J . Phys. Chem. 1994,98, 5984-5993
5984
Spherical-to-Wormlike Micelle Transition in CTAB Solutions Z. Lin, J. J. Cai, L. E. Scriven, and H. T. Davis’ Department of Chemical Engineering & Materials i Science and Center for Interfacial Engineering, University of Minnesota, 421 Washington Avenue S.E., Minneapolis, Minnesota 55455 Received: February 2, 1994’
Addition of methylsalicylic acid and hydroxybenzoicacid to 20 m M aqueous solutions of cetyltrimethylammonium bromide (CTAB) causes spherical micelles to undergo a transition to wormlike micelles. Shear rheometry and cryo-transmission electron microscopy (cryo-TEM) are combined to investigate the relationship between the colloid microstructures and rheology as a function of acid concentration. Cryo-TEM micrographs show that the spherical-to-wormlike micelle transition is not abrupt. Both types of micelles coexist over a range of acid concentration. When the majority of the surfactant is in wormlike micelles, the solutions exhibit viscoelastic behavior as a result of micellar entanglement. The shear and extensional viscosities of these solutions undergo flow thinning, flow thickening, and then flow thinning with increasing flow rates. W e interpret the micellar transition in terms of the effect the added acid has on the ratio of the effective area of the hydrophilic head group and the effective area of the hydrophobic chain of the surfactant. The effective area of the hydrophilic head group is decreased by association with the carboxylic acid group, and the effective area of the hydrocarbon chain is increased by penetration of the phenyl moiety of the acid. Both of these factors favor changing the surfactant monolayer from spherical to cylindrical curvature. From trends we observed for 3-, 4- and 5-methylsalicylic acid and 3- and 4-hydroxybenzoic acid, we conclude that increasing the effective area of the hydrocarbon chain is a n important factor in the transition mechanism.
The geometry of wormlike micelles in aqueous solution has been studied by small angle neutron scattering (SANS),22-14 and It is known that surfactant molecules self-assemble into by cryetransmission electron microscopy (cryeTEM).2526 Cumaggregates in aqueous solution above the so-called critical micelle mins and his colleague^^^^^^ concluded from their SANS data, concentration. At low concentration, the aggregates are generally taken from a solution undergoing significant shear, that the rotund, globular micelles.’ In some cationic surfactant systems micelles are rigid monodisperse rods. This is inconsistent with as well as some nonionic and anionic surfactant systems, long the cryo-TEM studies,25,26in which micelles have been found to wormlike micelles form at higher concentration and/or upon be polydispersed long flexible ones. Besides, one of the unanswered addition of salt or acid. The most extensively studied systems are questions is, what interactions cause wormlike or rodlike micelles alkyltrimethylammonium halide and alkylpyridinium halide.2-5 to grow from spherical micelles? Answering this could help us Halide anions associate only moderately with surfactant cations, understand the kinetics of micellar and mesophase growth. and micellar growth is gradual. However, with anions that The theory of the spherical-to-wormlike micelle transition12-13 associate strongly with surfactant cations, such as salicylate (Sal-), and the theory of the rheological behavior of wormlike micellar wormlike micelles grow rapidly a t low surfactant and salt solutions*presently lack strongly predictive powers, so we believe concentrations. The rheological behavior exhibited by these it is important to establish a good descriptive data base of systems systems is viscoelastic and analogous to that observed in solutions undergoing the spherical-to-wormlike micelle transition. Cryoof flexible polymers.&ll TEM is particularly suitable for visualization of microstructures It is well-known that the shape of the micelles depends strongly of the size scale of micelles.27 There is no staining or drying upon the actual packing parameters in micellar a s ~ e m b l y . ~ ~ - ~artifact ~ associated with the technique.28 Recently, cryo-TEM The counterion binding suppresses the micellar charge and has been used to explore the geometry of wormlike or rodlike decreases the surface area per surfactant molecule by reducing micelles in the cationic surfactant cetyltrimethylammonium the electrostatic repulsion between the head groups, thus promotchloride (CTAC) upon addition of NaSal and N a C P and in the ing the spherical-to-wormlike micelle transition. The addition nonionic surfactant hexaethylene glycol monohexadecyl ether of different types of molecules leads to large deviations of packing (C&6).26 Accordingly, in this paper, we report a cryo-TEM parameters. Many counterions and cosurfactants are strongly and rheology study of the spherical-to-wormlike micelle transition adsorbed at the micellar interface; depending on the amount of in aqueous solutions of CTAB upon addition of n-methylsalicylic penetration, this may change the mean distance between polar acid, n = 3, 4, 5, or 3- or 4-hydroxybenzoic acid. The purpose head groups or increase the volume of the micelle core. The of this study is to examine the micellar morphologies as a function salicylate and alkyl benzoate counterions are most efficient. These of concentration and type of added acid and to determine the additives are also often used in pharmaceutical products such as relationship of micellar morphology to solution rheology. ointments, creams, or lotions. N M R studies on the cetyltriA particular advantage of cryo-TEM for studying transitions methylammonium salicylate system reveal that the 1H lines for of micellar morphology is that the coexistence of spherical and the N(CH& group are shifted to higher fields, and the signals wormlike micelles can be directly visualized, whereas scattering are broadened.15-20 The salicylate anion orientates in such a way and rheology will not be very sensitive to mixed morphologies. that the negatively charged site (COO- group) stands perpendicular to the micellar ~urfactant.1~ This results in large reduction Materials and Methods of the net surface charge. Similar conclusions were also obtained from fluorescence measurements.21 Cetyltrimethylammonium bromide (CTAB) (greater than 99% purity) was purchased from Aldrich, Milwaukee, WI. All of the chemicals were used as received. 0 Abstract published in Advance ACS Abstracts, May 1, 1994. Introduction
0022-3654/94/2098-5984$04.50/0
0 1994 American Chemical Society
Micelle Transitions in CTAB Solutions All the solutions were prepared by adding doubly distilled water into weighed compounds to the desired molar fractions. All samples were stirred for about 3 h on a magnetic stirrer rotating at about 2 Hz. The compounds were always totally dissolved into the solvent. In the study here, the CTAB concentration was kept constant at 20 mM, while methylsalicylic acid concentration was varied. Samples for cryo-TEM were prepared in the CEVS, which has been described in detail elsewhere.29 The CEVS is an environmental chamber in which temperature can be controlled within 0.1 OC between -10 and +90 OC by a 600-W halogen quartz lamp and an insulated reservoir mounted on the outside rear wall of the chamber. The reservoir can be charged with a refrigerant. The atmosphere inside can be saturated by the solvent@). This is accomplished with porous sponges extending upward from liquid reservoirs. The air inside the chamber is recirculated across the sponges by a fan mounted on the back wall of the chamber to reduce temperature and composition gradients in the vapor. In the studies reported here, thin films of sample were formed by placing a 5 - p L drop of the surfactant solution on a holey polymer support film which had been coated with carbon and mounted on the surface of a 200-mesh standard TEM grid.”JThe grid was held by tweezers mounted on the plunge assembly. The drop was then blotted with filter paper from both sides of the grid to remove the excess liquid by moving the filter paper up and down while it was gently touching the grid surface. After most of the liquid had been removed, the rest formed a thin film spanning the holes in the holey polymer film. The hoies had typical sizes of 1-10 pm, and the suitable film thickness of less than 200 nm was sought. The desired film thickness was obtained by trialand-error. One way to make sure that the film formed on the grid had a suitable thickness was to leave some liquid on the meshes which were close to the tweezers so that it could be seen by eye while the other side of the grid was blotted harder so that there was a thickness gradient on the grid. About 20-30 s after the blotting, the entire assembly was then plunged through a synchronous shutter at the bottom of the environmental chaqber and into liquid ethane situated immediately beneath. The 2030-s delay was used in view of the finding by previous investigators25 that shear alignment from blotting of wormlike micelles relaxed within this time period, The plunge velocity is high (>2 m/s) and reproducible. Ethane was chosen asthe cryogen because the large temperature difference between its melting and boiling points helps prevent formation of a vapor which would envelop the specimen and slow down the freezing process during the fixation. The geometry of the specimen, which had large surface area and small thickness, the thermodynamic properties of ethane, and the high plunge velocity provide a cooling rate which is high enough to prevent the formation of ice crystals. This prevents possible structural rearrangements associated with thermotropic phase transitions within the system. The frozen specimen was transferred under liquid nitrogen into a liquid nitrogen-filled Dewar for storage. The frozen specimen was loaded under liquid nitrogen into the cryo-TEM transfer stage (Model 626, Gatan, Inc., Warendale, PA), and the holder was inserted into the microscope (JEOL 1010, JEOL U S A . , Boston, MA) for observation. The microscope was operated a t 100 kV. The cryo-holder temperature was maintained below -170 OC during imaging. The condenser lens aperture was set at 100pm in diameter, and theobjectiveaperture was 50 pm in diameter. The specimen was imaged at a nominal underfocus of 2-4 pm in order to achieve adequate phase contrast. The image was recorded on Kodak SO-163 film at 30 000 X (*5%), and 1-s exposure time. The film was developed with full-strength D-19 developer (Eastman Kodak Co., Rochester, NY) for 12 min. No special tricks were used in processing the prints.
The Journal of Physical Chemistry, Vol. 98, No. 23, 1994 5985 Steady shear, small-amplitude oscillatory, i.e, dynamic shear, and extensional viscosity measurements were performed on a Rheometrics fluid spectrometer RFS 11, and Rheometrics RFX fluids analyzer, which are supported by RHIOS software. The 10-gcm transducer was used to detect torque and normal force simultaneously. Dynamic shear measurements were taken over the frequency sweep from 0.1 to 100 or from 0.1 to 1000 rad/s. The rheometer has a built-in computer which converts the torque measurements into both G’(the storage modulus) and G”(the loss modulus) in dynamic shear experiments, or viscosity in steady shear experiments. For steady shear measurements, data were averaged for clockwise and counterclockwise directions. The measurements were performed a t room temperature. In Couette or concentric cylinder geometry, the cup radius was 17 mm, and the bob radius and length were 16 and 33 mm, respectively. The reason to use Couette geometry instead of parallel plate geometry is to increase the sensitivity of the instrument. In extensional viscosity measurement, RFX has opposed-nozzles flow field. It is the only instrument capable of measuring extensional viscosity for liquids with low viscosity and high extension rates. The diameters of nozzles range from 5 mm to 0.5 mm, depending on extensional rates.
Results Cryo-TEM. Figure 1 consists of a set of cryo-TEM images of the vitrified CTAB/5-methylsalicylic acid samples with the concentration ratio C A / C ranging ~ from 0.1 to 1.0. CA is the 5-methylsalicylic acid concentration, and CS is the CTAB concentration, which is constant at 20 mM. Figure l a shows the cryo-TEM image of CA/CS= 0.1 solution. Only spherical micelles appear. The diameter of the micelles appears to be about 60 A, although underfocusing creates some uncertainty in this value. Figure 1b shows the cryo-TEM image of a CA/CS= 0.2 solution. Besides spherical micelles of 60-A diameter, there are some wormlike micelles. The arrows in the micrograph point out two ends of one wormlike micelle. The wormlike micelles also appear to have a diameter of about 60A. However, the length of wormlike micellesvaries. At this particular molar ratio, only short wormlike micelles appear, their variable lengths ranging up to several hundred anstroms. Figure lcshows thecryo-TEM imageof a CA/CS= 0.3 solution. At this molar ratio, long wormlike micelles start to form in the solution, as shown in the micrograph. However, also shown in the micrograph are a lot of spherical micelles as well as some short wormlike micelles. Some ring structures also appear in the micrograph. Figure ldshows thecryo-TEMimageofa C*/Cs = 0.4solution. As shown in the micrograph, micelles have become very long and overlapping. It is very hard to trace one individual micelle from one end to the other. When observing apparent entanglements and other features, recall that electron micrographs are two-dimensional projections of microstructures seen by looking through three-dimensional specimens. Such a projection causes problems in the determination of micelle length and polydispersity in some cases. In Figure IC, because most wormlike micelles can be identified from one end to the other, it is relatively easy to identify the polydispersity of the micelles. However, in Figure Id, because micelles overlap, it is not possible to identify where they begin and end. Furthermore, without recording the same image at different tilt angles, it is often not possible to identify the corresponding incoming and outgoing portions of a micelle; therefore, the actual micelle length may be longer than it appears in the image. Most of the vitrified surfactant solutions are electron irradiation sensitive, so low-dose operation is necessary to preserve the
5986 The JournalofPhysical Chemistry, Vol. 98. No. 23.1994
Lin et al.
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Micelle Transitions in CTAB Solutions
The Journal of Physical Chemistry, Vol. 98, No. 23. 1994 5987
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that is driven by the film thickness dependence of the disjoining p0tentia1.j~ Although threadlike micelles can become aligned + G" with one another within liquid film, especially in unrelaxed samples, as demonstrated by the unrelaxed samples shown in Figure lf,g, many images in relaxed samples, like Figure ld,e, show no alignment. Parts f and g of Figure 1 illustrate the complexity of the flow + o * o alignment of threadlike micelles in unrelaxed samples. In Figure + o lf, there are some regions where micelles are highly disordered 0 0 and are trapped between the well aligned threadlike micelles. In 0.01 I J Figure lg, micelles are aligned along several different directions, 0.01 0. I I 10 IO0 which indicates that the flow pattern in the micellar solution Frequency (rnd/s) during the sample preparation is complicated. Figure 3. Dynamic shear measurement for (a) C A / C ~= 0.4 and (b) Figure l h shows the cryo-TEM image of the c A / c s = 0.7 cA/cs = 0.5. solution. Now few spherical micelles are seen in the micrograph, and wormlike micelles have begun to form an entanglement microstructures. Moreover, in this case, the electron microscope network. operator cannot see the micelles on the screen before taking the At 1.1 molar ratio of 5-methylsalicylic acid and CTAB, the picture. Thus, it is difficult to get micrographs of the same image solution becamevisibly turbid. Figure 1i shows the corresponding but with different tilt angles. cryo-TEM image. Vesicles are seen in the micrograph. Figure l e shows the cryo-TEM image of a relaxed sample of Rheology. Steady shear measurements with the Couette a cA/cS = 0.5 solution. Although all the wormlike micelles in rheometry were carried out on samples at all but one (0.1) of the the micrograph arevery long, there are still some spherical micelles concentration ratios ( c A / c s )listed in Figure 1. The torquelimits coexisting in the system. Such coexisting micelles, whose of the rheometers would not allow a reliable steady shear result prediction would pose a strong test of theory, would be hard to at c A / c s = 0.1 nor oscillatory shear results below c A / c s = 0.4. detect by any method other than the direct visualization provided by cryo-TEM. Even a t cA/c.= 0.4, the low-frequency parts of oscillatory shear results are unreliable. Thickness gradients in the liquid specimen film may cause microstructures to segregate by size or to order. Size segregation A typical set of data of steady shear measurements is shown is commonly observed in images of vesicles.25-33 Confinement in Figure 2, steady shear viscosity vs shear rate, with different shear and induced order or alignment are sometimes seen in ratios of cA/cS.For low concentration ratios, e.g., cA/cS= 0.2, systems of threadlike micelles.27.34 As the sample thins down to the solutions are nearly Newtonian liquids with low viscosity and less than micellar dimensions, flow may carry micelles away from nondetectable elastic response. Even though wormlike micelles the central region. Alternatively, micelles may be absent from occur at this concentration ratio, they are relatively short and are thecentral region of the biconcavesamplefilmof micellar solution not entangled (Figure 1b). With increasing the concentration as a result of a shift in equilibrium from micellar to monomeric ratios, CA/CS= 0.3-0.7, the most prominent feature is the strong
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F l p 6. Cry-TEM images of CTAB/3-mcthylsalicylic acid SdutiOM. G = 20 mM. (a) Ratio of 3-methylsalicylic acid concentration CAto Cs = 0.1. Only spherical micelles apprarrd (S). (b) Cr/Cs = 0.2. Short wormlike micelles (W). "dimers" (A), and spherical micelles (S) mexist. (c) CA/CS= 0.4. Wormlike micelles overlapped. They coexist with some spherical micelles (S).
shear thickening effect in the shear rate range 20-100 s-1, as found by Strivenlo in the salicylic acid/CTAB system. This feature was reproducible, wherever starting point on the shear rangescquencewasused. Thepeakpositionsofshearthickening do not change too much for CA/Csranging from 0.3 to 0.7. The storage modulus G'and the loss modulus C"have been measured as a function of the angular frequency o. Figures 3 and 4 show the curves of the storage modulus G'and the loss modulus C" with C+./Cs = 0.4, 0.5, and 0.7. At frequencies
above 100 rad/s, Wand G"drop to zero. This is attributed to the limitation of the transducer in the rheometer. In Figure 3, both Wand C"curves continue to slope upward until o = IO rad/s. From o = 2 tow = IO radJs, G'and G"are overlapping with competing elastic and viscous properties. At thefrequencyaboutS0 rad/s,G"suddenlydropand thesolution becomes nearly elastic. Samples with CA/C, = 0.4 and 0.5 have similarly shaped viscoelastic response curves, although their magnitudes are quite different.
5990 The Journal of Physical Chemistry, Vol. 98, No. 23. 1994
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Figure 7. CryeTEM images of CTAB/dmethylJalicylic acid solutions. Cs = 20 mM. (a) Ratio of dmethylsalicylic acid mnccntration CAto Cs = 0.2. Only spherical micelles appcared (S). (b) CA/CS= 0.2. Short wormlike micelles (W)and spherical micelles (S) coexist. (c) CA/CS= 0.4. Long. flexible wormlike micelles overlappd. They coexist with some spherical micelles (S) and short wormlike micelles (W). In the case of the CA/C, = 0.7 solution, Figure 4, the moduli the 5-methylsalicylic acid solutions behave as elastic solids, increase rapidly with frequency up t o o = 0.2 rad/s, where the whereas at low frequencies they behave as viscoelastic liquids. loss modulus W’reaches a maximum and the storage modulus This is consistent with what has been seen in other viscoelastic C‘crosses the C“curve, indicating a transition from a viscous to surfactant solutions?.9.’0.~’.~’ an elastic liquid. Then C’ continues to slope upward with In Figure 5, both extensional viscosity and shear viscosity of increasing rate; C“ goes through a minimum, increases with CA/Cs = 0.7 are plotted. The solution exhibits extensional increasing rate, and finally drops appreciably. In none of the thinning, thickening, and thinning with increasing extensional 5-methylsalicylic acid solutions did we observe a plateau in C’ rate. The transition of thickening and thinning is at about 8-9 of the kind found in CTAB/NaSal, CTAC/NaSal systems.7.25 s-1. Although shifted in frequency, the local maximum in the and salicylic acid (HSal)/CTAB ~ystems.3~At high frequencies extensional viscosity is probably caused by the same physical
Micelle Transitions in CTAB Solutions
The Journal of Physical Chemistry, Vol. 98. No. 23. 1994 599991
Cry-TEM imagesol (a) 2 1 molarratioofCTAB/3-hydroryknzOateaeidwith CS= 20 mM. Wormlike miallesoverlappcdandmexisled with spherical micslles. (b) 2 1 molar ratio of CTAB/4-hydroxybenmle acid with Cs = 20 mM. Only spherical micelled (S) appeared. Fl-8.
phenomenon as that which gives rise to the local maximum in the shear viscosity. For low ratios of CA/Cs,because of sensitivity limitationsofthe rheometer, no reliableextensional viscosity data could be obtained. Addition of 3- and 4-methylsalicylic acid to CTAB solutions transformmicells towormlikemicellsmuch thesameasaddition of 5-methylsalicylic acid. Figures 6 and 7 give images summarizing the trends. Again spherical and wormlike micelles are observed to coexist. Finally, addition of 4-hydroxybenzoic acid in a acid/CTAB ratio of CA/Cs = 0.5 produces only spherical micelle-s, whereas 3-hydroxybenzoic acid in the same ratio produces coexisting spherical and wormlike micelles (Figure E). This sensitivity to position of the hydroxy group is reminiscent of the differences observed by Rao et a1.” for added sodium salicylate and 3-hydroxybenzoic acid.
Discmsion In dilute solutions where interaggregate interactions can be neglected, the morphology of surfactant aggregates is a function of the degree of curvature of either the surfactant monolayer or bilayer.lzJf For hydrocarbon-bad surfactants, it has been argued”-“ that the curvature of the aggregate is strongly influencedbytheratio,ab/ac,whereahistheeffectiveheadgroup (hydrophilic) cross-sectional area and a, is the effective crosssectional area of the aliphatic chain. While decreasing the ratio of ahfa,, the aggregate shape is expected to follow the trend spheroidal micelle wormlike (rodlike) micelle bilayer structures inverse structures. In what follows, we will try to rationalize our experimental results in terms of the ratio ab/ao. In the context of this heuristic model, the behavior of the 5-methylsalicylic acid/CTAB solution can be rationalized if it isassumed that thesalicylicanion binds strongly to thesurfactant cation. This binding would reduce the ahof thecation. Also, the
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phenyl moiety of the salicylic anion might be embedded into the hydrophobic portion of the surfactant monolayer in a fashion such as that was demonstrated in the NaSal/CTAB/H20 system.l3-20J* This penetration may increase the volume of the micelle core, which is equivalent to increasing a,. Thus, addition of 5-methylsalicylic acid to the micellar solution will reduce ah, increase ac, and so decrease the ratio ah/(lc. The result then is the trend spheroidal micelles wormlike (rodlike) micelles with bilayer structures increasing the acid concentration. Our results offer strong evidence that penetrationof the benzene ring into the hydrophobic region of the surfactant monolayer is an important part of the transition. First wenote that the CA/CS ratio at which the spherical micelle to wormlikemicelle transition occurs is about the same for 3-, 4-, and 5-methylsalicylic acid. The methyl group is hydrophobic and so does not reduce the tendency of the benzene ring to penetrate the hydrophobic region of the surfactant monolayer. From previous work it is known that sodium salicylate and salicylic acid (2-hydroxybenzoic acid) are about equally effective in driving the transition, whereas 3-hydroxybenzoic acid is less e f f e ~ t i v e . * ~ JTo * , this ~ ~ we add our observation that 4-hydroxybenzoic acid is less effective than 3-hydroxybenzoic acid. We conclude that as the hydrophilic hydroxy group moves around the benzene ring away from the acid group, the hydrophobic part of the benzene ring is more effectively shielded, and the acid becomes less effective in driving the transmition because the phenyl moiety does not penetrate as well the hydrophobic region of the surfactant monolayer. Thus, it appears that changes in both ah and a, contribute importantly to the transition. Rehage and H ~ f f m a n nhave ~ ~ demonstrated that the viscoelastic solution property depends strongly upon the chemical structure of the counterion. They compared sodium salts of 0-, m-,and p-toluic acid. What they found is that a methyl group in the 4-position of sodium benzoate gave solutions with high viscosities. If the methyl group is situated in the 3-position, the viscous resistance decreases, and if the methyl group is positioned in the vicinity of the polar acid group, the solutions exhibit pure Newtonian flow properties. Therefore, they concluded that the viscoelasticity is associated not only with the counterion binding but also with the specific orientation of the added compounds. p-Toluic acid is adsorbed in such a way that the negatively charged site (COO-group) is oriented perpendicularly to the surface plane. That is exactly the same orientation as that observed for NaSal.17 However, cryo-TEM studies,25 including our results, do not support themodel proposed by Rao et al.,17in which thecounterion act as "cross-linkers", which are able to connect charged micelles by attractive electrostatic interactions. In this picture, the wormlike or rodlike micelle should consist of a string of spherical particles. Cryo-TEM images (Figure 1) show that at the concentration ratio C*/Cs 2 0.3, very long wormlike micelles formed in the system. At the same time, there is a trend of the shear thinning, thickening, and thinning when the shear rate is increased in the steady state measurements at the concentration ratio 1 0 . 3 . This trend is associated with the structure of the wormlike micelles and the flow field. In shear flow, micelles tend to deform, align, and tumble. At low and moderate shear rates, they deform and tumble, showing shear thinning like most flexiblechain molecular polymers. When shear rates are increased, shear thickening behavior probably comes from micellar overlapping and cluster formationloor so-called micellar aggregates.35 The fact that peak positions of shear thickening do not change much over a range of counterion concentrations indicates this is a characteristic frequency for the system. When shear rates are increased further, the structures of micellar overlapping and cluster formation are probably destroyed. Solutions exhibit shear thinning again. As far as the extensional viscosity is concerned, unfortunately, there are not many experimental data currently available for
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Lin et al. viscoelastic surfactant solutions. By comparing the nominal rates between shear and extensional viscosity data (Figure 5), extension flow, which is strong flow, makes micelles overlapping or aggregating a t far lower rates than shear flow. However, they all follow the trend thinning thickening thinning. More studies of this problem would be helpful. Interestingly, so far, only when the acid forms of salicylate or its derivatives was added to CTAB solutions, shear thickening effect was observed.1° When salicyclic acid was replaced with sodium salicylate, e.&, this effect was not observed, and so pH might play a special role in the thickening phenomena.
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