molecular shape of amphiphiles self-organised into bimolecular films

COLLOIDS AND A

Colloids and Surfaces SURFACES

ELSEVIER A: Physicochemical and Engineering Aspects 121 (1997) 23 26

Molecular shape of amphiphiles self-organised into bimolecular films 1

Elena V. Shumilina *, Yurii A. Shchipunov Institute of Chemistry, Far East Department, Russian Academy of Sciences, 690022 Vladivostok, Russia

Received 27 February 1996; accepted 14 July 1996

Abstract

The dependence of the appearance of the black spot in thick nonaqueous films and the formation of bimolecular lipid membranes on the shape of amphiphilic molecules has been studied using a wide-ranging homologous series of alkyl derivatives of diamines. The optimum molecular shape and limits to amphiphile self-organisation into the bimolecular state have been ascertained.

Keywords: Alkyl derivatives of diamines; Bimolecular films; Formation; Molecular shape

1. Introduction

Some of the essential factors itl amphiphile structural organisation and the phase transition are the geometric packing constraint [1,2]. These are mainly determined by the shape of the amphiphilic molecules. In this report we have studied how molecular shape variation in a wide-ranging homologous series of alkyl derivatives of diamines results in their ability to stabilise bimolecular films.

2. Experimental

Alkyl derivatives of diamines and alkyl derivatives with an acetyl group were synthesised through the reaction of ethylenediamine, 1,4-butanediamine or N-(2-aminoethyl ) acetamide with hexadecyl bro-

* Corresponding author. 1 Presented at the IXth European Colloid and Interface Society (ECIS) Conference, Barcelona, Spain, 17-22 September, 1995.

0927-7757/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PH S0927-7757 (96 )03758-2

mide as described in Refs. [3 -5] . All these reagents were purchased from Merck.

Diamine derivatives were dissolved in n-heptane (chemically pure). To generate a film, a nonaqueous solution was injected through an aperture (2 mm diameter) locating in the Teflon partition of a two- chamber cell filled with water.

The ability of diamine derivatives to stabilise the bimolecular films was characterised by the critical concentrations for black spot appearance in thick nonaqueous films (Cbl) and for transformation of the latter into the bimolecular state over the whole area (Cure). The method has been described in detail in Refs. [-6,7].

3. Results and discussion

The structural formulae of-the diamine derivatives examined, the conditions at which the formation of bimolecular films has been established and the parameters characterising bilayer formation

Tab

le 1

D

iam

ine

deri

vati

ves

exam

ined

and

co

nd

itio

ns

of

bila

yer

mem

bra

ne

form

atio

n

Am

phip

hile

M

olec

ular

str

uctu

re

Fo

rmat

ion

of

bila

yer

mem

bra

nes

Cbt

" (m

M)

Cbm

b (m

M)

pH

~

N-H

exad

ecyl

-l,2

-eth

aned

iam

ine

'VV

v'~/

VV

V~

N H

~

vN

H

2 20

5

-7

N,N

-Dih

exad

ecyl

- l,2

-eth

aned

iam

ine

'vV

V'v

Vk/

~ N

.,,

v N

H

MV

VV

VV

v~

2 0.

043

0.04

8 1.

5 3.

0

N, N

'-D

ihex

adec

yl- 1

,2-e

than

edia

min

e ~

N

H

4"

V',A

/VV

VV

'v N

H

5-7

N, N

, N '-

Tri

hexa

decy

l- 1

,2-e

than

edia

min

e v

vv

',/v

vx

~ N

H

5-7

N, N

, N ',

N '

-Tet

rahe

xade

cyl-

1,2

-eth

aned

iam

ine

VV

VV

VV

'Cv

Vxg

XA

A/x

.AA

, V

'VV

VV

VX

~ N

3.2

N, N

, N ',

N '

-Tet

rahe

xade

cyl-

1,4

-but

aned

iam

ine

VV

VV

',/V

'x/~

N

'x/x

gV

VV

VV

~ N

%

A,/

VV

VV

V'v

10

9.3

11

N, N

'-D

ihex

adec

yl- 1

,4-b

utan

edia

min

e 'v

'vN

'VV

'~

N

H

VV

VV

VV

'~

N

H

1-11

N-H

exad

ecyl

- 1,4

-but

aned

iam

ine

VVVV

VV~N

H~/

~.,N

H2

5-7

T (

° C) d

/~

r

(s) f

20

0.4

--

45

--

220

~.

30

0.2

--

20

1.0

--

20

0.5

30

0.5

e~

20

50

tu~

30

0.1

--

N-[

2-(

Hex

adec

yla

min

o)-

eth

yl]

acet

amid

e ^A

,'v'~

AN

~,N

H

"~,N

HC

OC

H

3 --

--

1-

11

20

60

--

--

E. V. Shumilina, Y.A. Shchipunov / Colloids Surfaces A. Physicochem. Eng. Aspects 121 (1997) 23-26 25

0 o4

05 3: 0 0 0 "r 7

Z

" 0

e~

0

"t-

O

0

0

Z ~

r ' a

oJ

g~

m

I

g

o o

©

¢1

¢-,

o o

o ~ . ~

.e ~ 9 ¢~ tao f2~

" ~ o "el

o ~.

~ . ~

~ . a g

.N.= ~

e~

o

m

. o ¢~

are summarised in Table 1. Further detailed data can be found in Refs. [-6,7].

Even a cursory examination of the experimental results presented shows that the bilayer-forming properties vary significantly in the homologous series of examined amphiphiles. To obtain an accu- rate assessment of their ability to self-organise into the bimolecular state, it is necessary to describe the process in quantitative terms, and this is partic- ularly the case with the molecular geometry. For this purpose, the molecular shape has been expressed in terms of a dimensionless packing parameter, S, suggested by Israelachvili et al. [8]:

S = V/(LA)

where V is the hydrocarbon chain volume per molecule, L is the chain length, and A is the effective headgroup area. Values for V, L, and A have been estimated from space-filling molecular models. It should be mentioned that in doing so the hydration of polar regions and the variability in conformations of amphiphilic molecules, which occur in reality, have not been accounted for. The trans-trans conformation has been assumed for the hydrocarbon chains. Values for S thus estimated have been used to plot a graph of the critical concentrations of black spot appearance Cb~ VS.

0.1 Lg Cb. Nonbilayer B l a c k Bimolecular Black

(M) structures ~ s p o t s membranes spots

O.Ol

o.ool

0.0001

a b c d

0.00001 1 3

Dimensionless packing parameter, S

Fig. 1. Plot of the logarithm of critical concentration of black spot appearance vs. dimensionless packing parameter. Space- filling molecular models correspond to the key points: (a) N,N'- dihexadecyl-1,4-butanediamine; (b) N-hexadecyl-l,4-butanediamine; (c) N,N-dihexadecyl-l,2-ethanediamine; (d) N,N,N',N'- tetrahexadecyl- 1,4-butanediamine.

26 E. V. Shumilina, Y.A. Shchipunov / Colloids Surfaces A: Physicochem. Eng. Aspects 121 (1997) 23-26

the dimensionless packing parameter. A V-shaped curve, as seen in Fig. 1, has been obtained. Regions corresponding to bimolecular membrane formation and black spot appearance are also indicated on the graph by approximate boundaries.

From the relationship in Fig. 1. the following conclusions can be drawn about the opt imum molecular shape and limits to amphiphile self- assembly into the bimolecular state.

(1) The minimum on the curve corresponds to the transformation of nonaqueous films into bimolecular membranes over the whole area and their maximal stability. We suggest that the minimum corresponds to the opt imum molecular shape. As evidenced from Fig. 1, the amphiphile should have a tendency to form very stable bimolecular membranes if the cross-sectional area of the nonpolar part of the molecule is nearly twice the cross- sectional area of the polar region.

(2) The ability of amphiphiles to self-organise into the bimolecular state is decreased when the molecular shape is approximately cylindrical. In the case of diamine derivatives with dimensionless packing p a r a m e t e r < l , the appearance even of short-lived black spots has been impossible to achieve. This finding enables one to infer that amphiphiles having only a truncated cone shape are capable of self-assembling into the bimolecular state.

(3) The upper limit of the ratio of cross-sectional area of hydrocarbon chains to cross-sectional area of the polar region has not been revealed because of the limited number of diamine derivatives examined. It has been established only that the black spots have appeared in nonaqueous films contain- ing amphiphiles for which the dimensionless packing parameter has ranged up to 2.5. From an extrapolation of the dependence in Fig. 1, one might expect a loss of the ability to form bimolecular structures by an amphiphile when the value of S approximates 2.7-3.0.

References

[ 1 ] S.H. White and G.I. King, Proc. Natl. Acad. Sci. USA, 82 (1985) 6532-6536.

[2] J.M. Seddon, Biochim. Biophys. Acta, 1031 (1990) 1-69. [3] F. Linsker and R.L. Evans, J. Am. Chem. Soc., 67 (1945)

1581-1582. [4] J.H. Fuhrhop, V. Koesling and G. Sch6nberg, Justus

Liebigs Ann. Chem., (1984) 1634-1640. [5] E.V. Shumilina, Russian Patent, 4839057/04, 1991. [6] Yu.A. Shchipunov, I.G. Maslennikova, A.F. Kolpakov and

E.V. Shumilina, Bioelectrochem. Bioenerg., 22 (1989) 45 54.

[7] Yu.A. Shchipunov and E.V. Shumilina, J. Colloid Interface Sci., 161 (1993) 125--132.

[8] J.N. Israelachvili, D.J. Mitchell and B.W. Ninham, J. Chem. Soc., Faraday Trans., 72 (1976) 1525-1568.

molecular shape of amphiphiles self-organised into bimolecular films

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