Effect of electrolyte on the molecular orientation in monolayers adsorbed at the liquid-liquid interface: studies by resonance raman spectra

Volume 55, number 3

EFFECT OF ELECTROLWE AT THE LIQUID-LIQUID

1 blay 1978

CHEhlICAL PHYSICS LETTERS

ON THE MOLECUJXR INTERFACE:

ORIENTATION

LN MONOLAYERS

STUDIES BY RESONANCE

RAMAN SPECTRA

ADSORBED

Tohm TAKENAKA Institute for Chemical Research, Kyoto Urziversiiy, @i. Kyoto-Fu 611. iapnn

Received 18 January 1978

Polarization measurements of resonance Raman spectra are made for monoIaycrs of a surface-active azo dye adsorbed at the interface between carbon tctrachloride and an aqueous solution and an effect of electrolyte on the molecular oricntation in the monolayers is examined.

1. Introduction In a previous paper, we have proposed a method of total reffection for recording resonance Raman spectra of adsorbed monolayers at the liquid-liquid interface and discussed the orientation of complexes of a cationic surfactant (cetyltrhnethylammonium bromide) with an anionic azo dye (methyl orange) in the monolayers [l] . The same method was applied to the study of monolayers of a surface-active anionic azo dye (Suminol Milling Brilliant Red BS, abbreviated as BRBS) adsorbed at the interface between carbon tetrachloride and the aqueous solution [Z] _ The most important result in this study may be a fmding of a correspondence between the physical state of the monolayer and the molecular orientation in it. It was found that when the concentration of BRBS in the aqueous solution was increased and therefore the amount of adsorbed molecules was increased, the BRBS molecules showed a tendency to stand up in the monolayer, accompanying the change in the physical state of the monolayer. Ohnishi and Tsubomura [3] have studied electronic absorption spectra of

monolayers of BRBS at the same interface using the multiple reflection method with polarized and unpolarized light, and discussed the orientation of the BRBS molecules_ In this study, an effect of electrolytes on the orientation of the BRBS molecules adsorbed at the interface between carbon tetrachloride and the aqueous solution is examined by polarization measurements of resonance Raman spectra using the total reflection method. Since it has been known that the addition of an electrolyte in an aqueous solution of a surfactant remarkably increases the amount of adsorbed molecules by the salting-out effect of the electroiyte 1451, it can be expected to attain a higher degree of orientation of the surfactant molecules in the monoIayer.

2. Experimental The sample of BRBS was the same as that previously reported [2]. Guaranteed reagent sodium chloride or barium chloride was used as the electrolyte, and dissolved hr a concentration of 1CP2 M into the aqueous solution of BRBS. The concentration of BRBS was changed in the rangeof lo-6 to IO-” M. Pure water was prepared by redistillation of distilled water which had been passed through an ion-exchange resin column. Carbon tetracbioride was the specially prepared reagent for spectroscopy by Nakarai Chemicals,

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CHEMICAL PHYSICS LETTERS

Volume 55, number 3

1 May 1978

and the aqueous solution were assumed to be 1.46, 1.43, and 134, respectively_ The subscripts fland 1 refer to the polarized exciting light with electric vector pamlIe and perpendicular, respectively, to the plane of incidence XV, and the subscripts X and Y refer to the polarized Raman line with electric vector parallel to the X and Y axes, respectively. A,, A,, and As are given by ptism Y

4lmous sdutlan

Fig. I. The total reflection method for Raman measurements of monolayers adsorbed at the interface between carbon tetrachloride and an aqueous soiution.

Ltd. and used without further purification. The interfacial tension was measured by the Wilhelmy method using a Teflon plate. The total reflection method for recording Raman spectra of adsorbed monolayers is shown in fig. 1. Details of the measurements have been described previously [ I] _ For polarization measurements of Raman spectra, a GlanThomson prism and a Polaroid were placed in the optical paths of the excit&g light and the Raman line, respectively_ The angle of incidence of the exciting light at the interface was 84”. The penetration depth of the evanescent wave in the aqueous solution was 1390 K for the 488.0 nm light of an Ar+ laser.

3. Theoretical background It has been reported that the orientation of BRBS is evaluated by the two Raman intensity ratios expressed by [2] r,, 0_72A, f 1.2/f, - 0.36A, -= (1) 0.3Z4, + 1.8A3 I UY and zfl -= IIY

3A, - 2A, + 0.6A, 4.4,

-2_4A3

’

(2)

under the assumption of uniaxial orientation with respect to the Y axis normal to the interfaced2 (fig_ 1). In the derivation of eqs. (1) and (2) the refractive indexes of carbon tetrachloride, the adsorbed layer, 516

A, = 1 -cod,,

(3)

A, = (1 - cos308) cos26,

(4)

A3 =(l

(5)

- cos5e,)cos4S,

where 0, is the maximum value of the angle 8 between the Y axis and the molecular axis z which lies in the plane of the chromophore of BRBS and is perpendicular to the Line connecting the two sulfonate groups (fig. 4 of ref. [Zj ). Here we assume that the planes of the chromophore are uniformly distributed between 8 = 0 and 8 = 8, _Thus the distribution function F(B) can be defined as

(6) - 6 in eqs. (4) and (5) is the angle between the z axis and the transition moment p of the electronic absorption band around 530 run which gives rise to the resonance Raman effect for the 488.0 nm exciting light [6] _Since the value of 6 has already been found to be 3 lo [2 ] , the angle 0, can be determined as a measure of the molecular orientation of BRBS from measurements of the two Raman intensity ratios_

4. Results and discussion In fig. 2, the interfacial tension 7 between carbon tetrachloride and the aqueous solution is plotted as a function of the logarithm of the concentration c of BRBS. The solid line shows the result obtained with sodium chloride, and the broken line shows the previous result obtained without electrolyte 121. Since the amount of adsorbed molecules is proportional to the tangent to these curves, it is apparently increased by the addition of sodium chloride_ Assuming the Gibbs adsorption isotherm for the divalent ion [73,

CHEMICAL

Volume 55, number 3

01

I

io-=

IO+

I

Id4

PHYSICS

LETTERS

I

lCF3

Concentrationof ERBS m c /M Fig. 2. Interfacial tension -y between carbon tetrachloride and aqueous solution as a function of logarithm of concentration c of BRBS. with sodium chloride; - - - without sodium chloride.

]r=_L

_!!?I_

3RT I aInc) c(electrolyte~

lo’=

I

Id4

,&

Concentration of BRSS, c/M Fig. 3. Raman intensity ratios f&ft y and Im/Ily for 1276 cm-l band as a function of logarithm of concentration c of BRES. with sodium chloride; - - - without sodium chloride.

(7)

obtain the maximum amount of adsorbed BRBS = 1.43 X lo-lo mol cm-2 (at a concentration r. otp6axX10-S M) from the solid line, while I’,, = 9.3 X lo-t1 mol cme2 (at a concentration of 7 X lo4 M) from the broken line. Resonance Raman spectra of the monolayer of BRBS adsorbed at the interface between carbon tetrachloride and the aqueous solution with sodium chloride is qualitatively the same as those obtained without sodium chloride (fig. 3B of ref. [21), suggesting that the structure of adsorbed BRBS does not change on the addition of sodium chloride. However, both the intensity ratios I ax/Ztl Y and ILu/IL Y are decreased by the addition of sodium chloride as shown in fig. 3 for 1276 cm-l band. Furthermore it is seen that the two intensity ratios decrease with increasing concentration of BRBS in the presence of sodium chloride. Other bands with relatively strong and medium intensity give rise to similar results. In fig. 4, the changes of the Raman intensity ratios are compared with that of the interfacial pressure z obtained from the interfacial tension y of fig. 2 by z = r. - 7, where -y. is the interfacial tension between carbon tetrachloride and pure water (43.8 dyn ~m-~). The abscissa of this figure is the surface area we

I

I

I

IO=

A occupied by each BRBS molecule calculated from the tangents to the y--log c curve (fig_ 2) at respective concentrations with the aid of the Gibbs adsorption

oco IO0

150

200

250

Surface area,

300

350

400

450

500

A / %? molecule-’

Fig. 4. Raman intensity ratios I&II

y and I&,y

and the

interfacial pressure H as a function of surface area A occupied by each BRBS molecule. with sodium chloride; - - without sodium chloride.

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isotherm, eq. (7). The solid and broken lines represent the results obtained with and without sodium chloride, respectively- It is obvious from comparison of the two x--A curves in fig. 4 that the monolayer is condensed in the presence of sodium chloride. Correspondingly, the two Raman intensity ratios obtained with sodium chloride give smaIler vaIues as compared with those obtained without sodium chloride. At the same time, they decrease almost linearly with decreasing surface area A. Substituting the respective values of the two intensity ratios into eqs. (1) and (2), we obtain the variation of the 0, values from 65 to 55” with the decrease of surface area from 220 to 120 A*, when sodium chloride is present_ When it was absent, on the other hand, the 0, values varied in the range from 86 to 80” as mentioned in a previous paper [2] _Thus it is concluded that the BRBS molecufes show a tendency to be packed with their planes ordered at much smalter angles to the vertical by the addition of sodium chioride. It is Jso seen from fig_ 4 that when sodium chloride is absent and the surface area is smaller than 210 AZ, the intensity ratio ILyjfLy increases with decreasing surface area but the intensity ratio Inx/ IB y decreases. Since this fact cannot be interpreted under the assumption of uniaxial orientation, other types of molecular orientation shouId be considered. In the presence of sodium chloride, however, such changes in the Raman intensity ratios are not observed even at smalIer surface areas than 2 10 AZ, Therefore it can be said that the effect of electroIyte appears not only on the amount of adsorbed mole-

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1 May 2978

CHEMICAL PHYSKS LETfERS

cules and the degree of orientation but also on the type of molecular orientation. The addition ofbarium chIoride gives nearly the same results of the interfaciat tension and of the Raman intensity ratios as those obtained with sodium chloride.

Acknowledgement The author wishes to thank Professor S. Ikeda of Nagoya University for his valuable suggestions about the Gibbs adsorption isotherm. This work was supported in part by a Grant in Aid for Scientific Research from the Ministry of Education, Japan, which is gratefully acknowledged.

References [I ] T. Takenaka and T. Nakana8a, J. Phys. Chem. 80 (1976)

415. [2f T_ Nakanaga [3]

645. T. OhGshi

and T. Takenaka, and H. Tsubomura,

(1976) 77. 141 N. Etengas and

J. Phys. Chem. 81(1977) Chem.

Phys. Letters 41

E. Rideal_ Trans. Faraday

Sot. 55 11959)

339. [S]

H. Fseundlich,

Kapibrchemie

(1930)

95.

J. Behringer, in: Raman spectroscopy, Vol. 1, ed. H.A. Szymamki (Plenum Press, New York, 19671 p. 205. [7] S_ Ikeda, BufL Chem, Sot. Japan 50 (1977) 1403; private communication f1977). 161