1987
Alkanoic Acid Melting Point-Krafft Point Correlation
ylaminoethylderivative IVb, is reflected by the higher limiting aggregation size, the higher individual KN values, and the increased free energy of association.
Acknowledgments. The author wishes to thank Lilly Research centre, Richardson-Merrill Ltd., A. H. Robins and Co. Ltd., Winthrop Laboratories, and G. D. Searle and Co. Ltd., for their generous gifts of drugs.
References and Notes (1) P. Mukerjee, J. Pharm. Sci., 63, 972 (1974). (2) D. Attwood, J. Pharm. Pharmacol., 24, 751 (1972). (3) D. Attwood and 0. K. Udeala, J. Pharm. Pharmacol., 26,854 (1974); 27, 395 (1975). (4) D. Attwood, J. Pharm. Pharmacol., 28,407 (1976). (5) D. Attwood and 0. K. Udeala, J. Phys. Chem., 79, 889 (1975). (6) D. Attwood and 0. K. Udeala, J. Pharm. Sci., in press. (7) E. W . Anacker and A. E. Westwell, J. Phys. Chem., 66,3490 (1964). (8) D. Att%ood, A. T. Florence, and J. M. N. Gillan, J. Pharm. Sci., 63, 988 (1974). (9) R. F . Seiner, Arch. Biochem. Biophys., 39, 333 (1952). (10) A. T. Florence and R. T. Parfitt, J. Phys. Chem., 75, 3554 (1971).
Correlation between Melting Points of Alkanoic Acids and Krafft Points of Their Sodium Salts Kbzb Shinoda, * Department of Chemistry, Faculty of Engineering, Yokohama National University, Ooka-2, Minamiku, Yokohama, Japan
Yutaka Mineglshl, and Haruhlko Aral Household Goods Research Laboratories, Kao Soap Company Limited, Bunka-2 Sumidaku, Tokyo, Japan (Received April 26, 1976) Publication costs assisted by Yokohama National University
In order to facilitate the use of longer chain surfactants which are more surface active, various ways of depressing the Krafft point have been studied. There is a close correlation between the melting points of alkanoic acids and the Krafft points of their sodium salts in water. Therefore, depressing the melting point also depresses the Krafft point and permits the use of long chain length surfactants at ordinary temperatures. Moderate branching of the lypophilic chain and the introduction of some groups, such as ethylene oxide, between the lypophilic chain and the hydrophilic group effectively depress the Krafft point as well as the melting point. An ionic surfactant dissolves in water in a singly dispersed state up to the saturation concentration. Above this concentration a hydrated solid surfactant phase separates a t temperatures below the Krafft point and micelles are formed above the Krafft p0int.l It is evident that the Krafft point is an important temperature above which hydrated solid surfactant melts and forms micelles, i.e., pseudophases in the liquid ~ t a t e . The ~ - ~ solubility of ionic surfactants increases abruptly due to micellar dispersion, and solubilization of oily materials occurs above this temperature. Minimum surface tension values are not attained below the Krafft point, because the concentration of singly dispersed species is smaller than the critical micelle concentration. Hence, in most practical applications, the Krafft point of a surfactant has to be lower than the experimental temperature. The surface activity of surfactants increases rapidly with the hydrocarbon chain length.5 There exists also a rough parallelism between the Krafft points of soaps and the melting points of the corresponding acids.6 Therefore, it is very important to find surfactants whose Krafft points are low yet the hydrocarbon chains are long. The experimental determination of the Krafft point, however, requires solubility7,8or electrical conductivity measurementsgof the saturated solution of the surfactant as a function of temperature. This process is time consuming. In order to estimate the Krafft point from some other more
easily accessible property of the surfactant the relationships between the melting points of alkanoic acids and the Krafft points of their sodium salts are plotted in Figure 1. Experimental details will be published elsewhere along with other properties.1° It is evident from Figure 1that the melting points of a-alkyl alkanoic acids are lower, and the Krafft points of their sodium salts are similarly lower, than the corresponding alkanoic acids and salts. The larger the side chain, the lower the melting and Krafft points. If the temperature at which a certain process has to be executed is 30 "C, the Krafft point has to be less than 29 "C or so. It is concluded from Figure 1that the hydrocarbon chain length has to be 12 or less in the case of straight chain soap, 14 or less in the case of Rn-LCH(CH3)COONa, but it can be as long as 19 in the case of Rn-1CH(C4Hg)COONa.This means much more surface active substances can replace ordinary surfactants. Data on Cn-1H2,-10CH2COQH and their sodium salts are also shown in Figure l.ll The Krafft points of these salts were not as low as one might have expected from the melting points of Cn-1H2,-10CH2COOH. Hence, the introduction of an oxygen atom into an alkanoic acid does not greatly depress the Krafft point. On the other hand, further introduction of ethylene oxide groups between the hydrocarbon chain and the polar group depresses the melting and Krafft points very much. The rough parallelism between the Krafft points and The Journal of Pttysical Chemistry, Vol. 80, No. 18, 1976
1988
K. Shinoda, Y. Minegishi, and H. Arai
TABLE I
I
Comwd n-C7F15COOH n-C7F&OONa n-C7F15COOK (CF3)2CF(CF2)4COOH (CF&CF( CF2)&OONa (CF3)2CF(CF2)4COOK ~-CGF~~COOH 80
r
I
I
MD. "C
KD."C
56.4-57.9
20
120
I
-
I
1
I
I
I
r'
8
25.6
Below 0 Below 0 Below 0 Below 0
13-14
27 I
I
I
the melting points and Krafft points were also larger (about 35-40 "C) than in paraffin chain compounds. As found above for paraffin chain compounds, branching leads to lower melting and Krafft points for fluorinated surfactants as well (see Table I).13 In conclusion, it was found that moderate branching of the hydrocarbon or fluorocarbon chain effectively depress the Krafft point and the melting point. Other means to depress the Krafft point include changing the kind of gegenions and introducing groups such as ethylene oxide in order to facilitate the use of longer chain surfactants. 9
II
1
1
I
1
I
13
15
17
19
21
hydrocarbon
chain
length
(n)
Figure 1: The effect of hydrocarbon chain length and the size of the side chain on the melting points of alkanoic acids and Krafft points (Kp) of their sodium salts: ( 0 )mp of R,COOH; ( 0 )Kp of R,COONa; (R) mp of R,-1CH(CH3)COOH; (a) Kp of Rn-1CH(CH3)COONa; (0)mp of R,-1CH(C2H5)COOH; (0) Kp of R,-1CH(C2H5)COONa; (A) mp of Rn-1CH(C4Hg)COOH; (A) Kp of R,-1CH(C4Hg)COONa;(a)mp of R,-,OCH&OOH; (0)Kp of Rn-10CH2COONa.
melting points of corresponding acids as a function of lypophilic chain length also holds in C,F~,+lCOONa(H). The relation is shown in Figure 2.12 In the case of fluorinated compounds, the melting and Krafft points increase much faster with chain length (about 30-35 "C per -CFpCF2- group) and the differences between
The Journal of Physical Chemistry, Vol. 80, No. 18, 1976
References and Notes (1) K. Shinoda, T. Nakagawa, B.Tamamushi, and T. Isemura, "Colloidal Surfactants", Academic Press, New York, N.Y., 1963, pp 7, 8. (2) K. Shinoda and T. Soda, J. Phys. Chem., 67, 2072 (1963). (3) K. Shinoda, S. Hiruta, and K. Amaya, J. Colloid lnferface Sci., 21, 102 (1966). (4) H. Nakayama, K. Shinoda, and E. Hutchinson, J. Phys. Chem., 70, 3502 (1966). (5) K. Shlnoda and K. Mashio, J. Phys. Chem., 64, 54 (1960); ref 1, pp 79, 80. (6) F. Krafft, and H. Wiglow, Berichte, 28, 2566 (1895). (7) R. C. Murrayand G. S.Hartley, Trans. Faraday Soc.. 31, 183 (1935). (8) N. K. Adam and K. G. A. Pankhurst, Trans. Faraday Soc., 42, 523 (1946). (9) 8.Yoda, K. Meguro, T. Kondo, and T. ino, J. Chem. Soc. Jpn., 77, 905 (1957) (in Japanese). (10) Y. Minegishi, K. Aigami, and H. Arai, J. Oil. Chem. Soc.Jpn., 24,237 (1975) (in Japanese). (11) M. Hato, K. Shinoda, and T. Miyagawa, Bull. Chem. SOC.Jpn., In press. (12) H. Kunieda and K. Shinoda, to be submitted for publication. (13) K. Shinoda, M. Hato, and T. Hayashi, J. Phys. Chem., 76, 909 (1972).
