Polymer Journal, Vol. 37, No. 5, pp. 368–375 (2005) Effects of Hydrophobic Chain Length on the Micelles of Heptaoxyethylene Hexadecyl C16E7 and Octadecyl C18E7 Ethers y Yoshiyuki EINAGA, Ai KUSUMOTO, and Akane NODA Department of Chemistry, Nara Women’s University, Kitauoya Nishi-machi, Nara 630-8506, Japan (Received January 4, 2005; Accepted February 1, 2005; Published May 15, 2005) ABSTRACT: Micelles of heptaoxyethylene hexadecyl C16E7 and octadecyl C18E7 ethers in dilute aqueous solu- tions were characterized at finite surfactant concentrations c by static (SLS) and dynamic light scattering (DLS) experi- ments at several temperatures T below the critical points, in order to examine the effects of hydrophobic chain length on size, shape, structure, etc., of the micelles. The SLS results were successfully analyzed by the thermodynamic theory formulated with wormlike spherocylinder model for SLS of micelle solutions, thereby yielding the molar mass Mw of the micelles as a function of c and the cross-sectional diameter d, indicating that the micelles assume a flexible spher- 2 1=2 ocylindrical shape. The radius of gyration hS i and hydrodynamic radius RH of the micelles as functions of Mw were found to be also well described by the corresponding theories for the wormlike spherocylinder or wormlike chain mod- els. The micelles grow in size with increasing T to greater length for longer hydrophobic chain, i.e., alkyl group of the surfactants. They were rather flexible compared to the micelles formed with other polyoxyethylen alkyl ethers. The cross-sectional diameter d and spacings s between adjacent hexaoxyethylene chains on the micellar surface were sim- ilar in the C16E7 and C18E7 micelles. [DOI 10.1295/polymj.37.368] KEY WORDS Polyoxyethylene Alkyl Ether / Micelle / Light Scattering / Diffusion Coefficient / Hydrodynamic Radius / Radius of Gyration / Wormlike Chain / Nonionic surfactants polyoxyethylene mono-alkyl lutions by SLS and DLS measurements. The SLS data ether H(CH2)i(OCH2CH2)jOH, hereafter abbreviated have been analyzed by using the thermodynamic theo- 17,18 as CiEj, form polymerlike micelles in dilute aqueous ry of light scattering for micellar solutions to yield solution (L1 phase) below the lower consolute phase the molar mass Mw of the micelle as a function of sur- boundary. The micelles are known to grow in size factant concentration, along with values of the cross- with increasing surfactant concentration and raising sectional diameter d. The results of the hydrodynamic 2 temperature, in particular when approaching the phase radius RH and mean-square radius of gyration hS i as boundary. The polymerlike micelles have certain sim- functions of Mw have been found to be well described ilarities to real polymers and have been, thus, charac- by the corresponding theory19–22 with wormlike spher- terized with the use of experimental techniques em- ocylinder or chain model, providing the information ployed in the polymer solution studies, such as static of the micellar size, structure, and intrinsic flexibility. (SLS) and dynamic light scattering (DLS),1–6,9 small- In this work, we have applied the same method to 10–12 12,13 angle neutron scattering (SANS), viscometry, micelles of heptaoxyethylene tetradecyl C16E7 and oc- 2–5 pulsed-field gradient NMR, and so forth. However, tadecyl C18E7 ethers, in order to investigate effects of the micelles possess the essential difference from the hydrophobic chain length of the surfactant mole- polymers that the micellar size and its distribution, cules on the micellar characteristics. in general, depend on surfactant concentration, tem- perature, intermicellar thermodynamic interactions, EXPERIMENTAL and other factors. It is formidably difficult to achieve separate evaluation of the micellar growth and the en- Materials hancement of the intermicellar interactions as func- High-purity C16E7 and C18E7 samples were pur- tions of surfactant concentration. Therefore, the inter- chased from Nikko Chemicals Co. Ltd. and used with- pretations given hitherto for the observed results out further purification. The solvent water used was especially by the SLS and DLS experiments are not high purity (ultrapure) water prepared with Simpli unequivocal. Lab water purification system of Millipore Co. In the previous papers, we have studied C12E6 and 14 15 C14E6 micelles, C14E8,C16E8, and C18E8 micelles, Phase Diagram 16 and C10E5 and C10E6 micelles, in dilute aqueous so- Cloud-point temperatures of given test solutions yTo whom correspondence should be addressed (E-mail: [email protected]). 368 Micelles of Heptaoxyethylene Hexadecyl and Octadecyl Ethers were determined as the temperatures at which intensi- measured at 25.0, 35.0, and 45.0 C for C16E7 micelle ty of the laser light transmitted through the solution solutions, and at 38.0, 42.0, and 46.0 C for C18E8 mi- abruptly decreased when temperature was gradually celle solutions, at 632.8 nm with a Union Giken R601 raised, by the fact that the originally transparent solu- differential refractometer. The results were 0.1314 3 3 tions became turbid. cm /g for C16E7 solutions and 0.1116 cm /g for C18E7 solutions, irrespective of temperature. Static Light Scattering SLS measurements were performed to obtain the Dynamic Light Scattering weight-average molar mass Mw of the C16E7 and DLS measurements were carried out to determine C18E7 micelles. The scattering intensities were meas- the translational diffusion coefficient D for the mi- ured for each solution and for the solvent water at celles in water at various temperatures in the one scattering angles ranging from 30 to 150 and at phase (L1 phase) region below or between the phase temperatures T ranging from 30.0 to 45.0 C for C16E7 separation boundaries shown below by the use of solutions and from 38.0 to 44.0 C for C18E7 solutions. the same apparatus and light source as used in the The ratio Kc=ÁR was obtained for each solution as a SLS studies described above. The normalized autocor- function of and extrapolated to zero scattering angle relation function gð2ÞðtÞ of scattered light intensity to evaluate Kc=ÁR0. Here, c is the surfactant mass IðtÞ, i.e., concentration, ÁR is the excess Rayleigh ratio at , hIð0ÞIðtÞi and K is the optical constant defined as gð2ÞðtÞ¼ ð2Þ hIð0Þi2 2 2 2 4 n ð@n=@cÞT;p K ¼ 4 ð1Þ was measured at scattering angles ranging from 30 NA0 to 150. with NA being the Avogadro’s number, 0 the wave- All the test solutions studied are the same as those length of the incident light in vacuum, n the refractive used in the SLS studies. From the data for gð2ÞðtÞ,we index of the solution, ð@n=@cÞT;p the refractive index determined D by the equation increment, T the absolute temperature and p the pres- ð2Þ 2 ð1=2Þ ln½g ðtÞÀ1¼ð1=2Þ ln f À K1t þÁÁÁ ð3Þ sure. The plot of Kc=ÁR vs. sin ð=2Þ affords a good 2 straight line for all the micelle solutions studied. The D ¼ lim K1=q ð4Þ apparent mean-square radius of gyration of the mi- q!0 celles was determined from the slope of the straight Here, f is the coherent factor fixed by the optical sys- line as described below. tem, K1 is the first cumulant, and q is the magnitude of The apparatus used is an ALV DLS/SLS-5000/E the scattering vector defined as light scattering photogoniometer and correlator sys- 4n tem with vertically polarized incident light of q ¼ sinð=2Þð5Þ 632.8 nm wavelength from a Uniphase Model 1145P 0 He–Ne gas laser. For a calibration of the apparatus, It should be noted that the D values should be regard- the intensity of light scattered from pure benzene ed as the z-average, since the micelles observed may was measured at 25.0 C at a scattering angle of 90, have distribution in size. where the Rayleigh ratio RUvð90Þ of pure benzene for unpolarized scattered light with polarized incident Density light at a wavelength of 632.8 nm was taken as The solution density required for the calculation 11:84  10À6 cmÀ1.23,24 of c and the partial specific volume v was measured The micellar solutions were prepared by dissolving at 25.0, 35.0, and 45.0 C for C16E7 micelle solutions, appropriate amount of the surfactant in water. Com- and at 38.0, 42.0, and 46.0 C for C18E7 micelle solu- plete mixing and micelle formation were achieved tions with a pycnometer of the Lipkin–Davison type. by stirring using a magnetic stirrer at least for one For both C16E7 and C18E7 micelle solutions, was day. The solutions thus prepared were optically puri- independent of w. Thus, we have used literature val- fied by filtration through a membrane of 0.20 mm pore ues of the density 0 of pure water at corresponding size and transferred into optically clean NMR tubes of temperatures for , and the v values of the micelles À1 10 mm diameter which was used as scatterring cells. have been calculated as 0 . The weight concentrations w of test solutions were de- termined gravimetrically and converted to mass con- RESULTS AND DISCUSSION centrations c by the use of the densities of the solu- tions given below. Phase Behavior The refractive index increment ð@n=@cÞT;p was Figure 1 depicts phase diagrams of the C16E7 + Polym. J., Vol. 37, No. 5, 2005 369 Y. EINAGA,A.KUSUMOTO, and A. NODA 60 3.0 ° ° : 30.0 C : 40.0 C C16E7 2.5 : 35.0 °C : 45.0 °C C E -1 55 16 7 2.0 /mol g ) 50 0 R ∆ 1.5 / Kc 45 ( 6 1.0 C 10 ° / T 40 0.5 C18E7 0.0 35 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 102c/g cm-3 30 Figure 2.