Quantitative determination of surface groups in silica : IR-analysis of isolated silanol groups in pyrogenic silica

JoumolofMolecularShccture,60(1980)409-414 0 Elsetier Scientific PublishingCompany,Amsterdam-Printedinl'heNetherlands

QUANTITATIVE

DETERMINATION

IR-ANALYSIS

Siegmar

OF

ISOLATED

OF

SURFACE

SILANOL

GROUPS

GROUPS

IN

IN

SILICA

PYROGENIC

:

SILICA

Laufer ,

Miissmattstr.47

D7888

Rheinfelden

(G.F.R.)

ABSTRACT The

quantitative

determination

isolated

silanol-

means

IF&spectroscopy

A of

of

(SiOH)-

silica

powder

independent

and

in

Ccl4

a clear

rapid

groups,

distinct

and

interchangeable.

can

be

phobic

silicon

of

between

silanol

determined

groups,

dioxide

especially

particles

showing

technique

using

This

method

developed_

determination

"bonded"

also

silicon

measuring

been

distinction

not

bound

by

described.

has

quantitative

Itsyields

surface

on

groups, is

reproducible

new,

of

"isolated"

the

that SiH

quantitatively,

are

the

groups.

"isolated"

groups

and

chemically

groups

in

highly

as

be

CH-bonds

can

allows

silanol

so-called

these

suspensions

active in

silica hydro-

dioxides.

INTRODUCTION The

applicative

pyrogenic

silica

Detailed fore

properties are

knowledge

decisively

of

this,

shows

so-called

schematically

"bonded"

silanol

silica

particles.

A

groups

is

possible

only

first

line

below

1958

(ref.

Virtually have

been

discs"). quantitative low

highly

dispersed

influenced

and

a rapid

the

so-called

by

silica

their

measuring

and

especially

surface

structure.

technique

are

there-

desirable.

Fig.1

in

of

groups

distinction

the

,which

"free" are

between

formulas

show

the

"isolated"

located

these

spectroscopically;

or

two

the

on

the

types

of

wave

assignment

the

surface

of

silanol

numbers

derived

and

in by

the

McDonald

1).

all

the

carried This

earlier

out

method,

routine

reproducibility.

using

spectroscopic pressed

however, measurement

has

measurements

silica

tabletts

proven

to

because

of

be

its

not

in ("self useful

tedious

this

field

supported for

procedure

a rapid and

410 Fig.

l-

3750 (3700

an-1

cm-l

From

in

CCILI)

earlier

works

silanol

"isolated" we

distinguish

as

a spectroscopist

and which A We

be

different found

the

"bands"

might

a chemist's for

spanning

21,

out

of or

clear,

and

cm"

not

in

CCL,)

words: silanol

decide

cm-'

"bonded"

other

isolated

clearly

permanent

3680

whether In

and

we

-

(3680

CC14)

bonded

and Can

groups,

between

bonded

interchangeable,

question? the

solution silica

to in

this

Ccl4

form

steadfast

cuvette

with

fixed

cuvettes are not R Aerosil manufactured

on

3660

quite

can

i.e.

of

a dismountable

-1

interchangeable.

ask,

groups,

suspensions

(ref.

never

are

Such

carried

in

became

suspensions

as

cm-l

groups

method

that

measured. In

it

silanol

would

cm

(3500

only

isolated

~3500

question

can

pastes,

a

was

needed.

reproducibly which

level-grinded

are

measured

distance

Measurements

suitable. by

be

have

ring been

Degussa.

RESULTS The for

band

water

have

been

(ref,

3)

the

band

and

is

-1 isolated silanol groups was found at 37OOcm R Aerosil as shown in Fig. 2 _ This frequency could

for

free,

free

expected

and

using at

found

simultanously

silica -1 3750cm at a

agrees discs

well

with

earlier

showed

that

in

results.

a gaseous

McDonald

atmosphere

is shifted to the red with increasing gas pressure -1 , if the disc 3702cm is immersed into liquid nitrogen;

strong

band

broadening

was

observed.

411 Fig.

2.

3

,

cm--

60

Ffg.3.

3500

I

3000

Pyrogenic water

silica

cm-’

with

adsox >ed

II

after

30 min

10 -3T0?Zr

20°C

III

after

6 h

10 -3Torr

zo*c

IV

after

15 h

10-3Torr

2ciOc

412 3 demonstrates that the extinction of the band at 37OOcm-' R in a CCL 4 paste is within a wide range independent from Aerosil R _ the water contents of the Aerosil Fig_

This the on

cannot

extinction water

In in

result

contents 4 the

Fig.

the

of

paste

be

obtained

by

the

corresponding

and

the

contents

reproducibility

is

at

use

of

band of

the

disc

at

375Ocm-I

bonded

silanol

different

method, is

of

where

both

dependent

groups.

concentrations

of

silica

shown.

Fig.4.

.

vSi OH ‘lJ

#’

I’

I

37aok

Pyrogenic curves

silica

with

different

Measurements which

have

value

for

possible groups

and the

seconds

(and

not and

to

only that

them.

SiOH

contents

of in

finely vacua

distributed for

times

(highest

extinction to

different

yield

corresponding

slopes.

samples

tempered

indeed,

changeable assigned

of

been

peratures)

having

at

temperature) -1 _ This 3700cm

distinguish between very

between their

different

highly

between

in

demonstrates and

that

they

functional

hours

resulted

isolated

bands),

disperse (low

importance

tem-

a constant that

bonded are

silica,

it

is

silanol

not

inter-

can

be

413 In

5 the

Fig.

(ref. Fig,

spectrum

of a specially

prepared

pyrogenic

silica

4) is shown. 5.

3.0

P

-90

-10

-60

-50

-40

-30

-20

-10

4000

This being The

3500

silica

particles

Cm-’

is exclusively

completely

spectrum

3000

free

of adsorbed 5 clearly

in Fig.

by use

characterized

of hydrogen

water

by and

indicates bonds

isolated

of bonded

that

obviously

silanol silanol

interactions do not

occnrr

groups, groups.

of the in the

CC14

matrix. Using groups

a topochemical of such

correlation groups

reaction

a specific

between

sample

at the particle

silica weight

surfaces.

(e.g.

alkylation)

it is possible and

the

amount

with to of

find

the

silanol

an exact

isolated

silanol

414

A direct amount could

proportionality

of isolated

extinction

groups

for

any

at 37OOcm -1 to the

employed

the

determination

same

method,

of other

It may be noted unnecessity

that

an eventually

groups,

e.g.

a valuable

compensation,

tatively,

too.

alkyl

simultanous groups,

advantage

of compensation

employing

of the

silica

it is possible

of the

to detect

R-S. McDonald, DP 2 000 847. R.S. McDonald, UsP 3 859 420.

J,Phys.Chem..

62(1958)1168-

J,Amer.Chem,Soc,,

79(1957)850.

quantitative

can be carried described

suspension

REFERENCES 1 2 3 4

pyrogenic

be found.

Applying

is the

silanol

of the

SiH

medium, groups

out

method However, quanti-