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Interactions of anionic surfactants with nonionic polymers. Comparison of guanidinium, tetraalkylammonium, and alkali metal ions as counterions
Interactions of Anionic Surfactants with Nonionic Polymers. Comparison of Guanidinium, Tetraalkylammonium, and Alkali Metal Ions as Counterions S H U J I SAITO, T A K U M I T A N I G U C H I , AND K A N J I K I T A M U R A Momotani Juntenl~an, Ltd., Minatolcu, Osaka, 552, Japan
Received November 5, 1970; accepted January 18, 1971 The interactions of anionic surfactants with nonionic polymers such as polyvinyl pyrrolidone (PVP), polyvinyl alcohol-acetate copolymers (PVA-Ac), and polyvinyl alcohol (PVA) in aqueous solution were studied with respect to guanidinium, four tetraalkylammoniums, sodium, and potassium as the counterions by means of electrical conductivity, dye solubilization, viscosity, and cloud point measurements. With sodium salts as the reference, guanidinium, a strongly water structure-breaking cation, lowers the critical micelle concentration (cmc) of the long-chain alkylsulfates, and in the presence of PVP and PVA-Ac it promotes the binding of the anions to them and enforces the attraction between the bound anions by ion-pairing, exhibiting such phenomena as shrinking of the polymer chains or depression of the cloud points. Similar phenomena have been observed and discussed previously in the interactions of these polymers and long-chain alkylammonium or symmetrical tetraalkylammonium ions with thioeyanate or iodide as counterions, both also strongly water structurebreaking anions. Sodium and potassium dodecylsulfates interact with PVP and polyvinyl acetate to a similar extent because both counterions are of weak effect on water. On the other hand, for the quaternary ions, which promote the water structure, as the counterions to dodecylsulfate, the cmc lowers with increase in size of the quaternaries but the polymer interactions weaken, and tetrabutylammonium dodecylsulfate in particular barely interacts with PVP and PVA in the presence of the micelles, and only weakly with PVA-Ac. Thus, in comparison with the sodium salts, only the strongly water structure-breaking counterions enhance the binding of the long-chain anions to the polymers. The role of the quaternary counterions has been discussed in terms of the effect on the equilibria between micelle formation and complex formation with the polymer. INTRODUCTION I n the system of nonionic polymers and surfactants in aqueous solution, a hydrophobic attraction arises between both cosolutes. W h e n the surfactants are ionic, the counterions are distributed around the surfactant ions bound to the polymers like polyelectrolytes, providing a locally concentrated complex domain. Preceding studies (1-4) showed that surfactant ions alone are not responsible for the interactions of ca~ tionic surfactants with nonionic polymers: the counterions classified as strongly water structure-breaking anions (thiocyanate >
iodide > nitrate) encourage interaction of long-chmn alkylammonium ions with some water-soluble polymers in comparison with chloride, a feebly water structure-breaking anion, whereas fluoride and acetate, both strongly water structure-making anions, are counterions that do not aid binding (4). With hydrophobic organic anions such as ethylsulfate and butyrate as the counterions, there is hardly an increase in the interaction of dodecylammonium with polyvinyl pyrrolidone (l). Such a counterion effect characteristic of the strongly water structure-breaking anions
Journal of Colloidand InterfaceScience,¥ol. 37, No. 1, September 1971
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INTERACTIONS OF ANIONIC SURFACTANTS WITH POLYMERS has been interpreted by the assumption that the incompatibilities of the local aqueous environment surrounding the hydrophobic cations and polymers ~x4th these anions may induce attraction between them accompanied by iota-pairing (3, 4). These facts prompted us to study extensively the polymer interactions of anionic surfaetants with various eounterions in which a strong binding to the polymers of dodecylsulfate ion with sodium as the counterion has been well recognized (5-11). Recently it was shown that whereas monovalent inorganic cations are rather inessential in the salting action on nonionie polymers in comparison with their anionic counterparts (12), guanidinium and large symmetrical tetraalkylammonium cations have remarkable effects of a different charaeter: the former by brealdng the ordered water in dose proximity to the polymers (13-16), and the latter, especially tetrabutylammonium, by direct contact or binding to the polymers (4, 12, 13). From this point of view, the polymer interactions of long-chain alkylsulfate ions having guanidinium, tetraalkylammoniums, and potassium as the eounterions are compared in this paper with those of the sodium salts. Since guanidinium ion is a unique organic cation in that it disrupts the aqueous order intensely (14, 17), it was of particular interest to know whether guanidinium alkylsulfates too could exert an effect on the polymers similar to that of alkylammonium thioeyanates and iodides (1-4). EXPERIMENTAL
Materials. Sodium dodeeylsulfate (NaDS)
was a product (Texapon L 100) of the Henkel & Cie, Germany, and was recrystallized from ethanol. The pure sodium decylsulfate (NaDeS) and octylsulfate (NaOS) were supplied by the Nihon Surfactant Co., Tokyo, and the Kao Soap Co., Tokyo, respectively, and the sodium hexylsulfate (NariS) was prepared from n-hexylalcohol (E.P. grade of the Tokyo Kasei Kogyo Co., Tokyo) in this laboratory. Potassium dodecylsulfate (KDS), a highly pure product (Nikkol KLS) of the Nikko Chemicals Co., Tokyo, was reerystallized once from water.
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Guanidinium dodecylsulfate (GuDS), decylsulfate (GuDeS), octylsulfate (GuOS), and hexylsulfate (GullS) were converted from the respective sodium salts by repeated reerystallizations from aqueous concentrated solutions of guanidinium sulfate (the Wako Pure Chemical Co., Osaka), and they were recrystallized several times from water. The melting point of GuDS was 144°-145°C. Anal. Caled for ClaHalNa04S: C, 47.97; H, 9.60; N, 12.91. Found: C, 47.59; H, 9.71; N, 12.85. The melting point of GuDeS was 144°-145°C. The melting point of GuOS was 135°-136°C. Anal. Caled for CgHlaNa04S: C, 40.13; H, 8.61; N, 15.61. Found: C, 40.32; H, 8.56; N, 15.89. The melting point of GuI-IS was 123°-124°C. Anal. Calcd for CTH19NaO4S: C, 34.84; H, 7.94; N, 17.41. Found: C, 35.17; H, 8.06; N, 17.29. Tetraalkylammonium dodecylsulfates (R4NDS) were made by a metathetical reaction of equivalent amounts of silver dodecylsulfate and tetraalkylammonium halides and dried in vacuo (18). Silver dodeeylsulfate was prepared from the sodium dodeeylsulfate and silver nitrate according to the method described (18). Silver nitrate was reagent grade of the Wako Pure Chemical Co. Tetramethylammonium and tetraethylammonium bromides were purchased from the Tokyo Kasei Kogyo Co., and tetra-n-propylammonium and tetra-n-butylammonium bromides were from the Eastman Organic Chemical Co., in the United States; they were reerystallized as described before (19). The critical micelle concentrations (emc) of these four R4NDS's at 30°C determined by the solubilization of Yellow OB, an oilsoluble dye, are in qualitative agreement with those determined by the conductivity method at 25°C (20), which are indicated in mM in parentheses, as follows. Tetramethylammonium dodecylsulfate (Me4NDS), 6.2 (5.52); tetraethylammonium dodecylsulfate (Et4NDS), 3.5 (3.85); tetrapropylammonium dodeeylsulfate (Pr4NDS), 2.5 (2.24); tetrabutylammonium dodecylsulfate (Bu4NDS), 1.1 at 25°C (1.3 (21)). As solubilization experiments for Bu4NDS were carried out at 12° =t= 1°C as described below, its cmc at 25°C was determined by surface tension measurements with a Du Nouy tensiometer:
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the surface tension at the cmc was 37 dynes/ cm.
Polyvinyl pyrrolidone (PVP), a product (Luviskol K90) of the Badische Anilin 4: Soda Fabrik, Germany, was precipitated twice from methanol-ether. Two kinds of polyvinyl alcohol-acetate copolymers (PVAAcI and PVA-AcII) with 30.0 % and 19.7 % acetate residues, respectively, and degrees of polymerization 2 0 0 0 both; polyvinyl alcohol (PVA), and polyvinyl acetate (PVAc), were the same as used previously (2; 4, 12, 13). The aqueous polymer solutions were treated by ion exchangers. Procedures. Electrical conductivity was measured under nitrogen gas atmosphere with a bridge equipped with a 1000 cps oscillator (Universal Bridge Type BV-Z 103A, Yokogawa Electric Works, Ltd., Japan) and a cell with blackened platinum electrodes at 25.00°C. The cell constant was 0.3164. The conductivity of the water used was 0.98 X 10-6 ohm-1, and that of 0.23 % PVP solution was 2.28 X 10-8 ohm-1, and they were subtracted from the conductivity of the surfactant solutions. Viscosity was measured with the same Ostwald viscometer used previously (12, 13). In a set of the measurements the reduced viscosity was calculated relative to the surfactant solution without considering the reduction of concentration due to binding to the polymers. This approach does not change the feature significantly as shown in Fig. 3. The solubilization of Yellow OB was done at 30°C in the same way as before (1, 5). In Bu4NDS, since the transparent solution turned opaque by the dye solubilization at 30°C, the experiment was made at 12° 4I°C. The cloud point of a PVA-AcI solution was determined as described before (12): RESULTS AND DISCUSSION 1. Solution Properties of Guanidinium Allcylsulfates. The equivalent electrical conductivity (h) and the limiting conductivity (A0)of GuOS and G u l l s are shown in Fig. 1 together with those of NaOS and Naris and also of NaDS (18). Although A0 of NaOS, 78.2, is lower than the literature value of 79 (22), those of NariS, 79.6, GuOS, 79.6, and GullS, 81.6, are nearly consistent: as A0 for Na + at 25°C is 50.1 (23(a)), A0 for OS- and HS- are, respectively, 28.1 and 29.5, and
those for Gu + obtained are 51.5 and 52.1 from GuOS and GullS, respectively. From the breaks of the A -- ~/M curves the cmc's for NaOS and GuOS are found to be 132 and 60 mM, respectively, at 25°C. The literature value of the cmc for NaOS is 0.13 M (22). The broken lines in Fig. 1 indicate the conductivity following the Onsager limiting equation for 1 - 1 electrolytes at 25°C (23(b)) A = A0 -- (0.229 A0 ~- 60.2) ~/M. Compared with the case of NaDS (18), the measured conductivities of these salts at dilute concentrations are in good agreement with the Onsager limiting slopes and deviate upward with increase of concentration more pronouncedly in N aOS than in NariS. NaDS, NaOS, and Naris are considered to be strong electrolytes, but they are supposed to be dimerized to some extent. The dimerization has been verified in several dodecylsulfate salts, and it also exists to decreasing degrees in salts of shorter alkyl chain lengths (18). The deviations from the Onsager slopes for both Gu + salts are rather small even around 10 mM. However, by assuming the same dimerization effect as the Na + salts to the respective Gu + salts and by subtracting the positive deviations in the respective Na + salts graphically from the curves for the Gu + salts, the corrected GuOS and GullS eonductivities show the negative deviations from the corresponding limiting slopes (Fig. 1), indicating that actually the Gu + salts may be slightly weak. The dissociation constants K for the Gu + salts are calculated from the corrected smoothed curves below 10 mM by the Shedlovsky equation (23(c)) 1/(AS,) = 1/ko q- (y2.S~MA)/(KAo2), whereS~ = 1 + z - g z2/2, z = (0.229A0-160.2) (MA)II2/A~/2, and --log y± = 0.509(S..MA/A0) 1/2. The plots are shown in Fig. 2, and h0's in Fig. 1 were taken as the best fits for the linear relation at dilute region. But, in view of the possible experimental errors of 0.2 in A0 and h in drawing the corrected curves, values of K obtained are 0.25-0.29 and
Journal of Colloid and Interface Science, Vol. 37, No. 1, September 1971
INTERACTIONS OF ANIONIC SURFACTANTS WITH POLYMERS i
.
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70
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100
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FIo. 1. E q u i v a l e n t c o n d u c t i v i t y A of solutions of NaOS, GuOS, NariS, and G u l l s in the absence and presence of 0.23% P V P p l o t t e d as a function of square root of molarity M at 25°C. The broken lines are the Onsager slopes. The corrected curves for GuOS and GullS (dotted lines), see text. The curves for NaDS were taken from reference 18.
//9 130 GuOS ~ /
j~"
~125
12(
0.4
0.8
1.2
y2SzMA
FIG. 2. The Shedlovsky plots for the calculation of the dissociation constants for GuOS and GullS solutions at 25°C. The plots were obtained from the corrected s m o o t h e d curves in Fig. 1 at 0, 2, 4, 6, 8, 10, 12, and 14 for 100x/M from left to right.
0.27-0.32 for GuOS and GullS, respectively, at 25°C, and the difference between them is rather ambiguous. The weakness of GuOS, its lower cmc and steeper conductivity slope above the cmc than those for NaOS, might arise from the ion-pairing due to the high polarizability which Gu + may have as this ion takes coplanar, resonance structures (24). However, the ion-pairing might arise supposedly from
a special, non-Bjerrum type mechanism due to the disturbances between the different vieinal aqueous media around the water structure-breaking cation and the hydrogenbond-promoting hydropbobic anion. A further study on the solutions of the other homologs would be of interest. 2. The Polymer Interactions of Guanidinum and Potassium Alkyl-sulfates. When a nonionic polymer is dissolved in a surfactant salt solution, some of the ions are confined to the polymer. In this case two processes are at work: binding or complexation of the long-chain ions to the polymer giving it a polyeleetrolytic nature, and counterion fixing or concentrating to the complexes. By the binding of ions to the polymers the conductance is decreased and the cmc break in the conductivity-concentration relation is blurred (25). By addition of 0.23 % PVP to NaOS and GuOS solutions (Fig. 1), the conductivity begins to decrease from that of the respective salt solutions far below the emc, suggesting that less conductive complexes are formed from this point on, and in line with the trend of the salt solutions, the downward deviations of the conductivity curve for the PVP-GuOS
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system take place at lower concentrations and the slope becomes steeper than that for the PVP-NaOS system. Thus, at the initial stage of the OS- binding the Gu + ion may make OS- more readily accessible to PVP than does Na +, and simultaneously the Gu + ion itself may be more tightly fixed to the bound OS- ion than to the free one, and this tendency of Gu + is more marked than that of the Na + ion. ]~eyond the cmc, the conductivity curves of the salts without and with PVP come close to each other and coincide eventually at high concentrations. This may suggest that with increase of binding the complex ions behave similarly to the ordinary micelles. As for OS-, the maximum binding may be similar in both salts, and this may be confirmed by the dye solubilization experiments. The formation of the polymer-surfactant complexes is usually associated with a different solubilization power from that of the surfaetant micelles alone depending upon the relations of the polymer and solubilizate (5, 6). This is illustrated in Fig. 3 in the systems of 0.23 % PVP-NaOS and - G u t s with Yellow OB as the solubilizate. The solubilization of the dye in the polymer solution starts at a lower surfaetant concentration than the cmc, and this point agrees with the point of deviation in the conductivity curve in Fig. 1. The dye solubilization in the polymer solution enhances more markedly with increase of the surfaetant concentration than that of the surfaetant solutions alone, and above a certain concentration the curves 10
i4 0,05
0,10 OCTYLSULFATE
M
0.15
0.20
FIG. 3. S o l u b i l i z a t i o n of Y e l l o w O B in s o l u t i o n s of N a O S a n d G u t S in t h e a b s e n c e a n d p r e s e n c e of 0.23% P V P a t 30°C. T h e a r r o w m a r k s t h e s u r f a c t a n t c o n c e n t r a t i o n S at t h e s a t u r a t i o n s o l u b i l i z a t i o n to t h e p o l y m e r c o m p l e x e s ; see t e x t .
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30
Fit. 4. Reduced viscosity ratio of 0.23% PVP and 0.16% PVA solutions in the presence of NaDS, KDS, and GuDS to the respective aqueous polymer solution plotted as a function of the surfaerant concentration at 42.5°C. The solid and broken lines represent the reduced viscosity ratio for PVP-GuDS system on the basis of the surfactant solutions and pure water, respectively, as the solvents; see text.
run straight and parallel to those of the respective surfaetant solutions without the polymer (Fig. 3), implying that at this concentration S the binding of OS- to the polymer nearly reaches saturation. It is considered that up to S the complexes are the primary solubilizing loci, and above S the ordinary mieelles join in the solubilization (5). Since OS- ion is common to both surfactants, the magnitudes of the dye solubilized in the complexes alone below S should be related, if not proportional, to the degree of occupancy on the polymer chains by OS- ions. The amount of the dye solubilization in the PVP-GuOS complexes at S seems to be similar to that in the PVP-I~aOS complexes despite the great difference in the cmc (the emc's for NaOS and Guts at 30°C determined by the dye solubilization method are 135 and 61 mM, respectively, as seen in Fig. 3), which may mean that the saturation amount of OS- binding is nearly the same in each case and that the eounterion effect may be related solely to the shift of the cme. However, the viscosity measurements
Journal of Colloid and Interface Science, Vol. 37, No. 1, September 1971
INTERACTIONS OF ANIONIC SURFACTANTS WITH POLYMERS revealed another aspect of the polymer interactions. The effect of addition of GuDS on the reduced viseosity of the 0.23 % PVP solution is indieated in Fig. 4, together with the effect of NaDS and KDS. As a result of the binding, the shape of the flexible polymer in solution undergoes changes which do not appear when the substrate is solid (5). The binding oeeurs mainly above the eme in mieellar form, but PVP binds NaDS sparsely on its chain below the eme (5), and this seems to hold also for KDS and GuDS. The Krafft points of KDS and GuDS are, respectively, 35°C (26(a)) and about 40°C, so the viscosity was measured at 42.5°C, in some cases in the supersaturated state: the eme and the solubility of GuDS at this temperature are about 5 and 8 raM, respectively. It is known that the solubility of a surfaetant is increased by the presence of polymers which bind the surfaetant (27). B y the way, the eme's for NaDS and KDS at 40°C are 8.9 and 7.8 mM, respeetively (26 (b)). Although the peak height in the reduced viscosity ratio-concentration curve for GuDS is much lower than for NaDS and KDS, the location of each peak for these surfaerants is at nearly the same concentrations, suggesting that in this ease an alteration in the eounterion fixing rather than in the D S binding may be the primary cause in the PVP-GuDS interaction. It is seen that, as in the usual salt effect on nonionic polymers in aqueous solution (12), Na +, a weak water structure-maker, and K +, a very weak water structure-breaker, as the eounterions to D S do not give any significant change. E u t Gu + ion is distinguished in its effect on proteins or some hydrophobie polymers by changing the water into a better solvent for them at high concentrations (13-16). The viscosity curve for the PVP-GuDS in Fig. 4 shows an abrupt change at 6 m M compared with those for NaDS and KDS, and it is likely that above this transition concentration the Gu + ions fix more firmly to D S ions directly bound to PVP, thereby producing a more compact polymer complex than the corresponding complexes of NaDS and KI)S. As shown later in Fig. 7, it is to be noted that Me4NDS, Et4NDS, and
159
GuDS, which have close cme's at the measured temperatures, give quite different, reduced viseosity patterns. Peeause of the poor solubility of GuDeS at room temperature, the viscosity of the 0.23% PVP-NaDeS and -GuDeS systems was measm'ed at 35°C, and a very similar viscosity trend to that of NaDS and GuDS was observed with a transition concentration for GuDeS at 26 mM, but the reduced viscosity increase by the DeS salts was Iess marked than that by the DS salts. The counterion effect of Gu + appears strikingly in the PVP-OS- system as an upward concave in the reduced viscosityconcentration curve at about 65 mM, falling below the value in aqueous solution, whereas NaOS raises the reduced viscosity weakly but smoothly (Fig. 5). The featm'e of the coneave viscosity eurve does not change much even if water is, instead of the surfactant solutions, supposed to the solvent in the other extreme ease (dotted line). The real reduced viscosity values are somewhere between both lines but probably closer to the solid line because the amount 2.0
O,.5
PVA-Acn-GuOS
PVA-AclI-Na O S ~ ,/@ /
,'"
PVP-GuOS
~1.0 u ~o.5
5'0OCTYLSULFATE ,;0 mM
~;o
FIo. 5. R e d u c e d v i s c o s i t y ratio of 0.23% P V P ,
0.16% PVA, and of 0.15% PVA-AeII solutions in the presence of NaOS and GuOS to the respective aqueous polymer solution plotted as a function of the surfaetant concentration at 25°C. The solid and broken lines represent the reduced viscosity ratio for the PVP-GuOS system on the basis of the surfactant solutions and pure water, respectively, as the solvents; see text.
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by addition of the shorter aikyl-chain anions than is PVP (12). Figure 6 exhibits the variation of the cloud point of a PVAAcI solution by addition of NaOS, NariS, GuOS, and GullS, and of guanidinium chloride (GuC1) and NaC1 for the sake of comparison. Despite the lowering of the cmc by substituting Gu + for Na + as the counterions, the cloud point-raising action of OS- is markedly offset in a certain dilute region and the rise resumes at a higher concentration. With CI- as the eounterion, 0 0.] 0.2 SALT M Gu + has a salting-in effect (13) but Na + FIG. 6. Cloud point of 0.21% PVA-AcI in and K + exert a similar saiting-out effect solutions of NaOS, GuOS, NariS, GullS, NaC1, on the polymer solution, although only and GuC1plotted as a function of the salt concen- weakly in the dilute region (12). Such behavior of the Gu + long-chain tration. anion salts in comparison with the Na + of binding here seems to be small (Figs. 4 or K + salts may be accounted for ad hoc by postulating that by ion-pairing Gu + and 5). promotes the binding of the long-chain In contrast to PVP, the reduced viscosity anion to the polymer and the mutual of a PVA solution is increased only slightly by GuDS whereas it is increased remarkably attraction between the hydrophobic anions by NaDS (Fig. 4), and the changes by directly bound to the polymer, rendering NaOS and GuOS are both quite small and the polymer more contracted and thus no concavity appears in the PVA-GuOS leading to the relative setback in the rise curve (Fig. 5). The great differences in the of the reduced viscosity or of the cloud peak position and height in the viscosity point and that with increasing total salt curves for NaDS and GuDS show that the concentration and with approaching to the amount of DS- binding to PVA is seriously micelle-wise saturation binding around the dependent on the counterions. This seems to cmc in the solvent phase, the counterion be related to the fact that, as the presence Gu + tends to be tess fixed followed by a of a number of - - O H side groups imparts rise in the reduced viscosity (PVP, PVAa strongly hydrophilic nature to the hy- AcII; Fig. 5) or in the cloud point (PVAdrophobic main chain, PVA binds the AcI; Fig. 6). In summing up, for raising the reduced surfactant anions less than does PVP and only above the cmc (5, 11), where GuDS viscosity or the cloud point of the polymer is less ionized, and therefore the PVA coils solutions (salting-in) : Gu + (salt-in) > Na + = K + (sait-out) can not develop their inner sites available for CI-: change in the solvent structure. for further binding. Na + = K + > Gu + for long-chain alkylFigure 5 demonstrates the reduced viscosity diagram of the PVA-AcII solution sulfate anions at dilute region: counterion with NaOS and GuOS additions. As in the fixing effect to the bound anions. As stated in the "previous paper (4), the PVP-GuOS system, the reduced viscosity cationic sequence for the C1- salts is concurve for this polymer solution on addition sidered as regular, so we can take that for of GuOS shows an upward concavity which the alkylsulfate ions as irregular. This is not observed on NaOS addition. A similar irregular influence of Gu + as the counterions viscosity behavior was also found in the solution of a polyvinyl pyrrolidone-acetate to long-chain anions may be what has been expected from the role of SCN- and I copolymer and GuOS. in the polymer interactions of tetrabutylPVA-AcI solution, which becomes cloudy ammonium and long-chain alkylammonlum by warming, is more susceptible to change Journal of Colloid and Interface Science, Vol. 37, No. 1, September 1971
INTERACTIONS OF ANIONIC SURFACTANTS WITI POLYMERS cations ( 1 4 ) , perhaps because Gu +, SCN-, and I - are all strongly water structurebreaking ions. Since the effect of the Gu + ion itself as the counterions to simple inorganic anions on the PVA-AcI is weaker than that of S C N - or I - in dilute solution (12, 13), the amount of saturated OSbinding seems to have little influence by change of the counterion from Na + to Gu + (Fig. 3). However, the effect of Gu + m " v corroborate the previously postv' =. . . . . anism of the interaction based on the incompatibilities of the local order of the solvent structure surrounding the strongly water structure-promoting hydrophobie ions and polymers with the structure-breaking eounterions (3, 4). A further study is needed to clarify whether such a hypothesis works or not. Since D S - and D e S - ions are bound to PVP more appreciably than are OS- ions, the counterion fixing of Gu + is reflected just as a transition in the reduced viscosity curve about the cmc (4.5 m M for GuDS, Fig. 4), which in the case of the PVP-GuOS may be attributable to the viscosity curve concavity. The behavior of GuOS and G u l l S in the PVA-AcI and PVA-AcII solutions (Figs. 5 and 6) may be explained in the same manner: as the cloud point-raising effect of OS- ions is still strong (12), the cloud point curve may run flat in the dilute range of GuOS solution instead of caving in. Though the effect of H S - ions on the cloud point of the PVA-AcI solution is weaker than that of OS- ions, the attraction between the bound GuHS's is not strong enough to cause the cloud point to fall below that of the aqueous solution. NaDS has been reported to cause a transition in the bound state on some polymer chains owing to mutual attraction between DS-'s (28), but in case of the Gu + salts the transition occurs as a result of the eounterion fixing. In line with the trend in the water-soluble polymers, the tightly coiled, aggregated polyvinyl acetate chains in water are disintegrated, unfolded, and dissolved as well by I4DS as by NaDS (29 30 10), but very poorly by GuDS.
3. The Polymer Interactions of Tetraalkylammonium Dodecylsulfates. The change
161
I-
PVP-Me4NDS O
O -~27
PVP~Pr4NDS
'PVA-Bu4NDS
0
rPVA-Pr4NDS
PVP-Bu4NDS
10 20 DODECYLSULFATE mM
30
FIG. 7. Reduced viscosity ratio of 0.23% PVP and of 0.16% PVA solutions in the presence of NaDS and four R4NDS's to the respective aqueous polymer solution plotted as a function of the surfactant concentration at 25°C. in the reduced viscosity ratio of the P V P and PVA solutions in the presence of NaDS and R4NDS's is shown in Fig. 7. The reduced viscosity of the PVP solution is increased instantly on addition of NaDS, Me4NDS and Et4NDS, but it was observed t h a t the reduced viscosity increment was lowered slowly with elapse of time (a small percentage in two weeks at the peak of the viscosity curves) and the change was not recovered by warming. In Fig. 7 the reduced viscosity curves measured immediately upon mixing are shown. In some cases the rate of decrease was irregular. Such a gradual decrease might be caused either by rearrangement of the salts in the bound state, perhaps in the eounterion fixing, or b y change in crosslinking of the polymers by the surfactants, but not by decomposition of the surfaetants because the phenomenon was reproducible by the same surfactant solutions stocked for several months. Actually in the dye solubilization experiments, a preliminary run in a PVP-Me4NDS system showed that the amount of Yellow OB solubilized was unchanged for at least one month. It is noted that the location of the peak in the reduced viscosity curves shifts to the lower concentrations from NaDS to the
Journal of Colloid and Interface Science, Vol. 37, No. i, September 1971
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SAITO, TANIGUCHI, AND KITAMURA
t~4NDS's with increasing cation size, in contrast to the addition of GuDS and KDS as exhibited in Fig. 4, and that the peak height also lowers in the same order, and the addition of ]~u4NDS in particular NaDS scarcely alters the reduced viscosity. Thus, I the order of the peak height is as follows: NaDS > Me4NDS > Et4NDS > 3 Pr4NDS > ]~u4NDS = no addition. > From the viscosity result only it is supposed P~NDS that either the binding of DS- decreases or g the counterion fixing increases with increase of the hydrophobieity of the cation, or both. t~elow the emc, R4NDS is un-ionized to some extent, whereas NaDS is completely dissociated, though DS- is somewhat dimerized (31). The dissociation constants of these surfactants in the highly dilute region are 0.080 for Me4NDS; 0.054 for 1'o 1'~ 20 Et4NDS; and 0.048 for Pr4NDS at 25°C DODECYLSULFATE mM (18). The increasing weakness of the surFIG. 8. Reduced viscosity of 0.21% PVA-AcI factant salts with the larger quaternary counterions arises from the increasing at- solution in the presence of NaDS, Pr4NDS, and traction between the hydrophobic cation Bu4NDS plotted as a function nf t.he surfactant concentration at 25°C. and anion. This situation is also true in the micelles (20); the eme decreases with increasing size supported by the dye solubilization experiof the quaternaries, and the degree of ments as shown below. On the other hand, ionization of the micelles, which greatly being more hydrophobie than PVP and PVA depends on the method of measurement, is as a whole, PVA-AcI is able to bind Pr4NDS estimated for each by electrokineties as and even Bu4NDS more markedly, and the follows: NaDS, 27.5%; Me4NDS, 25.9%; reduced viscosity of the solutions is raised Et4NDS, 23.9 %; Pr4NDS, 20.5 %; and for by them, but not so greatly as by NaDS ]~u4NDS it may be about 18 % by extrapo- (Fig. 8). In the Yellow OB solubilization in Fig. 9, lation to the Stokes law hydrated radius o 4.7 A for Bu4N + (32) in Fig. 2 of reference 20. the curves for PVP-NaDS, PVP-Me4NDS, However, at the same concentration and PVP-Et~NDS below 6 raM, and also below each cmc, the difference in the actual for PVP-Pr4NDS in very dilute region are degree of ionization between, for instance, similar to each other, despite the large Me4NDS and Pr~NDS calculated from their differences in the eme and in the solubilizing dissociation constants is rather insignificant, power of the surfactants themselves. As for say less than 5 %; and both salts are almost Bu4NDS, from the very small difference fully dissociated. In fact, the viscosity curves in the solubilization curves in the absence of the PVP solutions for NaDS, Me4NDS, and presence of PVP an inference may be Et~NDS, and Pr4NDS in the dilute region drawn that the DS- ion with this large below each cme overlap independently of hydrophobic cation, which is supposed to the eounterions, and the differences appear be attracted to the hydrophobic part of only above each cmc. From the flat reduced the polymers (12, 13), is bound to PVP viscosity lines for the PVP- and PVA- to quite a lesser extent than those with the Bu4NDS systems shown in Fig. 7, it is smaller ones and Na +. Consequently the plausible that this particular surfactant polymer has no intrinsic binding sites for with a very low cmc is barely bound to the long-chain anions, and the binding these polymers. This reasoning may be may be considered as a kind of eomicelliza-
it
Journal of Colloid and I~terface Science, ¥ o l . 37, N o . I, S e p t e m b e r 1971
'
'
'
INTERACTIONS OF ANIONIC SURFACTANTS WITH POLYMERS 20[
163
~ PVP-Me4NDS
16j-
s
~
~
, Pw-Noos
12
N
PVP-
Pr N D S
EI4NDS 4
Me4NDS
0
4
8
12 DODFCYLSULFATE
16 mM
20
24
FIG. 9. Solubilization of Yellow OB in solutions of NaDS and four R4NDS's in the absence and presence of 0.23% PVP at 30°C, except for the Bu4NDS system at 12° 4- I°C. The arrow marks the surfaetant concentration S at the saturation solubilization to the polymer complexes; see text. tion of the long-chain ions with the hydrophobic parts of the polymer. F r o m the binding d a t a b y the equilibrium dialysis technique on the P V P - N a D S system, the difference between the concentration S and the respective cmc m a y be a very rough assess of the m a x i m u m surfactant binding to the polymer chain (5). Thus, b y assuming the same principle for the other P V P - D S - systems, approximately 18 -- 7 = 11 m M for N a D S ; 12 -6 = 6 m M for Me4NDS; 9 -- 3.5 ~- 5.5 m M for Et4NDS; 7 -- 2.5 - 4.5 m M for Pr4NDS m a y be confined to 0.23 % or 21 m M monomer units P V P in solutions, and Bu4NDS m a y be bound only 2 -- 1 = 1 raM. To summarize, in the common dilute region in the absence of the mieelles, where the salts are supposedly almost ionized, the amount of the saturated D S - binding is probably similar independently of N a + and the quaternary counterions, but near the cmc in the solvent phase it reaches to each different saturation value in the increasing order of binding as in the following counterion sequence N a + > Me4N + > Et4N + > Pr4N + > B u r n +. This feature is in contrast to the cases of NaOS and GuOS, in which the saturated binding of OS- is considered to be similar around each cmc (Fig. 3). Usually the bulkier R4N + counterions are less fixed to polyelectrolytes with no
particular hydrophobic moieties (33, 34). This m a y not be related to the present cases because the bulkier, more hydrophobie R4N + ions are actually more firmly fixed to the long-chain anions (18) and the charge on the polymer chain is derived from the bound anions. Also, the lower dissociation of the mieelles of the larger R4NDS (18) m a y not correspond to the drastic change in the ability of binding. I n a series of homologous surfaetants, binding to the polymers increases with lowering of the emc (35), but the lowering of cmc by changes of counterion from N a + or Me4N +, both weak structure makers, to Bu4N +, a strong hydrogen-bond p r o m o t e r (19), and from N a + to Gu +, a strong hydrogen-bond breaker, affects the ability to bind differently. Hence, the counterion effect on the polymer interactions of longchain ions is neither a m a t t e r of cmc difference nor of mere direct relations of the counterions to the polymers. In a mixture of the polymer and surfaetant, the mieelle formation and the complex formation with the polymer are competitive. I n the equilibria a change in the counterion modifies the solution states not only of the salt but also of the micelles and the complexes. I t is known t h a t in the R4NDS's the free energy of micelle formation is lowered with increase of size of the quaternary ions: for example, the difference is
Journal of Colloid and Interface Science, VoI. 37, No. 1. September 1971
164
SAITO, TANIGUCHI, AND KITAMURA
about 17 % less for Pr4NDS in reference to NaDS, and in 1VaDS and K D S it is on the same level (20). I t is therefore supposed that, in comparison with N a + or Me4N +, B u r n + may work preferably for stabilization of the micelles rather than for the binding of the surfactant anions to P V P and PVA, and by addition of more hydrophobic PVAAcI it m a y shift the equilibrium to the binding, and that on the contrary Gu + m a y favor the binding to P V P and PVA-Ac by pushing the hydrophobic anions to them. ACKNOWLEDGMENTS The gifts of pure sodium decylsulfate and octylsulfate by Dr. N. Ohba, Nihon Surfactant Co., and by Dr. H, Arai, Kao Soap Co., respectively, are gratefully acknowledged. 1%EFE1%ENCES 1. SAITO, S., AND YUKAWA,M., dr. Colloid Inter-
face Sei. 30,211 (1969). 2. SAITO, S., AND YU~AWA, M., Kolloid-Z. Z. Polym., 234, 1015 (1969). 3. SAITO, S., dr. Polymer Sci. Part A-1 8, 263 (1970). 4. SAITO, S:, AND KITAMUnA, K., J. Colloid Interface Sei., 85, 346 (1971). 5. SAITO,S., Kolloid-Z. 154, 19 (1957). 6. SAITO, S., J. Colloid Interface Sei. 24, 227 (1967). 7. SCHWUGER,M. J., AND LANGE, H., Proc. 5th Internatl. Congr. Surface Active Substances, Barcelona, 1968, p. 955. 8. GaAvsHoLT, S., Thesis, Technical Univ. of Denmark, Lyngby, 1969. 9. LEWIS, K. E., AND ROBINSON, C. P., J. Colloid
interface Sci., 32, 539 (1970). 10. HORIN, S., AND A~AI, H., J. Colloid Interface Sci. 32, 547 (1970). 11. TOKIWA, F., AND MORIYAMA, N., Yukagaku 19, 236 (1970). 12. SAITO, S., J. Polymer Sei. Part A-1 7, 1789 (1969). 13. SAITO,S., ANDOTSUKA,W., J. Colloid Interface Sci. 9.5,531 (1967). 14. HAMMES, G. G., AND SWANN, J. C., Biochemistry, 6, 1591 (1967).
15. ERLANDER, S. R., AND TOBIN, R., Makromol.
Chem. 107,204 (1967). 16. GRATZER,W. B., AND BEAVEN, G. I-I., J. Phys. Chem. 73, 2270 (1969). 17. WETLAUFER,D. B., MALIK, S. K., STOLLER, L., AND COFFIN, 1%. L., dr. Amer. Chem. Soc. 86,508 (1964). 18. MUKERJEE, P., AND MYSELS, ]4~. J., dr. Phys. Chem. 62, 1400 (1958). 19. WEN, W. Y., AND SAITO, S., dr. Phys. Chem. 68, 2639 (1964). 20. MUKERJEE, P., MYSELS, K. J., AND KAPAUAN,
P., J. Phys. Chem., 71, 4166 (1967). 21. MEGURO,K., AND KONDO, T., Nippon Kagaku Zasshi 80,813, 823 (1959). 22. HAFFNER,F. D., PICCIONE, G. A., AND 1%OSEN-
BLUM,C., J. Phys. Chem. 46,662 (1942). 23. I-IARNED, IX. S., AND OWEN, B. B., "The Physical Chemistry of Electrolytic Solutions," 3rd ed. 1%einhold, New York, 1958. (a) ibid., p 231; (b) ibid., p 178; (c) ibid., p 289. 24. EDSALL,J. T., AND WYMAN, J., "Biophysical Chemistry," Vol. I, p 469. Academic Press, New York, 1958. 25. BA•KIN, S. M., Ph.D. Thesis, The Brooklyn Institute of Technology, New York, 1957. 26. "Handbook of Chemistry," Vol. II. The Chemical Society of Japan, Ed., Maruzen, Tokyo, 1966; (a) ibid., p 710; (b) ibid., p 707. 27. SAITO,S., Kolloid-Z. Z. Polym. 215, 16 (1967). 28. JONES,M. N., J. Colloid Interface Sci. 28, 36 (1967). 29. SATA,N., AND SAITO, S., Kolloid-Z., 128, 154 (1952). 30. EDELI-IAUSER, I'i. A., J. Polym. Sci. Part C, 27, 291 (1969). 31. MUKEnJEE, P., MYSELS, K. J., AND DULIN, C. I., J. Phys. Chem., 62, 1390 (1958). 32. ROBINSOn,1%. A., AND STOKES, !~. H., "Electrolyte Solutions," 2nd ed., p 125. Butterworths, London, 1959. 33. ERLANDER, S. R., J. Macromol. Sei.-Chem., A2, 1195 (1968). 34. TAMAKI, K., OZ&KI, M., OGIWARA,M., AND TAKEMURA, T., Nippon Kagaku Zasshi 88, 711 (1967). 35. SAITO,S., Kolloid-Z. 158,120 (1958).
Journal of Colloid and Interface Science, Vol. 37, No. 1, SeDtember1971
Received November 5, 1970; accepted January 18, 1971 The interactions of anionic surfactants with nonionic polymers such as polyvinyl pyrrolidone (PVP), polyvinyl alcohol-acetate copolymers (PVA-Ac), and polyvinyl alcohol (PVA) in aqueous solution were studied with respect to guanidinium, four tetraalkylammoniums, sodium, and potassium as the counterions by means of electrical conductivity, dye solubilization, viscosity, and cloud point measurements. With sodium salts as the reference, guanidinium, a strongly water structure-breaking cation, lowers the critical micelle concentration (cmc) of the long-chain alkylsulfates, and in the presence of PVP and PVA-Ac it promotes the binding of the anions to them and enforces the attraction between the bound anions by ion-pairing, exhibiting such phenomena as shrinking of the polymer chains or depression of the cloud points. Similar phenomena have been observed and discussed previously in the interactions of these polymers and long-chain alkylammonium or symmetrical tetraalkylammonium ions with thioeyanate or iodide as counterions, both also strongly water structurebreaking anions. Sodium and potassium dodecylsulfates interact with PVP and polyvinyl acetate to a similar extent because both counterions are of weak effect on water. On the other hand, for the quaternary ions, which promote the water structure, as the counterions to dodecylsulfate, the cmc lowers with increase in size of the quaternaries but the polymer interactions weaken, and tetrabutylammonium dodecylsulfate in particular barely interacts with PVP and PVA in the presence of the micelles, and only weakly with PVA-Ac. Thus, in comparison with the sodium salts, only the strongly water structure-breaking counterions enhance the binding of the long-chain anions to the polymers. The role of the quaternary counterions has been discussed in terms of the effect on the equilibria between micelle formation and complex formation with the polymer. INTRODUCTION I n the system of nonionic polymers and surfactants in aqueous solution, a hydrophobic attraction arises between both cosolutes. W h e n the surfactants are ionic, the counterions are distributed around the surfactant ions bound to the polymers like polyelectrolytes, providing a locally concentrated complex domain. Preceding studies (1-4) showed that surfactant ions alone are not responsible for the interactions of ca~ tionic surfactants with nonionic polymers: the counterions classified as strongly water structure-breaking anions (thiocyanate >
iodide > nitrate) encourage interaction of long-chmn alkylammonium ions with some water-soluble polymers in comparison with chloride, a feebly water structure-breaking anion, whereas fluoride and acetate, both strongly water structure-making anions, are counterions that do not aid binding (4). With hydrophobic organic anions such as ethylsulfate and butyrate as the counterions, there is hardly an increase in the interaction of dodecylammonium with polyvinyl pyrrolidone (l). Such a counterion effect characteristic of the strongly water structure-breaking anions
Journal of Colloidand InterfaceScience,¥ol. 37, No. 1, September 1971
154
INTERACTIONS OF ANIONIC SURFACTANTS WITH POLYMERS has been interpreted by the assumption that the incompatibilities of the local aqueous environment surrounding the hydrophobic cations and polymers ~x4th these anions may induce attraction between them accompanied by iota-pairing (3, 4). These facts prompted us to study extensively the polymer interactions of anionic surfaetants with various eounterions in which a strong binding to the polymers of dodecylsulfate ion with sodium as the counterion has been well recognized (5-11). Recently it was shown that whereas monovalent inorganic cations are rather inessential in the salting action on nonionie polymers in comparison with their anionic counterparts (12), guanidinium and large symmetrical tetraalkylammonium cations have remarkable effects of a different charaeter: the former by brealdng the ordered water in dose proximity to the polymers (13-16), and the latter, especially tetrabutylammonium, by direct contact or binding to the polymers (4, 12, 13). From this point of view, the polymer interactions of long-chain alkylsulfate ions having guanidinium, tetraalkylammoniums, and potassium as the eounterions are compared in this paper with those of the sodium salts. Since guanidinium ion is a unique organic cation in that it disrupts the aqueous order intensely (14, 17), it was of particular interest to know whether guanidinium alkylsulfates too could exert an effect on the polymers similar to that of alkylammonium thioeyanates and iodides (1-4). EXPERIMENTAL
Materials. Sodium dodeeylsulfate (NaDS)
was a product (Texapon L 100) of the Henkel & Cie, Germany, and was recrystallized from ethanol. The pure sodium decylsulfate (NaDeS) and octylsulfate (NaOS) were supplied by the Nihon Surfactant Co., Tokyo, and the Kao Soap Co., Tokyo, respectively, and the sodium hexylsulfate (NariS) was prepared from n-hexylalcohol (E.P. grade of the Tokyo Kasei Kogyo Co., Tokyo) in this laboratory. Potassium dodecylsulfate (KDS), a highly pure product (Nikkol KLS) of the Nikko Chemicals Co., Tokyo, was reerystallized once from water.
155
Guanidinium dodecylsulfate (GuDS), decylsulfate (GuDeS), octylsulfate (GuOS), and hexylsulfate (GullS) were converted from the respective sodium salts by repeated reerystallizations from aqueous concentrated solutions of guanidinium sulfate (the Wako Pure Chemical Co., Osaka), and they were recrystallized several times from water. The melting point of GuDS was 144°-145°C. Anal. Caled for ClaHalNa04S: C, 47.97; H, 9.60; N, 12.91. Found: C, 47.59; H, 9.71; N, 12.85. The melting point of GuDeS was 144°-145°C. The melting point of GuOS was 135°-136°C. Anal. Caled for CgHlaNa04S: C, 40.13; H, 8.61; N, 15.61. Found: C, 40.32; H, 8.56; N, 15.89. The melting point of GuI-IS was 123°-124°C. Anal. Calcd for CTH19NaO4S: C, 34.84; H, 7.94; N, 17.41. Found: C, 35.17; H, 8.06; N, 17.29. Tetraalkylammonium dodecylsulfates (R4NDS) were made by a metathetical reaction of equivalent amounts of silver dodecylsulfate and tetraalkylammonium halides and dried in vacuo (18). Silver dodeeylsulfate was prepared from the sodium dodeeylsulfate and silver nitrate according to the method described (18). Silver nitrate was reagent grade of the Wako Pure Chemical Co. Tetramethylammonium and tetraethylammonium bromides were purchased from the Tokyo Kasei Kogyo Co., and tetra-n-propylammonium and tetra-n-butylammonium bromides were from the Eastman Organic Chemical Co., in the United States; they were reerystallized as described before (19). The critical micelle concentrations (emc) of these four R4NDS's at 30°C determined by the solubilization of Yellow OB, an oilsoluble dye, are in qualitative agreement with those determined by the conductivity method at 25°C (20), which are indicated in mM in parentheses, as follows. Tetramethylammonium dodecylsulfate (Me4NDS), 6.2 (5.52); tetraethylammonium dodecylsulfate (Et4NDS), 3.5 (3.85); tetrapropylammonium dodeeylsulfate (Pr4NDS), 2.5 (2.24); tetrabutylammonium dodecylsulfate (Bu4NDS), 1.1 at 25°C (1.3 (21)). As solubilization experiments for Bu4NDS were carried out at 12° =t= 1°C as described below, its cmc at 25°C was determined by surface tension measurements with a Du Nouy tensiometer:
Journal of Colloid and Interface Science, ¥oL 37, No. 1, September 1971
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SAITO, TANIGUCHI, AND KITAMURA
the surface tension at the cmc was 37 dynes/ cm.
Polyvinyl pyrrolidone (PVP), a product (Luviskol K90) of the Badische Anilin 4: Soda Fabrik, Germany, was precipitated twice from methanol-ether. Two kinds of polyvinyl alcohol-acetate copolymers (PVAAcI and PVA-AcII) with 30.0 % and 19.7 % acetate residues, respectively, and degrees of polymerization 2 0 0 0 both; polyvinyl alcohol (PVA), and polyvinyl acetate (PVAc), were the same as used previously (2; 4, 12, 13). The aqueous polymer solutions were treated by ion exchangers. Procedures. Electrical conductivity was measured under nitrogen gas atmosphere with a bridge equipped with a 1000 cps oscillator (Universal Bridge Type BV-Z 103A, Yokogawa Electric Works, Ltd., Japan) and a cell with blackened platinum electrodes at 25.00°C. The cell constant was 0.3164. The conductivity of the water used was 0.98 X 10-6 ohm-1, and that of 0.23 % PVP solution was 2.28 X 10-8 ohm-1, and they were subtracted from the conductivity of the surfactant solutions. Viscosity was measured with the same Ostwald viscometer used previously (12, 13). In a set of the measurements the reduced viscosity was calculated relative to the surfactant solution without considering the reduction of concentration due to binding to the polymers. This approach does not change the feature significantly as shown in Fig. 3. The solubilization of Yellow OB was done at 30°C in the same way as before (1, 5). In Bu4NDS, since the transparent solution turned opaque by the dye solubilization at 30°C, the experiment was made at 12° 4I°C. The cloud point of a PVA-AcI solution was determined as described before (12): RESULTS AND DISCUSSION 1. Solution Properties of Guanidinium Allcylsulfates. The equivalent electrical conductivity (h) and the limiting conductivity (A0)of GuOS and G u l l s are shown in Fig. 1 together with those of NaOS and Naris and also of NaDS (18). Although A0 of NaOS, 78.2, is lower than the literature value of 79 (22), those of NariS, 79.6, GuOS, 79.6, and GullS, 81.6, are nearly consistent: as A0 for Na + at 25°C is 50.1 (23(a)), A0 for OS- and HS- are, respectively, 28.1 and 29.5, and
those for Gu + obtained are 51.5 and 52.1 from GuOS and GullS, respectively. From the breaks of the A -- ~/M curves the cmc's for NaOS and GuOS are found to be 132 and 60 mM, respectively, at 25°C. The literature value of the cmc for NaOS is 0.13 M (22). The broken lines in Fig. 1 indicate the conductivity following the Onsager limiting equation for 1 - 1 electrolytes at 25°C (23(b)) A = A0 -- (0.229 A0 ~- 60.2) ~/M. Compared with the case of NaDS (18), the measured conductivities of these salts at dilute concentrations are in good agreement with the Onsager limiting slopes and deviate upward with increase of concentration more pronouncedly in N aOS than in NariS. NaDS, NaOS, and Naris are considered to be strong electrolytes, but they are supposed to be dimerized to some extent. The dimerization has been verified in several dodecylsulfate salts, and it also exists to decreasing degrees in salts of shorter alkyl chain lengths (18). The deviations from the Onsager slopes for both Gu + salts are rather small even around 10 mM. However, by assuming the same dimerization effect as the Na + salts to the respective Gu + salts and by subtracting the positive deviations in the respective Na + salts graphically from the curves for the Gu + salts, the corrected GuOS and GullS eonductivities show the negative deviations from the corresponding limiting slopes (Fig. 1), indicating that actually the Gu + salts may be slightly weak. The dissociation constants K for the Gu + salts are calculated from the corrected smoothed curves below 10 mM by the Shedlovsky equation (23(c)) 1/(AS,) = 1/ko q- (y2.S~MA)/(KAo2), whereS~ = 1 + z - g z2/2, z = (0.229A0-160.2) (MA)II2/A~/2, and --log y± = 0.509(S..MA/A0) 1/2. The plots are shown in Fig. 2, and h0's in Fig. 1 were taken as the best fits for the linear relation at dilute region. But, in view of the possible experimental errors of 0.2 in A0 and h in drawing the corrected curves, values of K obtained are 0.25-0.29 and
Journal of Colloid and Interface Science, Vol. 37, No. 1, September 1971
INTERACTIONS OF ANIONIC SURFACTANTS WITH POLYMERS i
.
.
.
.
.
.
.
157
.
80
70
50 U
i
.
Z
x x "'-%
.
e
o~
50
60[
10
20
100
30
40
40
FIo. 1. E q u i v a l e n t c o n d u c t i v i t y A of solutions of NaOS, GuOS, NariS, and G u l l s in the absence and presence of 0.23% P V P p l o t t e d as a function of square root of molarity M at 25°C. The broken lines are the Onsager slopes. The corrected curves for GuOS and GullS (dotted lines), see text. The curves for NaDS were taken from reference 18.
//9 130 GuOS ~ /
j~"
~125
12(
0.4
0.8
1.2
y2SzMA
FIG. 2. The Shedlovsky plots for the calculation of the dissociation constants for GuOS and GullS solutions at 25°C. The plots were obtained from the corrected s m o o t h e d curves in Fig. 1 at 0, 2, 4, 6, 8, 10, 12, and 14 for 100x/M from left to right.
0.27-0.32 for GuOS and GullS, respectively, at 25°C, and the difference between them is rather ambiguous. The weakness of GuOS, its lower cmc and steeper conductivity slope above the cmc than those for NaOS, might arise from the ion-pairing due to the high polarizability which Gu + may have as this ion takes coplanar, resonance structures (24). However, the ion-pairing might arise supposedly from
a special, non-Bjerrum type mechanism due to the disturbances between the different vieinal aqueous media around the water structure-breaking cation and the hydrogenbond-promoting hydropbobic anion. A further study on the solutions of the other homologs would be of interest. 2. The Polymer Interactions of Guanidinum and Potassium Alkyl-sulfates. When a nonionic polymer is dissolved in a surfactant salt solution, some of the ions are confined to the polymer. In this case two processes are at work: binding or complexation of the long-chain ions to the polymer giving it a polyeleetrolytic nature, and counterion fixing or concentrating to the complexes. By the binding of ions to the polymers the conductance is decreased and the cmc break in the conductivity-concentration relation is blurred (25). By addition of 0.23 % PVP to NaOS and GuOS solutions (Fig. 1), the conductivity begins to decrease from that of the respective salt solutions far below the emc, suggesting that less conductive complexes are formed from this point on, and in line with the trend of the salt solutions, the downward deviations of the conductivity curve for the PVP-GuOS
Journal of Colloid and interface Science, Vol. 37, No 1, September 1971
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SAITO, TANIGUCHI, AND KITAMURA
system take place at lower concentrations and the slope becomes steeper than that for the PVP-NaOS system. Thus, at the initial stage of the OS- binding the Gu + ion may make OS- more readily accessible to PVP than does Na +, and simultaneously the Gu + ion itself may be more tightly fixed to the bound OS- ion than to the free one, and this tendency of Gu + is more marked than that of the Na + ion. ]~eyond the cmc, the conductivity curves of the salts without and with PVP come close to each other and coincide eventually at high concentrations. This may suggest that with increase of binding the complex ions behave similarly to the ordinary micelles. As for OS-, the maximum binding may be similar in both salts, and this may be confirmed by the dye solubilization experiments. The formation of the polymer-surfactant complexes is usually associated with a different solubilization power from that of the surfaetant micelles alone depending upon the relations of the polymer and solubilizate (5, 6). This is illustrated in Fig. 3 in the systems of 0.23 % PVP-NaOS and - G u t s with Yellow OB as the solubilizate. The solubilization of the dye in the polymer solution starts at a lower surfaetant concentration than the cmc, and this point agrees with the point of deviation in the conductivity curve in Fig. 1. The dye solubilization in the polymer solution enhances more markedly with increase of the surfaetant concentration than that of the surfaetant solutions alone, and above a certain concentration the curves 10
i4 0,05
0,10 OCTYLSULFATE
M
0.15
0.20
FIG. 3. S o l u b i l i z a t i o n of Y e l l o w O B in s o l u t i o n s of N a O S a n d G u t S in t h e a b s e n c e a n d p r e s e n c e of 0.23% P V P a t 30°C. T h e a r r o w m a r k s t h e s u r f a c t a n t c o n c e n t r a t i o n S at t h e s a t u r a t i o n s o l u b i l i z a t i o n to t h e p o l y m e r c o m p l e x e s ; see t e x t .
4
PVP-KDS
o
3
O
,""
/'
P~'P-GuDS
PVA-NaDS
~2
1
PVA-GuDS
0
]C) 2'0 DODECYLSULFATE rnM
30
Fit. 4. Reduced viscosity ratio of 0.23% PVP and 0.16% PVA solutions in the presence of NaDS, KDS, and GuDS to the respective aqueous polymer solution plotted as a function of the surfaerant concentration at 42.5°C. The solid and broken lines represent the reduced viscosity ratio for PVP-GuDS system on the basis of the surfactant solutions and pure water, respectively, as the solvents; see text.
run straight and parallel to those of the respective surfaetant solutions without the polymer (Fig. 3), implying that at this concentration S the binding of OS- to the polymer nearly reaches saturation. It is considered that up to S the complexes are the primary solubilizing loci, and above S the ordinary mieelles join in the solubilization (5). Since OS- ion is common to both surfactants, the magnitudes of the dye solubilized in the complexes alone below S should be related, if not proportional, to the degree of occupancy on the polymer chains by OS- ions. The amount of the dye solubilization in the PVP-GuOS complexes at S seems to be similar to that in the PVP-I~aOS complexes despite the great difference in the cmc (the emc's for NaOS and Guts at 30°C determined by the dye solubilization method are 135 and 61 mM, respectively, as seen in Fig. 3), which may mean that the saturation amount of OS- binding is nearly the same in each case and that the eounterion effect may be related solely to the shift of the cme. However, the viscosity measurements
Journal of Colloid and Interface Science, Vol. 37, No. 1, September 1971
INTERACTIONS OF ANIONIC SURFACTANTS WITH POLYMERS revealed another aspect of the polymer interactions. The effect of addition of GuDS on the reduced viseosity of the 0.23 % PVP solution is indieated in Fig. 4, together with the effect of NaDS and KDS. As a result of the binding, the shape of the flexible polymer in solution undergoes changes which do not appear when the substrate is solid (5). The binding oeeurs mainly above the eme in mieellar form, but PVP binds NaDS sparsely on its chain below the eme (5), and this seems to hold also for KDS and GuDS. The Krafft points of KDS and GuDS are, respectively, 35°C (26(a)) and about 40°C, so the viscosity was measured at 42.5°C, in some cases in the supersaturated state: the eme and the solubility of GuDS at this temperature are about 5 and 8 raM, respectively. It is known that the solubility of a surfaetant is increased by the presence of polymers which bind the surfaetant (27). B y the way, the eme's for NaDS and KDS at 40°C are 8.9 and 7.8 mM, respeetively (26 (b)). Although the peak height in the reduced viscosity ratio-concentration curve for GuDS is much lower than for NaDS and KDS, the location of each peak for these surfaerants is at nearly the same concentrations, suggesting that in this ease an alteration in the eounterion fixing rather than in the D S binding may be the primary cause in the PVP-GuDS interaction. It is seen that, as in the usual salt effect on nonionic polymers in aqueous solution (12), Na +, a weak water structure-maker, and K +, a very weak water structure-breaker, as the eounterions to D S do not give any significant change. E u t Gu + ion is distinguished in its effect on proteins or some hydrophobie polymers by changing the water into a better solvent for them at high concentrations (13-16). The viscosity curve for the PVP-GuDS in Fig. 4 shows an abrupt change at 6 m M compared with those for NaDS and KDS, and it is likely that above this transition concentration the Gu + ions fix more firmly to D S ions directly bound to PVP, thereby producing a more compact polymer complex than the corresponding complexes of NaDS and KI)S. As shown later in Fig. 7, it is to be noted that Me4NDS, Et4NDS, and
159
GuDS, which have close cme's at the measured temperatures, give quite different, reduced viseosity patterns. Peeause of the poor solubility of GuDeS at room temperature, the viscosity of the 0.23% PVP-NaDeS and -GuDeS systems was measm'ed at 35°C, and a very similar viscosity trend to that of NaDS and GuDS was observed with a transition concentration for GuDeS at 26 mM, but the reduced viscosity increase by the DeS salts was Iess marked than that by the DS salts. The counterion effect of Gu + appears strikingly in the PVP-OS- system as an upward concave in the reduced viscosityconcentration curve at about 65 mM, falling below the value in aqueous solution, whereas NaOS raises the reduced viscosity weakly but smoothly (Fig. 5). The featm'e of the coneave viscosity eurve does not change much even if water is, instead of the surfactant solutions, supposed to the solvent in the other extreme ease (dotted line). The real reduced viscosity values are somewhere between both lines but probably closer to the solid line because the amount 2.0
O,.5
PVA-Acn-GuOS
PVA-AclI-Na O S ~ ,/@ /
,'"
PVP-GuOS
~1.0 u ~o.5
5'0OCTYLSULFATE ,;0 mM
~;o
FIo. 5. R e d u c e d v i s c o s i t y ratio of 0.23% P V P ,
0.16% PVA, and of 0.15% PVA-AeII solutions in the presence of NaOS and GuOS to the respective aqueous polymer solution plotted as a function of the surfaetant concentration at 25°C. The solid and broken lines represent the reduced viscosity ratio for the PVP-GuOS system on the basis of the surfactant solutions and pure water, respectively, as the solvents; see text.
Journal of Colloid and Interface Science, Vol. 37, No. 1, September 1971
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SAITO, TANIGUCHI, AND KITAMURA
by addition of the shorter aikyl-chain anions than is PVP (12). Figure 6 exhibits the variation of the cloud point of a PVAAcI solution by addition of NaOS, NariS, GuOS, and GullS, and of guanidinium chloride (GuC1) and NaC1 for the sake of comparison. Despite the lowering of the cmc by substituting Gu + for Na + as the counterions, the cloud point-raising action of OS- is markedly offset in a certain dilute region and the rise resumes at a higher concentration. With CI- as the eounterion, 0 0.] 0.2 SALT M Gu + has a salting-in effect (13) but Na + FIG. 6. Cloud point of 0.21% PVA-AcI in and K + exert a similar saiting-out effect solutions of NaOS, GuOS, NariS, GullS, NaC1, on the polymer solution, although only and GuC1plotted as a function of the salt concen- weakly in the dilute region (12). Such behavior of the Gu + long-chain tration. anion salts in comparison with the Na + of binding here seems to be small (Figs. 4 or K + salts may be accounted for ad hoc by postulating that by ion-pairing Gu + and 5). promotes the binding of the long-chain In contrast to PVP, the reduced viscosity anion to the polymer and the mutual of a PVA solution is increased only slightly by GuDS whereas it is increased remarkably attraction between the hydrophobic anions by NaDS (Fig. 4), and the changes by directly bound to the polymer, rendering NaOS and GuOS are both quite small and the polymer more contracted and thus no concavity appears in the PVA-GuOS leading to the relative setback in the rise curve (Fig. 5). The great differences in the of the reduced viscosity or of the cloud peak position and height in the viscosity point and that with increasing total salt curves for NaDS and GuDS show that the concentration and with approaching to the amount of DS- binding to PVA is seriously micelle-wise saturation binding around the dependent on the counterions. This seems to cmc in the solvent phase, the counterion be related to the fact that, as the presence Gu + tends to be tess fixed followed by a of a number of - - O H side groups imparts rise in the reduced viscosity (PVP, PVAa strongly hydrophilic nature to the hy- AcII; Fig. 5) or in the cloud point (PVAdrophobic main chain, PVA binds the AcI; Fig. 6). In summing up, for raising the reduced surfactant anions less than does PVP and only above the cmc (5, 11), where GuDS viscosity or the cloud point of the polymer is less ionized, and therefore the PVA coils solutions (salting-in) : Gu + (salt-in) > Na + = K + (sait-out) can not develop their inner sites available for CI-: change in the solvent structure. for further binding. Na + = K + > Gu + for long-chain alkylFigure 5 demonstrates the reduced viscosity diagram of the PVA-AcII solution sulfate anions at dilute region: counterion with NaOS and GuOS additions. As in the fixing effect to the bound anions. As stated in the "previous paper (4), the PVP-GuOS system, the reduced viscosity cationic sequence for the C1- salts is concurve for this polymer solution on addition sidered as regular, so we can take that for of GuOS shows an upward concavity which the alkylsulfate ions as irregular. This is not observed on NaOS addition. A similar irregular influence of Gu + as the counterions viscosity behavior was also found in the solution of a polyvinyl pyrrolidone-acetate to long-chain anions may be what has been expected from the role of SCN- and I copolymer and GuOS. in the polymer interactions of tetrabutylPVA-AcI solution, which becomes cloudy ammonium and long-chain alkylammonlum by warming, is more susceptible to change Journal of Colloid and Interface Science, Vol. 37, No. 1, September 1971
INTERACTIONS OF ANIONIC SURFACTANTS WITI POLYMERS cations ( 1 4 ) , perhaps because Gu +, SCN-, and I - are all strongly water structurebreaking ions. Since the effect of the Gu + ion itself as the counterions to simple inorganic anions on the PVA-AcI is weaker than that of S C N - or I - in dilute solution (12, 13), the amount of saturated OSbinding seems to have little influence by change of the counterion from Na + to Gu + (Fig. 3). However, the effect of Gu + m " v corroborate the previously postv' =. . . . . anism of the interaction based on the incompatibilities of the local order of the solvent structure surrounding the strongly water structure-promoting hydrophobie ions and polymers with the structure-breaking eounterions (3, 4). A further study is needed to clarify whether such a hypothesis works or not. Since D S - and D e S - ions are bound to PVP more appreciably than are OS- ions, the counterion fixing of Gu + is reflected just as a transition in the reduced viscosity curve about the cmc (4.5 m M for GuDS, Fig. 4), which in the case of the PVP-GuOS may be attributable to the viscosity curve concavity. The behavior of GuOS and G u l l S in the PVA-AcI and PVA-AcII solutions (Figs. 5 and 6) may be explained in the same manner: as the cloud point-raising effect of OS- ions is still strong (12), the cloud point curve may run flat in the dilute range of GuOS solution instead of caving in. Though the effect of H S - ions on the cloud point of the PVA-AcI solution is weaker than that of OS- ions, the attraction between the bound GuHS's is not strong enough to cause the cloud point to fall below that of the aqueous solution. NaDS has been reported to cause a transition in the bound state on some polymer chains owing to mutual attraction between DS-'s (28), but in case of the Gu + salts the transition occurs as a result of the eounterion fixing. In line with the trend in the water-soluble polymers, the tightly coiled, aggregated polyvinyl acetate chains in water are disintegrated, unfolded, and dissolved as well by I4DS as by NaDS (29 30 10), but very poorly by GuDS.
3. The Polymer Interactions of Tetraalkylammonium Dodecylsulfates. The change
161
I-
PVP-Me4NDS O
O -~27
PVP~Pr4NDS
'PVA-Bu4NDS
0
rPVA-Pr4NDS
PVP-Bu4NDS
10 20 DODECYLSULFATE mM
30
FIG. 7. Reduced viscosity ratio of 0.23% PVP and of 0.16% PVA solutions in the presence of NaDS and four R4NDS's to the respective aqueous polymer solution plotted as a function of the surfactant concentration at 25°C. in the reduced viscosity ratio of the P V P and PVA solutions in the presence of NaDS and R4NDS's is shown in Fig. 7. The reduced viscosity of the PVP solution is increased instantly on addition of NaDS, Me4NDS and Et4NDS, but it was observed t h a t the reduced viscosity increment was lowered slowly with elapse of time (a small percentage in two weeks at the peak of the viscosity curves) and the change was not recovered by warming. In Fig. 7 the reduced viscosity curves measured immediately upon mixing are shown. In some cases the rate of decrease was irregular. Such a gradual decrease might be caused either by rearrangement of the salts in the bound state, perhaps in the eounterion fixing, or b y change in crosslinking of the polymers by the surfactants, but not by decomposition of the surfaetants because the phenomenon was reproducible by the same surfactant solutions stocked for several months. Actually in the dye solubilization experiments, a preliminary run in a PVP-Me4NDS system showed that the amount of Yellow OB solubilized was unchanged for at least one month. It is noted that the location of the peak in the reduced viscosity curves shifts to the lower concentrations from NaDS to the
Journal of Colloid and Interface Science, Vol. 37, No. i, September 1971
162
SAITO, TANIGUCHI, AND KITAMURA
t~4NDS's with increasing cation size, in contrast to the addition of GuDS and KDS as exhibited in Fig. 4, and that the peak height also lowers in the same order, and the addition of ]~u4NDS in particular NaDS scarcely alters the reduced viscosity. Thus, I the order of the peak height is as follows: NaDS > Me4NDS > Et4NDS > 3 Pr4NDS > ]~u4NDS = no addition. > From the viscosity result only it is supposed P~NDS that either the binding of DS- decreases or g the counterion fixing increases with increase of the hydrophobieity of the cation, or both. t~elow the emc, R4NDS is un-ionized to some extent, whereas NaDS is completely dissociated, though DS- is somewhat dimerized (31). The dissociation constants of these surfactants in the highly dilute region are 0.080 for Me4NDS; 0.054 for 1'o 1'~ 20 Et4NDS; and 0.048 for Pr4NDS at 25°C DODECYLSULFATE mM (18). The increasing weakness of the surFIG. 8. Reduced viscosity of 0.21% PVA-AcI factant salts with the larger quaternary counterions arises from the increasing at- solution in the presence of NaDS, Pr4NDS, and traction between the hydrophobic cation Bu4NDS plotted as a function nf t.he surfactant concentration at 25°C. and anion. This situation is also true in the micelles (20); the eme decreases with increasing size supported by the dye solubilization experiof the quaternaries, and the degree of ments as shown below. On the other hand, ionization of the micelles, which greatly being more hydrophobie than PVP and PVA depends on the method of measurement, is as a whole, PVA-AcI is able to bind Pr4NDS estimated for each by electrokineties as and even Bu4NDS more markedly, and the follows: NaDS, 27.5%; Me4NDS, 25.9%; reduced viscosity of the solutions is raised Et4NDS, 23.9 %; Pr4NDS, 20.5 %; and for by them, but not so greatly as by NaDS ]~u4NDS it may be about 18 % by extrapo- (Fig. 8). In the Yellow OB solubilization in Fig. 9, lation to the Stokes law hydrated radius o 4.7 A for Bu4N + (32) in Fig. 2 of reference 20. the curves for PVP-NaDS, PVP-Me4NDS, However, at the same concentration and PVP-Et~NDS below 6 raM, and also below each cmc, the difference in the actual for PVP-Pr4NDS in very dilute region are degree of ionization between, for instance, similar to each other, despite the large Me4NDS and Pr~NDS calculated from their differences in the eme and in the solubilizing dissociation constants is rather insignificant, power of the surfactants themselves. As for say less than 5 %; and both salts are almost Bu4NDS, from the very small difference fully dissociated. In fact, the viscosity curves in the solubilization curves in the absence of the PVP solutions for NaDS, Me4NDS, and presence of PVP an inference may be Et~NDS, and Pr4NDS in the dilute region drawn that the DS- ion with this large below each cme overlap independently of hydrophobic cation, which is supposed to the eounterions, and the differences appear be attracted to the hydrophobic part of only above each cmc. From the flat reduced the polymers (12, 13), is bound to PVP viscosity lines for the PVP- and PVA- to quite a lesser extent than those with the Bu4NDS systems shown in Fig. 7, it is smaller ones and Na +. Consequently the plausible that this particular surfactant polymer has no intrinsic binding sites for with a very low cmc is barely bound to the long-chain anions, and the binding these polymers. This reasoning may be may be considered as a kind of eomicelliza-
it
Journal of Colloid and I~terface Science, ¥ o l . 37, N o . I, S e p t e m b e r 1971
'
'
'
INTERACTIONS OF ANIONIC SURFACTANTS WITH POLYMERS 20[
163
~ PVP-Me4NDS
16j-
s
~
~
, Pw-Noos
12
N
PVP-
Pr N D S
EI4NDS 4
Me4NDS
0
4
8
12 DODFCYLSULFATE
16 mM
20
24
FIG. 9. Solubilization of Yellow OB in solutions of NaDS and four R4NDS's in the absence and presence of 0.23% PVP at 30°C, except for the Bu4NDS system at 12° 4- I°C. The arrow marks the surfaetant concentration S at the saturation solubilization to the polymer complexes; see text. tion of the long-chain ions with the hydrophobic parts of the polymer. F r o m the binding d a t a b y the equilibrium dialysis technique on the P V P - N a D S system, the difference between the concentration S and the respective cmc m a y be a very rough assess of the m a x i m u m surfactant binding to the polymer chain (5). Thus, b y assuming the same principle for the other P V P - D S - systems, approximately 18 -- 7 = 11 m M for N a D S ; 12 -6 = 6 m M for Me4NDS; 9 -- 3.5 ~- 5.5 m M for Et4NDS; 7 -- 2.5 - 4.5 m M for Pr4NDS m a y be confined to 0.23 % or 21 m M monomer units P V P in solutions, and Bu4NDS m a y be bound only 2 -- 1 = 1 raM. To summarize, in the common dilute region in the absence of the mieelles, where the salts are supposedly almost ionized, the amount of the saturated D S - binding is probably similar independently of N a + and the quaternary counterions, but near the cmc in the solvent phase it reaches to each different saturation value in the increasing order of binding as in the following counterion sequence N a + > Me4N + > Et4N + > Pr4N + > B u r n +. This feature is in contrast to the cases of NaOS and GuOS, in which the saturated binding of OS- is considered to be similar around each cmc (Fig. 3). Usually the bulkier R4N + counterions are less fixed to polyelectrolytes with no
particular hydrophobic moieties (33, 34). This m a y not be related to the present cases because the bulkier, more hydrophobie R4N + ions are actually more firmly fixed to the long-chain anions (18) and the charge on the polymer chain is derived from the bound anions. Also, the lower dissociation of the mieelles of the larger R4NDS (18) m a y not correspond to the drastic change in the ability of binding. I n a series of homologous surfaetants, binding to the polymers increases with lowering of the emc (35), but the lowering of cmc by changes of counterion from N a + or Me4N +, both weak structure makers, to Bu4N +, a strong hydrogen-bond p r o m o t e r (19), and from N a + to Gu +, a strong hydrogen-bond breaker, affects the ability to bind differently. Hence, the counterion effect on the polymer interactions of longchain ions is neither a m a t t e r of cmc difference nor of mere direct relations of the counterions to the polymers. In a mixture of the polymer and surfaetant, the mieelle formation and the complex formation with the polymer are competitive. I n the equilibria a change in the counterion modifies the solution states not only of the salt but also of the micelles and the complexes. I t is known t h a t in the R4NDS's the free energy of micelle formation is lowered with increase of size of the quaternary ions: for example, the difference is
Journal of Colloid and Interface Science, VoI. 37, No. 1. September 1971
164
SAITO, TANIGUCHI, AND KITAMURA
about 17 % less for Pr4NDS in reference to NaDS, and in 1VaDS and K D S it is on the same level (20). I t is therefore supposed that, in comparison with N a + or Me4N +, B u r n + may work preferably for stabilization of the micelles rather than for the binding of the surfactant anions to P V P and PVA, and by addition of more hydrophobic PVAAcI it m a y shift the equilibrium to the binding, and that on the contrary Gu + m a y favor the binding to P V P and PVA-Ac by pushing the hydrophobic anions to them. ACKNOWLEDGMENTS The gifts of pure sodium decylsulfate and octylsulfate by Dr. N. Ohba, Nihon Surfactant Co., and by Dr. H, Arai, Kao Soap Co., respectively, are gratefully acknowledged. 1%EFE1%ENCES 1. SAITO, S., AND YUKAWA,M., dr. Colloid Inter-
face Sei. 30,211 (1969). 2. SAITO, S., AND YU~AWA, M., Kolloid-Z. Z. Polym., 234, 1015 (1969). 3. SAITO, S., dr. Polymer Sci. Part A-1 8, 263 (1970). 4. SAITO, S:, AND KITAMUnA, K., J. Colloid Interface Sei., 85, 346 (1971). 5. SAITO,S., Kolloid-Z. 154, 19 (1957). 6. SAITO, S., J. Colloid Interface Sei. 24, 227 (1967). 7. SCHWUGER,M. J., AND LANGE, H., Proc. 5th Internatl. Congr. Surface Active Substances, Barcelona, 1968, p. 955. 8. GaAvsHoLT, S., Thesis, Technical Univ. of Denmark, Lyngby, 1969. 9. LEWIS, K. E., AND ROBINSON, C. P., J. Colloid
interface Sci., 32, 539 (1970). 10. HORIN, S., AND A~AI, H., J. Colloid Interface Sci. 32, 547 (1970). 11. TOKIWA, F., AND MORIYAMA, N., Yukagaku 19, 236 (1970). 12. SAITO, S., J. Polymer Sei. Part A-1 7, 1789 (1969). 13. SAITO,S., ANDOTSUKA,W., J. Colloid Interface Sci. 9.5,531 (1967). 14. HAMMES, G. G., AND SWANN, J. C., Biochemistry, 6, 1591 (1967).
15. ERLANDER, S. R., AND TOBIN, R., Makromol.
Chem. 107,204 (1967). 16. GRATZER,W. B., AND BEAVEN, G. I-I., J. Phys. Chem. 73, 2270 (1969). 17. WETLAUFER,D. B., MALIK, S. K., STOLLER, L., AND COFFIN, 1%. L., dr. Amer. Chem. Soc. 86,508 (1964). 18. MUKERJEE, P., AND MYSELS, ]4~. J., dr. Phys. Chem. 62, 1400 (1958). 19. WEN, W. Y., AND SAITO, S., dr. Phys. Chem. 68, 2639 (1964). 20. MUKERJEE, P., MYSELS, K. J., AND KAPAUAN,
P., J. Phys. Chem., 71, 4166 (1967). 21. MEGURO,K., AND KONDO, T., Nippon Kagaku Zasshi 80,813, 823 (1959). 22. HAFFNER,F. D., PICCIONE, G. A., AND 1%OSEN-
BLUM,C., J. Phys. Chem. 46,662 (1942). 23. I-IARNED, IX. S., AND OWEN, B. B., "The Physical Chemistry of Electrolytic Solutions," 3rd ed. 1%einhold, New York, 1958. (a) ibid., p 231; (b) ibid., p 178; (c) ibid., p 289. 24. EDSALL,J. T., AND WYMAN, J., "Biophysical Chemistry," Vol. I, p 469. Academic Press, New York, 1958. 25. BA•KIN, S. M., Ph.D. Thesis, The Brooklyn Institute of Technology, New York, 1957. 26. "Handbook of Chemistry," Vol. II. The Chemical Society of Japan, Ed., Maruzen, Tokyo, 1966; (a) ibid., p 710; (b) ibid., p 707. 27. SAITO,S., Kolloid-Z. Z. Polym. 215, 16 (1967). 28. JONES,M. N., J. Colloid Interface Sci. 28, 36 (1967). 29. SATA,N., AND SAITO, S., Kolloid-Z., 128, 154 (1952). 30. EDELI-IAUSER, I'i. A., J. Polym. Sci. Part C, 27, 291 (1969). 31. MUKEnJEE, P., MYSELS, K. J., AND DULIN, C. I., J. Phys. Chem., 62, 1390 (1958). 32. ROBINSOn,1%. A., AND STOKES, !~. H., "Electrolyte Solutions," 2nd ed., p 125. Butterworths, London, 1959. 33. ERLANDER, S. R., J. Macromol. Sei.-Chem., A2, 1195 (1968). 34. TAMAKI, K., OZ&KI, M., OGIWARA,M., AND TAKEMURA, T., Nippon Kagaku Zasshi 88, 711 (1967). 35. SAITO,S., Kolloid-Z. 158,120 (1958).
Journal of Colloid and Interface Science, Vol. 37, No. 1, SeDtember1971