Quantifying Acid-Base Properties of Organic Functional Groups at a

Reprinted from

the Journal of the American

Chemical Society, 1990, 112.

Copyright @ 1990

by the American

Chemical Society

and

reprinted

by

permission

of

the copyright

owner.

Quantifying

Acid-Base Properties

of Organic

Functional

Groups at

a

Polyethylene-Water

Interface

by Photoacoustic

Calorimetry

Lijian Zhang, Matthew

A. Shulman,

George M.

Whitesides,

and

Joseph J. Grabowski*

Department

of Chemistry, H art'ard L' nir ersi

t.t'

Camhridge, Ma.rsachusetts 02 I

38

Receired ,4pril 26, 1990

This

paper

describes

the use of

photoacoustic

calorimetry

(PAC)r

to characterize

acid-base

equilibria involving

organic

chromophores

covalently

attached

at the interphase between

surface-functionalized polyethylene

(PE)

film2'3 and

water. We

use

proton

transfer reactions

to define

the

polarity'

and solvating

capability

of the interphase

between

organic surfaces and

water.a

Characterizing

these

physical-organic

properties

of

interphases

is difficult:

light

scattering

at

interfaces

(especially

rctugh

in-

terfaces)

complicates

many

UV-vis absorption

and

fluorescence

methods;

infrared

spectroscopy

cannot

be applied routinely to

systems

involving water;

thermometric

and

conductometric

methods

are insensitive for

solids having

low surface areas. The

measurement

of the

contact angle of

buffered

water

on surfaces

as a function

of

pH-"contact

angle

titration"-yields

valuable

information

about

acid-base equilibria,3

but

depends

on unverified

assumptions.5

(l)

Rothberg,

L. J.;

Simon, J. D.;

Bernstein, M.; Peters, K. S.

J.

Am.

Chem. Soc. 1983, 105,3464-3468.

Braslavsky,

S.

E.; Ellul, R.

M.;

Weiss,

R.

G.; Al-Ekabi,

H.;

Schaffrier, K.

Tetahedron 1983, J9, 1909-1913.

Burkey, T.

J.; Majewski,

M.;

Griller, D.

J.

Am.

Chem. Soc.

1986,

/08,

22t8-222t.

(2)

Whitesides,

G. M.; Ferguson,

G. S. Chemtracts 1988, /, l7l-187.

Whitesides,

G. M.; Laibinis,

P. E,

Langmuir 1990, 6,87-96.

(3)

Holmes-Farley,

S. R.; Bain,

C.

D.; Whitesides,

G. M, Langmuir 1986,

4,921-931.

Wilson,

M.D.;Whitesides,

G. M.,/. Am, Chem. Soc. 1988, 110,

8718-8719.

Holmes-Farley,

S. R.;

Whitesides,

G. M. Langmuir 1987, J,

62-16.

(4)

Bordwell, F.

G. Acc.

Chem. Res. 1988. 21.456-463.

7070

J. Am.

Chem. Soc,,

Vol.

I12.

No. 19. 1990

Communications to the Editor

Table L Experimental

Values

of

pK172

and

pKu

for Organic

Functional

Groups in Aqueous

Solutibn

and at a Polyethylene-Water

I nterfaceo

R

NH{CH:)5NH

Dansyl

-ra

. :-,..,,:

J+g1-_,e'

______f_

a

L

c

q\R=pe

R=HJ

I

R

=

PE-COb

R=H

PKr/z

PK^

RNH(CH2)rNH-dansyl

0.56

+

0.1I

RNH(CH2)rNH-dansyl

1.2

*.

0,2'

RNHC6HT-3-NO2-6-OH

9.3

+

0.1

RNHNHCOC6H4-4-OH

12.4

+

0.2

3 3.52

*

0.03. 2

3 3.57

*

0.09.

2

3

7.00

+

0.10d 2

3 8.29

*

0.30

3

oAqueous

solutions

oi HCI

and NaOH were

used to

vary

the

pH

during

these

acid-base titrations.

The addition

of these

trace amounts

of acid

or base themselves

cause

no change in

the PAC signal

other

than that due

to degree

of

protonation

of the

substrate. The

errors

listed

are

I

standard deviation

of the number

of independent

experi-

ments indicated

by n.

DAll

films were prepared

from

PE-CO-CI

as

described

in rei 8. A fresh

film

was

prepared

for each

measurement.

Any

acid chloride

functionality

that

was

not converted

to the

appro-

priate

amide

is hydrolyzed

during

the synthetic

workup.

.See

refs

8

and 13

for

of these

values.

dThe

pK"

of R

=

COCH3, as determined

by PAC. is

6.02

+

0.03

(n

=

2\.

We

have

used as substrates

for

this

work

surface-oxidized

polyethylene

films

(p

=

0.92

glmL,l00

pm

thick;

pretreated

to

remove

additives)

to

which

were

covalently

attached

l-(di-

methylamino)naphthalene-5-sulfonyl

(dansyl)

or

phenolic

groups

(Table

I).6

These

functionalized polyethylene

films

were

sus-

pended

in

water

in a standard

UV-vis cuvette

and irradiated with

a low-energy,

unfocused light pulse

from

a nitrogen

laser

(337.1

nm;

ca. l0

pJ/pulse:

2-mm-diameter

spot).7

The

photoacousric

signal

generated

from

each

of the four films

exhibited

an

excellent

signal-to-noise

ratio

(Figure

l).

Control experiments

established

that these PAC

signals were

due to

the

attached

organic func-

tionality

and that each system was

stable

to irradiation:

(i)

Both

unfunction

alized

(PE-H)

and oxidized

(PE-COrH)

polyethylene

films

gave

weak, pH-independent

PAC

signals.

(ii)

The func-

tionalized

polyethylene

films

that

gave

strong,

pH-dependent

signals showed

contact

angle

titration

behavior

consistcnt with

that

reported

earlier,s

(iii)

The intensity

of the

PAC signal from

a functionalized

film

at constant pH was

linearly proportional

to

laser

energy.T'e

(iv)

The PAC

signal remained

invariant

over long

periods

of time at fixed pH,

indicating

that no

solvolytic

chemistrr

occurred.

(v)

Extended

irradiation

had

no

effect on

the

pAC

signal,

implying

that

no

beam damage

occurred.

(vi)

Upon re-

moval

of the films

from

the cuvette,

no PAC

signal was

detected

in

the

aqueous

solution.

The changes

in

the PAC

signal as

a

function

of

the degree

of

protonation

are due

to changes

both in

the

heat release

quantum

yield,/.,,

and

in

the extinction

coefficient

of the

protonated/de-

protonated

substrates.

For

example,

the dansyl

moiety

has

an

extinction

coefficient

of

-4000

M-r cm-r

at 337.1 nm while

the

protonated

dansyl

moiety

has

a value

of

-200

M-l cm-I.10

In

addition,

the fluorescence quantum

yields

of

these two

species

are

quite

different.

The

pH

titration

data

generated

in

these

pAC

experiments were

readily

fit by

a standard pK.

titration

expres-

sion.rr

The

similar

quality

of

the

fit for

the dita

from

the irlms

and

from solution

suggests

that

the PAC

experiment

samples

a

single

population

of functional

groups

in the

interphase.

.

The value

of

pKr12-the

solution

pH

at

which

the surface

ionization

of a functional

group

in the

interphase

appeared

to be

half

complete-differed

from

the

value

of

pK.

for

that

group

in

homogeneous

solution

in

the

direction

predicted

if one

issumes

it

is

more difficult

to create

a charge

in the

interphase

than

in

(5)

Bain,

C. D.; Whitesides,

G. M.

Angew.

Chem., Int.

Ed. Engt.19g9,

28,

506-5t2.

_

(6)

Jh9

polyethylene

film used

in

this study was

a

gift

from

Flex-Glas

Inc.

(

Flex-O-

Film DRT-600B).

(7)

Jain,

A.; Marzlufl

E.

M.; Jacobsen,

J. E.; Whitesides,

G.

M.;

Gra-

bowski,

J. J.

J. Chem.

Soc.,

Chem.

Commun.l9t9,

1557-1559.

(8)

Holmes-Farley,

S.

R.;

Whitesides,

G.

M. Langmuir

19t6,

2,266-2gt.

(9)

Grabowski,

J.

J.;Simon,

J. D.;

Peters,

K.

S.

/.

Am.

Chem.

Soc. 19g4.

r06,46t5-4616.

(10)

Measured

in

this

work.

(l

l)_The_procedure

used is

described

in

supplementary

materiar

in the

microfilm

edition.

E

o

LLJ

a

o

u

a

-cc

i:,

.

^i

T

,c

I

--j4:+=--%*

HO

A ):..

R'C

NH{

/)

NO

\

04

6

pH

Figure l. Acid-base titration curves

for the molecules

and

polyethylene

films indicated. The solid lines are the

calculated titration

curyes.r' S

I

Eo

refers to

the observed PAC signal normalized

to laser energy

and is based

on

the basic

PAC

equation. S

=

KfofL(l

-

l0-/). Inset

A: Photoacoustic

waves

at

pH

=

I

I 5

(-

- -)

and

pH

=

0.3

(-)

for the dansyl film

shown,

and for distilled

water

(--).

Inset B: Photoacoustic

waves

at

pH

=

10.0

(---)

and

ptl

=

0.8

(-)

ior the dansyl

derivative in homogeneous

solu-

tion. and ior distilled

water

(--).

For both insets,

the hatched

area

indicates

the

*integration-

pe

rformed

to obtain

S, the experimental value

of thc

photoacoustic

signal.

solurion.:

r'

T-hc

nragnirude of these

shifts

(2.3-4,1

pH

units)

is consistent

with

a

prei'ious

characterization of

a

PE-dansyl-water

interphase

as

hav'ing

an

effective dielectric constant

e

lu

9.8 For

all functionalizcd

films examined,

the fractional

change in the

amplitude of the PAC

signal for the functionalized

films

on

going

from low-

to high-pH

asymptotes

was

consistently

smaller than

that

for the

same

chromophore in

solution

(Figure

l). Experiments

are in

progress

to define the cause

of the changes in

relative

amplitudes.

Our

results demonstrate

that

PAC is a simple

and sensitive

method to

quantify

acid-base equilibria

at the interface

between

polyethylene

film

and

water.

This

photoacoustic

technique has

the advantages that it is insensitive

to scattering

of light from

the

interface and is easily

applicable to systems

in

which water

is

the

liquid

phase.

For PAC

to be useful

as a technique for

charac-

terization

of

interphases,

the signal

due to molecules

at the in-

terphase must be large compared

to

that

arising from

bulk solid

or liquid

phases.

The chromophore

used

to

label

the interphase

and to characterize the degree

of ionization should

therefore have

a high extinction coefficient relative

to that of the solid

and liquid

phases.

The system used here-with

optically

transparent

poly-

ethylene

and

water

as the two

phases

and

strongly absorbing

functional

groups

attached

at the

polyethylene-water

interphase-takes

advantage

of one

of the strengths

of

photoa-

coustic

spectroscopy: the

ability to detect very weak

absorption

against

a near-zero

background.l2

(

l2)

Harshbarger, W.

R.; Robin,

M. B. Acc.

Chem. Res. 1973,6,329-334.

Patel,

C. K. N.: Tam,

A. C. Reu.

Mod. Phys.198l,

5J, 517-550.

Sigrist, M.

W.

J. Appl. Phys.1986,60,

R83-Rl2l.

Tam, A.

C. Reu.

Mod. Phys.1986,

J8, 381-431.

Anderson,

V.

E.;Cheng,

H. Z.; Diebold,

G.

J.;

Mahmood, A.;

Sweigart,

D. A.

"/.

Am. Chem.

Soc. 1987, 109,6191-6193.

(13)

Strauss,

U. P.;

Vesnaver,

G. J. Phvs.

Chem.

1975.79.1558-1562.

707

|

We

believe that

PAC

has the

characterisrics to

be a highly

useful

tool for

studying interphases

between

optically transparent

solid

and liquid

phases,

provided

that an

appropriate reporter

group

with

a suitable

extinction

coefficient

can

be

localized

in thar

environmcnt.

Acknowledgment.

This

research

was

supported

by the Harvard

University

Materials

Research

Laboratorv

(\SF

Grant DMR-

86-14003). We

acknowledge

C. R.

Bertozzi.

.f

.

R. Jacobsen.

and

E. M.

Marzluff for

experimental

assistance

and

valuable

dis-

cussion.

Supplementary

Material

Available:

Methodologi' for

exrracting

pKr's

from the data

generated

in the PAC

studies described

in

the

paper

(2

pages).

Ordering information

is

grven

on

an\

current

masthead page.