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electron affinity of small particles. This will be observable in photoelectron emission spectra. With regard to the electron donor properties of metals this means also a stronger ability to form charge-transfer complexes.
The Electrochemical Properties of Small Metal Particles and the Surface Enhanced Raman Effect In the observation of the so-called surface enhanced Raman effect (SERS) some of the most fundamental experimental results have not yet found reasonable explanations. This included the following points: Why is the oxidation reduction cycle important? Why does silver deposition at moderate negative potentials result in no enhancement but deposition at high negative potentials18 does? Why does the enhancement effect irreversibly disappear after the polarization is increased above a limiting negative p ~ t e n t i a l ? ' ~ The answers to these questions may be found by considering the Outstanding electrochemical properties of microparticles especially of smallest size (e.g., 4-40 A for Ag15). The explanations described are further support for the idea that these particles are also responsible for the optical process of e n h a n ~ e m e n t . ~ , ~ The formation of clusters and microcrystallites on surfaces in metal deposition is favored (especially for silver) by rather rough electrochemical conditions with regard to the electrode potential as well as with regard to diffusion limitations. Special deposition forms (whisker, powder deposition) are obtained under very similar conditions as are necessary to activate an electrode for enhanced Raman scattering. No enhancement is found when the silver is deposited under moderate deposition conditions, regardless of the electrode roughness. In this case a fairly smooth deposit is, in general, obtained. Nevertheless, the fresh deposit strongly absorbs molecules and ions of the electrolyte. Thus, adsorption on a blank surface cannot explain the (19) W. J. Plieth, B. Roy, and H. Bruckner, Ber. Bunsenges. Phys. Chem., 85,273 (1981). (20) B. Honigmann, "Gleichgewichta- und Wachstumsformen von Kristallen", D. Steinkopf Verlag, Darmstadt, 1958.
enhancement mechanism. Ad-atoms are also expected on the surface under these conditions. The lack of enhancement despite the presence of these ad-atoms is a strong argument against the simple ad-atom theory for the enhancement. Clusters, especially silver clusters, have to be stabilized after formation. Otherwise they could not exist in equilibrium with the smoother, less energetic surfaces. The stabilization may be caused by complex formation with the substance under investigation (pyridine, CN-, etc.). The cluster properties will result in much stronger chemisorption bonds than on the bulk metal surface. The stabilization is lost after cleavage of the chemisorption or electrosorption bond when surpassing a limiting negative potential. The unstable cluster will rapidly transform into a stable surface form, that means it disappears from the surface and the enhancement effect is lost. Thus, in addition to discussing the optical properties of metal particles it seems helpful also to look at the electrochemical properties to derive a more complete understanding of the enhancement mechanism. As was pointed out by one reviewer, a formalism of the thermodynamics of small systems was developed by Hi1121 and was recommended to be used for the thermodynamic analysis of this paper. A reformulation of our analysis is in progress. Acknowledgment. The present paper was partly formulated and written during a sabbatical leave at the Lawrence Berkeley Laboratory of the University of California. I would like to thank my colleagues who made my stay such an enjoyable experience. I appreciate especially discussions with Rolf Muller, Phil Ross, Charles Tobias, Joe Farmer, and Felix Schwager. I appreciate also the funding by the German Research Foundation who made this stay financially possible. Preparation of the manuscript was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the US. Department of Energy under contract No. W-7405-ENG-48. (21) T. L. Hill,"Thermodynamics of Small Systems", W. A. Benjamin, New York, 1964, Vol. 1, 2.
Pressure Effect on the Krafft Points of Ionic Surfactants Nagamune Nlshikldo,' Hldekl Kobayashl, and Mltsuru Tanaka Depaftment of Chemistry, Facutty of Science, Fukuoka University, Fukuoka 814-01, Japan (Received: December 8, 1981; I n Final Form: March 29, 1982)
The pressure dependence of the Krafft temperature (or point) of typical anionic and cationic surfactants (sodium alkyl sulfates and cetyltrimethylammoniumbromide) has been determined at pressures up to 400 MPa by means of the electroconductivitymethod. The Krafft temperatures increased rapidly with pressure, and hence the range of temperature and pressure in which micelles can exist is rather restricted. In order to establish a rule describing the pressure dependenceof the Krafft temperature, the volume changes of transition from the hydrated surfactant solid to micellar state for sodium alkyl sulfates at atmospheric pressure have been calculated according to the model that a Krafft temperature is a melting point of the hydrated surfactant solid. Comparison of the calculated value with the directly determined one in the literature gave reasonable agreement, suggesting that the model is useful for describing the rule.
Introduction With increasing concentration of an ionic surfactant in water, the solid precipitates as a result of the limit of solubility at low temperatures at constant pressure. However, the solubility increases very rapidly at a certain temperature, called a Krafft point or temperature,' and
consequently the solutions of almost any composition become homogeneous at temperatures about 4O above the Krafft temperature. This phenomenon characteristic of (1) (a) F. Krafft and H. Wiglow, Ber., 28, 2566 (1895). (b) Recently, the Krafft temperature has been found for the aqueous solutions of nonionic surfactants under high pressure; see ref 9.
0022-365418212086-3 170$01.2510 0 1982 American Chemical Society
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surfactant solutions is attributed to the formation of aggregates called micelles. The micelle formation, which is an important aspect of the solution behavior of surfactants, does not occur below the Krafft temperature. That is, the Krafft temperature is one of the most important parameters which determine the property of surfactant solution, as well as a critical micellization concentration (cmc). Nevertheless, there are few reliable data for the pressure effect on the Krafft temperature? For this reason, among studies regarding the properties of surfactant solution under pressure, some authors have measured the properties in the metastable state since the surfactant solution is readily superpressed under p r e ~ s u r e . ~ In this report, we have measured the pressure dependence of the Krafft temperature of typical ionic surfactants in water and calculated the volume changes of transition from hydrated surfactant solid to micellar state a t atmospheric pressure according to the model that a Krafft temperature is a melting point of the hydrated solid for establishing a rule describing the pressure dependence of the Krafft temperature. Experimental Section The ionic surfactants used for the measurement of Krafft temperature are sodium alkyl (dodecyl, tetradecyl, and hexadecyl) sulfates and cetyltrimethylammonium bromide (CTAB). The anionic surfactants are abbreviated as SDS, STS, and SHS, respectively. SDS and STS were synthesized and purified according to the procedure described previ~usly.~ SHS was obtained from Tokyo Kasei Ind. and was recrystallized several times from ethanolwater, ethanol, and ether. CTAB was also obtained from Tokyo Kasei Ind. and was recrystallized three times from an acetone-ethanol mixture in which the volume fraction of ethanol was below 0.1.5 The high purities of SHS and CTAB were confirmed by no minimum near their cmc's in the plots of surface tension against concentration. The pressure vessel (prepared by Hikari Koatsu Kiki Co., Ltd.) for measuring the Krafft temperatures is schematically shown in Figure 1. This vessel is equipped with six electromagnets which make it possible to stir the solution in a conductivity cell at high pressures.e The pressure-transmitting medium (ligroin) is introduced into the vessel through pressure tubing at a right angle to the optical windows shown in Figure 1. By utilizing this vessel, we can measure the electroconductivity,transmittance, and turbidity of a solution at the same time. The Krafft temperatures at various pressures were determined by the electroconductivity method according to Ino.'v8 The variation of the specific conductance of the solution containing the precipitate of surfactant (the concentration in mol dm-3) with completely dissolved state: 1.7-2.9 X temperature was measured with a Conduct Meter Model CM-6A (TOA Electronics, Ltd.) and recorded together with temperature. During detection of the conductivity, the solution was heated at a constant rate of 0.05-0.08 "C m i d by circulating water from a temperature controller, holding the pressure constant. The temperature of the solution was measured by means of a thermocouple and (2)M. Tanaka, S. Kaneshina, T. Tomida, K. Noda, and K. Aoki, J. Colloid Interface Sci., 44,525 (1973). (3)S. D.Hamann, Reu. Phys. Chem. Jpn., 35, 109 (1965). (4)N. Nishikido, J . Colloid Interface Sci., 60,242 (1977). (5)T. Harada, N. Nishikido, Y. Moroi, and R. Matuura, Bull. Chem. SOC.Jpn., 54, 2592 (1981). (6)S.Ozawa, K. Kawahara, and Y. Ogino, "High-PressureScience and Technolow", Vol. 1. Plenum. New York and London. 1979.. D- 593. (7)T. !io; Nippon Kagakll Zasshi, 80, 456 (1959): (8)N. Nishikido, H. Akisada, and R. Matuura, Mem. Fac. Sci. K y w h u Uniu., C10, 91 (1977).
I
Ill
Flgure 1. Schematic drawing of the pressure vessel for the measurement of Krafft temperatwe under high presswe: (1) themKKxKlple; (2) lead connected to a conductivity meter: (3) copper ring; (4) (Mng; (5) Teflon ring; (6) watertirculating hole; (7) sapphire window; (8) syringe-type conductivity cell; (9) six electromagnets; (10) magnetic stirrer; (1 1) lead connected to the swkching device.
4300
20
30
10
1a"oture
50
60
I'C
Flgure 2. Some examples of the plots of specific conductance vs. temperature at constant pressure for determining the Krafft temperature.
was recorded. The generation and measurement of pressure were described in the previous papersg Examples of the specific conductance vs. temperature at constant pressure are shown in Figure 2. The plots exhibit a distinct break and the temperature at this break coincides with the Krafft temperature. Especially, the obtained Krafft temperatures at atmospheric pressure agreed with those determined by equilibrium methods.'&13 Conse(9)N. Nishikido, N. Yoshimura, M. Tanaka, and S. Kaneshina, J. Colloid Interface Sci., 78,338 (1980). (10)M. Hato, Nippon Kagaku Zasshi, 92,496(1971);M. Hato and K. Shinoda, J . Phys. Chem., 77,378 (1973). (11)K. Shinoda, S. Hiruta, and K. Amaya, J. Colloid Interface Sci., 21, 102 (1966). (12)K. Shinoda and T. Soda, J. Phys. Chem., 67,2072 (1963). (13)H. Lange and M. J. Schwuger, Kolloid-2. 2.Polym., 223, 145 (1968);M. Hato, M. Tahara, and Y. Suda, J. Colloid Interface Sci., 72, 458 (1979);K. Meguro and T. Kondo, Studies on Surfactants" (in Japanese), Saiwai Syobo, Tokyo, 1961,p 6.
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TABLE I: Heat and Volume Changes, AH, and A Vt, o f Transition from Hydrated Solid Surfactant t o Micellar State for Sodium Alkyl Sulfates at the Krafft Temperature and Atmospheric Pressure o n the Basis of the Melting Model A
Krafft temp,
AH,,
"C
kcalimol
SDeS
-5b
SDS STS SHS
9 26 36
10.2b 12.OC 13.F 15.6c
surfactant
v,,c m 3 / m o l
calcd 12 18 23 30
from pm?
r 3 r 3 t i.
3 3
26.8
Data from the measurement o f partial molal volume (pmv) in ref 1 2 . Presumed values in ref 10. Data from ref 11.
2i/,
I
-3
L A
20: Pressure
10 I
Wfl
Flgwe 3. Krafft temperatures of typical ionic surfactants as a function of pressure.
quently, the Krafft temperatures were determined easily, within an accuracy of rtl.0 "C.
Results and Discussion In Figure 3 are plotted the Krafft temperatures vs. pressure for the aqueous solutions of SDS, STS, SHS, and CTAB. The plots obtained are almost linear over the pressure range studied within experimental uncertainty, perhaps curving slightly downward. It is found from Figure 3 that the Krafft temperatures increase rapidly with pressure. Then, the range of temperature and pressure in which micelles can exist is rather restricted. For example, SDS micelles do not appear above 167 MPa at a constant temperature of 25 "C if the equilibrium state is completely realized. However, they can exist at least at pressures several tens of MPa above 167 MPa since the surfactant solution is readily superpressed and the metastable state is r e a l i ~ e d . ~ So far, various interpretations have been offered for the Krafft phenomenon.14-" Among these, the following mode1,18J9which is based on the pseudo-phase (in liquid state) separation model for micelle formation,'=' has given reasonable results in the thermodynamic treatment.'"-lz~~ The model says that the solubility increases suddenly since (14)P. H. Elworthy, A. T. Florence, and C. B. Macfarlene, "Solubilization by Surface-Active Agents", Chapman and Hall,London, 1968,p 36. (15)Y. Moroi, T.Oyama, and R. Matuura, J. Colloid Interface Sci., 60,103 (1977). (16)B. Lindman and H.WennerstrBm, Top. Curr. Chem., 87, 53 (1980). (17)M. Kodama and S. Seki, Yukagaku, 30,749 (1981). (18)K. Shinoda and E. Hutchinson, J . Phys. Chem., 66,577(1962). (19)K. Shinoda, 'Principles of Solution and Solubility" (P. Becher, trans.), Marcel Dekker, New York, 1978. (20) A. E. Alexander, Trans. Faraday Soc., 38, 54 (1942). (21)E. Matijevic and B. A. Pethica, Trans. Faraday SOC.,54, 589 (1958). (22)H.Nakayama, K.Shinoda, and E. Hutchinson, J. Phys. Chem., 70,3520 (1966).
the hydrated surfactant solid melts and transforms into liquid micelles at the Krafft temperature at constant pressure; in other words, the Krafft temperature is interpreted as a triple point at which hydrated solid, micelles (a liquid state), and singly dispersed surfactant are in equilibrium with each other. In order to establish a rule describing the pressure dependence of the Krafft temperature, we adopt this model from the viewpoint of this simplicity and ability to predict reasonably the effect of added alcohols,n inorganic salts,23and surfactants'o on the Krafft temperature. This model is expressed as "the melting model" below. According to the melting model, the volume change AV, of transition from hydrated solid to micellar state can be estimated by applying the Clapeyron-Clausius equation for the curve of the Krafft temperature vs. pressure if the transition enthalpy AH, is known. The calculated volume changes AV, for sodium alkyl sulfates at atmospheric pressure are tabulated in Table I, with the transition enthalpies A?It from ref 10 and 11. The volume change AV, for STS determined from the measurement of the temperature dependence of partial molal volume12is also listed in Table I. The agreement between the calculated and the directly determined values of AV, is reasonable if we take into account the experimental errors. Figure 3 also contains the Krafft temperature vs. pressure curve for sodium decyl sulfate (SDeS) predicted from the calculated values of AV,. The volume change AV, per methylene group was around 3.0 cm3 mol-' from the data in Table I. This value is comparable to the value estimated from the volume change during melting of long chain fatty acids, 2.7 cm3m01-l.~~This suggests that the micelles are in a liquid state, showing a self-consistency of the melting model. Acknowledgment. We thank Professor R. Matuura, Kyushu University, for furnishing the conductivity meter used in this study. (23)K.Tsujii and J. Mino, J. Phys. Chem., 82,1610 (1978);K.Tsujii, N. Saito, and T. Takeuchi, ibid., 84, 2287 (1980). (24)K. S. Markley, 'Fats and Oils", Wiley, New York, 1960,p 535.
