1804
HORSTW. HOYER, ANNEMARMO AND MARGARET ZOELLNER
J-01. 0.7
SOME COLLOIDAL PROPERTIES OF DECYL- AXD I>ODECYLTlXTMETHYLAMMOXIUPI/I DODECYL SULFATI;
w.HOYER,ANNEM A R M O AND n'fARGAItET Z O E L L N E R
BY HORST
Chemistry Department, Hunter College of Ihe City University of New York, New York 81, N . Y. Received June 27, 1960
Solutions of decyl and dodecyltrimethylammonium'dodecyl sulfate were prepared by mixing solutions of equivalent amounts of decyl or dodecyltrimethylammoniiim hydroxide and dodecyl hydrogen sulfate. Dilute solutions are stable in a limited concentration range up to and slightly above the c.m.c., then pass through a region in which coacervation occurs, and are again stable in Concentrated solutions. The dodecyl solutions show surface tension lowering, with increasing concentration, to 30 dynes/cm. at a concentration of 3.0 X M , a t which concentration micelle formation occurs. For the decyl compound the c.m c. is at 1.9 X IO-' M . Electrophoretic mobility determinations of the micelles of the latter compound gave a value of 1.~19f 0.02 X IO-' cm.*/volt-sec. Addition of excess dodecyl hydrogen sulfate causes the mobility to rise rapidly to a maximilin of 10 X IO-' cm.z/volt-sec. when a 100% excess is present and then to decrease slowly as more is ndded. The initial rapid rise is interpreted aa due to the incorporation of the dodecyl sulfate ion into the micelle with a rcsulting increase i n rnicellar charge. The gradual decline from the maximum value is a result of the normal repression of the ionic atniosphere u, ith increasing ionic concentration.
Introduction During the course of some exploratory work on the critical micelle concentrations and electrophoretic mobilities of colloidal electrolytes it was decided to examine the behavior of mixtures of dodecyltrimethylammonium chloride and sodium dodecyl sulfate. The work soon was expanded to include solutions consisting of the corresponding hydroxide and acid and also the decyltrimethylammonium chloride and hydroxide. Apparently little prwious work has been undertaken with these alkyl sulfate salts of the quaternary ammonium bases. Bolle and Bourgeois' have prepared several of these compounds from the sulfate esters but had not reported upon their properties. Anacker2 studied the light scattering of solutions of the related compounds, octyltrimethylammonium octane sulfonate and the corresponding decane sulfonate. Experimental
the stability of very dilute solutions. One of the first solutions of dodecyltrimethylammonium dodecyl sulfate prepared was 0.0685 A t and has been sitting without any sign of instability for five months. Dilution of this solution with tap or deionized water causes separation of a flocculent precipitate. The rate of precipitation increases as the solution is made progressively more dilute. Dilution to 1 X M causes separation of a precipitate within 3 to 4 days. Dilution to 6 X M will produce a precipitate within approximately 30 minutes. Dilution to 1 X loF6 or less does not produce any precipitate. The volume of precipitate is greater for the more concentrated solutions. For initial concentrations less than that of approximately 1 X 10-4 M the precipitate can be made to dissolve by heating the solution to 6080". If the hot solution is permitted to cool undisturbed to room temperature, the precipitate The preparation of the decyl- and dodecyltrimethyl- will reappear as a ring of coacervate floating half ammoniulr chloride is described in the following pa er.3 way up in the solution and with a radius half that The sodium dodecyl sulfate was supplied by Professor J. of the beaker. Using different size beakers changes Mysels and also haa been described previously.' The the diameter of the ring. The formation of these decyl- and dodecyltrimethylammonium dodecyl sulfate solutions normally were prepared by converting the cor- coacervate rings was observed for solutions of the responding quaternary salts to the hydroxides ltnd the decyl and dodecyl compounds in the concentration M with or withdodecyl sulfate to the acid form by passage through ap- range of 5 X M to 5 X propriate ion exchange columns. Each solution then was out an excess of either the quaternary compound standardized against HC1 or NaOH and equivalent amounts or the dodecyl sulfate. If some water-insoluble of each were mixed as needed. No changes were observed dye such as Sudan IV is solubilized in the hot in the standard solutions during the series of experiments. For the surface tension work a stable concentrated solution, solution then most of it will appear in the coacer0.0685 M , was diluted for the different measurements. A vate, coloring the ring a deep red. A photograph second set of measurements was taken on mixtures of equiva- of one such ring has been published.5 lent amounts of the sulfate and quaternary salts. Surface Tension and C.M.C. Determinations.Dye solubilization determinations and electrophoretic mobility measurements were made by the procedure recently The time lag in the formation of the precipitate described . 3 Surface tension measurements were made permits determination of the surface tension of with a Du Nouy tensiometer at 25 f I '. these solutions. Figure 1 shows the results obResults tained upon dilution of the concentrated (0.0685 Stability of Solutions.-One of the several un- M ) solution of dodecyltrimethylammonium dousual properties of solutions of decyl- and dodecyl- decyl sulfate. Low values of approximately 30 trimethylammonium dodecyl sulfate is the ap- dynes per em. were obtained in solutions as diiute parent stability of highly concentrated solutions, as 3 X loF5M . As a check, separate determinathe instability of relatively dilute solutions, and tions on dodecyltrimethylammonium chloride and sodium dodecyl sulfate solutions were made and ( 1 ) J. Boile and L. Bourgeois, Msm. sen. chim. clot (Park), S8, then on mixtures of these two solutions. These 159 (1953). (2) E. W,Anacker. J. Co2Zm.d Sei., 8, 402 (1953). results are indicated by the x's on the graph. These 13) R.W. Hoyer and A. Marmo, J . Phyr. Chem., 66, 1807 (1961). solutions are not identical with the first series since
8.
(4)
R. J. Wiiiams, 3. N. Phillip and K. J. Mysel..
Soc., 61, 728 (1955).
TTotIS. For&#
( 5 ) Chem.
[email protected], Sa, 104, July 4, 1960.
Oct., 1961
COLLOIDAL P R O P E R T I E S O F ~ E C T L T R I M E T H Y L A M M O N I U MD O ~ E C SY UL LFATE
they contain an equivalent amount of S a c 1 due to the manner in which they were prepared. This additional NaCl depresses the surface tension below that of the solution of the pure compound. This type of surface tension curve is typical of colloidal electrolytes. I n dilute solutions the siniple ions exist, and with increasing concentration the surface tension decreases to a minimum value. This mininium has generally been interpreted as the concent,ration range in which micelle formation occurs. Our surface tension data would require a c.m.c. for the dodecyl compound a t approximately 3 X M . To check this we attempted both dye absorption and dye solubilization determinations. Bot,h proved inadequate for the dodecyl compound and we then prepared the decyltrimethylammonium dodecyl sulfate. A spectral change wm observed with the dye absorption method but the apparently low micelle concentration made detection of the precise concentration region a t which this occurred extremely difficult. Fortunately the dye solubilization technique proved adequate and gave a value of the c.m.c. of 1.9 X lo-* M for the decyl compound. Surface tension measurements on this compound also gave values in this region. We concluded that the surface tension minimum of approximately 3 X low6M ior dodecyltrimethylammonium dodecyl sulfate does represent the c.m.c. for this compound even though this could not be confirmed by dye solubilization or absorption for this compound. Micelle Mobilities.-Since the preceding clearly demonstrated the existence of micelles at, concentrations considerably below those previousiy reported for colloidal electrolytes, we decided to investigate the electrophoretic mobility of these micelles. Since the compound is formed from equivdcnt aiiiounts of an anionic and a cationic detergent, one expects the formation of a neutral rnlccilc with little or no electrophoretic mobility. 0 1 1 the othcr hand, the extremely low ionic strength of the soltition should enhance the mobility of any ionic rnicc.llc n hirh might foim. k c a u s e of the higher sta I)I1 it y of the tlccyltri~netliylnn~monium dodwyl stilfatt solution, we dccidrd to work with I t r a t h c tmn with the dodecyl c~ompourid. -1san :idditional precaution agninst prccipitctit 'ion we madc our niouility dctcrrriinations at 3;". Prior TLoik6 11:td s h n n 11 that fcrnperature Pffects on the niohi1,rv r,E micclles nic slight, the incrcaseri er temperatures heing readily the increased fluidity of the
1805
2 4 G 8 10 Concn. X 105, moles/l. Fig. 1.-Surface tension measurements of dodecyltrimethylammonium dodecyl sulfate (DD) without and with an equivalent amount of KaC1: NaDDS = sodium dodecyl sulfate, DDTACl = dodecyl trimethylamrnonium chloritlc. 0
-
'1
~00002
2
0
-
-_
y _ L _ I
0 lox 20x 3ox lox Excess of dodecyl hydrogen sulfate Fig. 2.--blectrophoretic mobility of the beryltrirnc~tli:I ammonium dodecyl sulfate micelle 111 the presence ot m i excess of dodecyl hydrogen sulfate. The eoncentration of deryltrimethylammonium dodecyl sulfate is 2 5 X lo-* lf
an excess of dodecyl hydrogen sulfate. The resulting data are shown in Fig. 2 along with the average value obtained for the four determinations mentioned above. The mobility rises rapitil\r and reaches a maximum value of 0.001 m i . 2 pcir volt-sec. when approximately 5 1 0 0 ~ exwsb o ~i :IF wlfate is present and then falls ~ l o n l yab m n i e of the dodecyl hydrogen sulfate is added. An attempt to reproduce this lieiiavior ~i111 varying excesses of decy1trimethylaniiii~)riI~in~ h\ droxide proved impossible since soirit ions I\ it ti '1 50% excess of thi3 compouricl wcrc u1istal)lc C T e11 a t cievated temperatures. Ilo\ieever, a solutioi: \T iih a 25$7 excess proved stable. Xfobility rn urements show that in this rase !he m~ct-llr now positively charged and had n iiiobility o i X i o - 4 cm.*per volt-sec. f'our dr. ermiiiatioiis were madc at concentrations Discussion N . Two of these were on solutions of 2 5 >< The discovery of the inicellnr ualure of tniprepared \IS niixing equivaient amounts of decylfritiit~tbylrtrnnihrIliumhydroxide and dodecyl hy- clasi of compounds extends the plienoincnoti ot micelle formation to extremely dilute concentratic,iL. ( l i t gea sulfate and two by mixing equivalent a ~ n ( w i ~oft s the corresponding chloride and sndiuni The majority of association colloids so far studieii s:JtL 111 carh -me the micelle had a negative have critical micelle concentrations 111 thr range ,)f -11' We have found that the d o d t ~ ,ti,trge .is & o ~ : ?by the direction of miqration in IO-* to ttit c k c t i o p I ~ ~ * r - apparatus. ~:s The f'our de- yltrimethylamnionium dodecyl su1f:ttc mict i!c :i termi:,,:t:c E% gd-+r*' -" 31uec. of mobility (;qua; to exists above concentrations of 3 X - 4 m i . 2 per ;.olt+ec. f h e 17 "- However, at the ?lightly higher vonccntratio ib oC det,ermined ; P the presence )f approximately 6 k 1 0 - 5 171, the rticellr L
reer~fitl',d I ~ Z U RC b r a . 5: Tdi 19% I
I
,.L
tdtii1.3
J
i n OilCAemiutr'
1806
HORST
IT.HOYER, AWE
h f 4 R M O iV1)
in equilibrium with a colloid-rich coacervate. The micellar range of the corresponding decyltrin?ethylammonium dodecyl sulfate is somewhat greater, extending from 1.9 X A[ to approximately 5 X X . The equilibria existing in these solutions may be written
+
CiaHq~(”H3)~NH4+ C,?H?~SOC
hllcelle
Coacervate
Surfacc tension, electrical conductivity, dye soluhilization and dye adsorption determinations support this new. The formation of the coacervate rings upon cooling presented an interesting problem. Apparently they are produced by the convection currents generated as the solution cools. These will be downward near the glass wall and upward in the center of the beaker. They would thus tend to s \ ~ e c ~any p niaterisl with a density close to that of watcr into a ring shaped region half way up the solutiou and nith a radius half that of the beaker. These conditions were observed with containers of different cizes. An analogous phenomenon i u the manner in which a precipitatr collects in the center (If the bottom of a beaker when the liquid in the beaker is swirled. The surface tension behavior of these solutions, while unusual iiisofar as low values are obtained a t concentrations approximately 1/1OO as large as those usually observed for the surface active agents, is still rxplainable in terms of the action of such agents. It would appear that this group of compounds affords a transition from the soluble surface active compounds to the “insoluble” surface film-forming substances. It might be expected that thew hubstanves will possess unusual emulsif!-ing proprrt ies. Preliminary studies show that t h ( y form o i l in water emulsions of high staMity a?d water in oil emulsions of moderate stability. The mobility ni(yihurementb raise a number of theoretical quest iotis, only some of which can be answered at this time. Why did the apparently
hf 4 H G I R E T ZOELLXER
Vol. 63
neutral micelle possess such a high mobility? We arc inclined to believe that a number of factors, including the low ionic strength of the solution, the size of the micelle and the high surface activity of the decyltrimethylammonium hydroxide, are responsible. The mobility rises so rapidly with addition of dodecyl hydrogen sulfate that adsorption of a small fraction of the highly dilute quaternary compound on the surface of the glass could readily producr a negatively charged micelle with a slight excess of the dodecyl sulfate. The obperved formation of a negatively charged micelle upon addition of a known excess of dodecyl hydrogen sulfate and of a positively charged micelle with an excess of the quaternary compound is in agreement with this point of view. It is well established that addition of hydrocarbons or long chain alcohols to solutions of association colloids results in their incorporation into the micelle. The present work demonstrates that this is also true for amphipathic ions and shows that such a process can produce marked changes in the electrical properties of the micelIe. Upon the continued addition of an excess of hydrogen dodccyl sulfate to a micellar solution of decyltrimethylammonium dodecyl sulfate the electrophoretic mobility increases rapidly to a maximum of 10 X IOw4em.2per volt-sec. and then decwases gradually. The initial rapid increase can only be due to the incorporation of the dodecyl sulfate ion into the micelle. This results in a gradual increase in the negative charge on thc micelle. Iventually the micellar charge becomes great enough to prevent the further incorporation of the dodecyl mlfate into the niicelle. Continued addition of the dodecyl hydrogcri sulfate then decreases the mobility due to the repression of the ionic atniosphere by the added electrolyte. Acknowledgment.-We are indebted to the Nntioiial Science Foundation for the financigl support which made this vork possible and to Professor Karol J. Mysel:: for making available the high purity sodium dodecyl su1f:~te.
