L a n g m u i r 1991, 7, 604-607
604
Notes Anomalous Temperature Dependence in Monolayers at Water-Air Interface Gabriella Caminati,' Donatella Senatra,' and Gabriella Gabrielli'*+ Department of Chemistry, University of Florence, Via G Capponi 9, 50121 Florence, Italy, and Department of Physics, IJniuersity of Florence, L. go E. Fermi 2, 50125 Florence, Italy Received J u n e 11, 1990. I n Final Form: August 1, 1990
Introduction Anomalous thermal behavior in water-containing systems has been the subject of many investigations in the past years. Temperature discontinuities were found in several systems such as liquid crystals,' microemulsions,* membranes,3 and solid dispersion^.^ A previous work5 revealed an unexpected temperature behavior for tetradecanol and oleic acid monolayers a t the water-air interface; in particular the monomolecular films were found to undergo an area contraction with increasing temperature in the 25-35 "C interval. It was also shown that the aforementioned components exhibit bidimensional miscibility at the water-air interface.2 Furthermore this study pointed out that the mixtures with molar ratios in the 3:l to 5:l interval showed the greatest thermodynamic stability. Such stability was attributed to a difference in the energetic balance, depending on the temperature a t which the measurements were performed. For temperatures lower than 30 "C the 4 : l mixture was enthalpically favored, whereas a t 30 "C the greater stability seemed to depend on entropic factors. The experimental conditions adopted5 and the reproducibility of the s-A isotherms obtained with various compression rates and different amounts of spread substance excluded solubilization or evaporation of the monolayer in the experimental time scale; a transition was then suggested to occur in the 25-30 "C temperature range. A thermal anomaly for surface films of oleyl alcohol was found a t 30 "C also by other authors.6 Bois et aL7reported a discontinuity around 30 "C for monolayers of octadecanol at the water-air interface. Harkins and Copeland have evidence in their early works8p9of some peculiarities of fatty alcohol monolayers, namely anomalous temperature domains and abnormal dependence of surface viscosity on temperature. The temperature dependence of a-A isotherms is generally associated with monolayer expansion, due to the flexibility of the hydrophobic moieties, with increasing t
Department of Chemistry.
* Department of Physics.
(1) Dervichian, D. G. J . Colloid Interface Sei. 1978, 90, 71. (2) Eicke, H.-F.; Markovic, 2. J . Colloid Interface Sei. 1981, 79, 151. (3) Drost-Hansen, W.; Thorhaug, A. Nature 1967, 215, 506. (4) Drost-Hansen, W. J . Colloid Interface Sei. 1977, 58, 251. (5) Gabrielli, G.; Senatra, D.; Caminati, G.; Guarini, G. G. T. Colloid Polym. Sei. 1988, 266, 823. (6) Huhrenfuss, H.; Walter, W. J . Colloid Interface Sei. 1984,97,476. (7) Bois, A. G. J . Colloid Interface Sei. 1985, 105, 134. (8) Copeland, L. E.; Harkins, W. D.; Boyd, G. E. J . Chem. Phys. 1942, 10, 357.
(9) Harkins, W. D.; Copeland,
L. E. J . Chem. Phys.
1942, 10, 357.
0743-7463/91/2407-0604$02.50/0
temperature.1° Several examples are known where the surface film exhibits a reversal of temperature dependence,11J2 in some cases13 a thermally activated flip-flop transition was invoked, whereas other authors14postulated a mechanism that involves the breakdown of intramolecular hydrogen bonds between polar headgroups. A reversal of the temperature effect is commonly ascribed to the different effects that temperature exerts on the hydrophobic chains (increasing their mobility) and on polar headgroups (hydration-dehydration processes). The aim of the present work is to investigate the presence of a peculiar temperature discontinuity for tetradecanol, oleic acid, and their mixtures in bidimensional phase at a waterair interface around a critical temperature, namely 30 "C.
Experimental Section Tetradecanol (purity 99 50 ) was supplied by Aldrich;oleic acid (purity 965 ) was supplied by Merck. Chloroform (Merck) was used as spreadingsolvent. The subphase was an aqueoussolution of 0.01 N KC1 (supplied by Merck);water was twice distilled and further purified with a Milli-Qwater system (Millipore);the subphase pH was in the range 6.4-7. Surface pressure measurements were performed by using the Langmuir method on a computer-controlled Lauda Filmwaage balance elsewhere described.5 Monolayers were spread on the aqueous support from their chloroform solution with a 100-~L Pressure Lock microsyringe; 30 min was allowed for the solvent to evaporate and the film to spread homogeneously before the discontinuouscompression was started. The conditions of compression were optimized in a previous study on the same substance^.^ The temperature control was ensured by a Haacke PG 40 thermostat. Compression took place at 15 "C in all experiments; the barrier was stopped at a predetermined surface area and a. The temperature was then raised at a constant rate at 0.2 "C/ min. This velocity was found sufficient to reach equilibrium pressure at each temperature in a separate experiment. The surface pressure vs temperature curves were recorded in the temperature range 15-40 "C;temperature was then decreased with the same speed. Before each measurement,water cleanliness was checked by measuring the surface pressure in the entire area interval; surface pressure was always negligible (r< 0.1 mN/m). The accuracy in surface pressure and in temperature was respectively f0.07 mN/m and *0.05 "C. The plots reported are the average of at least two independent runs. The results obtained were reproducible within the experimental error. The r-T curves were recorded for the clean substrate. This contribution was subtracted from the r valuesobtained for spread monolayers. Results and Discussion Figure 1 shows the a-T relationship at two different surface concentrations for tetradecanol monolayers at an air-water interface, namely A = 0.55 and 0.69 m2/mg corresponding to molecular areas of 19.6 and 24.6 A2, respectively. These surface area values correspond re(IO) Adam, N. K. T h e Physics and Chemistry of Surfaces; Dover: New York, 1968. (11) Kellner, B. N. J.; Muller-Landau, F.; Cadenhead, D. A. J . Colloid Interface Sci. 1978, 66, 597. (12) Cadenhead, D. A,; Balthasar, D. H. J . ColloidInterface Sci. 1985, 107. 567. (13) Cadenhead, 1974, 49, 132.
D. A,; Muller-Landau, F. J . Colloid Interface Sei.
(14) Glazer, J.; Goddard,
E. D. J . Chem. Soc., Part 3 1950, 3406.
0 1991 American Chemical Society
Notes Tetradecanol
-
Tetradecanol Oleic Acid 4:l mixture
-
8-
161
E
*'."
\
E
6-
-.._.. ....
'*...
12-
e
.-......
4-
..... ".....
2-
-
. . * .* A
*
4
= 0.55 m2/mg
-- -
* A = 0.77 m2/mg ..... = 1.14 m2/mg 8
14
i
" " " " l a
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 15 20 25 30 35
Temperature ('C)
Temperature ("C)
Figure 1. Surface pressure vs temperature curves for tetradecanol at an water-air interface.
Figure 3. Surface pressure vs temperature curves for a tetradecanol-oleic acid bidimensional mixture with a molar ratio of 4:l.
Oleic Acid
16-
10
15
20
25
30
35
40
Temperature ("C)
Figure 2. Surface pressure vs temperature curves for oleic acid monolayers at an water-air interface. spectively to the liquid condensed and liquid expanded phases as reported in previous studies.5 A discontinuity centered around 30 "C is found for both surface concentrations: when the monolayer is in an expanded phase the A-T curve presents a maximum, whereas in the more condensed region a broad minimum is detected. For higher surface concentrations, i.e. more condensed monomolecular film, a reversal of temperature dependence is shown before the discontinuity point, centered around 30 O C , is reached. Such a phenomenon was already found for other substances15 and attributed to a different degree of hydration of the polar groups. The experimental measurements of A vs Tfor oleic acid are reported in Figure 2, a t surface concentrations of 0.72 and 1.12 m2/mg,or 33.8 and 52.5 A2/molecule. Oleic acid monolayers show at both surface concentrations a long plateau in the 25-35 OC temperature range. For low surface coverage ( A = 52.5 A2f molecule) the temperature effect is smaller but still significant: surface pressure is almost constant for T < 20 "C, then it decreases to reach the plateau, and for temperatures above 30 "C, a further increase in surface pressure is detected. A t smaller surface areas ( A = 33.8 A2/mo1ecule)a decreases with increasing temperature up to 25 "C after which a constant value of A is reached, but for T > 32 O C , ?r increases again. The trend is similar to that recorded for tetradecanol in the (15) Gabrielli, G.;Puggelli, M.; Baglioni, P. J. Colloid Interface Sci. 1982, 86,485.
more condensed phase, but in the case of oleic acid a superposition of a bidimensional phase transition, present a t every temperature in this range, must be considered. Time-dependent relaxation processes may take place and contribute at the pressure change even if they can be considered of minor relevance in the experiment time scale. When tetradecanol or oleic acid monolayers are cooled at constant area, surface pressure is a decreasing function of temperature for every surface concentration. Figure 3 reports the behavior of surface pressure with temperature for the monolayers of a mixture of tetradecanol and oleic acid with a molar ratio 4 to 1; as already mentioned this was a preferential ratio and these particular mixtures showed a maximum thermodynamic stability in the bidimensional phase. The surface concentrations studied where chosen to correspond to the liquid expanded (A = 1.14 m2/mg) and liquid intermediate ( A = 0.77 m2/ mg) phases of the film. A temperature discontinuity around 30 "C is again observed in the liquid expanded region. a-T curves are very similar to the plots for tetradecanol, which is indeed the major component in the mixture, even if the maximum is slightly shifted toward lower temperature as for oleic acid. For higher surface concentrations A decreases with increasing temperature, but a plateau is again found between 25 and 30 "C; condensing the monolayer, the contribution of oleic acid is more effective. A complete explanation of such behaviors may be hindered by the contemporary presence of several phenomena, such as partial loss of substance via evaporation and solubilization or time-dependent relaxation effects but the experimental procedure adopted excludes most of the previous factors. The decay of surface pressure with time for monolayers of fatty acids with one double bond was studied by other authors.16 The results showed that A relaxes a t a, and then remains constant for many hours. It seems thus possible to obtain reliable information on the temperature dependence of monolayers of tetradecanol and oleic acid. An anomalous temperature domain was already found for aliphatic alcohols a t both water-air and water-oil interfaces.7J7-20 Fatty acids do not generally show this (16) Hifeda, Y. H.; Rayfield, G. W. J. Colloid Interface Sci. 1985,104, 209.
(17) Petre, G.;Aza Azouni, M. J . Colloid Interface Sci. 1984,98,261. (18)Gericke, A.; Hahrenfuss, H. J. Colloid Interface Sci. 1989, 131, 588.
606 Langmuir, Vol. 7, No. 3, 1991
Notes
behavior unless a double bond is present in the hydrophobic tail.21 Moreover previous s t u d i e P showed that the surface pressure of tetradecanoic acid monolayers is almost entirely determined by the temperature of water a t the surface. Thermal anomalies have been described for many properties of water and aqueous solutions. Some investigators have proposed the existence of a t least four thermal anomalies between 20 and 110 "C, namely near 15,30,45, and 60 0C.23 Moreover it has been known for some time that thermal anomalies of water properties reflect a change of water structure near an interface.24 It might thus be expected that if water structure had any effect on the structuring of the surface film, the variation of such an effect with temperature could determine the surface properties of the film itself. It is, in fact, commonly accepted that the surface film introduces strong ordering effects on water m0lecules,~5which correspond to negative entropy within the water layers. Casilla et aL26founda marked change for stearic acid monolayers near the temperature of maximum density of water. This was ascribed to extrusion of water molecules from between polar headgroups into the interstitial volume of water structure. Huhnerfuss et al.,18p25who reported thermal anomalies in dielectric measurements, stated that the hydrophobic portion of the molecule may also induce ordering of the water layers below the film, with a penetration depth that for oleic acid is of the order of 25
m.
The strict analogy between thermal anomalies of both water properties and experimental spreading isotherms confirm that theories of molecular features of monolayers have to include both the surface film and the adjacent water layers. The properties of the interfacial region would then affect the temperature behavior of these systems where the interfacial region is predominant, such as microemulsions. Entropy effects within tetradecanol and oleic acid surface film can provide further insight on this phenomenon. An estimate of the entropy term can be determined from the temperature dependence of surface pressure AT at constant area A with respect to the change of temperature AT.6 This expression can be compared to hark ins'^^^^ expression for the entropy of expansion of a film on water
T,A,
AT
where S, is the entropy per unit area, S is the total entropy, Af is the molecular area of the film molecule, A , is the area of water, and yf is the surface tension of the film. The values of A - A r I A T computed in this way are summarized in Table I for tetradecanol and oleic acid monolayers. A different behavior again results for the two phases examined. For tetradecanol in the expanded region a negative entropy effect is revealed. This implies an increased ordering effect of the film-forming molecules on the water layers above 30 "C and is in agreement with a hypothesis of expulsion of water molecules from the (19) Caminati, G.; Senatra, D.; Gabrielli, G., submitted for publication in Langmuir. (20)Villers, D.; Platter, J. K. J.Phys. Chem. 1988, 92, 4023. (21) Sears, D. F.; Schulman, J. H. J.Phys. Chem. 1964,68, 3529. (22) Kellner, B. M. J. Colloid Interface Sci. 1980, 74, 308. (23) Drost-Hansen, W. Adu. Chem. Ser. 1967, No. 66. (24) Chi, R.; Loglio, G.; Ficalbi, A. J.Colloid Interface Sci. 1972,41, 287.
(25) Htihrenfuss, H.; Alpers, W. J. Phys. Chem. 1983,87, 5251. (26) Casilla, R.; Cooper, W. D.; Eley, D. D. J. Chem. SOC.,Faraday Trans. 1 1973, 251.
Table I. A.Au/AT Values (J/(g K)) for Tetradecanol and Oleic Acid Monolayers at a Water-Air Interface Tetradecanol
A = 19.6 .@/molecule
A = 24.6 A/molecule
-0.15 0.30
0.42 -1.09
T < 30 "C T < 30 "C
Oleic Acid
A = 33.8 A2/molecule A = 52.5 &/molecule
T < 25 "C 25 "C < T < 30 "C
-0.89 -0.11 0.25
T>35"C
-0.17 0.09
Table 11. A.Ar/AT Values (J/(g K)) for a 4:l Bidimensional Mixture of Tetradecanol and Oleic Acid at Water-Air Interface A = 0.17 m2/mg A = 1.14 m2/ma T T
< 25 "C > 25 "C
0.25 -0.40
T < 28 "C T>31"C
-1.16 -0.17
monolayer to the bulk phase with a correspondent packing of the molecules in the surface film. Similar conclusions were reached by Steinbach and Sucker2' for fatty acid monolayers. In the more condensed region, as the temperature is raised over 30 "C, thermal agitation produce a disordering effect and an increase in entropy. In this phase the interactions among the hydrophobic chains prevail over the polar groups-water interactions. The behavior of the entropy term for oleic acid shows similar trends in both the liquid expanded and liquid intermediate phases. For A = 33.8 A2/molecule the A ATIAT contribution is negative for T < 30 "C as for tetradecanol in a similar phase but greater in absolute value. In the expanded phase ( A = 52.5 A2/molecule), increasing temperature above 30 "C leads only to smaller changes in entropy: the molecules are far enough to be only slightly affected by an increase of chain mobility or a decrease in the number of water molecules coordinated at the polar head group. In this case we cannot neglect the contemporary presence of a phase transition to more expanded phases. A.Ar1A.T values for the 4:l mixture are reported in Table 11. In the expanded phase we again observe a decrease to negative values of the entropy term with increasing temperature, but the change is smaller than for tetradecanol. In the liquid intermediate phase ( A = 0.77 m2/mg) a discontinuity can again be found a t 30 OC. The entropic term remains negative although it decreases in absolute value as temperature increases.
Conclusions The experimental results showed that both the oneand two-component systems a t the water-air interface exhibit an anomalous behavior for temperatures around 30 "C. These anomalies are correlated to the peculiar properties of water layers near a surface-active substance. In particular surface entropy values indicated that the determining factor in the change of the monomolecular array and order is the balance between cohesive forces between the hydrophobic portions of the molecules, thermal agitation, chain mobility, and interactions between the hydrophilic groups and water molecules in the subphase. (27) Steinbach, H.; Sucker, C. H. R. Adu. Colloid Interface Sci. 1980,
14, 43.
Notes
Langmuir, Vol. 7, No. 3, 1991 607
The present results appear particularly interesting if compared with similar temperature-dependence studies performed by some of us on different systems.28 Striking thermal anomalies were found for some properties of a microemulsion system, viscosity and dielectric measurements. We recall that such a microemulsion was composed by water, dodecane or hexadecane, an aliphatic alcohol, and potassium oleate. The dielectric discontinuity found for these systems at 30 "C29 was explained with a change of shape of the droplets from elliptical to spherical with release of water molecules from the interface to the water core. In the present study, the most stable tetradecanol-oleic
acids mixture, corresponding to the actual ratio between surfactant and cosurfactant in the microemulsion, was studied in the liquid expanded and liquid intermediate phases, that are regarded as the most representative of the state of the mixed monolayer in the interphasal region of the microemulsion. The behavior of x vs T for the mixture in the more expanded phase reflects predominantly the tetradecanol contribution. The discontinuity found around 30 "C could then be correlated, as for tetradecanol, to a decrease to negative entropies above 30 "C. This was ascribed to a condensation of the monolayer at higher temperatures due to expulsion of water toward the subphase and to a different ordering of the molecules a t the interface.
(28) Gabrielli, G.; Caminati, G.; Guarini, G . G . T.; Senatra, D. Ann. Chim. 1987, 77, 297.
(29) Senatra, D.; Gabrielli, G.; Caminati, G.; Zhou, Z. IEEE Trans. Electr. Insul. 1988, 23, 579.
