Langmuir 1992,8, 1499-1500
1499
Notes Interaction of Cationic Surfactants with a Hydrophobically Modified Cationic Cellulose Polymer E. D. Goddard‘ and P. S. Leung Union Carbide Chemicals and Plastics Company, Znc., Specialty Chemicals Division, Tarrytown, New York 10591 Received November 5, 1991. Zn Final Form: February 13, 1992
Introduction The interaction often observed between polymers and surfactants in aqueous solution is recognized as being the result of one or both of two main driving forces. The first is an electrostatic attraction, usually reinforced by hydrophobic forces involved in the self-association of the bound surfactant ions. The second is a force involving an interaction between hydrophobic groups on the polymer and those of surfactant molecules in their incipient aggregation process.’ Illustrations of the former are the strong bonding, and also the precipitationreactions, observedwhen the polymer (in this case a polyion) bears a charge opposite to that of the surfactant ion; reduction in interaction affinity is observed when neutral salt is added to the system. When the polyion has a charge of the same sign as the surfactant, binding is generally not encountered. See below, however. There is a large number of illustrations of the second category,generally involving uncharged polymers reacting with anionic surfactants. In this respect it has been recognized that water-soluble polymers can be arranged in a reactivity series which appears linked to the extent of their “hydrophobic” character.’ In our laboratory it was recognized, furthermore, that a simple indicator of ”reactivity” (hydrophobicity) is the surface tension of an aqueous solution of the polymer by itself. Thus, the relatively inert polymer hydroxyethyl cellulose (HEC) is surface inactive, while the much more reactive polymer, methyl cellulose, has appreciable surface activity; a corresponding pair is polyacrylamide and N-alkyl (e.g. N methyl) polyacrylamide, respectively. A related, but readily distinguishable class of water-solublepolymers in which there is currently a great deal of interest is given the name “hydrophobic water soluble polymer” or “associative thickener”. These contain a small concentration ! by weight) of an alkyl group, in the range of chain (ca. 1% length from about 8 to 18 carbon atoms. These groups tend to self-associate in solution but also to attract surfactants by supplying “nuclei” for their aggregation. A question arises as to how a hydrophobic polymer of the above type that is a polyion will interact with ionic surfactants. Using a dodecyl-substitutedcationiccellulosic polymer (see below), we confirmed that precipitationlresolubilization reactions occur with a series of anionic surfactants. This work will be reported elsewhere. An unexpected result was that precipitation reactions were also observed with certain added cationicSurfactants.This latter finding forms the basis of this preliminary report. (1) Goddard,E. D. Colloids Surf. 1986, 19, 255 and 301.
0743-7463/92/2408-1499$03.00/0
Experimental Section Precipitation studieswere carried out by mixing equal volumes of polymer solution (usually 2% by weight) and surfactant (or salt) solution. Initial assessment was made after a few minutes and final assessment after at least one week’s standing. Surface tension measurements by the Wilhelmy plate technique were made shortly after mixing on freshly formed surfaces cleared of any foam bubbles by aspirating. To avoid wetting problems, a paper plate was employed as sensor. Materials. QuatrisoftLM200 (UnionCarbideChemicalsand Plastics Co., Inc.) is the chloride salt of a dodecyldimethyl(2hydroxypropylene-3-oxy)a”onium derivative*of hydroxyethyl cellulose of molecularweight ca. 100 OOO. Hexadecyl-,tetradecyl-, and dodecyltrimethylammonium bromides (c16Ti% ClrTAB, and C12TAB) were from Aldrich, and CloTAB was from Kodak. Tetramethyl-and tetraethylammonium bromides were from Aldrich.
Results and Discussion Mixing solutions of polymer and and CuTAB led to no visible change in appearance of the solutions. On the other hand, within a small, finite range of C12TAB, haziness of the mixed solutions was observed. A much broader concentration range of incompatibility was found in mistures of CloTAB and the polymer, but interestingly a region of “resolubilization”at high concentration (ca. 0.1 M) of the surfactant was again observed. See Figure 1. The above behavior, at least at the low end of the incompatibility range, invites speculation that a “salting out” phenomenon is involved. To test this, the experiments were repeated with two quaternary ammoniumsalta, viz., tetramethyl- and tetraethylammonium bromide, replacingthe surfactant salt. In both cases incompatibility with the polymer solution occurred at a concentration of added salt around 0.02 M, comparable to the initial concentrationof incompatibilityfor CloTAB. By contrast, however, no resolubilizationzone was found for these two salts. In a comparison, the “conventional”salt, sodium chloride,showedqualitativelysimilar behavior to the above two “structure making” salts, but led to a higher initial concentration of incompatibility, but a comparable “flocculation” concentration, signifying separation into two layers. It is noted that in the CloTAB case the entire incompatibility range involves a separation into two layers-a deep clear layer overlying a shallow hazy layer in which, presumably, the polymer is concentrated. The mechanism of resolubilization in this case evidently involves the building of a sufficient concentration of CloTAB micelles to associate with the alkyl groups of the polymer and enhance its solubility. This effect is similar to the solubilization of an insoluble alkyl-modified cellulosic polymer at a sufficiently high concentration of added SDS, as reported by Dualeh and Steiner.3 We note, in Figure 1,that the resolubilizationconcentration for the CloTAB system exceeds the nominal critical micelle concentration (CMC) of CloTAB by a factor of about 2. The Cl2TAB system evidently represents a borderline case in which the ionic strength built up by the monomeric surfactant salt is just sufficient to initiate phase separation. At a slightly higher concentration, micelles (2) Brode, G. L.; Goddard, E. D.;Harris, W. C.; Salensky, G . A. Polym. Mater. Sci. Eng. 1990, 63, 696. ( 3 ) Dualeh, A. J.; Shiner, C. A. Macromolecules 1990, 23, 251.
0 1992 American Chemical Society
Notes
1500 Langmuir, Vol. 8,No. 5, 1992
I
0 CLEAR “P HAZY -TURBID PPT. nTRANSLUCENT PPT.
+CMC
C,,TAB
CMC C,,TAB
NaCl 10’’
10‘
1 0’
MOLARITY
Figure 1. ‘Phase diagrams” of 1 % solutions of the polymer in the presence of varying concentrationsof added surfactantsand Salts.
form and resolubilization takes place. Notice that the resolubilization concentration and the nominal cmc are comparable in this case. An interesting qualitative difference in this case is that phase separation leads (eventually) to two clear layers, i.e. the phenomenon is akin to aqueous “biphase”formation observed with certain combinations of two polymers in aqueous s ~ l u t i o n . ~ Surface tension measurements were carried out on all the above mixtures, i.e. those containing the polymers and varying concentrations of the surfactants. Although the ! solution polymer by itself is quite surface active (its 1% has a surface tension of ca. 60 mN/M) and detailed interpretation of the surface tension data is difficult, we report the following differences. For Clo- and &-TAB a (4) Alberteson, P. A. Partition of Cells and Macromolecules; Wiley-Interscience: New York, 1971.
Figure 2. Surface tension curves for ClrTAE3and CloTAB in the presence (filled symbols) and absence (empty symbols) of 1 % Quatrisoft polymer, room temperature (24 1 “C).
*
crossover of the curves, corresponding to surfactant alone and surfactant-plus-polymer, occurred below the cmc signifying a loss of surfactant monomer by interaction. On the other hand, for c14- and C16TAB no crossover occurred and the with- and without-polymer surface tension curves for the surfactant merged below the cmc. Curves for two cases representative of each type of behavior, viz., (210-and C14-TAB, are presented in Figure 2. We believe this work represents the first study in which phase separation/ precipitation results have been reported for combinations of a polyelectrolyte and a like-charged surfactant. On the other hand there have been recent reports of interaction in such combinations based on evidence from fluorescence and surface tension6and from viscometric6studies on mixtures of SDS with certain hydrophobically modified polyanions. (5) McGlade, M. J.; Randall, F. J.; Techeurekdjian, N. Macromolecules 1987,20, 1782.
(6) Iliopoulos, I.; Wang, T. K.; Audebert, R. Langmuir 1991, 7,617.
