Peixun Li
University of Oxford
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Langmuir | 2013
Hui Xu; Peixun Li; Kun Ma; Robert K. Thomas; J. Penfold; Jian R. Lu
This is a second paper responding to recent papers by Menger et al. and the ensuing discussion about the application of the Gibbs equation to surface tension (ST) data. Using new neutron reflection (NR) measurements on sodium dodecylsulfate (SDS) and sodium dodecylmonooxyethylene sulfate (SLES) above and below their CMCs and with and without added NaCl, in conjunction with the previous ST measurements on SDS by Elworthy and Mysels (EM), we conclude that (i) ST measurements are often seriously compromised by traces of divalent ions, (ii) adsorption does not generally reach saturation at the CMC, making it difficult to obtain the limiting Gibbs slope, and (iii) the significant width of micellization may make it impossible to apply the Gibbs equation in a significant range of concentration below the CMC. Menger et al. proposed ii as a reason for the difficulty of applying the Gibbs equation to ST data. Conclusions i and iii now further emphasize the failings of the ST-Gibbs analysis for determining the limiting coverage at the CMC, especially for SDS. For SDS, adsorption increases above the CMC to a value of 10 × CMC, which is about 25% greater than at the CMC and about the same as at the CMC in the presence of 0.1 M NaCl. In contrast, the adsorption of SLES reaches a limit at the CMC with no further increase up to 10 × CMC, but the addition of 0.1 M NaCl increases the surface excess by 20-25%. The results for SDS are combined with earlier NR results to generate an adsorption isotherm from 2 to 100 mM. The NR results for SDS are compared to the definitive surface tension (ST) measurements of EM, and the surface excesses agree over the range where they can safely be compared, from 2 to 6 mM. This confirms that the anomalous decrease in the slope of EMs σ - ln c curve between 6 mM and the CMC at 8.2 mM results from changes in activity associated with a significant width of micellization. This anomaly shows that it is impossible to apply the Gibbs equation usefully from 6 to 8.2 mM (i.e., the lack of knowledge of the activity in this range is the same as above the CMC (8.2 mM)). It was found that a mislabeling of the original data in EM may have prevented the use of this excellent ST data as a standard by other authors. Although NR and ST results for SDS in the absence of added electrolyte show that the discrepancies can be rationalized, ST is generally shown to be less accurate and more vulnerable to impurities, especially divalent ions, than NR. The radiotracer technique is shown to be less accurate than ST-Gibbs in that the four radiotracer measurements of the surface excess are consistent neither with each other nor with ST and NR. It is also shown that radiotracer results on aerosol-OT are likely to be incorrect. Application of the mass action (MA) model of micellization to the ST curves of SDS and SLES through and above the CMC shows that they can be explained by this model and that they depend on the degree of dissociation of the micelle, which leads to a larger change in the mean activity, and hence the adsorption, for the more highly dissociated SDS micelles than for SLES. Previous measurements of the activity of SDS above the CMC were found to be semiquantitatively consistent with the change in mean activity predicted by the MA model but inconsistent with the combined ST, NR, and Gibbs equation results.
Langmuir | 2013
Peixun Li; Z. X. Li; Hsin Hui Shen; Robert K. Thomas; J. Penfold; Jian R. Lu
Four recent papers by Menger et al. have questioned methods of analysis of surface tension (ST) data that use the Gibbs equation to obtain the surface excess (Γ) of a surfactant at the air-water interface. There have been two responses which challenge the assertions of Menger et al. and a response from Menger et al. We use directly determined values of Γ from a range of neutron reflectometry (NR) data to examine some of the issues that are relevant to these seven papers. We show that there is excellent agreement between NR measurements and careful ST analyses for a wide range of nonionic adsorbents, including surfactants and polymers. The reason it is possible to obtain good agreement near the critical micelle concentration (CMC) is that nonionic surfactants generally seem to saturate the surface before the CMC is reached and this makes it relatively easy to determine the limiting slope (and hence Γ) of the ST-log(concentration) plot at the CMC. Furthermore, there is also generally good agreement between ST and NR over the whole range of concentrations below the CMC until depletion effects become important. Depletion effects are shown to become important at higher concentrations than expected, which brings them into the range of many experiments, including techniques other than ST and NR. This is illustrated with new measurements on the biosurfactant surfactin. The agreement between ST and NR outside the depletion range can be regarded as a mutual validation of the two methods, especially as it is demonstrated independently of any model adsorption isotherms. In the normal experimental situation NR is less vulnerable to depletion than ST and we show how NR and a single ST measurement can be used to determine the hitherto undetermined CMC of the nonionic surfactant C18E12, which is found to be 1.3 × 10(-6) M.
Langmuir | 2014
Peixun Li; Robert K. Thomas; J. Penfold
Neutron reflection (NR) and surface tension (ST) are used to show that there are serious limitations in applying the Gibbs equation accurately to ST data of cationic surfactants to obtain the limiting surface excess, Γ(CMC), at the critical micelle concentration (CMC). Nonionic impurities in C12TABr and C16TABr have been eliminated by extensive purification to give ST - ln(concentration) (σ - ln c) curves that are convex with respect to the ln c axis around the CMC, which is characteristic of a finite micellization width. Because NR shows that the surface excess often continues to increase at and above the CMC, this finite width makes it impossible to apply the Gibbs equation to obtain Γ(CMC) without knowledge of the effect of aggregation on the activity. NR data made it possible to apply the integrated Gibbs equation to the ST below the onset of the convex region of the σ - ln c curve and show that for C12TABr the micellization width causes the ST to underestimate Γ(CMC) by 12%. Hexadecyltrimethylammonium (C16TA) sulfate is used to show that divalent ion impurities are not a significant problem. For cationic surfactants, further errors are associated with ST methods that rely on complete wetting. Measurements using ring, plate, and bubble shape analyses indicate that with ring and plate incomplete wetting occurs at or above the CMC and may extend to lower concentrations and also causes the ST-Gibbs analysis to underestimate the surface excess. In combination with ion association and preaggregation in cationic gemini surfactants, this can cause errors as large as 100% in Γ(CMC). Comparison of ellipsometry and NR for C16TAX in 0.1 M KX (X = F or Cl) shows that ellipsometry cannot, as yet, be quantitatively modeled accurately enough for surface excess determination independent of NR calibration.
Langmuir | 2012
Ling Xiang Jiang; Jianbin Huang; Alireza Bahramian; Peixun Li; Robert K. Thomas; J. Penfold
The properties and phase diagrams of aqueous mixtures of dodecyltrimethylammonium bromide (C(12)TAB) with the sodium oligoarene sulphonates (POSn), POS2, POS3, POS4, and POS6 have been studied using surface tension and neutron reflectometry to study the surface, and neutron small angle scattering and fluorescence to study the bulk solution. The behavior of POS2 and POS3 is reasonably consistent with mixed micelles of C(12)TAB and POSn-(C(12)TA)(n). These systems exhibit a single critical micelle concentration (CMC) at which the surface tension reaches the usual plateau. This is contrary to a recent report which suggests that the onset of the surface tension plateau does not coincide with the CMC. In the POS3 system, the micelles conform to the core-shell model, are slightly ellipsoidal, and have aggregation numbers in the range 70-100. In addition, the dissociation constant for ionization of the micelles is significantly lower than for free C(12)TAB micelles, indicating binding of the POS3 ion to the micelles. Estimation of the CMCs of the POSn-(C(12)TA)(n) from n = 1-3 assuming ideal mixing of the two component surfactants and the observed values of the mixed CMC gives values that are consistent with the nearest related gemini surfactant. The POS4 and POS6 systems are different. They both phase separate slowly to form a dilute and a concentrated (dense) phase. Fluorescence of POS4 has been used to show that the onset of aggregation of surfactant (critical aggregation concentration, CAC) occurs at the onset of the surface tension plateau and that, at the slightly higher concentration of the phase separation, the concentration of POS4 and C(12)TAB in the dilute phase is at or below its concentration at the CAC, that is, this is a clear case of complex coacervation. The surface layer of the C(12)TA ion in the surface tension plateau region, studied directly by neutron reflectometry, was found to be higher than a simple monolayer (observed for POS2 and POS3) for both the POS4 and POS6 systems. In POS6 this evolved after a few hours to a structure consisting of a monolayer with an attached subsurface bilayer, closely resembling that observed for one class of polyelectrolyte/surfactant mixtures. It is suggested that this structured layer, which must be present on the surface of the dilute phase of the coacervated system, is a thin wetting film of the dense phase. The close resemblance of the properties of the POS6 system to that of one large group of polyelectrolyte/surfactant mixtures shows that the surface behavior of oligoion/surfactant mixtures can quickly become representative of that of true polyelectrolyte/surfactant mixtures. In addition, the more precise characterization possible for the POS6 system identifies an unusual feature of the surface behavior of some polyelectrolyte/surfactant systems and that is that the surface tension can remain low and constant through a precipitation/coacervation region because of the characteristics of two phase wetting. The well-defined fixed charge distribution in POS6 also suggests that rigidity and charge separation are the factors that control whether a given system will exhibit a flat surface tension plateau or the alternative of a peak on the surface tension plateau.
Langmuir | 2011
Peixun Li; Chu Chuan Dong; Robert J. Thomas; J. Penfold; Yilin Wang
We have measured the structure and properties of a series of dicationic quaternary ammonium compounds α,ω-bis(N-alkyl dimethyl ammonium)hexane halides (Cn-C6-Cn) for values of the alkyl chain length n of 8, 9, 10, 11, 12, and 16, and a series of α,ω-bis(N-alkyl dimethyl ammonium)diethylether halides (Cn-C2OC2-Cn) for values of n of 8, 12, and 16, as well as C8-C12-C8 and C12-C10-C12 at the air/water interface. Although the critical micelle concentration (CMC) in the two series decreases in the normal way, that is, logarithmically, with increasing chain length, the limiting surface tension at the CMC and the limiting area per molecule both increase with chain length, in the opposite direction from comparable single chain surfactants. The structures of the surface layers, which were determined by neutron reflectometry, indicate that the anomalous behavior of the surface tension and area are probably caused by poor packing of the gemini side chains between adjacent molecules. Comparison of the directly determined surface coverage using neutron reflectometry and the apparent coverage determined by application of the Gibbs equation to surface tension data gives an experimental measurement of the prefactor in the Gibbs equation, which should be 3 for these geminis. It was found to vary from about 3 for the two C16 geminis down to about 1.5 for the two C8 geminis. We have devised a simple quantitative model that explains this variation and earlier observations that the Gibbs prefactor for C12-Cn-C12 (n varying from 3 to 12) is around 2. The model is consistent with the conductivity, NMR, and fluorescence measurements of other authors. This model shows that both dimerization and ion association are required to explain the surface tension behavior of cationic gemini bromide surfactants and that, in many cases, the prefactor itself varies with concentration.
Soft Matter | 2013
Francesca Speranza; Georgia A. Pilkington; Thomas G. Dane; Philip T. Cresswell; Peixun Li; Robert M. J. Jacobs; Thomas Arnold; Laurence Bouchenoire; Robert K. Thomas; Wuge H. Briscoe
Despite extensive studies with many experimental techniques, the morphology and structure of the self-assembled aggregates of quaternary alkyl ammonium bromides (CnTABs; where n denotes the number of hydrocarbons in the surfactant tail) at the solid–liquid interface remains controversial, with results from atomic force microscopy (AFM) imaging pointing to a variety of surface aggregates such as cylinders and surface micelles, whilst surface force measurements and neutron reflectivity (NR) measurements reporting bilayer structures. Using a home-built liquid cell that employs the “bending mica” method, we have performed unprecedented synchrotron X-ray reflectometry (XRR) measurements to study the adsorption behaviour of a CnTAB series (n = 10, 12, 14, 16 and 18) at the mica–water interface at different surfactant concentrations. We find that our XRR data cannot be described by surface aggregates such as cylindrical and spherical structures reported by AFM studies. In addition we have observed that the bilayer thickness, surface coverage and the tilt angle all depend on the surfactant concentration and surfactant hydrocarbon chain length n, and that the bilayer thickness reaches a maximum value at approximately the critical micellisation concentration (∼1 cmc) for all the CnTABs investigated. We propose that CnTABs form disordered bilayer structures on mica at concentrations below cmc, whilst at ∼1 cmc they form more densely packed bilayers with the tails possibly tilted at an angle θt ranging from ∼40 to 60° with respect to the surface normal in order to satisfy the packing constraints due to the mica lattice charge, i.e. so that the cross-section area of the tilted chain would match that of the area of the lattice charge (As ≅ 46.8 A2). As the surfactant concentration further increases, we find that the bilayer thickness decreases, and we ascribe this to the desorption of surfactant molecules, which recovers certain disorder and fluidity in the chain and thus leads to interdigitated bilayers again. In light of our XRR results, previously unattainable at the mica–water interface, we suggest that the surface aggregates observed by AFM could be induced by the interaction between the scanning probe and the surfactant layer, thus representing transient surface aggregation morphologies; whereas the CnTAB bilayers we observe with XRR are intrinsic structures under quiescent conditions. The suggestion of such quiescent bilayers will have fundamental implications to processes such as lubrication, self-assembly under confinement, detergency and wetting, where the morphology and structure of surfactant layers at the solid–liquid interface is an important consideration.
Soft Matter | 2012
Wuge H. Briscoe; Francesca Speranza; Peixun Li; Oleg Konovalov; Laurence Bouchenoire; Jan van Stam; Jacob Klein; Robert M. J. Jacobs; Robert K. Thomas
We describe here the design of a liquid cell specific for synchrotron X-ray reflectometry (XRR) characterisation of soft matter nanofilms at the mica–water interface. The feature of the cell is a “bending mica” method: by slightly bending the mica substrate over an underling cylinder the rigidity of the mica sheet along the bending axis is enhanced, providing sufficient flatness along the apex of the cylinder as required by XRR measurements. Using this cell, we have performed XRR measurements for a number of systems and in this article we show example results: (1) a cationic surfactant, C16TAB; (2) a zwitterionic surfactant, C12H25PC; (3) a semi-fluorinated surfactant, F4H11(d)TAB; and (4) surface complex of an anionic fluorinated surfactant, CsPFN, and a positively charged polymer, PEI. For the data analysis we have taken into account the mica crystal truncation rod, i.e. the reflectivity from the mica substrate, and fitted the data with a custom Java™ based software package. Our results unravel detailed structural information of these soft nanofilms, indicating that this method is suitable for XRR measurements of a wide range of soft matter structures at the mica–water interface.
Journal of Physical Chemistry B | 2014
I. Tucker; Jordan T. Petkov; J. Penfold; Robert K. Thomas; Peixun Li; Andrew Richard Cox; Nick Hedges; John R. P. Webster
The synergistic interactions between certain ethoxylated polysorbate nonionic surfactants and the protein hydrophobin result in spontaneous self-assembly at the air-water interface to form layered surface structures. The surface structures are characterized using neutron reflectivity. The formation of the layered surface structures is promoted by the hydrophobic interaction between the polysorbate alkyl chain and the hydrophobic patch on the surface of the globular hydrophobin and the interaction between the ethoxylated sorbitan headgroup and hydrophilic regions of the protein. The range of the ethoxylated polysorbate concentrations over which the surface ordering occurs is a maximum for the more hydrophobic surfactant polyoxyethylene(8) sorbitan monostearate. The structures at the air-water interface are accompanied by a profound change in the wetting properties of the solution on hydrophobic substrates. In the absence of the polysorbate surfactant, hydrophobin wets a hydrophobic surface, whereas the hydrophobin/ethoxylated polysorbate mixtures where multilayer formation occurs result in a significant dewetting of hydrophobic surfaces. The spontaneous surface self-assembly for hydrophobin/ethoxylated polysorbate surfactant mixtures and the changes in surface wetting properties provide a different insight into protein-surfactant interactions and potential for manipulating surface and interfacial properties and protein surface behavior.
Langmuir | 2008
Mauro Moglianetti; Peixun Li; Fred L. G. Malet; Steven P. Armes; Robert J. Thomas; Simon Titmuss
The interactions between the weak polyelectrolyte, poly(2-(dimethylamino) ethyl methacrylate) or PDMAEMA, and the anionic surfactant sodium dodecyl sulfate (SDS) at the air-water interface have been investigated at pH = 3 and 9 using a combination of neutron reflectivity and surface tension measurements. By using deuterated PDMAEMA in combination with h-SDS and d-SDS, we have been able to directly determine the distribution of both the polymer and the surfactant at the air-water interface. At pH = 3, the polyelectrolyte is positively charged while at pH = 9 it is essentially uncharged. The enhancement in the adsorption of SDS at low coverage suggests that surface active polymer surfactant complexes are forming and adsorbing at the interface. This leads to close to monolayer adsorption of SDS, suggesting that it is surfactant monomers that are complexing with polymers that are in extended conformations parallel to the surface. As the concentration of SDS in the mixtures changes so does the surfactant content of the complexes, which affects the surface activity and hence the coverage of the complexes. Multilayer structures are formed at SDS concentrations of 0.1 and 1 mM, for pH = 3 and 9, respectively.
Langmuir | 2015
J. Penfold; Robert K. Thomas; Peixun Li; Jordan T. Petkov; I. Tucker; John R. P. Webster; Ann E. Terry
The Tween nonionic surfactants are ethoxylated sorbitan esters, which have 20 ethylene oxide groups attached to the sorbitan headgroup and a single alkyl chain, lauryl, palmityl, stearyl, or oleyl. They are an important class of surfactants that are extensively used in emulsion and foam stabilization and in applications associated with foods, cosmetics and pharmaceuticals. A range of ethoxylated polysorbate surfactants, with differing degrees of ethoxylation from 3 to 50 ethylene oxide groups, have been synthesized and characterized by neutron reflection, small-angle neutron scattering, and surface tension. In conjunction with different alkyl chain groups, this provides the opportunity to modify their surface properties, their self-assembly in solution, and their interaction with macromolecules, such as proteins. Adsorption at the air-water and oil-water interfaces and solution self-assembly of the range of ethoxylated polysorbate surfactants synthesized are presented and discussed.