| 1. |
- Folmer, B, et al.
(författare)
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The effect of surfactant-polymer association on the stabilities of foams and thin films: sodium dodecyl sulfate and poly(vinyl pyrrolidone)
- 2000
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Ingår i: Langmuir. - 0743-7463 .- 1520-5827. ; 16, s. 5987-5992
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Tidskriftsartikel (refereegranskat)abstract
- The foaming behaviour of the anionic surfactant Sodium Dodecyl Sulphate (SDS) has been studied in the presence and in the absence of the nonionic polymer Poly Vinyl Pyrrolidone (PVP). A current model of surfactant - polymer aggregation in bulk and at the air-water interface is related to the foam and thin film stability. Tensiometry, foaming tests and a thin film balance are used to obtain this relationship. It is found that at very low surfactant concentrations, where the surfactants are present as unimers in the bulk solution, there is association between surfactants and polymer at the liquid/air surface, giving an increased foam and thin film stability as compared to the same surfactant concentration but without polymer. As the surfactants and polymers associate in the bulk solution there is desorption of surfactants and polymers from the surface rendering a decrease in foam and thin film stability. At higher surfactant concentrations the bulk viscosity is significantly increased due to the presence of both micelles and saturated micelle-polymer complexes. Also the surfactant surface coverage at the liquid/air surface has reached its maximum value and is similar to the SDS solution above the CMC when no polymer is present. Both the increased surface viscosity and the increased bulk viscosity contribute to the observed foam and film stability. In the thin film studies several stratification steps are observed probably due to micelles, which are being pushed out of the film.
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| 2. |
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| 3. |
- Boschkova, K, et al.
(författare)
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Adsorption and frictional properties of Gemini surfactants at solid surfaces
- 2002
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Ingår i: Langmuir. - 0743-7463 .- 1520-5827. ; 18, s. 7930-7935
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Tidskriftsartikel (refereegranskat)abstract
- The adsorption and frictional properties of gemini surfactants at hydrophilic gold surfaces were measured using QCM-D (quartz crystal microbalance dissipation) and AFM. The molecular packing of a series of gemini surfactants was determined from QCM-D measurements, and the frictional behaviors of the surfactant films were characterized by employing atomic force microscopy (AFM). The results show that by changing the length of the spacer group from 3 to 12 a systematic change in the molecular packing at the surface is obtained. Furthermore, the molecular packing is seen to correlate to the frictional behavior of the surfactant film. A linear relation between the spacer group length, the adsorbed amount, and the frictional properties of the layer at the solid surface is found. This is discussed in terms of the critical packing parameter (CPP) of the surfactant, and a relation between CPP and frictional behavior is proposed. No correlation between spacer length and viscoelasticity of the adsorbed surfactant layer was detected using QCM-D. This indicates that the resolution of the dissipation factor from QCM-D measurements is not sufficient to describe the viscoelastic character of the thin surfactant film.
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| 4. |
- Boschkova, K, et al.
(författare)
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Frictional properties of lyotropic liquid crystalline mesophases at surfaces
- 2000
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Ingår i: Colloids and Surfaces A. - 0927-7757 .- 1873-4359. ; 166, s. 67-77
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Tidskriftsartikel (refereegranskat)abstract
- The lubricating effect of assemblies of amphiphiles at surfaces has been studied. Liquid crystalline mesophases were investigated in terms of frictional and wear properties in a pin on disc rig. It is shown that systems forming lamellar liquid crystals indeed serve as lubricating liquids, indicating that the lamellar liquid crystalline phase adsorb on the steel surface forming a lubricating tribofilm. Poor performance is obtained when the lubricating system is in a single phase, i.e. in this case a lamellar liquid crystalline region. However, good lubrication is found when the lamellar liquid crystalline phase is dispersed in the water. This is attributed to a low viscosity of the system rendering a fast relaxation of the system in order to form a new film after the disturbing action of the two sliding surfaces. The lamellar packing is altered by the addition of non-charged amphiphiles, such as short chain alcohols. Using different alcohols it was shown that aromatic short chain alcohols are detrimental to the wear. This was attributed to the Rebinder effect, i.e. a strong adsorption of the alcohol, which in turn facilitates crack formation and thus the wear. In systems with mixtures of cat- and anionic amphiphiles it was, not surprisingly found that the best lubrication is obtained with a slight excess of either surfactant.
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| 5. |
- Boschkova, K, et al.
(författare)
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Lubrication in aqueous solutions using cationic surfactants – a study of static and dynamic forces
- 2002
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Ingår i: Langmuir. - 0743-7463 .- 1520-5827. ; 18, s. 1680-1687
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Tidskriftsartikel (refereegranskat)abstract
- This paper concerns lubrication in aqueous surfactant systems where the surfactants adsorb at surfaces, in relative motion, forming either a surfactant monolayer or a multi- (liquid crystalline) layer. The surfactants were of two kinds, viz., a double chain cationic surfactant, didodecyldimethylammonium bromide, DDAB, and a single chain cationic surfactant, dodecyltrimethylammonium bromide, DTAB. Excellent film forming capability was shown for DDAB and interpreted as the result of good packing of the surfactant molecules at the surfaces, i.e., the inherent ability of the surfactant molecules to form liquid crystalline structures at the surface, resulting in good load-carrying capability. This is also reflected in the bulk properties of the surfactants, where DDAB show lamellar liquid crystalline phases at concentrations much lower than DTAB, which does not show good lubrication properties. The results are discussed in terms of film stability of a surfactant layer adsorbed at the surface, which in turn is correlated to the critical packing parameter of the surfactant, in analogy with the Kabalnov-Wennerström theory of emulsion droplet coalescence (Kabalnov, A.; Wennerström, H. Langmuir 1996, 12, 276). The systems were characterized using (i) the surface force apparatus determining the interaction forces between the adsorbed layers at the surfaces and (ii) the EHD rig (elastohydrodynamic rig) determining film formation under shear. The adsorption kinetics and composition at the surface were determined by a quartz crystal microbalance and X-ray photoelectron spectroscopy.
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| 6. |
- Boschkova, K., et al.
(författare)
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Study of thin surfactant films under shear using the tribological surface force apparatus
- 2001
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Ingår i: Tribology International. - 0301-679X .- 1879-2464. ; 34:12, s. 815-822
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Tidskriftsartikel (refereegranskat)abstract
- Static and dynamic behaviour of thin surfactant films in aqueous solution of hexadecyltrimethylammonium salicylate (C16TASal) were investigated using the tribological surface force apparatus. Normal force measurements show that 0.15 mM C16TASal builds up an innermost film of approximately 8-11 Angstrom thickness at each mica surface, indicating that the surfactant adsorbs in a flat conformation. Furthermore, the height of the force barrier at approximately 60 Angstrom is low (ca 2 mN/m) indicating that the second adsorbed layer is easily pushed out. Addition of salicylate salt to 0.15 mM C16TASal give rise to a more close packed structure, with a total thickness of 62-65 Angstrom. indicative of a micellar or bilayer arrangement at the surfaces. Furthermore, the frequency dependence of the shear modulus was investigated both at close separation at the innermost force barrier and at larger separations (up to 300-400 Angstrom). The visco-elastic measurements show that the elasticity modulus, G ', dominates over the loss modulus, G , for all studied cases, indicative of a more solid-like than liquid-like film. Finally, it is shown that shear at high contact pressures induces new aggregate structures at the surface.
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| 7. |
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| 8. |
- Kizling, J, et al.
(författare)
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On the formation of concentrated stable w/o emulsions
- 2001
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Ingår i: Advances in Colloid and Interface Science. - 0001-8686 .- 1873-3727. ; 89-90, s. 395-399
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Tidskriftsartikel (refereegranskat)abstract
- The conditions for the formation of concentrated w/o emulsions based on Aerosol OT and aliphatic hydrocarbons were studied. It was found that high stability is obtained if the attraction between the emulsion droplets is kept to a minimum. This is achieved by matching the refractive index of the dispersed phase and the organic phase. Furthermore, it is shown that the mere presence of salt does not render any stability. Hence, the speculated mechanism that salt stabilises against Ostwald ripening does not hold for these emulsions.
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| 9. |
- Liljekvist, P, et al.
(författare)
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Comparing decyl-β-maltoside and octaethyleneglycol mono n-decyl ether in mixed micelles with dodecyl benzenesulfonate. 1. Formation of mixed micelles
- 2000
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Ingår i: Journal of Colloid and Interface Science. - 0021-9797 .- 1095-7103. ; 222, s. 159-164
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Tidskriftsartikel (refereegranskat)abstract
- In this study the mixed micelle behaviour of an alkyl polyglycoside is compared to a surfactant of polyoxyethylene type, by means of surface tension measurements. The two nonionic surfactants are compared in mixed micelle systems together with an anionic surfactant. The surfactant mixtures are: decyl-b-maltoside (C10M) together with dodecyl benzenesulfonate (C12BS) and octaethyleneglycol mono n- decyl ether (C10EO8) together with C12BS. The mixture of C10M and C10EO8 is also studied. Critical micelle concentration (CMC) and the concentration at which the surface tension reduction is 20 mNm-1 (C20) are determined at different mixing ratios of the surfactant mixtures. By applying the non-ideal mixed micelle theory, interaction parameters at CMC (bCMC) and C20 (bC20) are calculated for the surfactant mixtures. The results show that the C10M - C12BS mixture has a bCMC parameter of -2.1, whereas the bCMC parameter for the C10EO8-C12BS mixture is -3.3, indicating a weaker net attractive interaction between C10M and C12BS than between C10EO8 and C12BS. This is attributed to a small negative and positive charge of the respective nonionic surfactants. This is supported by a slightly negative bCMC parameter obtained for the surfactant mixture C10M - C10EO8, indicating a small net attractive interaction between the two nonionic surfactants.
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| 10. |
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