IRVING COHENAND TONY VASSILIADES
1774
Voi. 65
CRITICAL PHENOMENA I N AQUEOCS SOLUTIONS OF LONG CHAIN QUATERNARY AMMONIUM SALTS. 11. SPECIFICITY AND LIGHT SCATTERING PROPERTIES BYIRVING COHENAND TONY VASSILIADES Department of Chemistry, Polytechnic Institute of Brooklyn, Brooklyn, N . Y . Received March 8, 1961
Aqueous solutions of a number of long chain quaternary ammonium salts at concentrations above the critical micelle concentrations have been studied as a function of added simple monovalent electrolyte and temperature. At a given temperature, there is a critical electrolyte concentration above which the system separates into two layers: the top layer is practically free of the quaternary ammonium salt and the bottom layer shows the characteristics of an oil. The volume of the bottom layer decreases with increasing electrolyte concentration. Before the onset of two phase formation, the turbidity and dissymmetry of light scattering rise sharply with increasing electrolyte concentration. An electrolyte transition range may be described, intermediate between zero electrolyte and the critical electrolyte concentration in which there is an a parent. reorganizat,ion of the micellar aggregate. The phenomenon of two phase formation in these cationic soap systems sfows a pronounced specificity to the anions and a lesser sensitivity to the cations of the added electrolyte. Small temperature changes produce marked changes in both the homogeneous systems and the soaprich layer of a two phase system. A method has been devised for predicting the critical electrolyte concentration of monovalent added anions from a graphical analysis of light scattering from homogeneous systems.
Introduction Aqueous solutions of some long chain quaternary ammonium salts at concentrations above the critical micelle concentration, in common with many “association” colloids,1,2 polyelectrolyte3 and polymer4 systems separate into two solution layers. In the case of these quaternary ammonium salts layering may be induced by the addition of small quantities of monovalent or polyvalent simple electrolytes such as NaCI, NaNOp, NaSCN or Na2S04. The layering phenomenon, classified as a form of simple coacervation by Bungenberg de Jong12may be rcgarded as an arrested precipitation of the colloidal species. Two solution phase formation begins with micelles aggregating to form submicroscopic clusters, these clusters coalesce to form microscopic droplets. Further coalescence produces macroscopic dropiets which tend to separate into a continuous phase. This phase appears as a top or bottom layer depending upon the density of the colloid-rich layer. At a fixed temperature, there is a critical electrolyte concentration above which the system separates into two layers. These layers are well defined; one layer is practically free of the quaternary ammonium salt and the other layer shows the characteristics of an oil. For a fixed soap concentration, the volume of the soap-rich layer decreases with increasing electrolyte concentration and is proportional to A f B/C%
+ D/Calt
(1)
where A , B and D are constants and C is the concentration of the added electrolyte. At high electrolyte concentrations, the colloidal species precipitates as a solid phase. In a two phase system, the electrolyte concentration in the coacervate layer is slightly less than in the soap-free equilibrium layer.’ This difference in electrolyte con(1) I. Cohen. C. F. Hiskey and G. Oster, J. Colloid Sci., 9, 243 (1954). (2) A. C. Bungenberg de Jong. ”H. R. Kruyt Colloid Sei. 11.” Elsevier, Amsterdam, 1949, Chap. X. (3) H. Elsenberg a n d G . R. Kohan, J . Phys. Chem.. 63, 671 (1959). (4) E. Turska a n d M. Lacekowki, Rocznrki Chem., 49, 941 (1955); J Polymer Set.. 9 8 , 285 (1957).
centrations in the two layers may be accounted foi in terms of displacement of solvent by soap with no apparent Donnan equilibrium effects. A precondition for layering is the tremendous growth of the micellar aggregate under the influence of added electrolyte. For the systems investigated, the onset of two-phase formation is characterized by a greater than one hundred-fold increase in micellar molecular weight as compared to the salt-free micellar systems. In addition, for each of these layering systems studied, there is a narrow range of electrolyte concentration intermediate between zero electrolyte and the critical electrolyte concentration, in which the micellar aggregate grows very rapidly. The rate of micellar growth for electrolyte concentrations above and below this transition range is Iess than the rate of micellar growth in the Characteristic electrolyte transition range. Small temperature changes profoundly effect both the homogeneous systems and two-phase systems. Lowering the temperature of a system a t constant electrolyte concentration produces effects comparable to increasing the electrolyte concentration at a k e d temperature. This may be illustrated in terms of the following: For a fixed electrolyte concentration and a futed soap concentration, a reduction of the temperature of the system results in a growth of the micellar aggregate. Above a minimum electrolyte concentration, a transition temperature may be defined, above which relatively small changes in micellar molecular weights occur with small temperature variations and below which large changes in micellar molecular weights occur with small temperature variations. I n addition, the electrolyte transition range for a fixed temperature, which is characterized by a large micellar growth, shifts to lower values as the temperature of the system is lowered. Over a wide range of electrolyte concentration, there is a unique critical temperature for each electrolyte concentration below which the system separates into two solution phases and above which the system is homogeneous. Below the lower limit of the electrolyte concentration range, gelation
LONGCH.4IN QTJATERNARY AMMONIUM SALTS
Oct., 1961
1775
H. 0
Fig. la.-Partial 1,ernary phase diagram-Hyamine 1622NaC1-H20. T =: 30". a = homogeneous region; b = 2phases, both liquid; c = indeterminant region (3-phsse, 2 liquid, 1 solid): d = 2-phase, liquid-solid.
Fig. 1b.-Expanded partial ternary phase diagram Hysmitir 1622-NaCi-Hz0, T 7 30".
occurs a t temperatures below -4' without two phase formation. At the upper limit of the electrolyte concentration range, the colloidal solute flocculates at the lower temperatures and is precipitated as an oil a t higher temperatures. The observed coacervation phenomenon shows a characteristic specificity wit,h regard to t'he added electrolyte. Cationic soap syst,ems are relatively insensitive to the cation of the added electrolyte. For monovalent anions of the added electrolyte, the relative effectiveness in inducing two phase formabion follows a lyotropic series. Two phase systems composed of aggregates of anionic soaps, i e . , alkali oleates, stearates and palmitates have been investigated by Bungenberg de Jong and associates.2 We hiave chosen to study quat,ern:try ammo nium cationic systems because coacervation occurs at relatively low electrolyte concentrations. 1,arge measureable changes in these systems are prod:iced with relatively small electrolyte and temperature increments. The quaternary ammonium s d t , ITyxmine 1622 was studied in detail because
layers. Theories for such interaction forces h a w been described for idealized models5--' and ha..r been developed further by Verwey and Overbeek* and Derjaguin, et aLY (b) Where the micelles possess 211 anisotropic dimension, lo entropy considerations for anisotropic colloidal species in solution developed by Flory" and Onsagerlz play a decisive role in the coacervation of the aqueous cationic soap solutions studied. Michelli, et al., have applied these crtricepts !r cwacervate systems. l 3 The theory predicts that in a tu o phase system of rod-like solute particles, one phase IS a highly ordered phase. In a well-defined system of this type, aqueous Tobacco Mosaic virus solutions, this is borne out by X-ray14 aiid light scatter~ng'~ studies. The data developed in the study of coacervation in quaternary ammonium salt scl1utior.s show dimensional anisotropy of the col1oid:~l aggregate in homogeneous solution prior to two phaw fornlation. In addition, the ~ 0 3 1ervntc luyrr of 3
IL
CII3
tions: 0027 A! for SaSC?; to 0.336 M for SaC1 26'. This stutiy is .onfined essentially to added elecI rolytil m d temperature effects upon homogeneous mi* nf.11 ' u t of the critical region. The conw - r j hcrc is ari ai tenipt to characterize the onsct of r) -cJution phase formation in dilute aqueous r.:-tt ionic. SCJ:LP SvLtems. Til(. i.xpt,r:nicntal results ohtamed can br exi ~ ' ~ i i i i ~1 1() t 'crnis of vurrcnt coiicepts of critical rirlnorrima wiutetf t o two set? of cnn,it
x i
I
1
CHt
i
I
CHI
t h i s d: foims (I' acwvates with a number. 01 mono7'31ent eleatrolyi es a t relatively low concentra-
attrai tion of the mic*ellcs to cwh u t h c I,oricion-\-m drr Waals forces, balanlvd by tt: : : P I t > > i ( d ! 1 repulsion tiuc t o their ic n i c tlouh!tL
$1;
CHJ
t.wo phase system show a high cicgrce of order of the micellar aggregatw. H. Xalnian and M . iTi!lstatter, S u ~ u r i c i s s . ,20, 95%:lg:Sei., 61. h':i tiC~49). (I?! I . Michelli. hf. ,J. Voorn nud .i.'PI,. (; Ov,rheek. J .I'O:(,WCsri. 23. 449 (1957). i W I. F'nnkurireq a n d 2 . 1;. Be;. A d I . : ; w . I ' h y s i o i o i ) y . $ 5 . ' 4 7 (1941) i!J!(1. i h t e r , i h i d , 33, 445 (19.57).
~ R V I N GCOHEN AND
1776
TONYVASSILIADES
Experimental Materials.-The this inveetigation
following cationic maps were used in
I-Hyamine
Vol. 65
Results and Discussion Phase Diagram.-The partial, constant temperature, ternary phase diagram of Hyamine
1622-di-isobutylphenoxyethoxyethyldimethylbenzylammonium chloride monohydrate
2-HDTAB-Hexadecyl
trimethylammonium bromide
CHr-( CH&.AH: chloride
3-Cetol-Hexadecyldimethylbenzylammonium CHI ca--fcH*~,,--+rS-cH L
AH*
Hyamine 1622 is a pure crystdine quaternary ammonium salt monohydrate. It was obtained from Rohm and H e and contained 1.5% HzO aa the only a preciable imunty. The hexadecyltrunethylammomum &omide (HD$AB), obtained from Eastman e g a n i c Chemcals and the Cetol, obtained from Fme Orgamcs, Inc. were both of technical grade. All inorganic electrolytes used in thb work were of C.P. grade. Apparatus. Light Scattering.-Light scattering measurementa were performed in a Briee-Phoenix hotometer, using incident unpolarized monochromatic Ugit of wave length 4360 A. The m w r e m e n t s were carried out in a cylindrical cell, with solutions which had been filtered through millipore filters of 0.45 p pore she. Since the 3component (eoap-electrolyte-H80) systems are sensitive to temperature changes, a brass housmg such as described by Boedtker and Doty16 waa constructed to maintain constant temperature in the optical cell. The temperature control wm within *0.lo. The cell correction was determined a t various angles by measuring the scattering from a fluorescein solution. Refractive index increments (dn/dc) were measured with a Z e i i dipping refractometer. For soap concentrations in excess of the critical micelle concentration, dn/dc is independent of electrolyte concentration. micellar size and micellar stam. The refractive index ihcrementa for the quaternary k o n i u m salts investigated are: Hyamine 1622, 0.1875; Cetol, 0.15; and HDTAB. 0.162. Experiments of the following nature were performed. (a) A partial ternary phase diagram waa constructed for the system, Hyamine 1622-NaCl-HIo. (b) The critical electrolyte concentration for a number of Hyamine 1622-electrolyte-H*0 systems was determined by the interpolation method of Der~ichian.1~The electrolytes used in this experiment were a number of monovalent salts, monovalent anion-divalent cation salts. and monovalent anion-trivalent cation ealts. (c) Micellar aggregate molecular weights were determined for the Hyamine 1622-NaC1-Hr0 systems aa a function of NaCl concent,ration and temperature from light scattering measurements. (d) Cherge ro rtiea of Hyamine 1622-NaC1-Hs0 systems for low a 1 concentrations (below 0.05 M NaCI) were eathated from light scattering data.18-" (e) A method has been devised for predicting the critical electrolyte concentration necesssry for two phase formation in systems composed of cationic soap-electrolyk-water. This method ie baaed upon a graphical analysis of the light scattering data of the homogeneous single phase system.
k
t?
--
(16) € Boedtker I. and P. Doty, J . Phys. Chsnr. 6% 968 (1954). (17) D. G. Derviehian. Tram. Faraday SOC.,U,231 (1954). (18) J. J. Hermana and W. Prins, KoninY. Ned. A M . Welcnachap. Proc.. B t S , 161 (1956). (19) J. J. Hermana and K. J. MJTwla. J. Cdl. Sei., if, 594 (1957). (20) E.W. Anaoker. J . Plrya. Chsn., 6% 41 (1958).
1622-NaC1-Hz0 (Fig. la) was constructed by the interpolation method of Dervichian. l7 The HzO apex of the phase diagram shows four regions: (a) homogeneous region; (b) two liquid solution phases; (e) three phases-2 solution and one crystalline phase (this region is not well defined); (d) two phases-solution and a crystalline phase. An essential feature of this partial phase diagram is the constancy of the critical electrolyte concentration (c.e.c.) necessary for two phase formation over a wide range of Hyamine 1622 concentration, Le., 0.27y0 (6 X 10-la M ) to 16.20% (3.50 X 10-i M ) . The NaCl c.e.c. for a 0.27y0 Hyamine 1622 solution a t 26' is 0.336 f 0.002 M and the NaCl c.e.c. for a 16.20% Hyamine solution is 0.314 f 0.002 M . A similar situation is indicated for the flocculation concentration of NaC1 for the Hyamine 1622 system. The NaCl flocculation concentration for a 0.27% Hyamine 1622 solution is 1.8 f 0.02 M and for 16.20% Hyamine 1622 solution is 1.71 f 0.02 M . Figure l b represents an expansion of the H 2 0 apex of the partial ternary phase diagram for dilute Hyamine 1622 soap solutions to 0.1% (0.0031 M ) slightly in excess of the critical micelle concentration. In this region of the phase diagram for soap concentrations less than 0.27%, the NaCl c.e.c. is dependent upon soap concentration and rises sharply as the solution is made more dilute in the quaternary ammonium soap. SpecScity.-Two phase formation of aqueous Hyamine 1622 solutions shows a pronounced specificity of the anions of the added electrolyte. This is indicated in Table I. This specificity follows a typical Hofmeister series for monovalent anions SCN-
< CI0,- < Br-
= NO,- < CI-
The c.e.c. is much less sensitive to the cationic species of the added electrolyte. Yor the alkali chlorides, the c.e.c. at 26" falls within the range of 0.325 M for LiCl to 0.425 M for CsC1 in the following order LiCl
< NaCl < IiCl < CsCl
With divalent and trivalent cations, there is a
LONGCHAINQUATERNARY AMMONIUM SALTS
Oct., 1961
small but significant increase in the critical anion concentration ;st a fixed temperature. For a 3% soap solution a t 26", the c.e.c. for the following salts are NaNO1-0.067 M in NOSNaC1-0.325 AT in C1Ba(NO&-0.077 M in NO,BaC1,-0.420 J4 in C1FeCL-0.470 Ai in C1Al(NOa)I-0.087M in NO,TABLE I CRITICAL COXCENTRATION NECESSARY FOR TWO PHASE FORMATION IN A 3% HYAMINE 1622 SOLUTION AT 26.0' OF MONOVALENT ANIONS A N D MONOVALENT, DIVALENT AND TRIVALENT CATIONS Monovalent cations
Critical concn.,
KSCN NaBr KCIOa
0.027 .OCA .059 .Ofi7 .250
NaNO, NaNOl
N
Divalent cations
Critical concn.,
N Cu(N03)~ 0.079
Ba(NOt)2 Ni(NOs)t Cd(NOJ8
Trivalent cations
Critical concn.,
1777
micellar aggregate is optically isotropic, the particle scattering factor (Pw)was taken as unity. For higher electrolyte concentrations for which the solutions show considerable dissymmetry of light scattering, a particle scattering factor (Pw)estimated for rod-shaped aggregates,21 was used to calculate micellar molecular weights. For the system investigated in detail, Hyamine 1622, the micellar charge p , the aggregation number m, and the ratio of charge to aggregation number ( p / m ) were estimated from the following expressions developed by Anacker.20 (3)
N
Al(N03)r 0.087
.078 .078 .073
Where p and m are the micellar charge and aggregation number, respectively, MI is the formula .:no cuc12 .430 FeClr .4iO weight of the detergent, A is the intercept of a plot .325 BaClz .420 o )C ] - Coand B its slope, n1 of [ H ( C - C o ) / ( ~ ~ vs. ,425 CaClt .370 and n3 the critical micelle concentration and NiClZ .400 electrolyte concentration both expressed in moles/ ml. and f is the ratio of molar refractive index TABLEI1 CHARGES A N D AGGREGATIONNUMBERS OF HYAMINE 1622 increment of the added salt to that of the detergent (f = 0.17 for NaCl concn. to 0.05 M). This inMICEL,LE:S A T Low NaCl CONCENTRATIONS~ formation is summarized in Table I1 for solutions NaCl (dm) MI 1/A x lo-' x lo-' m x 100 P (MI of low NaCl concentrations. For systems of higher 0.625 0.573 13.34 8.15 0 1.09 electrolyte concentrations where the solutions 3.45 16.35 3.50 75.4 0 . 0 1 12.31 showed tsheta solvent properties, no attempt was .02 16.30 78.0 20.0 3.63 3.64 made to estimate micellar charge. .03 16.2,s 83.0 19.6 3.87 3.94 The distinctive feature of the charge properties .04 11.7.5 88.0 13.35 4.10 4.06 of the micellar species derived from light scattering .05 10.00 100.0 10.00 4.65 4.62 data is the relatively low ratio of charge to agp = micellar charge as computed from eq. 4, m = ag- gregation number; derived p / m values decrettse to gregation number aa computed from eq. 3, MI = formula 0.10. The p / m values for a non-coacervating cetylweight of Hyamine 1622, L e . , 465.5, A = intercept of H pyridinium chloride-NaC1 systemz0range up to 0.27 ( C - Co)/(T T,,) us. c - ca plot. for NaCl concentration up to 0.4 M . PhilippoffZ2 A qualitative experiment that tends to indicate has summarized the charge properties of a number that specificity is related to the binding of counter- of anionic and cationic micellar systems deions to micellar species is the following: If coaeerva- termined by a variety of techniques. For typical ttion is produced by the addition of two electrolyte systems reported, the micellar ionization ranges species, the effects are not additive. The greater from 16 to 50%. the quantity of the more effective electrolyte in the The micellar molecular weights determined for system, the smaller the proportionate amount of Hyamine 1622 as a function of the NaCl concentrathe less effective electrolyte needed to produce tion (Table 111) show the following pattern: (a) a two phase system. For a mixed NaC1-NaBr at low NaCl concentrations, a relatively slow insystem a t 26" when the solution is 0.034 M in crease in hlMW occurs; (b) between 0.17-0.19 M NaBr which is '/2 the NaBr c.e.c. (0.067 M) NaC1 there is a sharp rise in MMW and (c) for coacervation may be produced with the addition of KaC1 concentrations above 0.19 M, the MMW in0.10 M NaCl. This NaCl concentration is very crease at a rate greater than the initial rate (low much less than '/z the NaCl c.e.c. (0.336 M ) . NaCl concn.) but lower than the rate of increase Light Scattering. Effects of Added Electro- in the interval 0.17-0.19 M NaCI. This pattern lyte.-Approximate micellar molecular weights of would indicate that in the electrolyte transition Hyamine 1622 in aqueous NaCl solutions were cal- range of 0.17-0.19 M NaCl, a profound reorganiculated from .the general relationship zation of the micellar structure occurs. A pertinent comparison may be made with two related 1 H(C - CO)(Pm)= jg 2Bc syst,cms. These systems are 7 - TO where the terms of this expression have the usual (CPC) [Cdfa-N + o] C l --NaCl-HtO and meaning. For those solutions where the medium behaved as a theta solvent (for NaCl concentration in excess of 0.05 M ) the second term of this expression becomes negligible. For low electrolyte concentrations where the
LiCl NaCl KC1 CSCl
.305
-'
+
IRVING COHENAND TONY VASSILIADES
1778
Light scattering studies by Anacker and DebyelOJO show pronounced differences between these systems. CPC micellar molecular weight as a function of NaC1 concentration shows a growth to a limiting value of 46,000 for NaCl concentrations in excess of 0.4 M. No further micellar growth occurs for higher NaCl concentrations. The data are consistent with a spherical model of the micelle. HDTAB a t KBr concentrations of 0.178 and 0.233 M shows micellar molecular weights of 795,000and 1,860,000, respectively. The data for the HDTAB system are consistent with a rod-shaped model of the micelle. A screening treatment of aqueous CPC solutions with a wide variety of monovalent and polyvalent anion salts failed to produce a coacervating effect. For a similar screening treatment of HDTAB, moderate amounts of KSCN (0.165 M ) induced two solution phase formation. Two observations may be made here: (a) for all of the coacervating systems investigated, the micellar species were dimensionally anisotropic prior to two phase formation and (b) not all anisotropic micellar systems formed coacervates. The HDTAB-KBr solutions were homogeneous in saturated KBr systems.
Vol. 65
. .
X
*
E i 6--
z
5-
4 -32
I-
1
TABLE I11 1 : MICELLARMOLECULAR WEIGHTSA N D DISSYMMETRIES OF 3.2 3.3 3.4 3.5 3.6 3.7 HYAMINE1632 AS A FUNCTION OF KaC1 CONCENTRATION^ I / T x 103 (OK.). AT T = 30.0" Fig. Pa.-Molecular weight of Hyamine 1622 as a function I160 /Ira0 NaCl (M) M M W x lo-' of temperature at fixed NaCl concentrations. 1 03 0 11 1 23 1 43 1 04 184 1 04 2 08 1 10 1; 3 06 1 11 I8 3 57 1 12 'XI 5 0 1 18 25 .!O 7 25 1 35 a I,,, and I 310 are the intensities of the scattered radiation at 45 and 135", respectively. 1.3
!5
1.4
h
' -
1 . 3 .-
LD
Dissymmetry of Light Scattering.-The dissymmetry as a function of electrolyte concentration shows the following characteristics. At loa electrolyte concentrations, external interferenw effects due to electrostatic intrrparticle interactions are evidenced by a small decrease in dissymmetry to d u e s of less than unity. At concentrations above the electrolyte transition region, the dissymrnetrics begin to rise and the increase becomes inore pronouiic~ed as the c.e.c. is approached. The dissyrnmet ries of the soap-rich layer of n two-phase system show an initial sharp drop (from 1 5 to I 18) for a 0.04 JI increment of S a C l from 0 10 to 0.44 3 IK:iCl. This sharp drop in dissymincrry would iiidicatc dcstructive interference of t h . svatterrd radiation from this phase. Temperature Effects. --Figure 2a is a plot of micellar molecular weights a t fixed electrolyte conceiitrntioiix (0 005 to 0 18 JI SdJ)as a function oi 1 I' 1% 7 i i the h w l u t c temperature of thc yystem, 111 tfx. trmper:Lturr range iiivcsiigatcd ,.> d0") d l of i h t , wlutilmc b t u d i c d n i t h the c'iception of t h c i i) 18 J I YaCl wlutioii~\irie Iiomogcncous o I r r l w rmtirt. ternpcraturc. r:uige 'Phlk ht.r.cS
"21) 1' Dot, mu R Stviner, J
Chsm f h u s 16, 1211 (l(r50 I'nrada,, SP. l i , 31, 1951,
1 Y
0 \
-
9
2 1.2 .1.1
LONGC:MN QUATERNARY AMMONIUM SALTS
OCL., 1961
1779
The zero angle points fall on a straight line. A linear extrapolation of the zero angle points to (c c')/Re = 0 yields an electrolyte concentration corresponding to the c.e.c. of the system. Essentially, the zero angle scattering is extrapolated to infinite turbidity. For very pure monodisperse systems, a very small fraction of the incident radiation is transmitted a t the critical point. For the quaternary ammonium salt-electrolyte solutions investigated, the character of these systems in terms of small amounts of impurities and polydispersity is such that the assumption that all of the incident radiation is scattered a t zero angle is a fair approximation of the situation. Figure 3a represents this treatment for a 0.0644 M (3%) Hyamine 1622 solution of KaCl concentrations ranging from 0.2 to 0.31 M . The linearity of (c c')/Re plot is obtained for NaCl concentrations above the electrolyte transition range (0.170.19 M NaCl) previously described. This method correctly predicts the relative constancy of c.e.c. over a wide range of soap concentration and was tested for several concentrations between 0.5 to 5% soap. Figure 3b represents a divalent cation system, Hyamine 1622-BaClzH20. The divalent cation effect of increasing the critical anion concentration necessary for two phase formation is predicted quantitatively by this method. Figures 3c and 3d represent the systems Cetol-NaClHzO and HDTAB-KSCN-H20, which form coacervates a t high and low electrolyte concentrations, respectively. The values of the c.e.c. check within 2% of the c.e.c. values obtained by the standard interpolation method. Conclusions A tentative model of the micellar aggregate in the quaternary ammonium salt solutions studied which will account for observed phenomena is the following: In a salt-free solution of Hyamine 1622 and at low NaCl concentrations, the micelles are experimentally isotropic and are of a lower charge density than is common in the many soap and polyelectrolyte systems previously investigated. This relatively lower state of ionization is indicated from the light scattering data, and the small quantity of added NaCl (-0.05 M ) necessary to achieve theta solvent properties in Hyamine 1622 solutions. StraussZ3has observed theta solvent effects in sodium polyphosphate solutions a t NaBr concentrations of 0.415 M . The polyphosphate systems were estimated to be 3040% ionized For cetylpyridinium chloride solutions2O in which the coacervation is not observed, theta solvent effects are observed for NaCl concentrations in excess of 0.4 M . The micellar aggregate in this system is 20-27% ionized. For anionic soaps such as potassium oleate for which the coacervation has been observed a t KC1 concentrations of the order of 1.6 M , the micellar ionization is 2030Yo2in salt free solutions. There is an apparent lower limit of ionization of the micellar species below which coacervation is not observed in the soap systems investigated. In the system Hyamine 1622-NaI-Hz0 flocculation
+
0.2 0.4 0.6 0.8 1.0 1.2 1.4 Fin2 e/2 C C'. Fig. 3a.-Critical electrolyte concentration derived from the angular dependence of the modified reciprocal reduced intensity. Hyamine 1622-NaCl-H& system. C = Hyamine conc?. = 0.0644 M ; C' = NaCl concn.; 5" = 30"; X = 4360 A.
+ +
tion range, large variations in micellar molecular weights were observed as the temperature was lowered. For a one degree temperature change, for the 0.18 M NaCl system, the micellar molecular weight increment between 30 and 29" was approximately 5 ;< lo3MW units. Figure 2b is
\
-
.
,
,
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 sin' 8/2 C C'. Fig. 3c.-Cntical electrolyte concentration derived from the angular dependence of the modified reciprocal reduced intensity, Cetol-NaC1-H20 system: C = molar concn. of Cetol = 0.005 M ; C' = molar concn. of NaCl; T = 30"; X = 4360&
+ +
t
.
.
.
,
.
.
,
0.2 0.4 0.6 0.8 1.0 1.2 1.4 sin' e/2 C C'. Fig. 3d.- -Critical electrolyte concentration derived From the angular dependence of the modified reciprocal reduced intensity, HDTAB-KSCN-HsO system: C = HDTAB concn. = 0.05487 M ; C' = KSCN concn., T = 29.0".
+ +
that two solut,ion phase formation in complex coacervate systems (polycation-polpanion mixtures) may be explained in terms of electrostatic interactions. The free energy of the system may be represented by an expression consisting of two terms, an electrostatic free energy term and an entropy of mixing term. The electrostatic free energy is independent of particle weight and may hr expressed in terms of electrostatic free energy per unit volume of polyion. The entropy term shows an inverse dependence on the length of the polyior? rod. For syPf.ern consisting of a rod-like assembly
Oct., 1961
VISCOSITY AND CHARGE PROPERTJES O F AMMO~U'IC~M SALTS
of quasi independent charges of low electrostatic free energy favorable conditions for coacervation require a low entropy of mixing. I n the Hyamine 1622-NaC1 solutions, assuming cylindrical micellar model, the large micellar growth that occurs prioir to the onset of two phase formation, with small NaCl increments, results in a marked elongation of the cylinder. Hence, a sharp decrease in entropy of mixing ensues without a concurrent large change in the charge properties of the system. Where the entropy of mixing decreases sufficiently, coacervation will occur. The smaller charge effects are in the direction of reducing the micellar charge density. These are cooperative effects that favor coacervation. The mechanism of Coacervation in cationic soap systems differs from complex coacervation in mixed polyelectrolyte systems. I n the Voorn-Overbeek interpretation of the mechanism of complex coa,cervation, the primary effect is the reduction of the electrostatic free energy of the system resulting from the mixing of oppositely charged p01yions.l~ It has been previously reported' that the coacervate layer, i.e., the soap-rich layer in a two phase system shrinks with further addition of electrolyte in excess of the c.e.c. A distinctive feature of the soap-rich phase is the apparent high degree of ordering of the micellar aggregates. Ordering is indicated by two effects: (a) the turbidity and (b) the dissymmetry of light scattering in the coacervate layer of Hyamine 1622
1781
systems show a sharp decrease with added electrolyte. These effects may be related to destructive interference of the scattered radiation, characteristic of a system with high degree of order. Current work in progress on viscosimetry, diffusion, flow birefringence and charge properties of cationic soap coacervate systems confirm the general character of the properties of the homogeneous systems indicated from the light scattering studies. The intrinsic viscosities for Hyamine 1622-NaC1-Hz0 solutions exhibit no shear dependence for NaC1 concentrations below the electrolyte transition range and are shear dependent for NaCl concentrations above the electrolyte transition range. The details of these studies will be published in a later paper. Acknowledgment.-The authors wish to express their appreciation for support provided by the Department of Health, Education and Welfare, Public Health S e r v i c e G r a n t No. A-2300 (C1 and C2). DISCUSSION H. L. ROOIJ (Leyden University).-Have you observed elasticity in your systems? I. CoHEN.-The viscous pro erties of the homogeneous phase of several cationic soap-etctrolyte-H?O systems have been detailed in the following paper of tl& group. These s stems shorn essentially the same properties as the oleateeTectrolyte-H~O systems investigated by Rungenburg de Jong, et aE. The term elasticity cannot properly be applied to t,hese systems. The homogeneous phase of the soapelectrolytRRg0 system a t relatively high electrolyte concentration is a free flowing solution of high viscosity.
CRITICA'L PHENOMENA IN AQUEOUS SOLUTIONS OF LONG CHAIN QUATERNARY AMMONIUM SALTS. 111. VISCOSITY, DIFFUSIOX AND CHARGE PROPERTIES BY IRVING COHENAND T o m VASSILIADES Department of Chemistry, Polytechnic Institute of Brooklyz, Brooklyn, N . Y . Rsceivcd July 6, 1961
The viscosity of several aqueous quaternary ammonium salt solutions was studied as a function of added electrolyte and temperature. The intrinsic viscosities of coacervating systems (two solution phase formation) as a function of simple univalent electrolyte (concentrationshow a discontinuity coincident with the electrolyte transition range previously determined from light scattering experiments. For the higher electrolyte concentrations, in excess of the transition electrolyte concentration, the intrinsic viscosity is a linear function of M ' h (micellar molecular weight) and L / M z h(where L is the long dimension of the anisotropic micellar particle) derived from light scattering data. The temperature de endence and the electrolyte specificity of the electrolyte transition range was investigated by viscosimetry techniques. %he self-diffusion coefficient of the cationic micellar soap system, Hyamine 1622, waa studied as a function of added sodium chloride by means of the capillary technique, using a dye as an indicator. The electrolyte transition range is characterized by a sharp drop in the diffusion coefficient. The charge properties of coacervating and non-coacervating cationic soap systems were studied a t low electrolyte concentration. There is a direct correlation between the charge pro erties of these systems and their subsequent behavior with further addition of electrolyte. The viscosity, diffusion a n 8 charge properties of the coacervating cationic soap systems are consistent with the micellar model previously proposed: at low electrolyte concentration the micelle is dimensionally isotropic and a t high electrolyte concentrations the micelle is a cylindrical rod-shaped particle.
Introduction The viscosity, diffusion and charge properties of dilute aqueous solutions of long chain quaternary amnioniiim sails have been studied as a, function of added simple electrolytes. A number of the micelhr systems behave as cortcervates in that they separate into TWO so!ution phases with the addition of electrolyte. 111 the coacervating systems, far each elcc-rolyte, st x constant tempera-
ture and constant soap concentration, there is a critical electrolyte concentration (CEC) necessary for two solution phase formation. The electrolyte specificity and the light scattering properties of these systems have been reported previously.' From these studies of the coacervating systems, an electrolyte transition range (ETR) may be described, intermediate between zero elec[I) I. Cohen and
T. Vassiliadea, J . Phys. Chsrn.,
66, 1774 (1961).
