Association of Cyclodextrin with Colloidal Electrolytes ferential Equations", American Elsevier, New York, N.Y ., 1969. (9) D. H. Everett, Trans. Faraday Soc., 81, 1637 (1965). (10) A. J. Ashworth and D.H. Everett, Trans. Faraday SOC., 56, 1609 (1960). (11) G. M. Janini and D.E. Martire, J. Chem. Soc., Faraday Trans. 2, 70, 637
2661 (1974). (12) M. L. McGlashan and A. G. Williamson, Trans. Faraday SOC., 57, 568 (1961). (13) C. L. Hussey and J. F. Parcher, J. Chromatogr., 92, 47 (1974).
Conductometric Studies on Association of Cyclodextrin with Colloidal Electrolytes Tsuneo Okubo, Hlromi Kitano, and Norio lse* Department of Polymer Chemistry, Kyoto University, Kyoto, Japan (Received April 13, 1976)
+
+
Conductance and surface tension measurements are carried out for the system, HzO colloidal electrolyte cyclodextrin. The apparent critical micelle concentrations (cmc) of the electrolytes, Le., sodium lauryl sulfate (NaLS) and cetyltrimethylammonium bromide (CTABr), are found to increase upon the addition of aand 0-cyclodextrins (aCD and PCD) in aqueous, ethanol-aqueous, and N-methylacetamide-aqueous media. It is concluded that the cyclodextrins form 1:lcomplexes with the colloidal electrolytes. The association constant, K , increases in the order NaLS-aCD < NaLS-pCD .( CTABr-aCD < CTABr-PCD. The free energy, enthalpy, and entropy of association for the CTABr-pCD system are -4.6 kcal mol-l, -3.4 kcal mol-I, and 4 eu at 35 "C, respectively. The equivalent conductances of monomeric (A,), micelle (Amic), and associated (with cyclodextrins) state electrolytes (A,,,,,) are evaluated. For both of NaLS and CTABr, A, is slightly larger than AaSSOC, and strikingly larger than Amic in aqueous media. By addition of ethanol, Am and A,,,,, decrease and Amic increases, whereas A,, Aassoc, and Amic decrease with increasing content of N-methylacetamide. The surface tension of a CTABr solution increased with addition of P-CD.
Introduction It has been well known that cyclodextrin forms complexes with a variety of molecular species.1,2 Schlenk and Sano3 demonstrated by solubility measurements that a- and 6cyclodextrins interact with long-chain aliphatic carboxylic acids. Therefore, ionic surfactants such as sodium lauryl sulfate and cetyltrimethylammonium bromide are expected to interact with cyclodextrin. The association constants of cyclodextrin with various molecular species were determined mainly by using changes of the absorbance of species in the uv region. Therefore, it has been difficult to determine the association constants for species having no chromophoric groups. We tried to overcome the difficulty by conductometric measurements. In the present report, we discuss complex formation of cyclodextrin with colloidal electrolytes by the conductometric method. Conductometric studies on surfactants have been carried out by various researcher^.^ However, the equivalent conductances of monomeric- and micelle-state electrolytes have not often been discussed. Such quantities are also studied in this paper.
Chemicals Co., Tokyo, was further purified by distillation (10.5 mmHg at 83 "C). Conductance Measurements. The conductivity was measured by a Wayne-Kerr autobalance precision bridge (B-331) a t a frequency of 1592 Hz and a recorder (Type SP-H,Riken Denshi Co., Tokyo). The capacitance correction was automatically effected and the precision of the conductivity is believed to be f0.01%. Two types of conductivity cells having platinum plates were used: a Jones-Ballinger type cell5 (cell constant = 4.97 cm-1) and a conductivity cell (cell constant = 4.80 cm-l) equipped with water-circulating jacket. Surface Tension Measurements. The surface tensions of aqueous mixtures of cyclodextrin and surfactant were measured by using a Wilhelmy type surface tensiometer (Shimadzu Type ST-1). A ground glass measuring plate was suspended from an electrobalance by a glass fiber wire into solution in a glass container, the inner diameter of which was 4 cm. The measurements were carried out at room temperature around 25 "C. Results and Discussion
Experimental Section
Materials. a- and P-cyclodextrins were obtained from Nakarai Chemical Co., Kyoto, and used without further purification. Cetyltrimethylammonium bromide (CTABr) and sodium lauryl sulfate (NaLS) were purchased from Nakarai Chemical Co. These two surfactants were further purified by recrystallization from water. Deionized water obtained with cation- and anion-exchange resins was used for the preparation of solutions. Spectroscopic grade ethanol was used. N Methylacetamide obtained from Tokyo Kasei Organic
+
+
The specific conductances, K , of the HzO NaLS aCD system as a function of NaLS concentration are given in Figure 1.The K values of NaLS solution in the absence of CD were in agreement with those reported by Goddard and Benson6 within experimental error. The characteristic feature of the surfactant] profile is that there can be drawn two straight lines having different slopes, as is clearly shown in the figure. The point of intersection corresponds to the apparent critical micelle concentration, m*. The m* of NaLS was clearly observed to become larger upon the addition of aCD. A similar feature was also obtained for other systems, i.e., HzO NaLS
+
The Journal of Physical Chemistry, Vol. 80, No. 24, 1976
2662
T. Okubo, H. Kitano, and N. lse I
I
I
TABLE 11: Equivalent Conductances of Surfactant Ions in Monomeric (Am), Associated (Aassoc), and Micelle States (Amic) and Association Constants of Surfactant Electrolyte and a- or j3-Cyclodextrin at 25 "C
I
1 M EtOH
CTABr-PCD
2M 3M 4M CTABr-PCD
0.5 M NMA 1M 2M
3M
V
I a01
0
I
I
I
0.02
0 3
a04
[NaLSI (MI
+
Figure 1. Conductances of the H20 f NaLS a C D system as a function of surfactant concentration at 25 OC: curve 1, initial concentration of aCD = 0 M; curve 2,0.0114 M; curve 3,0.0229 M; curve 4, 0.0343M; curve 5, 0.0457 M.
E
a002 0.004 [CDI (MI
0
0.W6
Figure 2. Apparent cmc, m " , as a function of CD concentration in the H20 CTABr +PCD (curve I), H20 CTABr a C D (2), H20 4 M EtOH CTABr PCD (3),and H20 4- 3 M NMA CTABr 4- PCD (4) systems at 25 QC.
++
+
+
+
TABLE I: Equivalent Conductances of Surfactant Ions in Monomeric (Am), Associated (AaSsOC),and Micelle States (Amic) and Association Constants of Micelle Electrolyte and a-and j3-Cyclodextrina 0-l cm2
Temp, "C
a
2000 1670 690 450
73 67 57 45
65 59 48 34
21
1190 1140 620 270
20 19 18
+ CD + S - C D
(1)
where S and CD indicate surfactant and cyclodextrin, respectively. When the concentrations of the monomeric-, associated-, and micelle-state electrolytes are denoted by m,, massoc, and mmic,respectively, the total concentration of the electrolyte, m , is given by m , masso, mmic. We assume here that m* is given by mm ma,,,,. Then, the association constant of monomeric electrolyte with cyclodextrin, K , is calculated by
8
CTABr-aCD NaLS-PCD NaLS-mCD
23 24 26 30
PCD. The cmc, m*,was increased in all systems by the addition of CD. Figure 2 demonstrates the change of m* in the various systems containing CTABr and CD. We assume here a 1:1type association as given by
+
CTABr-PCD
65 58 44 39
t PCD, H20 + cetyltrimethylammonium bromide (CTABr) + aCD, H20 + CTABr + PCD, H2O + ethanol + CTABr +
S
+
14 65 51 49
25 35 45 25 25 25
Am
84 101
118 79 57 58
A,,,,,
+
+
where CD and CDf denote the total concentration and the concentration of free-state cyclodextrin, respectively. The values of K obtained using eq 2 are compiled in Tables I and 11. The accuracy was within f 5 % under our experimental condition. It should be noted here that the values of K can be determined without knowledge of m* from conductometric measurements; when the association equilibrium given by eq l is valid, the following holds between K and K when CD >> m and m 5 m*: 1ooo(Ko - K )
= K.1000~- hassocmK (3) CD where K O , CD, and m are the conductance of the solution containing only S, the total concentrations of CD and S, respectively (in this case mmic= 0, or m = m , masso,). A,,,,c is the equivalent conductance of the electrolyte associated with cyclodextrin. From the 1000(~0 - K)/CDvs. 1 0 0 0 plots ~ can be obat constant value of m, the values of K and tained. Figure 3 gives plots for the system, HzO NaLS PCD, at 25 OC. The values of K and Aassocwere obtained to be 360 M-l and 50 0-1 cm2from the straight line. As is clear from the figure, the accuracy is not good (about ic20%), even if the experimental accuracy of K is high ( f o u l %under the present experimental condition). This is due to the small difference between the equivalent conductance of monomeric electrolyte, A,, and that of associated one, A,,,,,. Thus, we must stress here that the more accurate values of K are obtained by using m* (eq 2) rather than using eq 3.
+
+
Amic
78 90 108 I8 48 44
In HzO.
The Journal of Physical Chemistry, Vol. 80, No. 24, 1976
K , M-'
22 29 37
2240 1850 1560
21 22
1110
22
111
356
+
2663
Association of Cyclodextrin with Colloidal Electrolytes
1000 Y
-
Figure 3. 1000(~0 K)/CDvs. 1 0 0 0 ~ plots for the H20
system at 25 O C .
+ NaLS 4- PCD LCTABrl (M1
Next, the equivalent conductances of monomeric-, associFigure 4. Surface tension of aqueous solutions of CTABr in the presence ated-, and micelle-state electrolytes (Am, AaSSOc,and Amicy reof PCD at 25 O C . The values by the curves are the concentration of BCD. spectively) are evaluated. When m 5 m*, the conductance of solution, K , is given by the sum of the ionic conductances of monomeric (K,) and associated state electrolyte (K~~,,,). 20 and 35 R-l cm2 at 0.1 M and at 25 0C.4,7-10Thus, the Amic obtained by us may be reasonable. As is clear in the tables, A, Therefore, ha,,,, is determined from the slope of the line in the concentration region between 0 and m* by using of CTABr is larger than that of NaLS. Amic was strikingly smaller than A,, which is easily understood from the bulkiness K = Km ;I- Kassoc of micelle state electrolyte. A,,,, is comparativelysmaller than A, (about 5 to 30%)and strikingly larger than Ami* Am, Aassoc, AassocKCDOm Amm and Amic of the system H20 CTABr PCD increased with 1000(1+ KCDO) 1000(1+ KCDo) increasing temperature. Furthermore, the increase of K was - A m + AassocKCDom in the order NaLS-aCD < NaLS-PCD < CTABr-aCD < mlm* (4) lOOO(1 KCDo) CTABr-PCD. This order may be attributed to the difference of the hydrophobicitiesof the surfactant and cyclodextrin, Le., where CDo denotes the concentration of CD when the conNaLS < CTABr, aCD < PCD in the strength order. K incentration of surfactant is m. The value of Am is determined creased with temperature. The free energy, enthalpy, and from the slope when cyclodextrin is absent, i.e., CDo = 0. It entropy of the association process of 0-cyclodextrin with should be noted here that eq 3 is easily derived from eq 4, when CTABr were -4.6 kcal mol-1, -3.4 kcal mol-l, and 4 eu, reA, is equal to 1000Ko/m. When m 1 m*, the conductance of spectively, at 35 OC. The exothermic enthalpy observed in the solution is given by present system was also obtained for the association with dye K = Km + Kassoc + Kmic studied by Cramer et al.ll Both Am and Aassocdecreased upon A,m* AassocKCDom* the addition of ethanol or N-methylacetamide, It should be lOOO(1 + KCDo) 1000(1+ KCDo) noted here that Amic increased with ethanol, whereas it decreased with N-methylacetamide. This may imply that the A m i c ( m - m*) - A m + AassocKCDo - Amic m* aggregation number of the micelle-state electrolyte decreases 1000 lOOO(1 KCDo) on account of weakening of hydrophobic attractive forces +-m Amic m L m * (5) between hydrocarbon parts of CTABr by ethanol. In the case 1000 of N-methylacetamide, the mobility of the micelle-state electrolyte may be small because the micelle is solvated on Amic is, therefore, calculated from the slope of the K vs. m plots account of high dipole moment of the organic solvent. Finally, in the concentration region above m*. Amic means, of course, the K values sharply decreased with increasing fraction of the e'quivalent conductance of micelle-state electrolyte with organic solvent. This may be partly due to the fact that the reference to monomer unit. The real ionic equivalent conhydrophobic attractive interactions between cyclodextrin and ductance of the aggregated micelle as a whole can be obtained colloidal electrolyte are weakened by the organic solvent. if the aggregation number of the micelle is known. It should It should be mentioned here that the apparent increase of be noted that we assumed the changes of A m , Aassoc, and Amic cmc upon the addition of CD implies a portion of the available with surfactant concentration being negligibly small in eq 4 monomers in the micellization process is removed by comand 5. The concentration of monomer is not strictly constant plexation of monomers with CD. The surface tension, y, of the above the cmc by mass action theory of the micellization system H20 CTABr PCD as a function of [CTABr] is equilibrium. We furthermore assumed a constancy of monodemonstrated in Figure 4. The aqueous solution of cyclomer concentration above m*, which could be permissible for the micelles of large aggregation numbers. The changes of Am, dextrin did not show any surface activity. Upon the addition of small amounts of CTABr to the aqueous CD solution, y did Aassoc, and Amic with various concentrations of cyclodextrin not decrease. However, y was observed to decrease at higher were within f 5 %in our experiments. This constancy implies CTABr concentration and then remained constant. The apthe validity of eq 1of the assumption of a 1:l type reversible complex formation. In Tables I and 11,A,, Aassoc, and Ami, are parent cmc values, m*, obtained from the surface tension compiled. measurements were also increased upon the addition of CD and agreed with those from the conductance measurements The Amic values of CTABr and NaLS are close to each other within experimental error. If the associated state electrolyte (21 -22 C2-l cm2 at 25 "C). Equivalent conductivities of varhas no surface activity, the observed surface activity of the ious colloidal electrolytes have been reported to be between
+
+
+
+
+
+
+
+
+
The Journal of Physical Chemistry, Vol. 80, No. 24, 1976
2664
W. B. Williamson and J. H. Lunsford
HzO + CTABr + PCD system is attributed to the monomeric stateelectrolyte. The association constant, therefore, can be determined by using the y-[CTABr] profile in the presence of CD and that in its absence. The determination was, however, not carried out because of the comparatively large experimental error of y particularly in the concentration regions of CTABr where y sharply decreases.
(3) H. Schlenk and D. M. Sano, J. Am. Chem. Soc., 83, 2312 (1961). (4) See, for example, A. LottermoserandF. Puschel, KollokfZ, 83,175 (1933); E. L. McBain, W. B. Dye, and S. A. Johnston, J. Am. Chem. SOC.,61,3210 (1939); A. B. Scott and H. V. Tartar, ibid., 65,692 (1943); E. C. Evers and C.A. Kraus, ibid., 70, 3049 (1948); D. C. Robins and I. L. Thomas, J. Colloid Interface Sci., 26, 407 (1968); J. E. Adderson and H. Taylor, J. Pharm. Pharmacol., 23, 311 (1971). (5) J. Jones and M. Ballinger, J. Am. Chem. SOC.,53,411 (1931). (6) E. D. Goddard and G. C. Benson, Can. J. Chem., 35, 986 (1957). (7) A. W.Ralston and d. N. Eggenberger. J. Am. Chem. Soc., 70, 436 11948) - -,. (8) G. D. Parfitt and A. L. Smith, Trans. Faraday SOC.,61, 2736 (1965). (9) A. W.Ralston, D. N. Eggenberger, H. J. Harwood, and P.L. DuBrow, J. Am. Chem. SOC.,69, 2095
