Study of the heat of reversible adsorption at the air-solution interface. II. Experimental determination of the heat of reversible adsorption of some alcohols

Study of the Heat of Reversible Adsorption at The Air-Solution Interface II. Experimental Determination of the Heat of Reversible Adsorption of Some Alcohols R. VOCHTEN ANn G. P E T R E Laboratorium voor Algemene Scheikunde, Faculteit der Wetenschappen, Rijksuniversitair Centrum, Antwerp; and Service de Chimie Physique E. 1)., FacultO des Sciences A ppliquEes, UniversitE Libre de Bruxelles, Brussels, Belgium Received December 13, 1971; accepted May 15, 1972 The surface tension of aqueous solutions of higher alcohols is measured as function of temperature at constant pressure and mole fraction. For all the studied alcohols a minimum in the ~ (T) curves is obtained for sufficiently high concentrations, which minimum seems not to be explained by a solubility increase. The heat of reversible adsorption (or extension) is calculated for different temperatures following the procedure outlined in part I. The heat of reversible extension of the surface area vanishes at this minimum. INTRODUCTION

A. M A T E R I A L S

The surface tension of aqueous solutions of alcohols was measured in order to calculate the values of the heat of reversible adsorption for these systems. It was expected that the surface tension of aqueous solutions of alcohols should decrease with increasing temperature as for pure liquids and most known solutions. The first measurements were performed in the vicinity of 20°C and gave the expected decrease. Measurements performed on a more extended temperature range show a surprising phenomenon: the surface tension goes through a minimum above which it increases markedly with the temperature (Fig. 1). This effect was pointed out by us at the first European Biophysics Congress (1). This phenomenon was obtained with both normal and branched alcohols for sufficiently high concentrations. Accessory the influence of the structure of the alcohol on the molecular area was examined at surface saturation. The solubilities of the various alcohols were obtained from surface tension measurements.

All the studied alcohols were purified by distillation and preparative gas chromatography. They are listed in Table I with their boiling points. Triply distilled water from KMnO4 solution was used in all the experiments. B. APPARATUS The surface tension of the solutions was measured by means of the static method of Wilhelmy (platinum plate). The apparatus consisted of an electrobalance (R. G. Cahn) connected with a high impedance null detector (FLUKE type 845-AR). In order to keep the temperature constant (-4-0.1°C) a special all pyrex vessel was constructed (Fig. 2). C. STATISTICAL ANALYSIS OF N U M E R I C A L DATA

The results of the measurements have been analysed by standard statistical procedures. Linear and nonlinear least-square fitting has been used, providing standard errors bounds 320

Journal of Colloid and lngerface Science, Vol. 42, No. 2, February 1973

Copyright ~ 1973 by Academic Press, Inc. All rights of reproduction in any form reserved

HEAT OF REVERSIBLE ADSORPTION AT AIR-SOLUTION INTERFACE. II

321

TABLE I BOILING POINT Ot' THE ALCOHOLS Name

Formula

n-Butanol n-Pentanol n-Hexanol n-ileptanol n-Octanol n-Nonanol n-Decanol 2-Nonanol 3-Nonanol 4-Nonanol 5-Nonanol

Boiling point a

117.5 (oc) 137.8 157.0 175.8 195.0 2t2.0 231.0 193.0 194.5 192.0 193.0

CH3-(CH2) z-CH~OH

CH~- (CH2)3-CH2OH CH3-(CH2) 4-CH~OH CH~- (CH2)5-CH2OH CH3-(CH2)6-CH~OH CH~-(CH2)wCH2OH CH~-(CH2)8-CH2OH CH~- (CH2)r C H (OH)-CH3 CH3- (CH2)5-CH (OH)-CH2-CH~ CH3-(CH~),-CH(OIt)-(CH2)2-CH3 CH3- (Cil2) 3 Cil (OH)-(CH2)3-CH3 CH3

\ 2,6-Dimet hyl-4-heptanol

/

CH~

CH--CH2--CH (OH)--CH2--CH

/

\

CH3 CH2--CII~

CH3 CII~--CH2

\ 3,5-Dimethyl-4-heptano]

178.0

/

/ CII--CH (OH)--CH

CH3

171.0

\ CH3

C2H5

J 2,2-Diethyl-l-pentanol

CH~--(CH2)2--C-

CH2OH

192.0

i C2H~ CIl3

\ 7-Methyi-l-octanol

/

CH--(CH2)5--CH2OH

206.0

CH~ - At 760 mm Hg. (2). The computations were performed on the 1130-IBM computer of the University of Antwerp.

D-a. System n-Heptanol-Water

D. RESULTS The values of the molar heat of reversible adsorption were calculated by means of relation E36~ in part I (4). (~o)T,,,N~' =

r,,1

of 10-3 , while the experimental relative error exceeds 10-2 .

,,~,.

E13

The value of F3,1 was computed from Gibbs isotherms expressed in logCd instead of logNd. This leads to a relative difference of the order

First of all, the results for n-heptanol are given, this alcohol was the most fully investigated. Measurements of surface tensions at constant pressure and N3' for different temperatures ranging from 5 to 70°C were performed on solutions of different concentrations given at 20°C (Fig. 1). This figure shows that for concentrations higher than 10-3 M a minimum appears in the curves. The Gibbs isotherms were constructed for 5, 10, 15, 20,

Journal of Colloid and Interface Science, Vol. 42, No, 2, February 1973

322

V O C H T E N AND P E T R E (f d y n e . c m -1

ELECTRO BALANCE

21~

70

I ITHERMOSTATI

50 ~ .............

40-~

6

~

,

FIG. 2. Thermostated cell for surface tension measurements.

2'03oio soo'o7'0

oc

FIG. 1. Variation of the equilibrium surface tension with temperature, at constant pressure and mole fraction N J . At 20°C, the n-heptanol concentrations are (M): (1) water; (2) 6.31 X 10-4; (3) 8.00 X 10-*; (4) 1.00 X 10-8; (5) 1.30 X 10-3; (6) 1.59 X 10-8; (7) 2.00 X 10-8; (8) 5.00 X 10-*; (9) 7.60 X 10- 3.

Fig. 5. This was done by putting in Eq. F1] the values of Fa,1, obtained from Gibbs isotherms, and those of (O(r/OT);,v 3, calculated from Fig. 1 for a given temperature T (K). The solubility of 1-heptanol is given in Table II as a function of the temperature.

0-

30, 40, 45, 50, 55 and 60°C by means of independent dilution series. An example is given in Fig. 3. As a check one isotherm was derived from a set of curves such as these of Fig. 1 (points indicated X in Fig. 3). The values of F3.1 for n-heptanol were obtained from the Gibbs isotherms which are represented in Fig. 4. The values of the molar heat of reversible adsorption were calculated and represented in

70

6o

5o

40-

TABLE II SOLUBILITY Ole n - H E P T A N O L

dyn. cm "~

IN WATEI~ AS A

F U N C T I O N OF T E M P E R A T U R E

Temp (°C)

S (mole liter-1 4- 0.1 ; X 10-2)

15 20 30 40 50 60

1.7 1,3 1.3 1.1 1.2 1.3

30

FIG. 3. Equilibrium surface tension as function of log~0 Ca' for aqueous solutions of n-heptanoh (o) values obtained from a series of dilutions prepared at 40°C; (×) values obtained from surface tension measurements as function of temperature for different concentrations.

Journal of Colloid and Interface Science, Vol. 42, No. 2, February 1973

323

HEAT OF REVERSIBLE ADSORPTION AT AIR-SOLUTION INTERFACE. II

TABLE III RESULTS FOR MOLECULAR AREA, SOLUBILITY AND HEAT OF REVERSIBLE ADSORPTION FOR SOMENORMAL ALCOHOLS AT 15°C ~ A X 101~ (cm2) n-Alcohol

lb

n-Bu tanol n-Pentanol n-Hexanol n-Heptanol n-Octanol n-Nonanol n-Decanol

--28.8 4- 1 28.7 4- 2 26.4 -4- 2 26.6 4- 1 28.3 4- 1

S (mole liter-I ~ 0.1)

2" 27.7 27.4 27.4 27.3 27.6 27.2 27.4

4- 0.5 =t= 0.5 4- 0.5 4- 0.5 -4- 0.5 4- 0.5 -4- 0.5

1.1 2.6 6.6 1.7 4.1 1.0 2.0

a A = molecular area given b y A = U r N ; A = -- ( R T / N ) \

X X X X X X

(h~)2s8K,Iaun,sat. 3070 2920 2860 2480 1970 1350 910

10-~ 10-2 10 -~ 10-~ 10-~ 10-4

4- 90 4- 90 4- 90 4- 70 4- 50 :t= 40 4- 20

0 in C~ 0¢ ,/r,p; 37 = numbe r of Avogadro; S = solu-

bility (mole liter-~) at 15°C obtained from surface tension measurements of saturated solutions. b Values obtained from measurements by Hommelen (3) a t 20°C. Values obtained from our measurements a t 15°C.

TABLE IV

D-b. Effect of Chain Length in the Series of n-Alcohols

RESULTS FOR MOLECULARAREA, SOLUBILITYAND HEAT

For each of the n-Mcohols studied a curve similar to curve 5 in Fig. 5 has been calculated, following the procedure explained in D-a. These curves, shown in Fig. 6, represent the molar heat of reversible adsorption as function of the temperature for an alcohol concentration range near saturation. In that region, the values (0¢/0 logi0 C)T,p and (Oc/OT)p,z%, are both nearly independent of the concentration. The minimum of the function ~(T) shifts to lower temperatures if the number of the carbon atoms increases. This is clearly illustrated in Fig. 7 where the different curves represent the variation of the surface tension with the temperature for concentrations for which the surface tension is equal to 40 dyn cm -1 at 15°C. The surface molecular area A at saturation,

OF REVERSIBLE ADSORPTION FOR DIPFERENT

POSITIONS OF THE HYDROXYL GROUP IN NONANOL AT 15°C S l(mole

iter-1

Nonanol

A X 1016 (cm~)

1-Nonanol 2-Nonanol 3-Nonanol 4-Nonanol 5-NonanoI

27.2 32.8 41.7 49.6 51.9

44444-

0.4 0.4 0.6 0.6 0.7

±0.1; X 10s) (ha)288K,latm,sat. 1.0 1.8 2.2 2.6 3.2

1350 2200 3130 3570 3700

4- 40 4- 80 4- 80 4- 90 4- 100

the solubility S and the molar heat of reversible adsorption at 15°C, are given in Table III. A plot of logs versus the chain-length gives a straight line as Hommelen (3) has also obtained at 20°C. However, our measure-

TABLE V RESULTS FOR MOLECULAR AREA, SOLUBILITYAND HEAT OF REVERSIBLE ADSORPTION FOR SOME BRANCHED ALCOHOLS AT 15°C Alcohol 2,6-Dimethyl-4-heptanol 3,5-Dimethyl-4-heptanol 2,2 -Diethyl-l-pentanol 7-Methyl-l-octanol

A X 1016 (cm~) 53.6 50.5 45.7 30.2

-4444-

1.5 1,5 1.5 1.0

s (mole liter-~ =1=0.1 ; X 10-a) 3.1 5.0 3.8 3.2

(ha)2s8K,latm,sat. 2920 4760 1350 1820

4444-

90 140 40 60

Journal of Colloid and Interface Science, Vol. 42, No. 2, February 1973

,7

Z P

<

%

20

30

40

50 .

60

70

°C

Fro. 4. Relative adsorption of heptanol as function of temperature for various concentrations: (1) log C = - - 2 . 1 0 (8.00 X 10 -a 3//); ( 2 ) l o g C = - - 2 . 3 0 (5.00 X 10-a M); (3) log C = --2.70 (2.00 X 10-~ M ) ; (4) logC = --2.80 (1.59 X 10-a M); (5) logC = --3.00 (1.00 X 10-3 M) ; (6) log C = --3.10 (8.00 X 10-4 M) ; (7) log C = --3.20 (6.31 X 10-4 M).

10

Cm "2

F - . l O ~o

mole

0

1'0

2~0

300--

40

o

50

2

SO

°C

FIG. 5. Heat of reversible adsorption as function of temperature for various concentrations o5 heptanol: (1) l o g ( . ? = - - 3 . 2 0 ( 6 . 3 1 X 1 0 -4 M); (2) log C = --3.10 (8.00 X 10-4M); (3) l o g C = --3.00 (1.00 X 10-aM);(4) log

- 3000

- 2000

-1000

+1000

+ 200(

t 3000

* 4001

[ha] T . p ~1; Cal.mole -1

z

P~ :<

o (3

HEAT OF REVERSIBLE ADSORPTION AT AIR-SOLUTION INTERFACE. I[

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HEAT OF REVERSIBLE ADSORPTION AT AIR-SOLUTION INTERFACE. II rnents lead to somewhat higher values of the solubilities.

D-c. Effects of the Position of the Hydroxyl Group in a Nine Carbon Straight Chain Alcohol The molar heat of reversible adsorption near saturation in function of the temperature for 1-nonanol to 5-nonanol is illustrated in Fig. 8. The temperature at which the minimum surface tension was reached increases with the shift of the OH-group from position 1 to position 5. The surface area at saturation, the solubilities and the heat of reversible adsorption, at 15°C are reported in Table IV.

D-d. Effect of Branching Table V is analogous to Tables I I I and IV for some branched alcohols with nine carbon atoms. Figure 9 represents for the branched alcohols studied the values of (ha)T,p,N3' as function of the temperature for concentrations near saturation.

327

For the alcohols studied the most important phenomenon was the unexpected presence of a minimum in the surface tension-temperature curves in a wide concentration range. For all the alcohols investigated this minimum leads to the fact that the molar heat of reversible adsorption decreases from positive to negative values.

For n-heptanol, this phenomenon was studied more extensively as a function of the alcohol concentration. A minimum appears for concentrations ranging from saturation to one seventh of the saturation value. Complementary investigation seems to be necessary in order to explain the fact that the observed minimum in the a(T) curves can be ascribed either to a surface or to a bulk rearrangement and m a y be to a combination of the two phenomena. ACKNOWLEDGMENTS The authors are indebted for computation facilities to the computer center of Professor Dr. J. De Vreese and to L. Cnudde for the technical assistance. REFERENCES

E. CONCLUSIONS Near saturation, we observe the following expected qualitative effects: 1. the molar area increases with the shift of OH-group to the center of the carbon chain, and also with the branching of the chain. 2. the solubility increases with the shift of the OH-group to the center of the chain.

1. VOCHTEN,R., ANDP~TRE, G., in "Proceedings of the First European Biophysics Congress, Wien 1971," (E. Broda and A. Locker, Eds.) Vol. 4, p. 417. H. Sprlnger-Lederer, Wien, 1971. 2. BRANDT, S., in "Statistical and Computational Methods in Data Analysis," North-Holland, Amsterdam, 1970. 3. HOMMELEN,J. R., J. Colloid Sci. 14, 392 (1959). 4. VOCttTEN~R., PETRE, G., AND DEt'AY, R., J. Colloid Interface Sci. 42, 310 (1973).

Journal of Colloid and Inlerface Science, Vol. 42, No. 2, February 1973