Note: Descriptions are shown in the official language in which they were submitted.
METHOD FOR PRODUCING DUAL ZONE MATERIALS
BY USE OF AN ORGANOSILANE MIXTURE
The present invention relates to a method for
producing dual zone porous materials having an external zone
bearing silyl groups of a first type immobilized on the
external surfaces of the porous material and internal zone
having silyl groups of a second type immobilized on the
internal surfaces of the porous material and the dual zone
materials.so produced. More particularly, it relates to a
method for contacting a porous hydroxyl-bearing support
simultaneously with two different organosilanes which
differentially react with the external and internal hydroxyl
groups on the porous support and production thereby of a dual
zone material.
In United States Patent No. 4,782,040 of Revis et al.
issued November 1, 1988, there are disclosed dual surface (more
properly termed dual zone) porous materials made by treating
a porous hydroxyl-bearing support, such as porous silica,
alumina, zirconia, etc., with substoichiometric amounts of an
ultrafast silylating agent. This agent is chosen from those
which are so reactive that the resulting surface group is
immobilized in the external zone of the porous support before
the agent has had time to migrate deeply into the porous
internal zone. A subsequent silylation reaction can be
employed to convert residual hydroxyl groups, which reside
predominantly in the internal zone, to a second immobilized
group of another type. See also, Williams & Tangney,
Silanes, Surfaces & Interfaces, D. E. Leyden, ed., Gordon &
Breach Publisher, 1986, P. 471 ff_-
In U.S. Patent No. 4,782,040, the
disclosed ultrafast_silylating agents are reactive silane
intermediates. Also known as
A
_2_
ultrafast silylating agents are silanes having "leaving
groups" such as i) substituted amides, ii) substituted
amines or iii) thioethers. It is believed that these facile
leaving groups lower the activation energy required for
reaction with surface hydroxyl groups and thus enhance the
extent to which the silane can be captured by covalent bond
formation in the external zone of the porous material, that
is, captured early during its diffusion path into said
material.
As stated in U.S. Patent No. 4,782,040, traditional
silylation reactions are generally not fast enough to permit
preferential silylation of the external surface of the porous
support. "Traditional silylation" is described in
Plueddemann, Encyclopedia of Chemical Technology, 3rd ed.,
Vol. 20, page 962 et seq. Plueddemann states that silylation
is the displacement of active hydrogen from an organic
molecule by silyl groups where "The active hydrogen is
usually OH, NH or SH and the silylating agent is usually a
trimethylsilyl halide or a nitrogen-functional compound. A
mixture of silylating agents may be used; a mixture of
trimethylchlorosilane and hexamethyldisilazane is more
reactive than either reagent alone and the by-products
combine to form neutral ammonium chloride."
Neither consecutive nor simultaneous treatment.of
porous supports with two such traditional silylating agents
has produced a dual. zone porous material of the type
described in U.S. Patent No. 4,782,040. For
example, Abbott in U.S. Patent No. 4,298,500 discloses
sequentially treating a porous silica gel with an organo-
silane reagent to form a "first residue" and, then, an
organosilane-containing diol, diol precursor or amide to form
a "second residue". However, the resulting product is a
A
-3-
mixed phase composition which shows negligible dual zone
characteristics.
Likewise, Marshall et al. in "Synthesis of LC
Reversed Phases of Higher Efficiency by Initial Partial
Deactivation of the Silica Surface", Journal of
Chromatography Science, Vol. 22, June 1984 pp. 217-220,
disclose first treating silica with a small amount of
end-capping reagent (such as trimethylchlorosilane) followed
by exhaustive octadecylation. Again the result is a
homogeneous distribution of surface bound molecules.
In terms of simultaneous treatment with a mixture
of reactants, reference is made to the Plueddemann
publication mentioned above and to M. L. Hunnicutt and
J. M. Harris, "Reactivity of Organosilane Reagents on
Microparticulate Silica", Anal. Chem., Vol. 58, April 1986,
pp. 748-752. Hunnicutt and Harris discuss the results of
competitive surface reactions between binary organosilane
mixtures and silica gel. The organosilane m~.xtures used
include mixtures of two haloalkylsilanes such as
(1-bromomethyl)dimethylmonochlorosilane, (1-chloromethyl)-
dimethylmonochlorosilane or (3-chloropropyl)dimethylmono-
chlorosilane, as well as mixtures of a haloalkylsilane with
an alkylsilane such as trimethylchlorosilane (TMCS) or
hexamethyldisilazane (HMDS). In a number of instances, a
catalyst such as pyridine was added to the silica slurry
prior to silane addition for base catalyzed reactions.
Hunnicutt and Harris showed that their reaction did not
display pore diffusion control. Thus they could not have
produced dual zone materials (DZMs) with respect to
differential distribution of their chosen immobilized groups.
This outcome is believed to be due to several factors. Most
importantly, mixtures of chlorosilanes of the type used by
Hunnicutt and Harris do not react with sufficient speed and
A'
'" ~Q~
differentiality even when the reaction is catalyzed with
pyridine.
Furthermore, the reaction conditions were not
adjusted so as to produce DZMs even from the point of view of
selective capture of both chlorosilanes together in the
external zone. Firstly, the solvent they used was chloroform
which is highly polar and is known to be a proton donor in
hydrogen'-bonded complexes. Such solvents have been found to
reduce pore diffusion control, probably by sequestering the
surface reactive sites (silanol) and thus slowing down the
reaction rate. Protic solvents such as ethanol are even more
deleterious since the halosilane is solvolyzed and
transformed into the less reactive ethoxysilane. Secondly,
the rate of silane addition to the silica slurry was
excessively fast at about 0.3 molecules/nM2/minute.
Accordingly, individual silica particles would be subjected
to unusual doses of silane and the resultant particle-to-
particle heterogeneity would overcome any intraparticle
inhomogeneity (dual zone structure) that might otherwise
occur. Accordingly, even though Hunnicutt and Harris
conducted what could be described as a mixed halosilane
reaction, Hunnicutt and Harris do not teach one of ordinary
skill in the art how to produce dual zone materials by means
of such reaction mechanisms.
And yet, it is known to be desirable to produce
dual zone porous materials having silyl groups of one type
predominantly on the external surface and silyl groups of
another type predominantly on the internal surface in order
to provide on the external and internal surfaces
differentially selective adsorbents, for example, for
specific chromatographic and catalytic applications. It
would also be desirable to use a mixture of organosilanes
A
1
a~
because of the ease and lower cost involved. To date,
however, it has not been possible to do so.
Accordingly, the need remains for a method for
simultaneously contacting a porous hydroxyl-bearing support
with a mixture of organosilanes in the production of dual
zone porous materials.
That need is met by the present invention which
utilizes a mixture of organosilanes of a specific type under
defined reaction conditions in order to produce a dual zone
porous material: The mixture includes a first and a second
organosilane. The first organosilane has silyl groups of a
first type such as haloalkylsilyl, vinylalkylsilyl and
aminoalkylsilyl groups and has a leaving group such as an
alkyl or aryl sulfide, di-substituted amino, catalyzed
non-fluorine halogen group or substituted amido group, which
is capable of rapidly reacting with the hydroxyls on the
external surfaces of a hydroxyl-bearing porous support such
as a porous metalloid oxide, a porous metallic oxide or
mixtures thereof (preferably in particulate form and most
preferably particulate silica). For the case of a non-fluorine
halogen 7,eaving group the reaction must_be catalyzed as
described in U.S. Patent No. 4,950,635 dated August 21, 1990.
The second organosilane has silyl groups of a
second type such as trimethylsilyl or dimethyi alkyl silyl
groups and is capable of diffusing into the interior of the
porous support. It has a leaving group such as an alkoxy,
siloxy, carboxy or chloro group which will not substantially
exchangelwith the leaving group of the first organosilane,
but which will react slowly with the hydroxyls on the
internal surfaces of the porous support. For the chloro
leaving group case on the second organosi~.ar~e) the reaction
must not be catalyzed in U.S. Patent No. 4,950,635.
A
._ ~~~'~8~1
-6-
In the mixture, the first organosilane should be
present in a substoichiometric amount, i.e. less than the
stoichiometric equivalent based on the reactable hydroxyl
content of the porous support. On the other hand, the second
organosilane can be in any convenient amount. The sum of the
amounts of both silanes need not be sufficient to treat all
of the internal and external reactive silanols (silica case).
The material may be used as is or the residual reactive
silanols may be converted to additional individualized group
by subsequent silane additions.
The organosilane mixture is added to a porous
support suspension formed by mixing the porous support with a
solvent. The preferred solvent is an aprotic solvent. Most
preferred are non-polar solvents such as hexane, octane,
decane, toluene or mixtures thereof since such non-polar
solvents will not retard the reaction rate due to their
minimal interaction with the surface reactive sites.
The organosilane mixture is added at a rate which
is rapid with respect to the reaction rate of the second
organosilane, but slow with respect to the rate of mixing of
the suspension with the organosilane mixture. Generally, the
organosilane mixture is itself predissolved in the same
solvent as is used for the suspension in order to facilitate
said slow addition.
The organosilane mixture is maintained in contact
with the porous support for a sufficient period of time to
permit formation of organo silyl groups of the first type
predominantly on the external surfaces of the porous support
and to permit formation of organo silyl groups of the second
type predominantly on the internal surfaces of the porous
support. Thereafter, the dual zone porous material may be
isolated by conventional separation techniques, such as
filtration.
~~~H~~~~
7
The result is a dual zone porous material having
organo silyl groups of the first type predominantly in the
external zone and organo silyl groups of the second type
predominantly in the internal zone.
Actually, the amount of surface area of the porous
support which is considered to be in the external zone versus
the amount considered to be in the internal zone may vary.
Preferred for purposes of this invention is the situation
where the internal zone approximately comprises the internal
90 percent of the surface area and the external zone
approximately comprises the external 10 percent of the
surface area of the porous material. Use of a larger amount
of first organosilane will result in a greater degree of
penetration into the interior of the porous support, will
cause reaction with a greater number of hydroxyl groups on
the surfaces of the porous support and will create a larger
external zone occupying a greater percentage of the surface
area of the porous support. Thus, the external zone may
range from 0.5% to 50°/ of the surface area and the internal
zone may range from 50-99.5% of the surface area.
In any event, it is possible to control by
selection of the organosilanes used in the mixture and the
reaction conditions the size of the external zone formed.
This enables one to produce dual zone materials having
varying degrees of capacity for separation, such as for use
as packing materials in liquid chromatography or otherwise.
Accordingly, it is an object of the present
invention to provide an improved method for production of
dual zone porous materials by simultaneous application of a
mixture of organosilanes. Other objects and advantages of
the invention will become apparent from the following
detailed description and the appended claims.
~~3~'~8 1
_8_
The porous materials found useful in this invention
are those materials which are porous solids having hydroxyl
groups on their surfaces. Such materials, for example, are
silica, silica gel, alumina, stannia, titania, zirconia and
the like. Also, these materials can be porous glass, porous
ceramic or plastic as long as the material has, or will form,
hydroxyl groups on its surface.
The form of the porous material is not overly
critical. Particulate porous materials, as well as
filaments, slabs, discs, blocks, spheres, films and other
such forms can be used in this invention. Also contemplated
within the scope of this invention is the treatment of
particulate materials by the process of this invention and
the subsequent forming of the treated particulate materials
into slabs, discs, blocks, spheres, films, membranes, sheets
and the like.
Preferred for this invention are the porous
metalloid oxides, metallic oxides or mixtures thereof such as
silica, alumina, zirconia and titania in all of their related
forms. Most preferred are particulate silicas which may vary
in pore diameter from 50 to 2000 angstroms and in particle
size from 3 to 1000 micro-meters, for example.
As mentioned, the first step in the production of
the dual zone porous material of the present invention is
forming a suspension of the porous support in a solvent. The
solvent is preferably an aprotic solvent and most preferably
a non-polar solvent, for example, hexane, octane, decane,
toluene or mixtures thereof. Preferably, porous silica
particles having a particle size of from 3 to 1000
micro-meters are mixed with the solvent in the amount of 0.1%
to 40 w/v7.
The organosilane mixture is slowly added to this
suspension. Preferably, the mixture is a solvent-based one,
using the same solvent as that used in the porous support
suspension. The rate of addition must be adjusted to be
faster than the reaction rate of the second organosilane but
sufficiently slower than the rate of mixing with the
suspension particles to receive a uniform dose of the said
first organosilane.
The first organosilane in the mixture preferably
has the formula:
LmSi(R")3-mR
wherein L is a catalyzed non-fluorine halogen group as in
U. S . Patent No. 4, 950, 635 or is NRI°~ or an
RIV-substituted sulfide or amido group wherein RIV is methyl,
ethyl, phenyl, Si(R~~)3-mR, or, when L is NRIV2, RIV may be a
cyclic amino group such as imidazoyl or piperidinyl, m is
1-3, R" is methyl, ethyl or phenyl and R is selected from
hydrogen, allyl, vinyl and Q wherein Q is
(a) alkyl groups having less than twenty carbon
atoms,
(b) aryl groups having less than twenty carbon
atoms,
(c) -CH2CH2CnFZn +1 wherein n is 1 or more,
(d) -CH2~pH2pCH2SR', _
(e) -CH2CPH2pCH2NR'2,
(f) -CH2CPH2pCH2N(R')(CH2)2N(R')2,
(g) -CH2CpH2pCH20CH2CHCH2
0
0
(h) -(CHZ)2~
(i) -CHZCpH2pCH20C(0)C(CH3)=CHZ or
A
-lo-
R'-0
N
( j ) - CH2 C HZ CH2 ~
P P
(k) -CH2CPHZpCH20(CH2CH20)aZ wherein Z is an alkyl
or aryl hydrocarbon having less than seven
carbon atoms or is acetyl or is the group
-CH CH-CH
2I I 2
0 0
_ /,C .
Me ~ Me
wherein Me is methyl and wherein a is 0=10.
(1) CH2CPH2pCH2X wherein X is a halogen, R' is
methyl, ethyl or phenyl and p is 1 or 2.
The preferred leaving group is either an N-methyl-
acetamido group or a silylimine and the preferred silyl
portion of the compound is haloalkylsilyl, vinylalkylsilyl or
aminoalkylsilane. A preferred first organosilane is
Me Si-CH
2 Z~CHCH3
N-CHZ
CH3
Fluoroalkylsilanes are also preferred in that they
can be used to produce a dual zone porous material having
fluoroalkylsilyl groups in the external zone. The fluoro-
alkylsilyl groups serve as a lipophobic phase which is less
adsorptive to proteinaceous substances when the dual zone
porous material is used as a reverse phase packing material
for high-pressure liquid chromatographic blood serum ~n~.lysis
as~disclos_ed irk U.S. Patent No. 4,778,600 of D.E. Williams,
issued in October, 1988.
The second organosilane in the mixture preferably
has the formula
A'
-11-
LmSi (R")3-mR
wherein L is alkoxy, siloxy, carboxy or uncatalyzed halogen
as described in U.S.patent No. 4,950,635, R" is as
defined above, m is 1-3, and R is as defined above,~and wherein
L, m, R" and R are chosen so that the second organosilane is
not the same as the first organosilane. The preferred
leaving group is an acetoxy group and the preferred silyl
portion of the second organosilane is trimethylsilyl or
dimethylalkylsilyl.
The amounts of first organosilane and second
organosilane used in the mixture vary depending on the amount
and hydroxyl content of the porous support found in the
suspension. In any event, the amount of first organosilane
used should be less than a stoichiometric equivalent of the
reactable hydroxyl content of the porous support present and
the amount of second organosilane used should be any
convenient amount, depending on how much residual reactable
hydroxyl one may wish to leave in the material. Preferably,
the first organosilane is used in an amount varying from 0.05
to 2.0 m/nM2, i.e. molecules of organosilane per square
nanometer of surface area of the porous support and the
second organosilane is used in an amount exceeding 0.05
m/~2. -
As mentioned, the organosilane mixture is added to
the porous support suspension at a rate which is rapid with
respect to the reaction rate of the second organosilane, but
slow with respect to the rate of mixing of the suspension
with each aliquot of the said mixture. Acceptable addition
rates must be determined empirically for each organosilane
mixture. Prefezably, however, the addition is completed
within 8 hours.
A
_12_
Thereafter, the organosilane mixture is maintained
in contact with the porous support for generally from about
minutes to 24 hours and preferably from about 1 to 6
hours. The temperature during this step of the process is
not narrowly critical and can range from 0°C. to 400°C. Most
preferred is the reflux temperature of the reaction mixture
at about 70° to 175°C.
The amount of first organosilane present in the
mixture will determine the depth of external zone produced in
the dual zone porous material. Generally, the external zone
can be the exterior 0.5% to 50% of the surface area of the
porous support. However, there are often minor differences
between the external and the average surface composition due
to random scatter in analytical results. Furthermore, real
differences between two compositions must be large enough to
significantly affect properties of the material. In view of
these considerations, meaningful dual zone character is
attained only when either of the following conditions are
met:
(a) T 1 (E) > 1.5, rl (E) > 0.3 molecules/nM2
and T (A) >_ 0.1 molecules/nM2
2
1 (A)
(b) t 1 (E) ? 1.5, Tl (E) ? 15% of saturation
of the surface of the porous
T 1 (A) material; and T2 (A) > 5% of
saturation of the surface of the
porous material
wherein t (E) is the external surface density in
Molecules/nM2 as inferred by Electron Spectroscopy Chemical
A
, -13-
Analysis and wherein Tl (A) and T2 (A) are like measurements
of average surface density measured by bulk analysis and
wherein subscripts 1 and 2 refer to the groups immobilized in
reaction with the first organosilane and, then, with the
second organosilane respectively.
Concentration is expressed in appropriate units
such as molecules per square nanometers (m/nM2).
T 1(A) - 6 Xl
106
lOS
T 1(E) - R1 (E) T°1(A)
R°1(A)
Rl(E) Aj / Ar
where Xl is the amount of some group (labeled 1) as measured
in moles per gram of silica by bulk elemental analysis, S is
the specific surface area expressed in square meters per gram
of silica and R1(E) is the ESCA-measured ratio of the surface
atomic composition of element j, Aj, to that of element r,
Ar. Element j is chosen to be unique to group 1 and element
r is chosen to come primarily from the porous substrate.
Silicon was chosen as the reference element, r, since the
contribution to ASi by the immobilized silane is relatively
small. The analogous quantities, T°1(A) and R°1(A), are
found by measurements on a silica possessing immobilized
group ~E1 that does not have a compositional gradient. Such
materials are easily prepared by exhaustive treatment with a
single silylating agent.
-14- r~ ,~
Such exhaustive treatment also defines the amount
of reactable hydroxyl initially present in the porous
substrate. This amount is dependent on the size of the
immobilized organosilyl group as well as the reaction
temperature.
When it is determined that the reaction is
essentially finished, the product is typically isolated from
the reaction mixture. Thus, the final step of this process
is the isolation of such products from the reaction mixture.
This can be accomplished in a number of ways. For example,
the liquid can be decanted, the porous material washed and
the liquid decanted successively, or the reaction mixture can
be filtered to remove the liquid from the solid product. If
the final product is other than particulate in form, it can
be used as is or it can be further shaped and formed without
losing beneficial properties. If the material is in a
particulate form, it can be used as is or it can be
compressed, sintered or otherwise formed.
So that those skilled in the art may appreciate and
understand the invention described herein, the following
examples are offered for illustration purposes only. The
examples should not be construed as limiting the invention as
defined in the claims.
I. Preparation of -SiMe2CH2CH2CF3/-SiMe3 dual zone material
1.00 g (1.196 x 10 3 eq. OH @ 2.4 OH/nM2) dry Baker
silica gel and 100 cc decane were added to a 250 cc round
bottomed flask equipped with a thermometer, an air motor
driven paddle, an addition funnel and a condenser topped with
an N2 sweep. This mixture was heated to 60°C. 15 micro-
liters (6.975 x 10 5 eq. or 0.14 molecules/nM2) of
CF3CH2CH2Me2SiN(Me)C(0)Me (TFSA) in 15 cc decane and 0.39 cc
(2.65 x 10 3 eq. or 5.3 molecules/nM2) of Me3Si0Ac in 15 cc
decane were then mixed with the addition funnel and added to
2~0'~8~1.
-15-
the reaction flask over two minutes with vigorous stirring
(Me represents the methyl group). The reaction mixture was
then heated at solvent reflux (170°C.) for three hours
without any additional stirring. After cooling, the silica
was isolated from the solution via filtration and washed once
with the decane and three times with ethyl ether. Finally,
the silica was dried i.n a vacuum oven for four hours at 80°C.
prior to ESCA and bulk elemental analysis. Bulk analysis
gave values of 0.16 wt.% F and 3.76 wt.% C, corresponding to
average surface concentrations of 0.07 m/nM2 for
-SiMe2CH2CH2CF3 and 2.11 m/nM2 for -SiMe3. External surface
analysis by ESCA gave a value of 0.0689 F/Si, corresponding
to a concentration at the external surface of 0.36 m/nM2 for
-SiMe2CH2CH2CF3. The value of 300 M2/g for the specific
surface area of the silica was used to convert bulk
analytical values to average surface concentrations.
Calculation of the external surface concentration was made
using reference values of 0.41 F/Si (by ESCA) and 5.19 wt.% F
corresponding to 2.16 m/nM2 of the bound group. These values
were obtained for silica that had been thoroughly treated to
saturation by vapor phase TFSA so that no concentration
gradient was present.
The surface concentration of the trifluoropropyl
group for the exterior was 0.36 m/nM2, i.e., five times
greater than for the average and the average for trimethyl
group was 2.11 m/nM2, proving that a dual zone material had
been prepared.
II. Preparation of -SiMe2CH2CHMeCH2NHMe/-SiMe3 dual zone
material
A material was prepared similarly to Example I
above with the following exceptions:
1) 25 microliters (1.65 x 10 4 eq. or 0.33
molecules/nM2) of cyclic silylimine, having the formula
SiMe2CH2CHMeCH2NMe was used instead of the TFSA.
-16-
2) Octane was used as the solvent instead of
decane and the reactio~~ mixture refluxed at 117°C.
Bulk analysis gave values of 0.10 wt. % N and 3.68
wt. % C, corresponding to average surface concentrations of
0.17 m/nM2 for -SiMe2CH2CHMeCH2NHMe and 1.86 m/nM2 for
-SiMe3. External surface analysis by ESCA gave a value of
0.0146 N/Si, corresponding to a concentration at the external
surface of 0.33 m/nM2 for the amine. Calculation of the
external surface concentration was made using reference
values of 0.0786 N/Si (by ESCA) and 1.07 wt.% N corresponding
to 1.78 m/nM2 of the bound group. These values were obtained
for silica that had been treated thoroughly to saturation
with excess cyclic silylimine (no concentration gradient
could occur in that case).
The surface concentration of the amine for the
exterior was 0.33 m/nM2, i.e., two times greater than for the
average and the average for the trimethyl group was 1.86
m/nM2, proving that a dual zone material had been
successfully prepared. An earlier preparation of this
material, made using a less preferred "Teflon"* coated magnetic
bar for stirring (which is thought to degrade the silica
particles to expose some of their interior), also proved to
be a-DZM. It possessed a milder concentration gradient as
shown by an external surface concentration of 0.33 m/nM2 and
an average surface concentration of 0.20 m/nM2 for the
-SiMe2CH2CHMeCH2NHMe group.
III. Preparation of -SiMe2Vi/-SiMe3 dual zone material
A material was prepared similarly to Example I with
the following exceptions:
1) 25 microliters (1.65 x 10 4 eq. or 0.33
molecules/nM2) of ViMe2SiN(Me)C(0)Me was used instead of
TFSA, where Vi represents the vinyl group.
* Trademark
A
-17-
2) 150 cc octane was used as a solvent instead of
100 cc decane.
3) A "Teflon"* coated stir bar was used instead of
the air motor driven paddle.
The ViMe2Si/Me3Si dual zone material was
derivatized as follows prior to spatial distribution
analysis.
0.5 g of the treated silica was added to a 1 oz.
vial along with 10 cc CC14 and sonicated 1 minute. 10 cc of
a 2% (wt./vol.)-ICI solution in glacial acetic acid was added
to the vial which was shaken in the dark for 2 hours. The
silica was then isolated by filtration and washed twice with
CC14 and twice with ethyl ether. Finally, it was vacuum oven
dried for two hours at 80°C. prior to ESCA and bulk elemental
analysis.
Bulk analysis gave values of 0.31, 1.17 and 3.33
wt.% for C1, I and C respectively, corresponding to average
surface concentrations of 0.23 m/nM2 for -SiMe2Vi and 1.71
m/nM2 for -SiMe3. External surface analysis by ESCA gave a
value of 0.0258 C1/Si and 0.0190 I/Si, corresponding to a
concentration at the external surface of 0.49 m/nM2 for
-SiMe2Vi. Calculation of the external surface concentration
was made using reference values of 0.1008 and 0.0848 for
C1/Si and I/Si and using the measured values of 2.30 Cl wt.%
and 9.14 I wt.% corresponding to 2.05 m/nM2 for the bound
group, all measured for a silica which had been thoroughly
treated to saturation with excess -SiMe2Vi prior to the IC1
derivatization reaction, (hence insuring no concentration
gradient).
The external concentration of the vinyl group was
0.49 m/nM2, i.e., two times greater than its average value
and the average for the trimethyl group was 1.71 m/nM2,
proving that a dual zone material had been made.
*Trademark
A
~~~1'~~~1
, . _18_
Having described the invention in detail and by
reference to the preferred embodiment thereof, it will be
apparent that modifications and variations are possible
without departing from the scope of the invention defined in
the appended claims.
