烷基表面活性剂的性能-main

A family of alkyl sulfate gemini surfactants.1.Characterization of surface propertiesBo Gao ⇑,Mukul M.SharmaDepartment of Petroleum and Geosystems Engineering,The University of Texas at Austin,Austin,TX 78712,United Statesa r t i c l e i n f o Article history:Received 24February 2013Accepted 25April 2013Available online 3May 2013Keywords:Sulfate gemini surfactants Critical micelle concentration Electrical conductivity Surface tension MicellizationSurface adsorptiona b s t r a c tThe fundamental aqueous and surface properties of a family of sulfate gemini surfactants have been char-acterized.The critical micelle concentrations (cmc)were determined by both electrical conductivity and surface tension methods.The cmc values were found to be two orders of magnitude lower than those measured for single tail surfactants.The cmc values depend primarily on the surfactant tail length,and relatively little on the spacer length and solution temperature.The surface tension measurements suggest that current family of gemini surfactants have higher tendency to spontaneously adsorb at the air–water interface and thus are much more efficient in reducing surface tension than conventional sin-gle-chain surfactants.Thermodynamic calculations of Gibbs free energies for micellization and adsorp-tion indicate surface adsorption is promoted more than micellization for these sulfate gemini surfactants.This type of molecules may therefore be very efficient and cost-effective in applications that require ultra-low interfacial tensions and high interfacial activities.Ó2013Elsevier Inc.All rights reserved.1.IntroductionGemini surfactants represent a class of surfactants made up of two amphiphilic moieties connected at or very close to,the head groups by a spacer group [1].The current interest in such surfac-tants arises from their distinct properties [1–6].First and foremost,gemini surfactants have critical micelle concentration (cmc)values that are several orders of magnitude lower than those of the corre-sponding conventional (monomeric)surfactants.These molecules are also more efficient and effective in reducing the surface/inter-facial tension.Moreover,aqueous solutions of some gemini surfac-tants exhibit strong viscosifying capability even at relatively low concentration [4–6].While there have been many papers studying the aforemen-tioned properties for cationic gemini surfactants,very few reports deal with the surface and aqueous properties of anionic gemini surfactants [7–11],which are of particular interest in the energy sector [12].Unfortunately,at present there is not enough pub-lished information/data to establish structure performance rela-tionships for anionic geminis.It is thus important to look into anionic gemini surfactants of potential interest and study their rel-evant properties for practical applications.In the current study,a family of sulfate gemini surfactants was prepared.The solution and surface properties were systematically investigated using elec-trical conductivity and surface tension measurements.2.Experimental 2.1.MaterialsA family of seven alkyl sulfate gemini surfactants was synthe-sized in our group,following a two-step reaction scheme proposed by Rist and Carlsen [13].To the best of our knowledge,the funda-mental surface properties of these molecules have not been fully characterized,not to mention any application specific properties.The general structure of the synthesized gemini surfactants is illus-trated in Fig.1.As a general feature,the synthesized molecules contain double chains each consisting of hydrophobic alkyl chains that are terminated by ethylene sulfate (CH 2CH 2O–SO 3Na)head groups.The chains are interconnected by alkyloxy spacer groups.The surfactants are named ‘m –s –m ’,where m and s represent the number of carbon atoms in the tail and spacer groups of the mol-ecule (m =14,16,18and 20+;s =2and 4).Note here m of 20+rep-resents a mixture of surfactants with chain lengths ranging from 20to 30(by starting with a C 20–C 30epoxide mixture).All products exhibited spectroscopic properties that were in agreement with those expected for the desired structures.Synthe-sis scheme of current family of sulfate gemini surfactants,and 1H NMR spectra are giving in the Supplementary material .2.2.MethodsAll of the gemini surfactant solutions were prepared by diluting a stock (concentrated)solution with ultrapure Milli-Q water (resis-tivity =18.2M O cm),and stirred on a magnetic stirrer at desired temperature for an hour.0021-9797/$-see front matter Ó2013Elsevier Inc.All rights reserved./10.1016/j.jcis.2013.04.043⇑Corresponding author.Present address:3319Mercer Street,URC-URC-SW204,Houston,TX 77027,United States.Fax:+17134316360.E-mail address:b.robert.gao@ (B.Gao).2.2.1.Electrical conductivityElectrical conductivities were measured using a digital conduc-tivity meter(OaktonÒECTestr11+)from Eutech Instruments.Mea-surements were carried out in a water bath set at desired temperature and repeated twice for every sample.To obtain critical micelle concentration(cmc)of gemini surfac-tants,the conductance of the solution was recorded at various con-centrations.And then the cmc was calculated as the intersection of linear parts in the dependence of conductivity versus surfactant concentration.The degree of ionization a was calculated as the ra-tio of slopes above and below the cmc in the plot.2.2.2.Surface tensionThe surface tensions of aqueous solutions of the gemini surfac-tants were measured with a ring tensiometer(CSC DuNouy Tensi-ometer)using the du Nouy ring technique.All measurements were carried out at lab temperature of25°C.Sets of measurements were taken at10min intervals until the change in surface tension was less than0.05mN mÀ1.The cmc and the surface tension at the cmc(c cmc)were determined from the breakpoint of the surface tension versus the logarithm of the concentration curve.The sur-face tension method can be used for checking the sample purity and the absence of a minimum is a good sign.3.Results and discussion3.1.Aqueous solubilityThe solubility of ionic surfactants is commonly characterized by the Krafft temperature T K,which is the minimum temperature at which surfactants form micelles.As described by Hato and Shinoda [14],aqueous solutions of all sulfate gemini surfactants were pre-pared at a controlled temperature of20°C and concentration of 1.0wt.%.Upon sufficient mixing,all solutions were visually exam-ined to be clear and parably low Krafft temper-atures were also reported for other gemini surfactants[7,15,16].It should emphasize here that only the range of T K(620°C)is deter-mined here by visual inspection method described above.More definitive and accurate estimate can be obtained using more quan-titative techniques including electrical conductivity and spectrophotometry.The low Krafft temperatures of these surfactants might be attributed to the hydrophilic nature of their spacer and head groups[17–19].The hydrophilic–lipophilic balance(HLB)of a sur-factant is a measure of the degree to which it is hydrophilic or lipo-philic.Stronger hydrophilic(or weaker lipophilic)characteristics of the molecule will promote its affinity with water molecules and might facilitate its solubility in aqueous environment.Table1 shows a comparison of calculated HLB numbers(using Becher’s method[20])between current sulfate geminis and several sodium alkylsulfate surfactants of equivalent chain length.It is clearly seen that gemini surfactants have much higher HLB numbers and thus exhibit better aqueous solubility,compared to their conventional counterparts.3.2.Conductivity dataThe cmc of the surfactants was taken as the concentration at the intersection of the linear portions of the conductivity versus con-centration plots above and below the breakpoint,according to the Williams method[21],as shown in Fig.2for the single-chain surfactant sodium dodecyl sulfate(SDS,from Sigma–Aldrich)and in Fig.3for the gemini surfactant14–2–14at three different tem-peratures(30°C,40°C and60°C).As expected,the slope above the cmc is lower than the one below the cmc,as micelles are worse charge carriers when compared to surfactant monomers.The average degree of micellar ionization,a,was taken as theratio of the slopes above and below the cmc in the electrical con-ductivity versus concentration plots.The standard Gibbs energy of micellization(per mole of surfactant molecule),D G oM,for gemini surfactants with monovalent counterions can be calculated using (derivation provided in Supplementary material):D G oM¼2ð1:5ÀaÞRT ln cmcþ2ð1ÀaÞRT ln2ð1ÞMoreover,from the dependency of cmc and a to absolute tem-perature,the standard enthalpy and entropy of micellization D H oM and D S oMcan be estimated from(see Supplementary material):D H oM¼À2ð1:5ÀaÞRT2@lnðcmcÞ@Tð2ÞD S oM¼D H oMÀD G oMð3Þwhere R is the gas constant and T is absolute temperature.The micellar parameters obtained from conductivity data are summa-rized in Table2.The cmc,a and D G o M values for SDS obtained from current study are in good agreement with those reported in the literatureGeneral chemical structure of sulfate gemini surfactants investigated study.Table1HLB values for gemini and conventional surfactants.Gemini surfactant HLB Conventional surfactant HLB14–2–1473.45C12–SO4Na40.00 18–2–1872.50C14–SO4Na39.05 14–4–1469.65C16–SO4Na38.10 16–4–1668.70C18–SO4Na37.15 18–4–1864.90and Interface Science404(2013)80–8481[22–25],demonstrating the accuracy of our measurements.Forboth surfactants,D G oM remains negative while D S oMremains positiveover the temperature range investigated,confirming that micelleformation occurs spontaneously when the surfactant concentra-tion reaches the cmc value.The magnitudes of the energy terms for gemini surfactant14–2–14are in line with reported values for some cationic gemini sur-factants[26].The significantly more negative D G oMvalue for14–2–14surfactant(as compared to values for SDS)is the driving force for its micellization.The ultralow cmc is a direct result of the large negative free energy.Micellization of14–2–14is primarily an en-tropy driven process,whereas SDS experiences a transition from entropy driven to enthalpy driven as solution temperature in-creases.As can be seen in Table2,the entropy term,ÀT D S oM,dom-inates over the enthalpy term,D H oM,over a wide temperature range.On the other hand,for SDS the dominant energy term switches from entropy to enthalpy at midpoint of the temperature range.This is intuitively understandable considering the much lar-ger disruption to the water structure(entropic effect)caused by double tail groups from a gemini surfactant.Table3above summarizes all the cmc values for the current family of sulfate gemini surfactants,compared with values for three single-chain surfactants.The cmc values for gemini surfac-tants are two to three orders of magnitude lower than those for the conventional single-tail molecule.These cmc measurements fully demonstrate the strong tendency of gemini surfactants to self-aggregate and form micellar structures in aqueous solutions at ultralow concentrations.Moreover,the cmc values depend pri-marily on the surfactant tail chain length,and relatively little on the length of spacer and solution temperature.Fig.4shows the plots of the cmc logarithms against the alkyl chain length,m.A fairly linear relationship was observed,with R2determined to be0.9740and0.9556for the m–2–m and m–4–m series respectively.In general,an empirical equation that relates the cmc to surfac-tant alkyl chain length can be expressed as[27]:log cmc¼AÀBmð4Þwhere A is a constant for a particular homologous series,and B rep-resents the energy contribution of the methylene group[28].The B values were estimated to be0.0999and0.1013for m–2–m and m–4–m,respectively,which were smaller than most reported values for cationic gemini surfactants($0.430)[2].This indicates that sul-fate gemini micelles have a comparatively weaker cohesive force(or stronger electrical repulsive force).3.3.Surface tension dataFrom surface tension measurements the cmc and the surface tension at the cmc were determined from the breakpoint in the surface tension versus logarithm of surfactant concentration pro-file,as exemplified in Figs.5and6for the gemini surfactants stud-ied(m–2–m and m–4–m series respectively).The maximum surface excess,C max,at the air-solution interface and the minimum area per molecule,a min,were calculated by applying the Gibbs adsorption isotherm[29],C max¼À12:303nRTd cd log Cmaxð5ÞTable2Critical micelle concentration(cmc),degree of micellar ionization(a)and standardenergies of micellization(D G oM ;D H oMandÀT D S oM)obtained from conductivity data atdifferent temperatures.Surfactant T(°C)cmc(l mol LÀ1)a D G oM(kJ molÀ1)D H oM(kJ molÀ1)ÀT D S o M(kJ molÀ1)SDS308.30Â1030.2192À21.49À4.64À16.85 408.63Â1030.2415À21.75À8.63À13.126010.28Â1030.2623À22.02À16.92À5.1014–2–1430 5.140.1591À79.43À22.50À56.8740 6.020.1710À80.21À29.52À50.70607.990.1852À82.35À44.05À38.30Table3CMC values for the family of anionic gemini surfactants.Surfactant cmc(l mol LÀ1)30°C40°C60°C14–2–14 5.14 6.027.9918–2–18 1.53 1.78 2.3020+–2–20+0.620.90 1.2314–4–14 4.79 5.10 5.4616–4–16 2.28 2.53 2.9118–4–18 1.22 1.44 1.8020+–4–20+0.540.570.62C12–SO4Na8.22Â1038.63Â10310.24Â103 C14–SO4Na 2.13Â103 2.19Â103 2.80Â103 C16–SO4Na–0.59Â103–82 B.Gao,M.M.Sharma/Journal of Colloid and Interface Science404(2013)80–84a min ¼1016N A v C maxð6Þwhere c is the surface tension in mN m À1,R is the gas constant (8.314J mol À1K À1),T is the absolute temperature,N Av is the Avoga-dro’s number,C max is in mol cm À2,and a min is the in Å2.For gemini surfactants,various investigators have used 2or 3for n [1].In all calculations that follow,n value of 2will be used based on the understanding that it is merely indicative of the change in C max within the family of compounds.A convenient measure of the efficiency of adsorption is the neg-ative logarithm of the concentration of surfactant in the bulk phase required to produce a 20mN m À1reduction in surface tension:p C 20=Àlog C 20.p C 20can be calculated using the following relation [29]:pC 20¼Àlog C 20¼p cmc À202:303nRT C maxÀlog cmc ð7Þwhere p cmc is the surface pressure defined by p cmc ¼c 0Àc cmc .The standard Gibbs free energy of adsorption D G o ad tells us whether adsorption is spontaneous or not and the magnitude of the driving force:D G o ad ðin J =mol Þ¼2:303nRT log ðC p =x ÞÀ6:023p a minð8Þwhere C p is the molar concentration of surfactant in the aqueous phase at a surface pressure of p cmc and x is moles of water per liter.Micellar parameters obtained from surface tension measurements are summarized in Table 4.The cmc values obtained from surface tension data agree well with the ones obtained from conductivity measurements.The small differences may be related with specifics of the methods used,since surface tension measurements are sensitive primarily to the surfactant monomers,as micelles are not generally thought to be surface active,while electrical conductivity method depends on the conductivity of all the ionic species present.Also seen from the table is the fact that micellization of gemini surfactants takes place at a concentration below 10l mol L À1,which is about three orders of magnitude lower than that of SDS.The extraordinary ability to self-aggregate is an important property of gemini surfac-tants.Lower p cmc values are also observed.For ionic surfactant molecules in water,the micelle formation is a direct result of two opposing forces,an attractive force favoring aggregation and closer packing and an electrostatic repulsive force that prevents larger size micelles.The attractive force arises from the hydrophobic effect [30]acting upon the hydrocarbon chains of the surfactants.The hydrophobic effect is primarily an entropic effect [31]originating from the disruption of highly dynamic hydrogen bonds between molecules of liquid water by the hydro-carbon chains.The repulsive force in micelle formation comes pri-marily from electrostatic interactions between the head groups.Longer hydrocarbon tails will thus induce stronger hydrophobic attraction which results in a tighter packing and lower surface ten-Table 4Critical micelle concentration (cmc),surface tension at the cmc (c cmc ),saturation adsorption (C max ),area per molecule at interface (a min ),p C 20,cmc/C 20,and standard Gibbs adsorption energy obtained from surface tension data at 25°C.Surfactant cmc (l mol L À1)c cmc (mN m À1)C max Â1010(mol cm À2)a min (Å2)p C 20cmc/C 20D G o ad (kJ mol À1)14–2–14 5.2833.70 2.1379 5.99 5.22À99.3618–2–18 1.7326.53 3.5048 6.39 4.14À100.1920+–2–20+0.6820.87 5.0133 6.70 3.33À102.0014–4–14 4.9232.59 1.9884 6.12 6.47À101.4116–4–16 2.2028.02 2.2872 6.537.38À104.7118–4–18 1.0424.00 3.4349 6.70 4.98À103.9620+–4–20+0.5318.34 4.11417.00 5.01À106.47SDS8.54Â10338.733.08532.422.22À54.70Table 5Standard free energy of micellization and adsorption for gemini surfactants studied.Surfactant D G o M (kJ mol À1)D G o ad (kJ mol À1)14–2–14À79.43À99.3618–2–18À90.61À100.1920+–2–20+À97.68À102.0014–4–14À78.54À101.4116–4–16À84.97À104.7118–4–18À90.82À103.9620+–4–20+À97.07À106.47B.Gao,M.M.Sharma /Journal of Colloid and Interface Science 404(2013)80–8483sion at an air–water interface,provided that the electrostaticrepulsion remains unchanged.Since surface tension reduction depends on the replacement ofwater molecules at the interface by surfactant molecules,the effi-ciency(e.g.p cmc and p C20)of a surfactant should reflect the inter-facial surfactant concentration relative to that in the bulk liquidphase.The ratio of surfactant concentration at surface to that inbulk phase,C S/C,is,therefore,a suitable measure.The surface con-centration C S(in mole LÀ1)in turn can be related to the surface ex-cess concentration C(in mole cmÀ2)by the relation C S%1000C/d, where d(in cm)is the thickness of surface/interfacial region.C S/ C=1000C/C d.When the tension has been reduced by20mN mÀ1, C is close to C max,and most surfactant molecules are lying tilted at the interface[32].Assuming that the thickness of the interfacial re-gion d is determined by the height of surfactant normal to the interface,then d is inversely proportional to the minimum surface area per molecule a min.A larger value of a min generally indicates a smaller angle of the surfactant with respect to the interface;a smaller value of a min indicates an orientation of the surfactant more perpendicular to the interface.All the above discussions explain why we see a general trend ofdecreasing a min,increasing C max,and resulted decreasing p cmc as the hydrocarbon chain of geminis becomes longer.The much high-er p C20values of gemini surfactants over SDS represent the supe-rior efficiency of gemini molecules in reducing surface tension. The cmc/C20ratios decease as the alkyl chain length increases.The standard free energies of adsorption D G oadfor all gemini surfac-tants are negative;indicating that adsorption of these compoundsat the air–water interface is spontaneous.The values of D G oM and D G oadare compared side by side in Table5.The negative values of both free energy terms illustrate once againthe great potential for gemini surfactants to form micelles in solu-tion and to adsorb onto the air–water interface.With D G oMshowingconsistently smaller magnitudes than D G oad,current dataset sug-gests adsorption is energetically favored over micellization for this family of surfactants.This result is supported by the large values of p C20reported above.4.ConclusionsIn this article,we prepared a family of alkyl sulfate gemini sur-factants with systematically changing structures and investigated their fundamental properties such as their cmc,surface tension, and adsorption/micellization parameters in order to understand their structures and performance relationships.With two hydro-philic head groups introduced into the structure,they showed high water solubility.The relationship between the cmc and hydrocar-bon chain lengths gave fairly linear trends,similar to what would typically be observed for conventional surfactants[27],despite the double head double tail structure.It is worth emphasizing that the cmc values of the sulfate gemini surfactants were orders of magnitude lower than those of single chain surfactants with the same alkyl chain length.All the surfactants studied show great effi-ciency in surface tension reduction and are more likely to adsorb at the air/water interface rather than to form micelles,a behavior also found for some other anionic gemini surfactants[33,34].Until now,although a large number of novel surfactants have been designed and synthesized by many researchers[2,35],to the best of our knowledge,a systematic study of structure surface/aqueous property relationships of the titled anionic gemin-is,with clear end use in the energy sector[12],have not been re-ported.Therefore,the current article serves to set the stage for subsequent studies focused on more application oriented topics: the water–oil interfacial tension reduction potential of titled sul-fate geminis will be reported in a subsequent paper;their rheolog-ical properties in solution will be the subject of further study. AcknowledgmentsInsightful inputs from Dr.Fredric M.Menger(Emory Univer-sity),Dr.Lei Shi(Princeton University),Dr.Gary A.Pope and Dr. Upali P.Weerassoriya(The University of Texas at Austin)are grate-fully acknowledged.Appendix A.Supplementary materialSupplementary data associated with this article can be found,in the online version,at /10.1016/j.jcis.2013.04.043. 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