynamics of the +SiCH2CH2CH2NH,moiety not provided by the 29Si NMR data, e.g., the nature of the amino groups’ interactions or reactions with other functional groups (e.g., see Figure 1). The most apparent trends seen in spectra displayed in Figure 3 relate to the intensities of the CH3 and CH2peaks of the ethoxy group at 17 and 56 ppm, respectively. The spectra in Figure 3 show definite trends to lower ethoxy contents as the curing temperature is increased within a given series or progressing from the 200 series to the 110 series to the R T series to the AQ series for a given curing temperature. There is no evidence of any unreacted ethoxy groups for the AQ series of samples. Variable-contact-time 13C CP/MAS experiments were carried out on representative examples of the modified silica gel samples of this study. The results on TCH, TlpH, and I * / I (1ms), defined by eq 1for T = 1ms, are summarized in Table IV. One sees that for the silylated silica gel samples of this study, the I * / I (1 ms) values are within about 21% of each other for the C1, C2, and C3 carbons of the (aminopropy1)silane moiety and the a and p carbons of the ethoxy groups of these particular samples. Hence, it seems reasonable to use the relative I3C NMR intensities shown in Figure 3 directly, without correction by eq 1,for qualitatively examining trends in these samples. The fact that no ethoxy carbon signals are seen in the spectra in the AQ series, the fact that only relatively small ethoxy carbon signals are observed in the spectra of samples prepared from silica gel dried at only room temperature, and the general “horizontal” trend stated above for a given curing temperature, show that reactivity of the +Si-OEt moieties to surface silylation, >Si-O-Si+ condensation, and/or hydrolysis is enhanced by increased hydration of the silica gel surface. Of course, +Si-OEt hydrolysis to +Si-OH may be an important step in the silylation or condensation processes occurring at the silica surface. Some of these issues are addressed in more detail below. The “vertical” trend summarized above for the depletion of ethoxy carbon intensity with increasing curing temperature for a given series implies that one or more of the following four processes is (are) responsible: (a) hydrolysis of +Si-OEt groups, (b) surface silylation of surface silanols by >Si-OEt groups, (c) surface condensation of +Si-OEt groups by K>Si-OH groups, and (d) the desorption of surface-adsorbed ethanol that has been generated during the surface modifi-

ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988

Table IV. TcHand

TlpH

1781

Values of Modified Silica Gel"

TCH(5, ms) c3

Si0,-X(A,B)

c1

c2

SiO,-APTS(RT,RT) Si02-APTS(110,RT) SiO,-APTS(llO,RT)H* Si02-BTMS(RT,RT)dte Si0,-DAP(RT,RT) Si02-NPA(RT,RT)

0.16 f 0.02 0.06 f 0.01 0.04 f 0.01 0.20 f 0.02 0.50 f 0.01 0.76 f 0.05

0.14 f 0.02 0.05 f 0.01 0.05 f 0.01 0.20 f 0.02 0.27 0.03 1.02 f 0.02

0.15 f 0.02 0.04 f 0.01 0.04 f 0.01 0.20 f 0.02 0.50 f 0.01 1.92 f 0.15

*

Ca

CB

0.097 f 0.011 0.075 f 0.001 0.08 f 0.01

0.32 f 0.07 0.34 f 0.01 0.36 f 0.01

tip^ (ms) SiO,-X(A,B) Si02-APTS(RT,RT) SiO,-APTS(llO,RT) Si0,-APTS( 11O,RT)H+ Si02-BTMS(RT,RT)dre Si02-DAP(RT,RT) Si02-NPA(RT,RT)

c1

c3

c2

4.3 f 0.6 14.3 f 2.0 13.5 f 2.0

>500

3.4 f 0.1 2.9 f 0.1

5.0 f 0.6 8.51 f 0.18 15.0 f 2.9 >500 5.0 f 0.3 5.0 f 0.2

5.4 f 0.6 15.8 f 0.4 14.9 f 0.4

c1

c2

SiO,-APTS (RT,RT) Si02-APTS(l10,RT) Si02-APTS(llO,RT)H+ Si02-BTMS(RT,RT)d*e Si02-DAP(RT,RT) Si0,-NPA(RT,RT)

1.26 1.07 1.08 1.01 1.55 1.93

1.22 1.12

1.07 1.01 1.25 1.96

CB

7.3 f 0.5 15.8 f 0.4 41.2 f 10.5

7.1 f 1.2 36.6 f 1.2 24.0 f 1.0

>500

3.4 f 0.1 14.4 f 2.7 Z*/Z(I mdb c3

Si02-X(A,B)

Cff

1.21

1.06 1.07 1.01 1.55 2.64

Cff

CP

1.15 1.06 1.02

1.20 1.08 1.11

aThe numbering system for APTS and BTMS identifies C3 as the CH2 group adjacent to silicon. For APTS, NPA, and DAP, C1 is defined to be the CH2group adjacent to amino group. C3 in the case of NPA is the methyl group. The CH3group for BTMS is not assigned a number. b1*/Z(7)calculated from eq 1 for 7 = 1ms. CAPTS-derivatizedsilica sample: Si0,-APTS(ll0,RT) protonated by gaseous HCl in toluene suspension. The sample of Figure 9B. dFor the methyl carbon TCH= 0.30 f 0.04, T l o > ~500 ms. eC1 and C2 peaks overlap. cation process by any combination of a, b, and c and would, according to this argument, be present primarily in samples represented toward the bottom-left regions of Figures 2 and 3. The desorption process seems unlikely, on the basis of the following fact, to be the main driving force for the vertical trend of ethoxy depletion. Although a 13CCP/MAS spectrum of adsorbed ethanol can readily be obtained on a sample of silica gel that has been treated as a slurry with absolute ethanol, only very faint CH3CH20- signals can be found in the 13C CP/MAS spectrum obtained on a ethanol-treated sample of silica gel that is more closely related to other samples of this study. This sample was prepared by first drying the silica gel at 25 OC (at lo-' Torr) and then treating it with a dry toluene solution of ethanol in a concentration (about 5% (w/w)) that could be produced by a stoichiometric reaction of APTS in the silylation systems of the present study and Torr). finally "curing" the sample a t 25 "C (at The lack of substantial CH3CH2-0- 13C NMR signals for a Si0,-EtOH(RT,RT) sample, prepared at ethanol concentrations comparable to those that could result from the Si02-APTS(A,B) samples of this study, indicates that the ethoxy signals seen in the spectra of Figure 3 are not due to physisorbed ethanol and hence must be due to unreacted EtO-SifR moieties. The CH2and CH3chemical shifts found in the 13C NMR spectrum of neat, liquid APTS are 58.2 and 17.0 ppm, respectively, very close to the ranges observed for adsorbed ethanol (not shown), 58.3-60.2 ppm and 16.0-17.4 ppm, respectively. It is perhaps unfortunate that the positions of the ethoxy signals in the AP'rS reagent and its derivatized systems are apparently so relatively insensitive to the detailed structural environment that the 13C spectra do not appear capable of delineating fine structural detail a t this time. Assuming that the I * / I (1ms) corrections given in Table IV for the APTS-derivatized silica samples are nearly constant (within experimental error) for all such samples of this study, one can readily calculate the ratio of the integrated intensity

of CLY to that of C1, which should give the average number of residual ethoxy groups per silane moiety in these APTSderivatized silicas. Then, assuming from results discussed in the preceding paragraph that the entire ethoxy 13C intensity is due to unreacted R-SifOEt moieties, we can obtain values for the average number of unreacted ethoxy groups per silane as determined by 13CNMR. The results are moiety, Arav(13C), summarized in Table IV; those values relevant to the samples of Table I11 are also included there. Close examination of the spectra in Figure 3 reveals that, although the shapes and positions of the C1 and C3 resonances are rather invariant among the samples, the C2 resonance shows significant variations, especially among the samples of the R T and AQ series. One sees for the C2 peaks in those series a shift of intensity from higher to lower shielding as the amount of hydration of the surface is decreased, say moving horizontally from the AQ series to the R T series or vertically upward within either series. In order to examine this situation more closely, 13C CP/MAS spectra were obtained on APTSderivatized silica samples that were "cycled" through various stages of hydration; the results are summarized in Figures 4 and 5 . Figure 4 shows explicitly the effects of the state of hydration on samples prepared by APTS derivatization of a silica gel Torr-i.e., the realm of sample first dried at 25 OC and the R T series of Figure 3. Parts B and C of Figure 4 correspond to Si0,-APTS(RT,RT) and Si02-APTS(RT,200), respectively; they show a dramatic change in line shape and corresponding shift in intensity for the C2 peak to lower shielding, centered a t about 27 ppm for the Si02-APTS(RT,200) sample (Figure 4C), compared to the C2 peak of the Si0,-APTS(RT,RT) sample (Figure 4B). Parts D and E of Figure 4 show the effects of saturating the Si02-APTS(RT,200) sample with liquid water (4D) and then drying the water-saturated sample a t 200 "C and lo-' Torr (4E). The water-saturated sample yields a I3C CP/MAS spectrum in

1782

ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988

-60

YO

20

0

Flgure 6. I3C CPIMAS spectra employing a 250-ps extension of 'H spin locking prior to cross polarization: A, spectrum of SO,-APTS(AQ,RT); B, spectrum of SO,-APTS(RT,RT); C, spectrum of SO2APTS( 110,RT).

I

BO

"

~

I

'

"

I

60

' ' YO

'

I

'

"

I

'

"

0

2D

i

-20

Flgure 4. 13C CP/mAS spectra of APTSderivatized silica gel samples: A, sample SiO2-APTS(25,25) treated with 50 % relative humidity; B, SiO,-APTS(25,25) sample; C, SiO,-APTS(RT,200) sample; D, sample SiO2-APTS(RT,200) saturated with H,O; E, the sample of D heated at 200 "C and lo-, Torr.

1

YO

'

'

'

20

l

~

~

'

I

0

Figure 5. I3C CPIMAS spectra of APTSderivatized silica gel samples from the AQ series: A, sample Si0,-APTS(AQ,RT); B, sample SiO,-APTS(AQ,200); C, sample SiO2-APTS(AQ,200) treated with 50 % relative humidity.

which the C2 resonance is shifted dramatically to higher shielding (centered at about 21 ppm), and in which the peaks for all three carbon sites are substantially narrowed, relative to the Si02-APTS(RT,200) case. Heating the water-saturated sample a t 200 "C and Torr restores the 13C CP/MAS spectrum (Figure 4E) to essentially that of the initial Si02APTS(RT,BOO) sample, i.e., imparts a low-shielding shift to the C2 resonance and broadens the peaks. Placing the Si02-APTS(RT,RT) sample (4B) into an environment of 50% relative humidity produces a spectrum (4A) in which the C2 resonance is markedly sharpened and shifted to higher shielding (with a maximum a t about 21 ppm). The other feature that is apparent in Figure 4 is the fact that either saturating sample Si02-APTS(RT,200) with water or subjecting sample Si02-APTS(RT,RT) to 50 % relative humidity eliminates ethoxy peaks from the corresponding 13C CP/MAS spectra, demonstrating the completion of Si-OEt cleavage in the +Si-OEt moieties of these samples by excess H,O. Similar results were obtained on samples derived by silylation of silica gel in aqueous media (the AQ series). Figure 5 shows the effects of curing the sample at 200 "C (Figure 5B) rather than at 25 "C (Figure 5A) and the effect of subjecting the Si02-APTS(AQ,200) sample to an environment of 50% relative humidity. The cycling of the C2 peak from higher shielding (Figure 5A) to lower shielding (Figure 5B) and again to higher shielding (Figure 5C) is again apparent in this case.

Therefore, irrespective of the degree of hydration of the silica gel employed in the silylation process, 200 series or AQ series, it appears that decreasing the amount of water on the surface of the silylated surface brings about a reversible shift of the C2 resonance to lower shielding and a corresponding increase in the width of the C2 peak. This increase in line width is probably a consequence of superimposing an additional resonance at about 2 1 ppm (e.g., Figure 4A,D, Figure 5A,C) on a peak a t about 27 ppm that dominates the C2 region for highly dehydrated samples (e.g., Figure 4C,E, Figure 5B). Since the chemical shift of the carbon two bonds removed from the nitrogen is known to be very sensitive to the electronic environment of nitrogen in an aliphatic amine (26,27), this C2 pattern is interpreted here in terms of structural changes occurring in the amino group. The presence of an additional C2 peak a t about 21 ppm in the I3C CP/MAS spectra of partially hydrated samples was confirmed by the application of a double-exponential-multiplication technique (17) to the free induction decay (results not shown), as well as a simple modification of the CP/MAS technique (28). In this modification a 250-ps time extension is introduced during the 'H spin lock period, before cross polarization. During this extended period, those protons that are most strongly affected by rotating-frame 'H spin-lattice relaxation (e.g., those close to paramagnetic impurities or 'H fields that fluctuate at 50 kHz) will experience a preferential attenuation of magnetization and will correspondingly contribute less effectively to cross polarization in a later stage of the experiment. Then, if those protons whose magnetizations are selectively attenuated are largely associated with I3C nuclei that are mainly responsible for the broad features of the 13C resonance for C2, introduction of the spin-lock extension period should yield a narrower 13C peak in the resulting CP/MAS spectrum. Figure 6 shows the results of applying this technique to three of the samples represented in Figure 3: Si0,-APTS(AQ,RT), Figure 6A; SiO,-APTS(RT,RT), Figure 6B; and Si02-APTS(llO,RT), Figure 6C. These spectra indicate that the broad C2 peak in the 21-30 ppm range is composed of at least two components, one centered a t about 21-22 ppm and one at about 25-27 ppm, and that the relative contributions of these two peaks (and the corresponding structures) depend upon the amount of water present a t the silica surface. Observation of a water-dependent 13Cpeak at about 21 ppm has previously been reported in the solid-state 13C NMR spectra of APTS-derivatized Cab-0-Si1 (8), prepared in an aqueous medium. The authors attributed the high-shielding shoulder at about 21 ppm to hydrogen bonding of the +SiCH2CH2CH2NH2amino group with silanols of the silica surface and/or of hydrolyzed R-SifOEt groups; they also reported that the high-shielding shoulder a t 21 ppm disappeared after heating the silylated Cab-0-Si1 sample a t 130 "C for 24 h. Their interpretation was that heating the sample

ANALYTICAL

60

PO

20

,

0

Figure 7. 13C CPlMAS spectra of silica gels derivatized by n-butyltrimethoxysilane: A, spectrum of Si02-BMTS(200,RT); B, spectrum of SO,-BMTS(RT,RT); C, spectrum of SiO,-BMTS(RT,200).

under vacuum (curing) produced condensation of silane silanols with silica silanols, forming siloxane bonds that destroy the hydrogen bonding in which the amine group participated. An alternate or more explicit explanation emerges from the results that follow here. Implication of the amino group in determining the waterinduced shifts in the C2 resonance of APTS-derivatized silica gel is further indicated by contrasting the 13C NMR spectral patterns described above with those obtained for the analogous silylating agent, n-butyltrimethoxysilane (BTMS), CH3CHzCHzCHzSi(OCH3)3.The 13C CP/MAS spectra shown in Figure 7 correspond to samples prepared from the reaction of a dry toluene solution of BTMS (5% (v/v)) with silica gel that had been previously dried at 200 "C (Figure 7A) and 25 "C (Figure 7B and Figure 7C). The samples were dried at 25 "C (Figure 7A and Figure 7B) or at 200 "C (Figure 7C) at Torr prior to obtaining the spectra. Starting at low shielding, the peaks in Figure 7A are assigned to unreacted methoxy groups (50.4 ppm), the second and third methylene carbons relative to the silane silicon atom (C2 and C1, 25.6 ppm), the terminal methyl carbon (11.9 ppm), and the methylene carbon (C3) adjacent to the silicon atom (this appears as a highsheilding shoulder at 10.0 ppm). These chemical shift assignments were determined from the solution 13C chemical shifts of freshly distilled BTMS (in deuteriated benzene with 1%TMS as a chemical shift reference). The spectra in parts B and C of Figure 7 show decreases in the relative intensity of the methoxy resonance, it being virtually absent in Figure 7C, and a slight low-shieldingshift of the C3 shoulder relative to the spectrum in Figure 7A. These changes are attributed to an increase in reaction of the methoxy groups of BTMS in the samples corresponding to parts B and C of Figure 7. Most important, however, is the indication that the carbon resonance at 25.6 ppm in Figure 7B,C does not shift in position or undergo a change in observed line width relative to that observed in Figure 7A. This is in contrast to the behavior of the C2 resonance of APTS-modified silica prepared in dry toluene solution (Figure 3). The spectra of Figure 7 show that increasing the relative amount of water present at the silica surface prior to the silylation reaction between silica and BTMS, or after curing, does not alter the chemical environment of the alkyl carbons of BTMS (with the exception of the slight effect on C3). These results show that changes in the C2 carbon resonance of APTSmodified silica observed in the patterns discussed above are

VOL. 60,NO. 17,SEPTEMBER I, 1988 1783

CHEMISTRY,

I

60

,

,

,

I

I

,

20

YO

S

, ,

,

0

Figure 8. 13C CP/MAS spectra of silica gels on which n-propylamine is adsorbed. A, SiO2-NPA(200,RT); B, Si0,-NPA( 110,RT); C, sample of B exposed to air for 6 h; D, SO,-NPA(RT,RT); E, sample of D saturated with H,O.

associated with chemical interactions occurring with the amino groups of the +SiCHzCHzCHzNHzmoiety. The nature of the amine interactions responsible for the behavior of the C2 peak in the 13C CP/MAS spectra was explored by examining silica gel systems that had been treated with aliphatic amines that can interact with the silica surface in an adsorption process, participating, say, in hydrogen bonding, but for which silylation is not possible. Figure 8 shows the I3C CP/MAS spectra obtained on silica gel samples, initially dried at 200 "C (Figure 8A), 110 "C (Figure 8B), or 25 "C (Figure 8D), on which n-propylamine (NPA) was adsorbed, followed by subsequent treatment as indicated. The three samples specified above were dried at 25 "C and Torr. The sample represented in Figure 8C was obtained by exposure of the sample of Figure 8B to air for 6 h. Figure 8E represents a sample prepared from the sample of Figure 8B by saturation with liquid water, followed by drying in a desiccator under a flow of dry Nz. Analysis of the spectra in Figure 8 shows the same kind of water-induced shifts of C2 signal intensity between about 25-27 ppm (less hydrated) and about 21-22 ppm (more hydrated) that was described above the APTS-derivatized silica samples. The likely explanation for the common patterns of HzO dependence of the C2 resonance in the 13CCP/MAS spectra of APTS-derivatized silica gel and NPA-modified silica gel emerges from the recognition that the protonation of NPA with 1equiv of acid in solution produces a P-carbon (C2) shift from 27 to 21 ppm (26, 27). Therefore, the shift of C2 13C resonance intensity for APTS-derivatized and NPA-modified silica gel in the spectra shown in Figures 3,4, 5,6, and 8 are identified with Bronsted protonation of the NH2 group, promoted by the presence of water. According to this view, although not explicitly indicated in eq 2, the amino group on OH****H2NCH2CH2CH2-

+

~HzO

VI1

VI11

the left side is involved in various types of hydrogen bonding interactions. The removal of H20 from the system would shift this reversible equilibrium to the hydrogen-bonded amine on

1784

ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988

!LdL‘ ,

‘I VIjl ‘L A

60

YO

UNPROTONATED

, ,+

2C

0

-20

PPM

Flgure 9. Effect of protonation on the 13C CP/MAS spectrum of APTSderivatized silica gel: A, SiO,-APTS(llO,RT). B, sample of A

treated with HCI(g). the left (VII), with a C2 chemical shift of 25-27 ppm. The addition of H 2 0 shifts the equilibrium to the hydrated Bronsted complex (VIII), with a C2 chemical shift a t 21-22 ppm. The transfer of a proton to the -NH2 group from a surface silanol, rather than from water, is supported by the relative pK, values, 5.0-9.5 (24, 25) and 14 (29, 30),respectively. In order to test this protonation hypothesis, we obtained the 13C CP/MAS spectrum of a Si02-APTS(l10,RT) sample before and after treatment with gaseous HCl. The results are given in Figure 9, which shows unequivocally a shift of C2 signal intensity from about 26 ppm to about 21 ppm, in accordance with the hypothesis stated above. Shinoda and Saito (10) obtained the 13C NMR spectra of a Si02-APTS suspension, using a liquid-sample spectrometer. As HC1 was added to the deuterium oxide suspension, the C2 peaks in the spectrum became sharper and shifted to higher shielding. Those results, along with 13Cspin-lattice relaxation and NOE measurements, indicated that protonation enhanced the reorientational mobility of the amino group. These authors postulated that the amino group of APTS was hydrogen bonded to surface silanol groups and/or other amino groups prior to protonation. Sudholter and co-workers (9) also have identified a +SiCH2CH2CH2NH3+ moiety as responsible for a peak a t about 21 ppm in the 13C CP/MAS spectra of APTS-derivatized silica gel, and Chiang and co-workers (8) have implicated acid-base transformations in shifting the C2 resonance of this system between 27 and 2 1 ppm. Additional information on the APTS-derivatized and NPA-modified silica systems was sought by examination of the cross polarization dynamics of these and related systems. Variable-contact-time 13CCP/MAS data on samples pertinent to this issue are included in Table IV.One sees from the table that the T C H values for Si02-BTMS(RT,RT) are generally larger than for the Si02-APTS(RT,RT) sample. Furthermore, the TcH value for the CH3 carbon of the SiO,-BTMS(RT,RT) sample is the largest among the carbons of these two samples. This latter trend is consistent with the fact that the cross polarization rate constant, TcH,depends monotonically on the second moment of the 13C-lH interaction, which in turn is decreased by molecular motion, such as -CH3 rotation. These kinds of effects have been reported previously for silica gel systems derivatized with C8 and C18 chains (31, 32). For the Si02-NPA(RT,RT) sample, the observed T C H values are generally larger than for the silica gel samples modified with APTS, BTMS, or diaminopropane (DAP), H2NHC&H2CH2NH2. This qualitatively reflects the larger degree of motion (more rapid and/or more isotropic) in Si02-NPA(RT,RT) than in the other samples and is consistent that the fact that the only “attachment” of NPA to the silica

surface involves the interactions of the single amino group. The fact that T C H in this system increases from C1 to C2 to C3 indicates that the -CH2NH2 carbon (Cl) has the smallest degree of mobility and the -CH3 carbon (C3) the greatest mobility and demonstrates a strong interaction between the amino group and the silica surface. For the Si02-DAP(RT,RT) sample, the T C H values are uniformly smaller than for the Si02-NPA(RT,RT) sample, suggesting a generally lower degree of mobility for diaminopropane than for npropylamine on the silica surface. The T C H value for C2 in SO2-DAP(RT,RT) is smaller than the C1 (C3) value. This fact implies that each -NH,/surface interaction is a rather mobile one, so that considerable mobility is experienced by the -CH2NH2 carbons. It also implies that, at any given instant, not more than one of the two amino groups in a typical DAP molecule is strongly interacting with the surface. These results suggest a generally lower degree of mobility for diaminopropane than for n-propylamine on the silica surface. A confusing aspect of the results shown in Table IV is the relationship between the TCHvalues of the Si02-APTS(110,RT) and SO2-APTS( 11O,RT)H+ samples. One might expect that the effect of protonation by an acid HB, such as HCl, could be explained in terms of some combination of the following types of transformations:

+

&-OH..*H2NCH2CH2CH2-

+

HB I

VI1

B-

IX (3)

H

\ *O-H***NCH~~HZCH~H ’

+

HB

-

C N H -,~ C -H $ H ;~ -I;$ ~ O .

H\+

VIIa

0-

X

(4)

XI1

The C2 13C NMR peak of the Si02-APTS(l10,RT) sample shown in Figure 3 appears to indicate contributions from both hydrogen-bonded forms, e.g., VI1 or VIIa, and Bronsted acid-base forms, e.g., X. If the main effect of HC1 protonation could be represented by eq 3 or eq 5, then one might expect to find some evidence of increased mobility of the N-C-C-C chain, as implied by the results of Shinoda and Saito (10). However, the data of Table IV indicate no significant changes in the T C H values for C1, C2, C3, Ca, or Cp upon protonation of Si02-APTS(l10,RT). This pattern would seem to imply no significant change in the mobilities of any of these CH2 or CH3 moieties upon protonation of the sample. Another interesting feature of Figure 9 is the fact that treatment with HCl does not affect the relative amounts of -CH2CH2CH2NH, and -OCH2CH3 moieties as dramatically as one might have expected. Hence, the main I3C NMR evidence of the effects of HC1 treatment at the present time is the effect on the C2 peak position and shape. More detailed studies of these issues appear to be warranted. Chiang and co-workers (8)reported that the line width of the C2 resonance was smaller with heat-treated samples than with uncured samples and attributed this narrowing to an increased amino-group mobility that they associated with increased curing. In our results, obtained on samples prepared differently, we see no general trend of this sort. Furthermore, our results suggest that the width of the resonance pattern for the C2 carbon is due not to mobility but primarily to the

ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988

inhomogeneous broadening associated with contributions from at least two different species, corresponding to the hydrogen-bonded and protonated amine structures (e.g., VII-XI in eq 2-5). The spectra shown in Figure 4 suggest that the width of the C2 pattern is larger for situations in which both types of contributions are present in substantial proportions (say, Figure 4B) than in either extreme (compare with parts A and C of Figure 4). This implies that the equilibrium represented by eq 2 is not rapid; if it were rapid, a sharper peak corresponding to the a weighted-average of resonances for structures VII-XI would be observed for the intermediate cases. 4. Combining Results from 29Siand 13CNMR. In the discussion above it has proved beneficial to compare 13Cand %Si NMR results on the same sets of samples. In this section this approach is extended. The 29SiNMR results given in Table I11 on the relative amounts of structures of the types I, II/III, and IV/V/VI are in agreement with the qualitative trends apparent in the 13C NMR spectra of Figure 3 and the Nav(13C)values tabulated in Table 111. The combination of 29Siand 13C NMR results indicate that (1) for a given curing temperature, a larger number of siloxane bonds have been formed during the preparation of samples of the AQ and RT series than for samples of the 110 and 200 series, and (2) increasing the curing temperatures increases the number of siloxane attachments to the silane silicon atom (or decreases the number of unreacted ethoxy groups). The 13C NMR spectra of samples of the AQ series are void of intensity at 58 and 16 ppm, peaks associated with unreacted ethoxy groups. Therefore, the intensity at -58 ppm in the %i NMR spectra of the AQ series shown in Figure 2 originates exclusively from attached silane moieties with structures II/III having R' = H. The sample of the RT series dried at 25 OC, Le.,, Si02-APTS(RT,RT), contains a small number of unreacted ethoxy groups, as shown by its 13C NMR spectrum in Figure 3 and by the data given in Table 111. Based on this evidence, the intensity at -58 ppm in the 29SiNMR spectra of the RT series samples shown in Figure 2 is attributed to molecular structures II/III with R' = H and R' = Et, primarily the former. The ratio of the integrated 29SiNMR intensity at -49 ppm to the total integrated silane silicon intensity is the fraction of attached silyl groups with one siloxane bond and either two ethoxy groups or one ethoxy group and one hydroxy group (structure I in Figure 2). After deconvolution, integration, and intensity correction (eq 1) of the overlapped silane resonances in the 29SiCP/MAS spectra of the Si02-APTS(110,RT) and Si02-APTS(200,RT) samples, one calculates that 81% and 71%, respectively, of the attached silane moieties exist in the form of structure I. The average number of unreacted ethoxy groups per attached silane moiety in these two samples was determined by 13C NMR to be 1.4 and 1.6, respectively (see Table 111). These values indicate that roughly half of the attached silyl groups in the samples of the 200 and 110 series contain two unreacted ethoxy groups, roughly half bearing one. On the basis of these results, one may propose that (1)the majority of the unreacted ethoxy groups in the samples of the 200 and 110 series are associated with the structure I with R' = E t and (2) the majority of the intensity at -49 ppm is associated with this structure. If one proceeds under the assumptions proposed, then the average number of residual ethoxy groups per attached silane moiety can be calculated as follows: (integrated intensity a t -49 ppm) x 2 Nav(29Si) = (total integrated silane intensity)

(6)

By use of eq 6, N,,('%i) values of 1.6 and 1.5 are calculated for the SiO,-APTS( 110,RT) and Si02-APTS(200,RT) sam-

1785

~~

Table V. Residual (Unreacted) Ethoxy Content, NaV(13C), Determined by 13C NMR Data"

postreaction

curing temp, "C 200 150

110 65

RT

200

series

0.92 1.2 1.2 1.4 1.3

110 series

RT series

AQ series

0.54 0.94 1.2 1.3 1.4

0 0.06

0 0 0 0 0

0.13

0.13 0.26

"Defined as number of unreacted -OCH2CH3 groups per R-Sit group. Based on values uncorrected via eq 1. ples, respectively. These results compare roughly to the NaV(I3C)values given in Table I11 for these samples. The agreement between the data obtained by 13Cand 29SiNMR for these two samples suggests that the relative intensity at -49 ppm in the 29Sispectra of APTS-modified silica can be used to estimate the number of residual ethoxy groups. On the basis of the 13C results given above, there is only a small difference (if any) between the number of unreacted ethoxy groups in samples prepared from silica dried at 200 "C and 110 OC, despite a larger amount of surface water in the latter. 5. Conclusions. The combination of 13Cand ?3i CP/MAS NMR data provides a powerful approach for studying APTS-derivatized silica gels. Information on residual, unreacted ethoxy groups is provided by both the 13C and 29Si spectra, the former most directly. Information on the number of siloxane (Si-0-Si) attachments in which the silane moiety participates is provided by analysis of the three peaks contributing in the -49 to -66 ppm region of the 29SiCP/MAS spectra. From these data one can estimate the fraction of ethoxy groups that have reacted, the ratio of unreacted ethoxy groups remaining per silane moiety, and the number of siloxane attachments to the silane moiety. These kinds of structural details were determined in relation to the details of preparation of the silica gel prior to silylation, the conditions of the silylation reaction and the curing temperature. The data show that the reaction of ethoxy groups and formation of siloxane bonds are promoted by the presence of water at the silica surface during the silylation reaction and by higher curing temperatures. Maximum curing (formation of siloxane bonds to the silane moiety) is not achieved at a curing temperature below about 150 OC. The highest silane loading level is achieved with silica gel that has been dried at 25 "C and Torr and silylation conditions involving a dry toluene medium at 25 OC. The fact that this higher loading level is obtained, relative to that obtained with silylation of silica dried at 200 OC and Torr, is understandable in terms of the important role of surface water. The fact that the silylation reaction carried out in an aqueous medium yields smaller silane loading than the optimum case described above indicates that, although some surface water is desirable for optimizing the silylation reaction, too much water can interfere, presumably by prior hydrolysis of +Si-OEt moieties (6). ACKNOWLEDGMENT G. S. Caravajal acknowledges the technical assistance of B. Hawkins, G. Hatfield, and C. Bronnimann. Registry No. NPA, 107-10-8; DAP, 109-76-2.

LITERATURE CITED (1) Grushka, E. Bonded Stationary Phases in Chromatography; Ann Arbor Science Publications: Ann Arbor, MI, 1974. (2) Weetal, H. H. Sep.Purif. Methods 1973, 2 , 199. (3) Leyden, D. E.; Luttrell, G. H. Anal. Chem. 1975, 4 7 , 1612. (4) Burwell. R. L. Chem. Techno/. 1974, 370. ( 5 ) Chiang, C.; Ishida, H.; Koenig, J. L. J . Colloid Interface Sci. 1980, 7 4 , 396.

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Anal. Chem. 1988, 6 0 , 1786-1791

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1984. (7) Caravajal, G. S.;Leyden, D. E.: Maciel, G. E. “Solid State NMR Studies of Aminopropylsiiane Modified Silica,” in Si/anes, Surfaces and Interfaces: Leyden, D. E., Ed.; Gordon and Breach Science Publishers: New York, 1986; p. 383. (8) Chiang. C.; Liu, N.; Koenig, J. L. J . Colloid Interface Sci. 1982, 8 6 , 26. (9) Sudholter, E. J. R.; Huis. R.; Hays, G. R.; Alma, N. C. M. J. Colloid Interface Sci. 1985, 103, 554. (IO) Shinoda, S.;Saito, Y. J. CoNoid Interface Sci. 1985, 103, 554. (11) Schaefer, J.; Stejskal, E. 0. Topics in Carbon- 13 NMR Spectroscopy; Levy, G. C., Ed.; Wiley: New York, 1979; Vol. 3, p 284. (12) Stejskal, E. 0.; Schaefer, J. J. Magn. Reson. 1975, 18, 560. (13) Frye, J. S.;Maciel, G. E. J. Magn. Reson. 1982, 4 8 , 125. (14) Tegenfeklt, J.; Haeberlen, U. J. Magn. Reson. 1979, 36, 453. (15) Maciel. G. E.; Sindorf, D. W. J. Am. Chem. Soc. 1980, 102, 7606. (16)Maciel, G. E.; Sindorf, D. W.; Bartuska, V. J. J. Chromatogr. 1981, 205. 438. (17) Sindorf. D. W.; Maciel, G. E. J. fhys. Chem. 1982, 8 6 , 5208. (18) Sindorf, D. W.; Maciel, G. E. J . Am. Chem. SOC. 1983, 105, 3767. (19) Sindorf, D. W.; Maciel, G. E. J . fhys. Chem. 1983, 87, 5516. (20) Mehring, M. Principles of High ResolutionNMR in Solids; Springer-Verlag: New York, 1983; p 153. (21) Pleuddemann. E. P. ”Chemistry of Silane Coupling Agents”, Sily/ated Surfaces: Leyden, D. L., Collins, W. T., Eds.; Gordon and Breach Science Publishers: New York, 1980: p 31. (22) Marsmann, H. NMR: Basic Princ. frog. 1981. 17, 65.

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RECEIVED for review December 16, 1987. Accepted April 4, 1988. The authors gratefully acknowledge project support from National Science Foundation Grants CHE-8210014, CHE-8306518, CHE-8513247, and CHE-8610151 and assistance of the Colorado State University Regional NMR Center, funded by National Science Foundation Grants No. CHE8208821 and CHE-8616437.

Pulsed Laser Resonance Ionization Mass Spectrometry for Elementally Selective Detection of Lead and Bismuth Mixtures B. L. Fearey and C. M. Miller Isotope and Nuclear Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545

M. W. Rowe,‘ J. E. Anderson, and N. S. Nogar* Chemical and Laser Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545

Pulsed laser, single-color, (2 4- 1) multiphoton ionization is used to achieve elemental selectlvity wMle concurrently eliminatlng Isobaric interferences for lead (Pb) and bismuth (BI) mixtures detected via resonance ionization mass spectrometry. Experimental resuns are compared with theoretical calculations by using a slmple rate equation formalism. The following oscillator strengths were determined: Pb, 6p7p 3P0 6p7s 3P,0, f = 0.4 f 0.1; Bi, 6p2 (3P0)7p J = 6p27s 4P,,2, f = 0.07 f 0.02. I n addition, the Bi ionlzatlon cross section at 64412 cm-’ was estimated to be (5 f 2) X cm2. Relative efficlencles of the lonlzatlon processes for these elements and a comparison between pulsed and continuous laser excitation are dlscussed.

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Interest in measuring large isotope ratios for rare isotopes with small sample sizes continues to increase ( I , 2). Resonance ionization mass spectrometry (RIMS) overcomes the problem of isobaric interferences frequently encountered in standard thermal ionization mass spectrometry ( I , 3). The particular driving force for the present studies is the need to measure large isotope ratios in bismuth (Bi), including specifically the neutron-deficient isotopes, the production of which is a measure of high energy neutron fluences. In standard thermal mass spectrometric analysis of Bi, lead (Pb) Current address: Chemistry Department, Texas A&M University, College Station, TX 77843.

isotopes are particularly ubiquitous and are the primary isobaric interferences, i.e. natural zos-20sPbobscure zffi-208Bi, The elementally selective technique of RIMS is a logical solution to this problem. In addition, there is a need to measure the small but finite quantity of the radioactive isotope 210Pbin lead ( 4 ) . Lead solder is used extensively in interconnections of computer chips, where the decay of minute amounts of zloPband its daughters (zlOBiand 210Po)can damage the integrated circuit. This problem may become acutely important for future generations of supercomputers, where smaller chips will lead to more intimate contact between the solder and the integrated circuit. Here, the sensitivity of RIMS may be used to advantage. In this work, one consideration was to limit experimental complexity, which suggested the use of a single dye laser for excitation and ionization. For the high ionization potentials of the elements in question, the number of ionization pathways available was limited. One alternative was to utilize a frequency-doubled dye laser operating in the ultraviolet region to give a ( I + 1) (photon to resonance plus photon to ionize) resonance ionization process (3). A simpler and largely overlooked second alternative is the use of a visible dye laser in a (2 + 1)process (5). The latter was chosen for the present case.

EXPERIMENTAL SECTION The basic experiment consisted of a pulsed dye laser, tuned t o resonance with an atomic transition and focused into a quadrupole mass spectrometer. The particular species then is

0003-2700/88/0360-1786$01.50/06 1988 American Chemical Society