Structure of the Cetyltrimethylammonium Surfactant at Fused Silica/Aqueous Interfaces Studied by Vibrational Sum Frequency Generation

Abstract

Utilizing vibrational sum frequency generation (SFG), we characterized the structure of adsorbed cetyltrimethylammonium chloride (CTAC) at the silica/aqueous interface in the presence of 10 to 500 mM NaCl and as a function of surfactant surface coverage. For low ionic strengths (10 mM NaCl), results indicate that adsorbed aggregates do not change conformation with increasing surface coverage. Instead, the surfactant adsorbs as micelle-like structures at concentrations considerably lower than surface saturation and the CMC. At high ionic strengths (300-500 mM NaCl), the structure of the adlayer is considerably different: The SFG results indicate that for 30 microM bulk CTAC the surfactant packs with fewer gauche defects in the hydrocarbon backbone, which is attributed to reduced Coulomb repulsion between the positively charged surfactant headgroups, and the results also indicate that CTAC adsorbs as monomers at low surface coverage but then rearranges into aggregates at higher surface coverage.

Macromolecular Structure of Dodecyltrimethylammonium Chloride at the Silica/Water Interface Studied by Sum Frequency Generation Spectroscopy

“…A similar trend was also observed for experiments carried out at a DTAC concentration of 10 mM that will be included in a future publication that examines the SFG spectra as a function of DTAC concentration. The increase in the SFG intensity with the addition of base is consistent with a study carried out by Tyrode et al 20 at near-neutral conditions where CH resonances were not observed for CTAB adsorbed at the silica/water interface in which the lack of peaks was attributed to the centrosymmetric structure of the aggregate, as well as a later study by Hayes et al 25 carried out at pH 11 that observed CH peaks in the SFG spectra for the same surfactant and interface. The SFG spectra shown in Figure 2 indicate that the surfactant macromolecular structure at the silica/water interface becomes more non-centrosymmetric with the addition of acid or base.…”

“…The peak assignment was facilitated by comparing SFG spectra for DTAC and DTAB-d 25 as shown in Figure 3. For DTAB-d 25 , which 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Focusing on the symmetric modes, the SFG spectra were fitted with Eq 4 in the range between 2750 cm -1 and 2890 cm -1 in order to determine the amplitude of these modes. The results are presented in Figure 4.…”

“…Changes in the SFG spectrum have been attributed to an excess of OH -at the interface. 47 However, recent modeling work has alternatively attributed these changes to two phenomenon: (1) the formation of a buried electrical double layer in which 25 the Na + is slightly more buried at the interface and (2) the solvation of OH -. 48…”

“…SFG spectra for DTAB-d25 and DTAC in the presence of 100 mM NaCl and at pD 11. The spectra were collected at the silica/D 2 O interface.…”

Molecular dynamics simulations of surfactants at the silica–water interface: Anionic vs nonionic headgroups

“…Molecular-level information, obtained from both experiments and simulations, is necessary for achieving the desired understanding. Vibrational sum frequency generation (VSFG) experiments have been widely used to study water [44,45] and surfactants [46,47] at interfaces. Adsorption [48,49] and scattering experiments [22,25,26,[50][51][52][53] are performed to determine the structure of the adsorbed surfactant layers.…”