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Crystal phases of dodecyl sulfates obtained from aqueous solutions: structure of the rubidium salt
| Content Provider | Semantic Scholar |
|---|---|
| Author | Coiro, Vincenza Maria Mazza, Fernando |
| Copyright Year | 1989 |
| Abstract | Rubidium dodecyl sulfate, Rb+.C12H2504 S-, M , = 3 5 0 . 8 6 , triclinic, Pi , a = 7 . 1 3 7 (2), b = 7.492(2), c=30-685 (10)A, et= 96.26 (2), t = 90.57(2), y = 8 4 9 1 ( 2 ) ° , V = 1 6 2 4 . 5 ( 8 ) A 3, z = 4 , D x = 1.43 g cm -3, 2(Mo Ka) = 0.71069/~, /z(Mo Kct) = 3 3 . 5 cm -~, F(000) = 728, R = 0.057 for 4665 independent reflections. The molecules form a lamellar structure with alternate polar and apolar regions. Monolayers with facing polar heads are held together tightly by Coulombic forces whereas weak van der Waals interactions are present in the tail-to-tail apolar region. The hydrocarbon chains are tilted on the polar plane but do not interdigitate each other. At variance with the sodium salt crystal phases, no water molecule is present in this crystal phase. Introduction. The knowledge of the structure of micellar aggregates is necessary for the comprehension of their chemical and physical properties and related phenomena. The available models of micellar aggregates in aqueous solution do not give satisfactory accounts of the experimental data and often give rise to contradictions (Dill, Koppel, Cantor, Dill, Bendedouch & Chen, 1984; Menger & Doll, 1984). In fact the techniques employed so far have not been able to provide precise information on the micellar structure. There are micellar systems which change to solid phases (gels, fibers and crystals) by variation of thermodynamic parameters such as concentration, temperature or ionic strength. In the case where the solid and liquid phases give rise to X-ray intensity minima or maxima i n the same regions of the diffraction angle, it is reasonable to suppose that similar structural subunits characterize them. Then, the structural model obtainable from the study of the solid phase can be checked in the investigation of micellar aggregates (Campanelli, Candeloro De Sanctis, Giglio & Petriconi, 1984; Conte, Di Blasi, Giglio, Parretta & Pavel, 1984; Esposito, Zanobi, Giglio, Pavel & Campbell, 1987; Esposito, Giglio, Pavel & Zanobi, 1987; Giglio, Loreti & Pavel, 1988). * Also at Dipartimento di Fisica, Universita' di L'Aquila, 67100 L'Aquila, Italy. 0108-2701/89/081132-05503.00 With the aim of obtaining models to be successively tested in the study of micellar solutions with different techniques we have undertaken the elucidation of crystalline phases of dodecyl sulfates which can be grown from aqueous solutions at a concentration above the critical micellar concentration (Coiro, Mazza & Pochetti, 1986; Coiro, Manigrasso, Mazza & Pochetti, 1987; Coiro, Mazza & Pochetti, 1987). Furthermore, since the micellar growth is very sensitive to the nature of the counterion (Missel, Mazer, Carey & Benedek, 1982), in order to investigate the effect of the counterion on the molecular aggregation of surfactants we are also examining the crystal packing of the same anionic amphiphile with different cations. Here we report the crystal structure of rubidium dodecyl sulfate (RDS). Experimental. The title compound was prepared by sulfation of 1-dodecanol with chlorosulfonic acid; the product obtained was then neutralized with rubidium hydroxide (McIntire, Chiappardi, Casselberry & Blount, 1982). RDS was purified by repeated crystallizations from ethanol and water. Single crystals in the form of colourless prisms were obtained from water. A crystal of 0.8 × 0.5 x 0.4 mm was mounted on an automatic Syntex P21 diffractometer equipped with graphite monochromator and M o K a radiation. The unit-cell parameters were determined from a leastsquares fit of the angular settings of 15 reflections in the range 12 < 0 < 24 °. Because of the X-ray decay three different crystals of about the same dimensions as that used for crystal data were successively used to complete the intensity data collection. Three data sets at increasing intervals of 0 were recorded by the 0-20 scan technique, with a scan rate varying in the interval 29.3-1 .0min -1 up to a maximum 20 value of 56 ° . Background counts were taken for a time equal to half of the scan time. For each set of data, ~scan curves obtained for two reflections with Z ~ 90° were used to take care of anisotropy of absorption and the correction for X-ray decay was performed by three standard reflections (020, 003 and 115) measured after every 100, which showed a steady decrease: when their intensities were less than 40% of © 1989 International Union of Crystallography V. M. COIRO AND F. MAZZA 1133 their initial value a new crystal was mounted for intensity data collection. The three sets of data were subsequently scaled by averaging the intensities of common reflections. After merging equivalent reflections, Rin t was 0.08. A total of 4665 independent x reflections with I > 2 . 5 t r ( / ) were obtained (h-9-.9, Rb(l ) 0.0876(1) S(l) -0.0141 (2) k-9-,9 , 10-,40), and corrected for Lorentz and O(11) 0.1197(8) polarization factors. 2013 unobserved reflections. 0(12) -0.1589 (8) O(13) -0.0825 (7) The structure was solved by the semiinvariant o(~4) 0.1047(8) representation package SIR (Cascarano, Giacovazzo, c(11) 0.167(1) C(12) 0.352 (l) Burla, Nunzi, Polidori, Camalli, Spagna & Viterbo, C(13) 0.404(1) 1985) using 400 reflections with the largest E's (> 1.89) c(14) 0.599 (l) C(15) 0.652 (l) and 100 reflections with the smallest E's (< 0.057). For c(16) 0.848 (1) C(17) 0.902 (1) the convergence-divergence and phase expansion proc(18) 1.097 (I) cesses, 4000 triplets, 13 two-phase semiinvariants and c(19) 1.146 (2) C(110) 1.341 (2) seven one-phase semiinvariants accepted as known C ( I I I ) 1.387(2) phases, among the strongest, were used. The E map C(112) 1.580(2) computed with the phases of the second set having the Rb(2) 0.5080 (1) S(2) 0.6055 (2) highest figures of merit revealed molecular fragments: o(21) 0.4612(6) the location of all the non-H atoms was completed by o(22) 0.7451 (8) 0(23) 0.6748 (7) Fourier recycling, o(24) 0.4917 (8) After isotropic and anisotropic refinement of the c(21) 0.394(1) C(22) 0.230 (1) heavy atoms by block-diagonal least-squares method, C(23) 0-158(1) all the H atoms were found from successive AF c(24) -0.022(1) C(25) -0.091 (1) syntheses: their positional parameters were varied in the C(26) -0-273 (1) C(27) -0.341 (1) last cycles of refinement keeping fixed their isotropic c(28) -0.522(1) temperature factors deduced from the carrier atoms, c(29) -0.587(1) C(210) -0 .768 (1) The function minimized was ~ . w ( I F o l IFcl) z where c(211) -o.83o(2) w = (a + I Fol + e lFol2) -1 with a and e equal to 2Folmi, I c(212) -1.010(2) and 2/Fo(max), respectively. The atomic scattering factors were corrected for anomalous dispersion. In the last refinement cycle (A/Cr)ma× was 0-2 and 0.3 for the positional and thermal parameters of the methyl groups respectively, while much smaller values were obtained for the parameters of the other atoms. The maximum and minimum heights in the final Fourier difference synthesis were 0.2 and 0 . 1 e A -3, respectively. The final R and wR were 0.057 and 0.082, respectively, and S = 0 . 4 . Scattering factors were taken from International Tables for X-ray Crystallography (1974). All the calculations were carried out on the DG ECLIPSE MV/8000II of the CNR Research Area (Montelibretti) using local programs (Camalli, Capitani, Cascarano, Cerrini, Giacovazzo & Spagna, 1986). The final atomic parameters are reported in Table 1.* Discussion. Bond lengths and angles found for the two independent anions are reported in Fig. 1. For each sulfate group the ester S O bond is longer than the other S O bonds and all the O S O valence angles are larger than those involving the ester O atom. These * Lists of structure factors, anisotropic thermal parameters, H-atom parameters and anion valence bond lengths and angles have been deposited with the British Library Document Supply Centre as Supplementary Publication No. SUP 51766 (26 pp.). Copies may be obtained through The Executive Secretary, International Union of Crystallography, 5 Abbey Square, Chester CH 1 2HU, England. Table 1. Final fractional coordinates and Beq values of the non-H atoms with e.s.d.'s in parentheses Beq = ~Zi Zfl~ua,.a j. |
| Starting Page | 1132 |
| Ending Page | 1136 |
| Page Count | 5 |
| File Format | PDF HTM / HTML |
| DOI | 10.1107/S0108270189000636 |
| Volume Number | 45 |
| Alternate Webpage(s) | http://journals.iucr.org/c/issues/1989/08/00/mn0774/mn0774.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
