bility. As the complexity and heterogeneity of the niolccules i n the solid carboxylate phase are increased, a point is reached at which thc niixture is in equilibriuni with ZL higher concentration of dissolved species than is required for the formatjon of niicellcs. The
solid phasc then passes over completely to the micellar state unless the system is below the critical solution ternperature, in which case a solvent-swollen “solid” phase equilibrates with the inicellar solution, i.e., there is limited miccllar solubility.
Molecular Association in Pairs of Long-chain Compounds.
11.
Alkyl Alcohols and Sulfates
by H. C. Kung and E. D. Goddard Research CentPr. Lever Brothers Company, Edgewater, New Jersey
(Reccieed April 8,1 ,Ng,
In our previous studies, which described the formation of I : 2 association complexes hetween alkyl alcohols and sulfates, sample preparation was by a dry nielt method. The present work involves studies on samples prepared from aqueous or aqueous-ethanol solutions of lauryl alcohol and sulfate and of myristyl alcohol and sulfate. I t is shown that, over a range of concentration, mixing ratio, atid solvent composition, the spccinicns which settle out of solution have a coiiiposition corresponding to a 1 : 2 alcohol-sulfate ratio and that their differential thermal, infrared, and X-ray patterns correspond to those of the previously reported eomplexes. No evidence of free alcohol was obtained. Difficulty was encountered in preparing the “1: 1” adducts reported by other workers; however, although some samples having approximately the l : l coinposit ion were obtained, thc above techniques showed the adducts to be of an ill-defined nature and to contain uricombined alcohol under 1he test conditions used. I t is also shown that reaction between alkyl alcohols and sulfates is rapid when the alcohol is melted.
I t has been shown in a previous paper’ that alkyl alcohols and sodium alkyl sulfates combine to form 1:2 association coniplexes with the release of a considerable amount of energy. In these studies samples were prepared by a melt method in order to avoid the effects of solvcnt inclusion. As there is interest in the interaction of the above species when present in aqucous solution or at aqueous interfaces, it seemed desirablc to establish whether or not the same cornplexes form in an aqueous environment. Work of this type has been done previously. In their studies on phase relations in the system sodium lauryl or myristyl sulfate, lauryl alcohol, and water, Rpstein,2
et al., isolated crystalline adducts in which the ratio of alcohol to sulfate was 1 : 2 ; later 1 : l adducts were i ~ o l a t e d . ~However, little information on these adducts, other than their composition, was published. The purpose of this work was to prepare such adducts and to establish their identity or nonident ity to the melt-prepared complexes by us(’ of the differential thermal analysis (DTA), infrared, and X-ray tech(1) ET. C. Kung arid E. D. Goddard, J . Phga. Chen.. 6 7 , 1965 (1963). (2) M. B. Epstein. A. Wilson, C. W. Jakob, L. E;. Conroy, and J. Rous, ibid.. 58, 860 (1954). (3) M . B. Epstein, A. Wilson. J. Gershmari, and .J. Itoss, ibid.. 6 0 ,
1051 (1956).
niques used previously.1 In addition, further experlments have been carried out to establish the conditions under which interaction occurs in the absence of solvent.
Experimental The specimens of lauryl alcohol (LOH), myristyl alcohol (NOH), sodium lauryl sulfate (NaLS), and myristyl sulfate (NaMS) have been described previously.1 hdducts were prepared as follows : appropriate quantities (see Tables I and 11) of long-chain al-
Table I : Composition of Solutions and Adducts. LOH-NaLS % by -Initial SaLS
1 2 3 4 5 6
0.805 0.810 0.801 0.800 0.813 0 803
solution. g./100 ml. of water--LOH Pu'aC1
0.154 0 414 0,542 0 605 0.425 0.553
.. 0 12 0.12 0.12 0.12 0.12
wt,.
of LOR in
adduct
24.1 258 25.1 26.1 38.3 39.8
Results Composition of Adducts. Composition data for the
Table I1 : Composition of Solutions and Adducts. MOH-NaMS % by at. --Initial NaMS
7 8
0.812 2.4 2.3" 0.801
9 10 a
solution, g./100 ml. of water-MOH NaCl
0.207 0.41 0.68 0,409
0.12 ..
..
0.12
of M O H in adduct
24.7 24.3 24.9 36.2
Water replaced by 50: 50 water-ethanol.
cohol, long-chain sulfate, and water or mater-ethanol were stirred together at room temperature. The solvent was generally added last. In some experiments a small amount of sodium chloride was included.3 Stirring was continued while heating t o 75-80' and maintaining at this temperature for a few minutes. The systenis were more or less turbid at this point and were then aIlowed to cool slowly to room temperature. Crystals of adduct grew as small, thin plates. After separation and drying over phosphorus pentoxide, their content of long-chain alcohol was determined by the method of Epstein, et aL2 This involves transferring a weighed amount of the crystals into a fritted glass filter funnel and extracting with successive small portions of heated petroleum ether to constant The Journal
of
Physical Chemistry
weight. The water content of the crystals, where tested, was found to be 97y0). There have been several more complete studies of ternary detergent-amphiphile-water systems. 111 briefly describing some of these it is appropriate to start with the water-free systems studied by LMcBain, et al.,8v9 who obtained evidence of 1:1 and 1:2 palmitic acid-sodium palmitate and of 1: 1 oleic acid-potassium oleate associations. Furthermore, their phase diagrams bear a striking resemblance, in the areas of relevance, to the partial phase diagram we constructed for the MOH-NaMS system. McBain and Stewartlo also point out that in the oleic acidpotassium oleate-water system the boundary confining the water-rich isotropic region corresponds to the 1:2 acid-soap ratio. Similarly, Dervichianll has found an upper limit of solubility of octanol and potassium caproate (or laurate) in water to correspond to the 1:2 molar ratio and indicates that such stoichiometry is encountered only if the chain length of the amphiphrle is sufficiently long. In the extensive work of Lawrencel2>l3and recent work of Ekwall’4 no significance is apparently attached to changes taking place at or near stoichiometric ratios. Most of Lawrence’s work has been concerned with amphiphiles of chain length lower than twelve carbon atoms. He has, however, reportedla the separation, from LOHXaLS-water mixtures, of crystals of a “solid solution” of these three components whose composition varies
3469
with that of the starting system. Unfortunately, details of the compositions involved were not given. It remains to consider how widespread the existence and formation of stoichiometric complexes between long-chain polar and long-chain ionic materials are. That such formation occurs under certain conditions is, as we have shown, now well established; and we will show in another publication that complex formation is by no means limited to the pairs of compounds discussed above. However, further work, both in bulk phase and a t interfaces, is required to establish to what exteat it is responsible for the diverse phenomena which have led various workers to suspect and postulate its existence. Such phenomena are spontaneous emulsification, mesomorphic phase formation, the position of phase boundaries, inhibition of precipitation of anionic detergents by heavy metal or longchain cations, abnormal viscosity and surface tension effects, foam stability, slow draining films, monolayer penetration, and others. That attraction and interaction of the two species are involved in several of the above phenomena there seems little doubt. In conclusion, evidence has been brought forward th&t the same 1 : 2 alkyl alcohol-sulfate complex is formed whether the association is allowed to take place via the melt or via aqueous solution. The raison d’Nre of the complexes, which can form under such a wide variety of conditions, we believe is associated with the limited stability of the alkyl sulfate structure. Stabilization is achieved in the complex by hydrogen bonding and charge separation. As soon as it is molten, an alkyl alcohol can react with a solid alkyl sulfate with the formation of the complex, and the release of ti considerable amount of energy.
Acknowledgments. The authors wish to thank Messrs. H. Lemaire and E. Mecheski for preparation and purification of materials and the Lever Brothers Co. for permission to publish this paper. (7) K. J. Mysels, personal communication. (8) J. W. McBain and M. C . Field, J. Chem. Soc., 920 (1933). (9) J. W. McBain and A. Stewart, ibid., 924 (1933). (10) J. W. McBain and A. Stewart, ibid., 928 (1933). (11) D. G. Dervichian, Proc. Intern. Congr. Surface Activity, Ond, London, 1, 327 (1957); Discussions Faraday ~ o c . 2, 5 , 68 (1958). (12) A. J. Hyde, D. M. Langbridge, and A. S. C. Lawrence, ibid., 18, 239 (1954). (13) A. 9 . C. Lawrence, ibid., 25, 51, 70 (1958). (14) P. Ekwall, I. Danielsson, and L. Mandell, Proc. Intern. Congr. Szrrfuce Activity, J r d , Cologne, 1, 189, 193 (1960).
Volume 68,Number 1% December, 1964
