Contact Angles Of Surfactant Solutions Research Articles

Modulation of the wettability of excipients by surfactant and its impacts on the disintegration and release of tablets

Objective: To investigate the modulation of the wettability of excipients by different types of surfactants and its impacts on the disintegration of tablets and drug release.Materials and methods: The critical micelle concentration (CMC) of surfactants, including sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS), dodecyl trimethyl ammonium bromide (DTAB), cetyltrimethyl ammonium bromide (CTAB) and polysorbate (Tween-20 and Tween-80), was obtained using the platinum ring method. Contact angles of surfactant solutions on the excipient compacts and double-distilled water on the mixture of surfactant and the other excipient (magnesium stearate (MgSt) or sodium alginate (SA)) were measured by the sessile drop technique. Besides, surface free energy of excipients was calculated by the Owens method. Finally, the disintegration of tablets and in vitro dissolution testing were performed according to the method described in USP.Results and discussion: The wettability of excipients could be enhanced to different extent with low concentration of surfactant solutions and maintained stable basically after CMC. For MgSt (hydrophobic excipient), the shorter the hydrophobic chain (C12, including SDS and DTAB), the better the wettability with the addition of surfactant in the formulation, leading to the shorter disintegration time of tablets and higher drug release rate. In contrast, the wettability of SA (hydrophilic excipient) was reduced by adding surfactant, resulting in the longer disintegration time of tablets and lower release rate.Conclusion: The modulation of the wetting of pharmaceutical excipients by surfactant had changed the disintegration time of tablets and drug release rate to a greater extent.

Cleaning of Starchy Soils in Clean-in-Place (CIP) Systems: Relationship Between Contact Angle and Detergency

The cleaning of hard surfaces soiled with starch films using nonionic (alkylpolyglucosides and polyoxyethylene alkyl ethers) and zwitterionic (lauramine oxide) surfactant solutions has been investigated. Response surface methodology was applied to determine the effect of the composition of the cleaning solution on detergency, as well as the relationship between the contact angle of surfactant solutions on starch films and the removal efficacy of such films. The surfactants with a higher capacity for the removal of starch films were lauramine oxide and alkylpolyglucosides, and their mixtures. The effect of polyoxyethylene alkyl ethers was also statistically significant; however, their contribution to soil removal was very low. A correlation between contact angle and detergency for starchy soils was found, and the importance of the hydration process of the starch film on its removal was established.

Contact angles of surfactant solutions on heterogeneous surfaces.

Using Gibbs' adsorption equation and a literature isotherm, a new general model to predict the contact angle of surfactant solutions on (smooth or rough) chemically heterogeneous surfaces is constructed based on the Cassie equation. The model allows for adsorption at the liquid-vapor, solid-liquid, and solid-vapor interfaces. Solid-vapor adsorption is allowed in order to model the autophobic effect on hydrophilic surfaces. Using representative values for the coefficients which describe adsorption at each interface, model predictions for contact angles as a function of f parameters (area fractions) and surfactant concentration are made for heterogeneous surfaces made up of different materials. On smooth surfaces, the f parameters serve as weighting factors determining how to combine the effects of surfactant adsorption on each material to predict the behavior on the heterogeneous surface. Due to the non-linear nature of the model, the inclusion of a small amount of hydrophobic material has a greater effect on a predominantly hydrophilic material than vice versa, explaining the result seen in literature that a small amount of hydrophobic contamination (such as oil) significantly increases contact angle on a hydrophilic surface. The fact that even a small amount of heterogeneity can greatly change experimental results could lead to incorrect experimental conclusions about surfactant adsorption if a surface were wrongly assumed to be homogeneous. Model predictions rapidly become more complex as the number of differently wettable materials present on the surface increases. Also, an approximately equal weighting of different materials generally leads to more complex behaviors compared to heterogeneous surfaces composed largely of a single material. Rough heterogeneous surfaces follow previous results for surfactant wetting of rough homogeneous surfaces, leading to an amplification/attenuation of surfactant effects for penetrated/unpenetrated wetting, and further increasing the complexity of predictions. These potential complexities point to the importance of characterizing the heterogeneities of any surface under consideration. With proper characterization, the model described in this paper will allow for prediction of contact angles on all types of heterogeneous surfaces, and design of surfaces for specific interactions with surfactant solutions.

Model and experimental studies for contact angles of surfactant solutions on rough and smooth hydrophobic surfaces

Despite the practical need, no models exist to predict contact angles or wetting mode of surfactant solutions on rough hydrophobic or superhydrophobic surfaces. Using Gibbs' adsorption equation and a literature isotherm, a new model is constructed based on the Wenzel and Cassie equations. Experimental data for aqueous solutions of sodium dodecyl sulfate (SDS) contact angles on smooth Teflon surfaces are fit to estimate values for the adsorption coefficients in the model. Using these coefficients, model predictions for contact angles as a function of topological f (Cassie) and r (Wenzel) factors and SDS concentration are made for different intrinsic contact angles. The model is also used to design/tune surface responses. It is found that: (1) predictions compare favorably to data for SDS solutions on five superhydrophobic surfaces. Further, the model predictions can determine which wetting mode (Wenzel or Cassie) occurred in each experiment. The unpenetrated or partially penetrated Cassie mode was the most common, suggesting that surfactants inhibit the penetration of liquids into rough hydrophobic surfaces. (2) The Wenzel roughness factor, r, amplifies the effect of surfactant adsorption, leading to larger changes in contact angles and promoting total wetting. (3) The Cassie solid area fraction, f, attenuates the lowering of contact angles on rough surfaces. (4) The amplification/attenuation is understood to be due to increased/decreased solid-liquid contact-area.

Contact angle of surfactant solutions on precipitated surfactant surfaces. III. Effects of subsaturated anionic and nonionic surfactants and NaCl

AbstractThe contact angles of saturated calcium dodecanoate (CaC12) solutions containing a second subsaturated surfactant on a precipitated CaC12 surface were measured by using the drop shape analysis technique. The subsaturated surfactants used were anionic sodium dodecylsulfate (NaDS), anionic sodium octanoate (NaC8), and nonionic nonylphenol polyethoxylate (NPE). Comparing at the critical micelle concentration (CMC) for each surfactant, NaC8 was the best wetting agent, followed by NaDS, with NPE as the poorest wetter (contact angles of 320, 420, and 620, respectively). Surface tension at the CMC increased in the order NaC8<NPE<NaDS, and subsaturated surfactant adsorption increased in the order NPE≪NaDS (1.4 vs. 84 μmole/g); adsorption of the NaC8 was not measurable. Interfacial tension (IFT) reduction at the solid‐liquid interface due to subsaturated surfactant adsorption is an important contribution to contact angle reduction, in addition to surface tension reduction at the air‐water interface. Surfactant adsorption onto the soap scum solid is crucial to solid‐liquid IFT reduction and to good wetting. The fatty acid was the best wetting agent of the three surfactants studied, probably because calcium bridging with the carboxylate group synergizes surfactant adsorption onto the solid of the higher molecular weight soap. NaCl added to NaDS surfactant results in depressed CMC, lower surface tension at the CMC, decreased NaDS adsorption onto the solid, and decreased reduction in solid‐liquid IFT. The contact angle is not dependent on the NaCl concentration for NaDS. The NaCl causes an increased tendency to form monolayers, which decrease air‐water surface tension, but a decreased tendency to form adsorbed aggregates on the solid; the two trends offset each other, so wettability is not affected by added salt. The Zisman equation does not describe the wetting data for these systems well except for NaDS, further emphasizing the danger of ignoring solid‐liquid IFT reduction in interpreting wetting data in these systems.

Contact Angle of Surfactant Solutions on Precipitated Surfactant Surfaces. III. Effects of Salt Fatty Acid Salt, and Nonionic Surfactant

Contact Angle of Surfactant Solutions on Precipitated Surfactant Surfaces. III. Effects of Salt Fatty Acid Salt, and Nonionic Surfactant

Contact angle of surfactant solutions on precipitated surfactant surfaces. II. Effects of surfactant structure, presence of a subsaturated surfactant, pH, and counterion/surfactant ratio

AbstractThe contact angle of a saturated aqueous surfactant solution on the precipitate of that surfactant was measured by using the sessile drop method. The sodium and calcium salts of alkyl sulfates (C12, C14, and C18) had advancing contact angles higher than those of alkyl trimethylammonium bromides (C14, C16, and C18). The measured advancing contact angles for several surfactant solutions did not substantially change with varying surfactant/counterion ratios; therefore, the precipitating counterion concentration (e.g., water hardness) had little effect on the wettability. The contact angles of fatty acid (C12 and C16) solutions did not show any dependence on pH between a pH of 4 and 10. The contact angles of saturated calcium dodecanoate (CaC12) solutions containing a second subsaturated surfactant (sodium dodecyl sulfate: NaDS) decreased with increasing NaDS concentrations until reaching the critical micelle concentration of the surfactant mixture. These results show that the second suractant can act as a wetting agent in this saturated surfactant system. Application of Young's equation to contact angles showed that the solid/liquid surface tension can change substantially with surfactant concentration and be important in addition to the liquid/vapor surface tension in reducing contact angles. Application of the Zisman equation results in a “critical” surface tension for the CaC12 or soap scum of 25.5 mN/m, which is comparable to difluoroethene.

Contact angle of surfactant solutions on precipitated surfactant surfaces

AbstractIn this study, the contact angle of a saturated aqueous surfactant solution onto the surface of a precipitate of that surfactant is investigated. Those precipitates include fatty acids (C10, C12, C14, C16, and C18), sodium salts of fatty acids (C14, C16, and C18), calcium salts of fatty acids (C8, C10, C12, C14, C16, and C18). On virgin surfaces, free fatty acids and calcium salts of fatty acids have advancing contact angles (θA) between 77 and 92°, with little dependence on alkyl chain length for C12 and higher alkyl chains. The sodium salt of a fatty acid has a lower θA than the free fatty or the calcium salt of the soap. The calcium salt of dodecyl sulfate has a lower θA than the calcium salt of dodecanoic acid (θA = 46 vs. 82°), but the calcium salt of the 18‐carbon hydrophobes showed nearly the same contact angle for the soap and the alkyl sulfate. Greasiness, or slipperyness, or a scummy feel of a precipitated surfactant does not necessarily correspond to a hydrophobic surface.

Contact angles of surfactant solutions in oil solvents on low energy solid surfaces

We have measured contact angles for a range of pure oils of different cohesive energy density and surfactant solutions in oil on polyethylene (PE) and polytetrafluoroethylene (PTFE) solid surfaces. For the pure oils, the measured contact angles are in approximate agreement with values calculated using dispersion force theory. Using measurements of surface tension and contact angle as a function of surfactant concentration, we have estimated surfactant adsorption isotherms at the oil/vapour surface and the ratio of the extents of adsorption at the oil/vapour and solid/oil surfaces. For alkylbenzenes in di-iodomethane (DIM) it is found that the extents of adsorption at the DIM/vapour and PTFE/DIM surfaces are similar whereas extents of adsorption at the PE/DIM surfaces are lower. n-Dodecyl pentaoxyethylene glycol ether (C12E5) adsorbs at the DIM/vapour surface but not at either the hexadecane (HD)/vapour or squalane (SQ)/vapour surfaces. For all the oils tested, C12E5 shows virtually zero adsorption at the solid/oil surface.

Measurement of contact angles of spherical monodisperse particles with surfactant solutions

Small hydrophobic particles are often used as antifoam agents, and in connection with the mechanism by which particles rupture foam lamellae, it is necessary to have a knowledge of the contact angle between particles and the foaming solution. In the bridging—dewetting process of film rupture, the minimum contact angle requirement is dependent on particle geometry. There is thus an interest in using particles with a well-defined geometry in the study of foam breakdown, and monodisperse spheres are favoured candidates. It is usual to assume that the contact angle between particles and solution is the same as that between the solution and a flat plate with a coating similar to that of the particle. Tests have been made in the past to confirm such equivalence using packed beds of large particles. This method requires relatively large samples, and is not convenient therefore for the study of the much smaller particles of interest in foam breaking. In this paper we describe a method for the determination of contact angles of surfactant solutions with monodisperse spheres (diameters of approximately 6 nm and 2.5 μm have been used) employing a Langmuir trough technique. The particles are spread as insoluble monolayers on the aqueous surfactant solutions and compressed to monolayer collapse. Using an approach devised by Clint and Taylor (J.H. Clint and S.E. Taylor, Colloids Surfaces, 65 (1992) 61), it is then possible to calculate contact angles from the collapse pressures and a knowledge of the surface tensions of the substrates.