Langmuir 1993,9, 894-898
a94
A New Strongly Flow Birefringent Surfactant System B. K. Mishra,+S. D. Samant,* P. Pradhan,§ Sushama B. Mishra,ll and C. Manohar'lt Bhabha Atomic Research Centre, Trombay, Bombay 400 085, India, and University Department of Chemical Technology, Matunga, Bombay 400 019, India Received June 1,1992. In Final Form: December 1,1992
In the present paper we report a new surfactant-hydrotope mixture of cetyltrimethylammoniumbromide and sodium 3-hydroxynaphthalene-2-carboxylatewhose aqueous solutions show two viscoelastic and two nematic liquid crystalline phases symmetrically placed about the equimolar ratio. It is demonstrated that the nematic to optically isotropic viscoelastic phase transition is driven by an increase of surface charge of the micelle. It is argued that the surface-active nature of both the molecules is the primary driving force for these transitions and the symmetry observed. Comparisons are made with the viscoelastic phases, spontaneous vesicles, and lyotropic nematic phases reported in the literature to bring out the unifying principles underlying the formation of these phases.
Introduction In the present paper we report investigations on a strongly viscoelastic and flow birefringent surfactanthydrotrope system which not only is a new example but contributes significantly to our understanding of the connection between vesicles, liquid crystals, and threadlike micelles ("living polymers"). The motivation for this work came from several observations. An extensively studied viscoelastic system14 is the mixture of a cationic surfactant such as cetyltrimethylammonium bromide (CTAB) and sodium salicylate (SS). This system has been shown to consist of long threadlike micelles. These micelles are elongated, ranging from lo00 to 2000 A in length having an overall high flexibility. The micelles are composed of regions ranging in length from 100to 200 A which are rigid, these sections can break and re-form, and thus the micelles have been called a living polymer Alternate models like a string of beads of micelle suggested earlierlOJ1have been shown to be incorrect from the subsequent studies. Some aspects of the system which attracted the interest of the present authors were the following. The SS molecules are strongly adsorbed on the micellar surface with the carboxylic and hydroxyl group protruding out of To whom the correspondence should be addressed. + Chemistry Division, Bhabha Atomic Research Centre. t
University Department of Chemical Technology.
I Bio-organic Chemistry Division, Bhabha Atomic Research Centre. 11
Heavy Water Division, Bhabha Atomic Research Centre.
(1) Hoffmann, H.; Rehage, H.; Reizlein, K.; Thurn, H. In Proceedings
of the ACSsymposium on Macro- and Microemulsions; Shah, D. O.,Ed.; American Chemical Society: Washington, DC, 1985; p 41. (2) Hoffmann, H.; Platz, G.; Rehage, H.; Schoor, W. Adu. Colloid Interface Sci. 1982, 17, 275. (3) Gravsholt, S. J. Colloid Interface Sci. 1976,57, 575. (4) Wunderlich, I.; Hoffmann, H.; Rehage, H. Rheol. Acta 1987, 26, 532. (5) Rehage, H.; Hoffmann, H.; Wunderlich, I. Ber. Busen-Ges. Phys. Chem. 1986,90, 1071. (6) Ulmius, J.; Wennerstrom, H.; Johansson, L. B. A.; Lindblom, G.; Graveholt, S. J. J. Phys. Chem. 1979, 83, 2232. (7) Cates, M. E. Macromolecules 1987,20,2289. Europhys. Lett. 1987, 4,497; J. Phys. Chem. 1990,94,371. (8)Kern, F.; a n a , R.; Candau, S. J. Langmuir 1991, 7, 1344.
the micelle.1° This orientation suggested that SS was surface active," which was confirmed by the systematic study of the surface active, aggregative, and solubilization properties of SS, and it was shown that this molecule belonged to the category of hydrotropes.12 More importantly Hoffmann et al observed13that when one investigates the effect of addition of SS to cetylpyridinium chloride (CPC) on rheology, one finds that the viscosity of the system rises sharply until slightly above a 1:l molar ratio of CPC/SS and then the viscosity drops off drastically. On continued addition of SS the viscosity rises again until the ratio reaches about 1:4 and then drops off again with continued addition of SS. This feature was observed for several concentrations of CPC and to date there does not seem to be any satisfactory explanation of this phenomenon. We strongly felt that this feature gives extremely important structural information which has deeper s i p nificance but is not understood. A clue to a possible solution came from the recent observation that when one mixes a cationic surfactant with an anionic surfactant, vesicles are formed s p o n t a n e ~ u s l yand ~ ~ that the phase diagram of the water-cationic surfactant-anionic surfactant system is approximately symmetrical with respect to the line joining the water corner to the equal molar ratio of the two surfactants. In view of the earlier comments on the surface-active properties of SS, it was natural to make a comparison of the CTAB-SS system with those reported in ref 14 on a mixture of two oppositely charged surfactants. This strongly suggested a connection between vesicles and viscoelastic solutions. We decided to increase the surface activity of molecules structurally similar to SS. For this purpose we selected sodium 3-hydroxynaphthalene-2-carboxylate(SHNC), commonly known as Bon Acid, as the hydrotope. The presence of naphthalene ring in SHNC was expected to confer more hydrophobicity on the molecule as compared to SS. The system thus investigated was CTAB-SHNC. Even the preliminary observations showed that this system was extremely interesting with the most striking property being the exhibition of strong flow birefringence. Samples viewed through the crossed polaroids were optically isotropic, but a small tilt or a mild tapping of the container
(9)Ott, A.; Bouchaud, J. P.; Lanaevin, D.: Urbach. W. Phvs. Reu. Lett.
1990,65, 2201. (10) Manohar, C.;Rao, U.R. K.;Valaulikar,B. S.;Iyer,R. M. J. Chem. Soc., Chem. Commun. 1986, 379. (11) Rao, U. R. K.; Manohar, C.; Valaulikar, B. S.; Iyer, R. M. J.Phys. Chem. 1987, 91, 3286.
(12) Balasubramanian, B.; Srinivas, V.; Gaikar, V. G.; Sharma, M. M.
J. Phys. Chem. 1989,92, 3865. (13) Rehage, H.; Hoffmann, H. J. Phys. Chem. 1988,92, 4712. (14) Kaler, E. W.; Kamalakara Murthy, A.; Rodriguez, B. 2.; adzinski, J. A. N. Science 1989, 145, 1371.
0743-1463/93/2409-0894$04.00/00 1993 American Chemical Society
Zas-
A New Strongly Flow Birefringent Surfactant System
A STR0W-Y FLOW ~-
a
~I
hngmuir, Vol. 9, No. 4, 1993 895
BIREFRINGENT GEL OF C T A 8 - m
~
SAMPLE VIEWED BETWEENTWO ~NCROSSEDPOLAROIDS
C
NOTE
SAMPLETILTED
THE DARK BAN0 IN
THE MIDDLE
Figure 1. The extreme sensitivity of the birefringence to stress or flow is demonstrated. (a) Sample H (Table I) is viewed between the polaroids which are not crossed. (b) Same sample is now viewed between crossed polaroids. It can be seen that the sample is opticallyisotropic. (c)The sample is tilted but continued to be viewed between crossed polaroids. The strong tilt-induced birefringence is seen. The stress distribution in the sample is seen clearly and the dark line in the middle indicates the zero stress and hence no molecular orientation.
produced very strong birefringence which lasted several milliseconds. Figure 1shows the dramatic observation of very strong birefringence introduced by smallstresseswhen a tilted sample is viewed through the crossed polaroids.
Materials and Methods CTAB was purchased from Sigma Chemicals and 3-hydroxynaphthalene-Zcarboxylicacid (HNC), a dye intermediate, was agift from M/Atul Products, Ltd. HNC was purified by dissolving in NaHCOR and regenerating with HCl and subsequently crystallizingfrom ethanol. Its sodium saltwas prepared by adding a requisite amount of alcoholic NaOH to the alcoholic solution
of HNC. The NMR studies were carried out using a Briiker-200 NMR at 200.13 MHz- surface tension was “ u ~ u r e dby AC 8 I - h “ n interfacial
Results Titration results of different concentrations of CTAB solutions showed the transitons
-
gel +liquid crystal --., precipitate liquid crystal gel (1) Here the gel implies a strongly Viscoelastic liquid which is optically isotropic and highly viscous. The liquid 4
896 Langmuir, Vol. 9, No. 4, 1993 Table I. Constant Concentration of CTAB = 60 m M (CTAB/SHNC) mole ratio 2 1.7 1.5 1.2 1 0.75 0.67 0.6
remarks (A) clear, isotropic, single phase gel, strongly flow birefringent (B)2-phase, top phase nonviscous isotropic, lower phase similar to remark A (C) single phase liquid crystalline, turbid (D) single phase liquid crystalline, more turbid than C (E) thick precipitate formed (F) single phase liquid crystalline, turbid similar to C (G) 2-phase system similar to B (H) single phase gel similar to A
crystalline phase is very likely a nematic phase (see the arguments below). Typical concentrations of CTAB and SHNC are shown in Table I. Experiments were also done for other concentrations of CTAB but the transitions in eq 1were always observed. The liquid crystalline phase appeared approximatelyat the same molar ratio of CTAB/ SHNC equal to 1.9, and always equimolar concentrations produced a thick yellowish precipitate. The liquid crystalline phase showed a turbidity with a yellowish tiqge (muddy) which is quite different from milkish turbidity that one comes across in emulsions. On the other hand this turbidity was reminiscent of the turbidity that one comes across in thermotropic nematic crystals.15 All these liquid crystalline samples showed well-defined, reversible transition temperatures when heated to optical isotropy. These turbid samples did not show any phase separation when they were allowed to stand for few months and even after centrifuging. The gel phase was transparent and optically isotropic but showed a strong flow birefringence. As mentioned above, even a slight tilting or mild tapping produced a strong burst of light when viewed between crossed polaroids. The authors are not aware of any cited example which showed produces such a strong effect. Figure 2 shows the oscilloscope trace of the response from two samples (light pulse monitored using a photodiode) produced by a slight tapping of the sample. The two examples showed the rich variety of properties indicating possible applications, e.g., vibration sensors, drag reducing agents, etc., which are being investigated separately. Results of surface tension measured as a function of the concentration of SHNC in distilled water are shown in Figure 3. NMR spectra of the SHNC are shown in Figure 4 and show the same trend as is observed in the corresponding CTAB-SS system,ll showing penetration of SHNC into CTAB micelles. NMR spectra of the CTAB are shown in Figure 5 and the line shapes are reminiscent of the CTAB-SS system,ll which shows the existence of long threadlike micelles of length -1000 A. Discussions It is clear from the results in Figure 3that SHNC reduces surface tension indicating that SHNC is mildly surface active and, in view of the concentrations, could be regarded as a hydrotope. The length of the aromatic portion of SHNC is about 4.8 A and this could be regarded as the hydrophobic portion of the molecule. This is further strengthened by the comparison of the NMR spectrum of SHNC only with that of SHNC with CTAB (Figure 4). This figure shows that the protons at the 4, 5, 6, and 7 positions are present in a nonpolar environment inside (15)de Cennes, P. C. The Physics of Liquid Crystals; Clarendon Press: Oxford, 1974.
Mishra et ai.
the micelles of CTAB. In other words, the molecule SHNC, just like SS,l0is oriented on the micellar surface keeping the naphthalene moiety penetrated into the micelle as shown in Figure 6 (schematic). This orientation is consistent with the surface active nature of SHNC Gust as in the case of SS). The NMRspectrum of CTABshowed a strong resemblance with that of CTAB-SS and the line shapes were approximately fitted using the prescription given by Ulmius and Wennerstrom.'6 The lengths of micelles obtained by these methods were about lo00 A. However this CTAB-SHNC system differed in a major way from the CTAB-SS system through the presence of a sequenceof phaaes shown in eq 1. The increased surface activity of SHNC played a significant role in these observations. The surface activity of SHNC implied that the CTABSHNC system should be regarded as a mixture of two oppositely charged surfactants (more appropriately a surfactant-Hydrotope mixture) one with a chain length of 17A (CTAB) and the other one with a much shorter chain length of about 4.8 A (SHNC). Once this view was taken, the system became directly comparable with two important systems reported in the literature, namely, ref 13 and ref 14, and suggested that the properties of the present system should be a combination of these two systems. The main similarities are as follows: (a) The symmetry of the transitions observed around equimolar concentrations of CTAB:SHNC (eq 1) was comparable to the phase diagram reported in ref 14 (of course, for equal chain length surfactants which form vesicles). (b) Existence of liquid crystalline phases as one approaches the equimolar concentrations (Hoffman et al. in ref 17 have incorporated into a single surfactant two different chain lengths, one of the chains is about 4 A or less, and obtained the nematic phase.) The first comparison (with ref 14) should be made with care because of the difference in chain lengths. If v is the volume of the hydrophobic portion of a surfactant, 1 is its length and a the area of the polar head. The combination of SHNC-CTAB could be regarded as a single surfactant with the Mitchell-Ninham parameter's (vlla) less than for a combination of surfactants,for example CTAB with an approximately equal chain length anionic surfactant (example: sodium dodecyl benzenesulfonate, SDBS).The latter combination has been shown to form vesicles and, therefore, the CTAB-SHNC combination having a lower vlla value should have a tendency to form long cylindrical micelles, which it seems to do in view of the formation of the viscoelastic gel and the liquid crystalline phase. It is, in fact, arguments of this kind which lead Hoffmann to design lyotropic nematics." The liquid crystalline samples mentioned in Table I were turbid with weak birefringence and, therefore, we took thick (1cm) samples in glass cells and viewed them through crossed polaroids. The samples showed typical schliren-like textures indicating a nematic type ordering confirming the suggested analogy with the Hoffmann system." Polarized microscopy using a mixture of powders of CTAB and SHNC with the ratios in Table I also showed the threadlike textures. With these comparisons, the sequence of transitions in eq 1 becomes qualitatively understandable. With the addition of SHNC to a CTAB micellar solution, the former (16) Ulmius, J.; Wennerstrom, H. J . Magn. Reson. 1977,28, 309. (17) Hertel, G.; Hoffmann, H. Liq. Cryst. 1989, 5, 1883. (18) Israelachivli, J. N.; Mitchell, D. J.; Ninham, B. W. J . Chem. SOC., Faraday Trans. 2 1976, 72, 1525.
A New Strongly Flow Birefringent Surfactant System
Langmuir, Vol. 9, No. 4, 1993 897
1
c
Figure 2. Oscilloscopetrace of the birefringence induced on mild tapping is shown for two samples: (a, left) decay of the birefringence; (b, right) birefringence decays off after characteristic vibrations of the sample.
E
cn
z
W
701
*
* *
%
%
0 v)
* %
z
!i!60
**** *
I
50
-2.00
-1.80 -1.60 -1.40 -1.20 LOG CONCENTRATION ( MOLAR)
-1.00
Figure 3. Surface tension vs concentrationof SHNC indicating the surface active nature. SHNC is therefore a hydrotope.
is adsorbed on to CTAB micelles converting the micelles into cylindrical (or a polymeric) micelles. The transformation is assisted by the decrease in the area of the polar head group (electrostatic interactins reduced) of the new surfactant entity thus increasing the parameter vlla. A t this stage the micelles are positively charged. Further addition reduces the Coulomb repulsion (asSHNC adsorbs onto a surface neutralizing the charge) and the cylinders are able to come closer to form a nematic phase. As one approaches the equimolar ratio, the surface charge becomes so small that the micelles coagulate producing a thick precipitate. Continued addition of SHNC again increases the charge of micelles (but this time the charge is negative) and stabilizes the nematic phase. Continued addition again drives the system to the gel state as before and this mechanism explains the symmetry of the transitions observed in eq 1,similar arguments were invoked in ref 14 to explain the transitions in the case of vesicles. For a given concentration of CTAB and SHNC the system can be looked upon as a mixture of an equimolar concentration of two surfactants to which the excess of one of the components has been added. The equimolar mixture could be expected to form a nematic liquid crystal because the combination could be regarded as a single electrically neutral surfactant with one long chain and the other short-just as the one reported by Hoffmann et al.17 This equimolar mixture could also form precip-
I
I
I
I
I
I
I
I
I
I
1
8.80 8.60 8.40 8.20 8.00 7.80 7.60 7.40 7.20 7.00 PPM
Figure 4. NMR spectrum of SHNC: (a) SHNC in heavy water; (b) SHNC in presence of 10 mM CTAB. The shift in the protons in positions 4, 5, 6, and 7 indicate that they have moved to a nonpolar medium in the presence of the micelle.
itates. Now, when an excess of one of the molecules, CTAB or SHNC, is added, it would get adsorbed on the cylindrical micelles of the nematic phase and increase the Coulomb repulsion between the two cylinders. In the nematic phase the two cylindrical micelles are almost parallel, but an increase in the surface charge of the micelles tends to destroy the parallel orientation-it is more favorable energetically for the cylinders to lieperpendicular to each other to reduce the Coulomb energy. Therefore, the system is driven to an optically isotropic phase which, unlike the thermally induced isotropic phase, is very strongly correlated and, therefore, shows strong flow birefringence. It is of interest to recall two important results which hinted these possibilities but unfortunately were not stressed. Olsson et al.I9concluded that with the increased concentration of SS in the CTAB-SS system the micellar (19) Olsson, U.; Soderman, 0.; Guering, P. J. Phys. Chem. 1986,90, 5223.
Mishra et al.
898 Langmuir, Vol. 9,No.4, 1993
:TAB
+SHNS
:TAB A
4.00
3.50 3.00
2.50
2.00
1.50
1.00
0.50
PPM
Figure 5. Comparison of line shapes of CTAB with and without SHNC. The broad lineshapes are typical of elongated micelles (see ref 16).
u
Figure 6. Schematic orientation of the SHNC on the micellar surface (corresponding orientation of SS is shown in ref 10).
charge was changed from positive to negative and they commented that the system was similar to a mixture of cationic and anionic surfactants! Similarly Baker et al. observed that in sodium dodecyl sulfate (SDS)-octyltrimethylammonium bromide (CsTAB)-water systems the
two phase and the liquid crystallinephaee were surrounded by viscoelastic phases just like the ones reported here.20 This was also true of SDSzwitterionic-water systems. They also reported the observation of a distinct turbid phase between the viscoelastic phase and the two phase regions in the SDS-CaTAB-water system. The structure of the turbid phase was not clear. In view of these comments it appears as though the gel phase should be regarded as a highly correlated isotropic phase of the corresponding nematic phase-driven to isotropy by an increase in Coulomb interactions! Conclusions The present paper introduces a new system of CTAB + SHNC which shows the following: (a) T w o strongly viscoelastic flow birefringent (gel) phases possibly consisting of micelles of opposite surface charge. (b) On slight decrease of the micelle charge the gel changesover to a nematic liquid crystallineordering. There are very few lyotropic nematic phases which have been reported. (c) The symmetry of the transitions and the surface active nature of SHNC suggest comparisons with the spontaneous vesicular systems produced by mixing two Surfactants of opposite charges.14 (d)The short length of SHNC molecule and the existence of a nematic liquid crystalline phase make the system comparable to single surfactant systems reported by Hoffmann.17 (e) The strong flow birefringence coupled with the observation that this gel state is obtained by increasing the surface charge of the micelles in the nematic phase seems to suggestthat the gel consists of stronglycorrelated (i.e. induced by Coulomb repulsion) rods driven to optical isotropy by Coulomb repulsions.
Acknowledgment. We are grateful to Miss Girija, Mr. Nadkami, Mr. Manimaran, and Dr. Naik for help in setting up birefringence experiments. We thank Drs. V. K. Kelkar, S. V. G. Menon, S. S. Bhagwat, Usha Deniz, and J. P. Mittal for discussions. We are grateful to the reviewers for their constructive suggestions which have improved the quality of the paper and for drawing our attention to important references (refs 18 and 19). (20) Barker, C. A.; Saul, D.;Tiddy, C. J. T.; Wheeler, B. A.; Willis, E.
J. Chem. SOC.,Faraday Trans. 1 1974, 70, 154.
