Bubble Profile Analysis Research Articles

A Multistate Adsorption Model for the Characterization of C13DMPO Adsorption Layers at the Aqueous Solution/Air Interface.

Experimental data for tridecyl dimethyl phosphine oxide (C13DMPO) adsorption layers at the water/air interface, including equilibrium surface tension and surface dilational viscoelasticity, are measured by bubble and drop profile analysis tensiometry at different solution concentrations and surface area oscillation frequencies. The results are used to assess the applicability of a multistate model with more than two possible adsorption states. For the experiments with single drops, the depletion of surfactant molecules due to adsorption at the drop surface is taken into account. For the assessment, the same set of model parameters is used for the description of all obtained experimental dependencies. The agreement between the proposed model and the experimental data shows that for the nonionic surfactant C13DMPO, the description of the adsorption layer behavior by three adsorption states is superior to that with only two adsorption states.

A Multistate Adsorption Model for the Adsorption of C14EO4 and C14EO8 at the Solution/Air Interface

The dynamic and equilibrium properties of adsorption layers of poly (oxyethylene) alkyl ether (CnEOm) can be well described by the reorientation model. In its classical version, it assumes two adsorption states; however, there are obviously surfactants that can adsorb in more than two possible conformations. The experimental data for C14EO4 and C14EO8 (dynamic and equilibrium surface tensions and surface dilational visco-elasticity as measured by bubble profile analysis tensiometry) are used to verify if a reorientation model with more than two possible adsorption states can better describe the complete set data of CnEOm adsorption layers at the water/air interface. The proposed refined theoretical model allows s different states of the adsorbing molecules at the interface. The comparison between the model and experiment demonstrates that, for C14EO4, the assumption of s = 5 adsorption states provides a much better agreement than for s = 2, while for C14EO8, a number of s = 10 adsorption states allows an optimum data description.

Thermodynamics, Kinetics and Dilational Visco-Elasticity of Adsorbed CnEOm Layers at the Aqueous Solution/Air Interface

The adsorption behaviour of linear poly(oxyethylene) alkyl ether (CnEOm) is best described by a reorientation model. Based on a complete set of experimental data, including the adsorption kinetics, the equilibrium surface tension isotherm and the surface dilational visco-elasticity, the thermodynamic and kinetic adsorption parameters for some CnEOm at the water/air interface were determined. For the study, six CnEOm surfactants were selected (n = 10, 12 and 14 and m = 4, 5 and 8) and were studied by bubble profile analysis and maximum bubble pressure tensiometry. A refined theoretical model based on a reorientation-adsorption model combined with a diffusion-controlled adsorption kinetics and exchange of matter allowed us to calculate the surface layer composition by adsorbing molecules in different orientations. It turns out that at larger surface coverage, the adsorption rate decreases, i.e., the apparent diffusion coefficients are smaller. This deceleration can be explained by the transition of molecules adsorbed in a state of larger molar surface area into a state with smaller molar surface area.

Surface tension and dilational rheology of mixed β-casein – β-lactoglobulin aqueous solutions at the water/air interface

Bubble profile analysis tensiometry was used to study the surface tension and dilational visco-elasticity of individual and mixed solutions of β-casein (BCS) and β-lactoglobulin (BLG) at various mixing ratios of the components. The results for the individual protein solutions were analysed based on a new theory, which is applicable in the pre-critical and post-critical concentration range of the surface tension isotherm. It is shown that these proteins are capable of displacing each other from the adsorption layer, which affects the surface tension and visco-elasticity modulus of the adsorbed layers. The model developed for protein mixtures agrees well with the experimental data and the calculated dependencies of the surface properties on the solution composition confirm the mutual influence of added proteins.

Surface Properties and Solubility Enhancement of Anionic/Nonionic Surfactant Mixtures Based on Sulfonate Gemini Surfactants

AbstractAs a class of novel surfactants, Gemini surfactants usually exhibit fairly excellent interfacial properties in aqueous solutions on account of the unique structure. They have significant application and development potential for industrial production. However, the mixing properties of Gemini surfactants with conventional surfactants are the key to their application. The equilibrium surface tension curves of anionic/nonionic surfactant mixtures based on the sulfonate Gemini surfactant (SGS‐12) were measured using the Wilhelmy Plate method. The parameters of surface adsorption, the interaction parameters between anionic and nonionic surfactants, and the thermodynamic parameters of micelle formation were calculated from the corresponding equations. In addition, the dynamic surface tension (DST) curves of anionic/nonionic surfactant mixtures were examined through bubble profile analysis, and the diffusion performance parameters were acquired from empirical formulas. The solubilization of pyrene in micelle solutions was studied using UV–vis absorption spectroscopy. The results show that the interaction parameters of all anionic/nonionic surfactants are negative, indicating that there is a synergistic effect on reducing the surface tension. For the SGS‐12/OP‐10, SGS‐12/Tween 80, SGS‐12/AEO9, and SGS‐12/APG0810 mixtures, the optimum mixing ratios are 6:4, 7:3, 7:3, and 8:2, respectively. The thermodynamic data of micelles show that the formation of mixed micelles for SGS‐12/APG0810 mixtures is an enthalpy‐driven process. The tendency of DST curves of the SGS‐12/APG0810 mixture is similar to that of SGS‐12. In comparison with single‐surfactant solutions, the anionic/nonionic surfactant mixtures show stronger solubilization capacity toward pyrene.

Fluorocarbon Exposure Mode Markedly Affects Phospholipid Monolayer Behavior at the Gas/Liquid Interface: Impact on Size and Stability of Microbubbles

Although most phospholipid-shelled microbubbles (MBs) investigated for medical applications are stabilized by a fluorocarbon (FC) gas, information on the interactions between the phospholipid and FC molecules at the gas/water interface remains scarce. We report that the procedure of introduction of perfluorohexane (F-hexane), that is, either in the gas phase above dimyristoylphosphatidylcholine (DMPC) or dipalmitoylphosphatidylcholine (DPPC) Langmuir monolayers, or in the aqueous subphase, radically affects the compression isotherms. When introduced in the gas phase, F-hexane is rapidly incorporated in the interfacial film, but is also readily desorbed upon compression and eventually totally expelled from the phospholipid monolayers. By contrast, when introduced in the aqueous phase, F-hexane remains trapped at the interface. These dissimilar outcomes demonstrate that the phospholipid monolayer acts as a barrier that effectively hinders the transfer of the FC across the interfacial film. F-hexane was also found to significantly accelerate the adsorption kinetics of the phospholipids at the gas/water interface and to lower the interfacial tension, as assessed by bubble profile analysis tensiometry. The extent of these effects is more pronounced when F-hexane is provided from the gas phase. The size and stability characteristics of DMPC- and DPPC-shelled microbubbles were also found to depend on how the FC is introduced. As compared to reference MBs prepared under nitrogen only, introduction of F-hexane always causes a decrease in MB mean radius. However, while for DMPC this decrease depends on the F-hexane introduction procedure, it is independent from the procedure and most pronounced (from ∼2.0 μm to ∼1.0 μm) for DPPC. Introducing the FC in the gas phase has the strongest effect on MB half-life (t1/2 = ∼1.8 and 6.8 h for DMPC and DPPC, respectively), as compared to when it is delivered through the aqueous phase (∼0.8 and ∼1.7 h). Fluorocarbonless reference DMPC and DPPC bubbles had a half-life of ∼0.5 and 0.8 h, respectively. The effects of F-hexane on MB characteristics are discussed with regard to the interactions between phospholipids and F-hexane and monolayer fluidization effect, as revealed by the Langmuir and tensiometric studies.

Adsorption and surface dilational visco-elasticity of CnEOm solutions as studied by drop profile analysis tensiometry

The surface tension isotherms and dilational visco-elasticity for three nonionic surfactants (C10EO8, C12EO5 and C14EO8) were comparatively studied using the bubble and drop profile analysis tensiometry. The experiments based on drop profiles were analysed assuming the depletion of surfactant molecules from the bulk of the drop due to adsorption. To process the experimental results, two theoretical adsorption layer models (Frumkin and reorientation) were applied, while the reorientation model provides a better description of the experimental results. In addition, the visco-elasticity moduli obtained from drop and bubble profile experiments were compared. It was shown that at higher surfactant concentrations the drop profile method provides visco-elasticities systematically larger than those obtained from bubble profile experiments, even though the adsorption-related depletion has been correctly considered. The proposed correction protocol provides the option for direct comparison between data obtained by the drop profile analysis method with those from other methods.

Equilibrium and dynamic surface properties of cationic/anionic surfactant mixtures based on bisquaternary ammonium salt

The interaction behavior and the process of mixed micellization for binary mixtures of bisquaternary ammonium salt (BQAS) and sodium dodecyl sulfate (SDS) were investigated using the surface tensionmeter, and the synergistic properties of these mixtures were evaluated. The interaction parameters and thermodynamic parameters of BQAS/SDS mixtures were calculated based on the regular solution theory. Moreover, the dynamic surface tension curves of these mixtures were examined using bubble profile analysis method, and the corresponding kinetic parameters were obtained. Results indicate that the critical micelle concentrations of the BQAS: SDS mixtures with the molar ratios of 3:7, 5:5, and 7:3 are 4.0 × 10−5 mol/L, 5.0 × 10−5 mol/L and 6.0 × 10−5 mol/L, respectively, which are two orders of magnitude lower than that of either component. Despite the mixing ratios vary from 3:7 to 7:3, these mixtures have almost equal surface tensions. Consequently, the component of adsorption layer is dominated by the catanionic surfactant complexes, rather than the mixing ratios in the bulk phase. The interaction parameters βσ and βm of binary systems with the BQAS: SDS molar ratios of 5:5 are −18.11 and −15.56, respectively, indicating the strong synergistic effects in the reduction of surface tension and the formation of micelle. The thermodynamic parameters suggest that the formation of micelles is an exothermic and entropy increase process. Moreover, results of dynamic surface tension indicate that the BQAS/SDS mixtures exhibit a faster adsorption process than either pure component, resulting from the enhanced hydrophobicity of catanionic surfactant complexes. In addition, the initial stage of adsorption of BQAS/SDS mixtures exhibits a mixed kinetic-diffusion mechanism.

Surface Tension Measurements with the Drop Profile Analysis Tensiometry—Consideration of the Surfactant Mass Balance in a Single Drop

In drop profile analysis tensiometry, the ratio of drop surfaces area S to volume V is large, i.e., S/V >> 1. In such a case, the concentration of a surfactant within the drop bulk decreases due to adsorption at the drop surface. In contrast, in bubble profile analysis tensiometry, we have S/V << 1 so that depletion due to adsorption is negligible. A protocol is presented to determine the correct adsorption parameters of surfactants from surface tension data measured by bubble and drop profile analysis tensiometry. The procedure is applied to experimental data measured for selected surfactants of different adsorption activities: C10OH, CTAB, Tween 20, and the equimolar mixture SDS + DoTAB. The results show that for surfactants with higher surface activity, the differences between the surface tensions measured with the drops and bubbles profile analysis tensiometry, respectively, are larger, while for less surface-active surfactants, such as SDS, the results obtained from drop and bubble profile experiments are very close. The correction procedure is based on the same set of adsorption parameters used to fit both the experimental data obtained from drop-based measurements (which involve the depletion effects) and those data measured in a way that depletion effects are negligible.

Dilational Viscoelasticity of Adsorption Layers Measured by Drop and Bubble Profile Analysis: Reason for Different Results.

The dilational viscoelasticity of adsorption layer was measured at different frequencies of drop and bubble surface area oscillations for aqueous C12EO5 solutions. The modulus values obtained by the two experimental protocols are the same for Π < 15 mN/m, while for higher surface pressures the values from drop experiments exceed those from bubble profile analysis. The nature of this phenomenon was studied using stress deformation experiments. At high surfactant concentrations the magnitude of surface tension variations is essentially higher for drops as compared with bubbles, leading to an increased viscoelasticity modulus for oscillating drops. The observed effects are analyzed quantitatively using a diffusion controlled exchange of matter model. The viscoelasticity moduli for a number of surfactants (different CnEOm and Tritons, C13DMPO, and SDS) are reported, and it is shown that the discrepancies between the data obtained by the two methods for many surfactants agree well with the predictions made here.

β-Lactoglobulin adsorption layers at the water/air surface: 1. Adsorption kinetics and surface pressure isotherm: Effect of pH and ionic strength

The dynamic surface pressure Π of air bubbles aging in buffered β-lactoglobulin (BLG) solutions containing various protein concentrations CBLG (10−9–10−4M), pH (3–7) and buffer concentrations Cbuff (1, 10 and 100mM) was measured as a function of time t by bubble profile analysis tensiometry. Adsorption kinetics was studied by recording Π(t) data for 80000s and the final Π-values were used to construct the surface pressure isotherm Π(CBLG) for the pH values of 3, 5, 7 (Cbuff=10mM) and 6.3 (in pure water). On the basis of obtained kinetic and concentration dependencies the effect of pH on the protein surface activity was qualitatively analyzed. At constant Cbuff=10mM and relatively low protein concentrations (CBLG<10−8M), BLG exhibits the shortest induction time τind and highest Π-values at pH 7 (negative net charge), in comparison to pH 5 and 3, whereas at sufficiently high protein concentrations (ca. CBLG>10−6M) BLG is the most surface active at pH 5 (negligible net charge in the vicinity of the isoelectric point, pI ≈5.1). At constant Cbuff, BLG solutions with pH 3 show the lowest surface activity. The influence of the ionic strength (buffer concentration) of the solution on the dynamic Π was studied at a selected protein concentration of CBLG=10−5M. The kinetics of adsorption is weakly affected by Cbuff for solutions with pH 5 and significantly enhanced by increasing Cbuff for solutions with pH≠pI. The effect of the chemical nature of the electrolyte was obtained for non-buffered aqueous BLG solutions (natural pH ∼6.3) at different concentrations of NaCl or CaCl2. It is demonstrated that the adsorption kinetics and the surface pressure isotherm of BLG at the water/air surface depend strongly on the protein effective charge, which is dictated by the solution pH and ionic strength.

Using profile analysis tensiometry for monitoring auto-oscillations caused by the hydrolysis of 3-(trimethoxysilyl)propyl methacrylate when contacting water

3-(trimethoxysilyl)propyl methacrylate (TPM) hydrolyzes via the alkoxy groups attached to silicon when it comes into contact with an aqueous phase and forms surface active 3-(trihydroxysilyl)propyl methacylate (THPM) molecules. Their formation and adsorption at the water/air interface, i.e. decrease in the surface tension, can be used as a sensor to follow the hydrolysis process and can help to explain the role of these molecules in the emulsion formation and stabilization process. The target of this contribution is to demonstrate the experimental details of a surface tension sensor using a bubble profile analysis tensiometer for monitoring the chemical reaction of TPM with water and to explain the peculiarities observed in the measured signal, i.e. irregularities in the dynamic surface tension. Auto-oscillations of the surface tension were observed during the measurements, which are connected to a complex of different types of flows in the volume of the aqueous phase. The kinetics of the reaction and hence of the surface tension can be controlled by changing the pH of the aqueous phase. It is also possible to prevent auto-oscillations by means of a continuous gentle stirring of the aqueous phase, which then allows a quantitative monitoring of the rate of the TPM/water reaction.

Recruitment and Immobilization of a Fluorinated Biomarker Across an Interfacial Phospholipid Film using a Fluorocarbon Gas

AbstractPerfluorohexane gas when introduced in the air atmosphere above a film of phospholipid self‐supported on an aqueous solution of C2F5‐labeled compounds causes the recruitment and immobilization of the latter in the interfacial film. When the phospholipid forms a liquid‐condensed Gibbs monolayer, which is the case for dipalmitoylphosphatidylcholine (DPPC), the C2F5‐labeled molecule remains trapped in the monolayer after removal of F‐hexane. Investigations involve bubble profile analysis tensiometry (Gibbs films), Langmuir monolayers and microbubble experiments. The new phenomenon was utilized to incorporate a hypoxia biomarker, a C2F5‐labeled nitrosoimidazole (EF5), in microbubble shells. This finding opens perspectives in the delivery of fluorinated therapeutic molecules and biomarkers.

Recruitment and Immobilization of a Fluorinated Biomarker Across an Interfacial Phospholipid Film using a Fluorocarbon Gas.

Perfluorohexane gas when introduced in the air atmosphere above a film of phospholipid self-supported on an aqueous solution of C2F5-labeled compounds causes the recruitment and immobilization of the latter in the interfacial film. When the phospholipid forms a liquid-condensed Gibbs monolayer, which is the case for dipalmitoylphosphatidylcholine (DPPC), the C2F5-labeled molecule remains trapped in the monolayer after removal of F-hexane. Investigations involve bubble profile analysis tensiometry (Gibbs films), Langmuir monolayers and microbubble experiments. The new phenomenon was utilized to incorporate a hypoxia biomarker, a C2F5-labeled nitrosoimidazole (EF5), in microbubble shells. This finding opens perspectives in the delivery of fluorinated therapeutic molecules and biomarkers.

Perfluoroalkylated poly(oxyethylene) thiols: Synthesis, adsorption dynamics and surface activity at the air/water interface, and bubble stabilization behaviour

New amphiphiles with a poly(oxyethylene) spacer inserted between a thiol function and a F-alkylated chain have been synthesized. The F -alkylated POE thiols adsorb rapidly at the air/water interface and decrease the tension of water to values as low as ∼16 mN m −1 . They form microbubbles in aqueous solutions with diameters smaller than 10 μm. These microbubbles were stable for days when a fluorocarbon gas is present in their gas phase. • New amphiphiles with a poly(oxyethylene) spacer inserted between a thiol function and a F -alkylated chain have been synthesized. • The F -alkylated POE thiols adsorb rapidly at the air/water interface. • They decrease the interfacial tension of water to values as low as ∼16 mN m −1 . • They form microbubbles in aqueous solutions with diameters smaller than 10 μm. • The microbubbles were stable for days when a fluorocarbon gas is present in their gas phase. We report the synthesis of four new perfluoroalkylated thiols, C n F 2 n +1 (CH 2 ) 2 (OCH 2 CH 2 ) y SH ( FnH 2(OE) y with n = 4 and 6; y = 4 and 6, that have a hydrophilic poly(oxyethylene) (POE) segment inserted between the hydrophobic fluorinated chain and the thiol functional group. The dynamics of adsorption and surface activity of these compounds at the air/water interface were studied using bubble profile analysis tensiometry. The adsorption kinetics profiles show that all compounds diffuse rapidly to the air/water interface and decrease the interfacial tension of water to values ranging from 16.5 to 31.6 mN m −1 , which qualifies them as potential components of microbubble shells. Their ability to self-assemble and stabilize gaseous microbubbles in the presence or absence of a fluorocarbon gas (perfluorohexane) was investigated by optical microscopy. Three out of the four compounds (the two F 6 derivatives and F 4 H 2(OE) 6 ) were able to form and stabilize microbubbles. Microbubbles based on both F 6 PEG thiols are more stable than those based on the F 4 derivative. The number of oxyethylene (OE) groups substantially influences bubble stability; the higher the number of OEs, the more stable the microbubbles. Finally, the most important stabilising effect was obtained by introducing an internal fluorocarbon gas, whose presence in the bubble increases bubble stability from 6 h to 5 days for the most promising candidate, F 6 H 2(OE) 6 SH.

Adsorption isotherm and equation of state for β-Lactoglobulin layers at the air/water surface

The adsorption behaviour of β-Lactoglobulin (BLG) at the air/water surface and the visco-elastic properties of the BLG adsorption layers are discussed in terms of quasi-equilibrium adsorption isotherms and equation of state gained by comparing a model with a set of experimental data obtained by bubble profile analysis tensiometry. The presented simultaneous characterization of the adsorption isotherm and the dilational visco-elasticity under dynamic and equilibrium conditions is based on measurements of the dynamic surface pressure and its response to sinusoidal bubble area variations. The thermodynamic model was applied to the set of experimental data under the assumptions of monolayer compressibility, surface dimerization and development of a secondary layer. The effect of the bubble surface age on the adsorption isotherm and equation of state, and also on the visco-elastic characteristics is discussed and it allows concluding that the existence of a bilayer structure, which is formed beyond a certain protein adsorption, is responsible for the observed relaxation processes.

Effect of water hardness on surface tension and dilational visco-elasticity of sodium dodecyl sulphate solutions

The complementary drop and bubble profile analysis and maximum bubble pressure tensiometry are used to measure the dynamic surface tension of aqueous SDS solutions in the presence of hardness salts (CaCl2 and MgCl2 in the ratio of 2:1 at concentrations of 6 and 40FH). The presence of hardness salts results in an essential increase of the SDS adsorption activity, which indicates the formation of Ca(DS)2 and Mg(DS)2 in the SDS solutions. The surface tension isotherms of SDS in presence of Ca(DS)2 and Mg(DS)2 are described using the generalised Frumkin model.The presence of hardness salts accelerates the ageing of SDS solutions as compared with the addition of 0.01M NaCl due to a faster hydrolysis and hence formation of dodecanol. These results are used to estimate the possible concentration of dodecanol in the studied SDS solutions.The buoyant bubble profile method with harmonic surface oscillations is used to measure the dilational rheology of SDS solutions in presence of hardness salts in the frequency range between 0.005Hz and 0.2Hz. The visco-elasticity modulus in the presence of hardness salts is higher as compared with its values in the presence of 0.01M NaCl additions. The ageing of SDS solutions leads to an essential increase of the visco-elastic modulus.

Characterisation of alkyl amines at the water/air surface with the drop and bubble profile analysis tensiometry

Pendant drop and buoyant bubble methods have been used to study the surface characteristics of alkyl amines at the water/air surface. The investigated alkyl amines, triethylamine and octylamine, showed unusual changes in the surface tension as a function of time: an initially steep drop and a subsequent steady increase in the surface tension until a value close to the one of the pure water/air system was observed. This phenomenon is explained by the evaporation of the alkyl amines, for which several sets of experiments have been conducted with the pendant drop and buoyant bubble methods. Using an appropriate experimental protocol, the equilibrium adsorption behaviour of the two amines can be quantitatively measured.

A Nonpolar, Nonamphiphilic Molecule Can Accelerate Adsorption of Phospholipids and Lower Their Surface Tension at the Air/Water Interface

The adsorption dynamics of a series of phospholipids (PLs) at the interface between an aqueous solution or dispersion of the PL and a gas phase containing the nonpolar, nonamphiphilic linear perfluorocarbon perfluorohexane (PFH) was studied by bubble profile analysis tensiometry. The PLs investigated were dioctanoylphosphatidylcholine (DiC(8)-PC), dilaurylphosphatidylcholine, dimyristoylphosphatidylcholine, and dipalmitoylphosphatidylcholine. The gas phase consisted of air or air saturated with PFH. The perfluorocarbon gas was found to have an unexpected, strong effect on both the adsorption rate and the equilibrium interfacial tension (γ(eq)) of the PLs. First, for all of the PLs, and at all concentrations investigated, the γ(eq) values were significantly lower (by up to 10 mN m(-1)) when PFH was present in the gas phase. The efficacy of PFH in decreasing γ(eq) depends on the ability of PLs to form micelles or vesicles in water. For vesicles, it also depends on the gel or fluid state of the membranes. Second, the adsorption rates of all the PLs at the interface (as assessed by the time required for the initial interfacial tension to be reduced by 30%) are significantly accelerated (by up to fivefold) by the presence of PFH for the lower PL concentrations. Both the surface-tension reducing effect and the adsorption rate increasing effect establish that PFH has a strong interaction with the PL monolayer and acts as a cosurfactant at the interface, despite the absence of any amphiphilic character. Fitting the adsorption profiles of DiC(8)-PC at the PFH-saturated air/aqueous solution interface with the modified Frumkin model indicated that the PFH molecule lay horizontally at the interface.

Equilibrium adsorption layer characteristics of mixed sodium dodecyl sulphate/Triton solutions

The adsorption of the anionic surfactant SDS in mixtures with non-ionic surfactants of the Triton type (X-45, X-100, X-165, X-405) is studied by bubble profile analysis tensiometry at the solution/air interface. The surface tension isotherms are described by a thermodynamic model for mixed surfactants which allows consideration of different adsorption models for the two compounds: a reorientation model for the Tritons and a Frumkin model for SDS. The experimental data are very well described by the model while for the mixtures the characteristic parameters of the single compounds have been used.