Critical Micelle Concentration Of Surfactant Research Articles

Scaling of binding affinities and cooperativities of surfactants on carbon nanotubes

High-end applications of single-walled carbon nanotube (SWNT) require detailed understanding of their binding affinity (Kd) with surfactants. Here, we quantitatively determined the comprehensive Kd and aggregation number (γ) of nine nonionic and anionic surfactants according to SWNT chirality. Photoluminescence (PL)-based titration using flavin mononucleotide (FMN)-SWNT complexes showing the largest redshifted optical transition enabled quantitative comparison of the surfactants and displayed distinct first and second regimes which correspond to partial and full replacements of FMN, respectively. Especially, the second transition exhibited sigmoidal PL change whose middle point denoted by the inverse of Kd slightly differs from critical micelle concentration of surfactant alone, depending on its functional groups. Moreover, nonionic surfactants displayed larger Kd and lower γ values than the anionic did. Specifically, Kd values between Pluronic F108 and sodium dodecyl sulfonate differ by four orders of magnitude. Such differences were rationalized in terms of the occupied volume of the micelle based on the hydrodynamic volume and γ. During the replacement, Kd and γ were affected by the degree of surfactant rearrangement induced by aging process, leading to more stable complex. Scaling the interactions between surfactant and SWNT provides useful guidelines to design novel SWNT sorting method using more than two surfactants.

Cocrystal solubility-pH and drug solubilization capacity of sodium dodecyl sulfate – mass action model for data analysis and simulation to improve design of experiments

This review discusses the disposition of the anionic surfactant, sodium dodecyl sulfate (SDS; i.e., sodium lauryl sulfate), to solubilize sparingly-soluble drugs above the surfactant critical micelle concentration (CMC), as quantitated by the solubilization capacity (k). A compilation of 101 published SDS k values of mostly poorly-soluble drug molecules was used to develop a prediction model as a function of the drug’s intrinsic solubility, S0, and its calculated H-bond acceptor/donor potential. In almost all cases, the surfactant was found to solubilize the neutral form of the drug. Using the mass action model, the k values were converted to drug-micelle stoichiometric binding constants, Kn, corresponding to drug-micelle equilibria in drug-saturated solutions. An in-depth case study (data from published sources) considered the micellization reactions as a function of pH of a weak base, B, (pKa 3.58, S0 52 μg/mL), where at pH 1 the BH.SDS salt was predicted to precipitate both below and above the CMC. At low SDS concentrations, two drug salts were predicted to co-precipitate: BH.Cl and BH.SDS. Solubility products of both were determined from the analysis of the reported solubility-surfactant data. Above the CMC, in a rare example, the charged form of the drug (BH+) appeared to be strongly solubilized by the surfactant. The constant for that reaction was also determined. At pH 7, the reactions were simpler, as only the neutral form of the drug was solubilized, to a significantly lesser extent than at pH 1. Case studies also featured examples of solubilization of solids in the form of cocrystals. For many cocrystal systems studied in aqueous solution, the anticipated supersaturated state is not long-lasting, as the drug component precipitates to a thermodynamically stable form, thus lowering the amount of the active ingredient available for intestinal absorption. Use of surfactant can prevent this. A recently-described method for predicting the solubility product of cocrystals (coupled with predicted k values described here) allowed for simulations of solubility-pH speciation profiles of cocrystal systems in the presence of SDS. Well in advance of any actual measurements, these simulations can be used to probe conditions favorable to the design of cocrystal experiments where SDS stabilizes cocrystal suspensions against drug precipitation over a predicted range of pH values.

Aggregation Behavior of Monomeric Surfactants and a Gemini Cationic Surfactant by NMR and Computer Simulation Data

Aggregation of decyltrimethylammonium bromide and cetyltrimethylammonium bromide (CTAB) in D2O has been studied. Spin–lattice relaxation time and self-diffusion coefficient of surfactant molecules were measured at concentrations below and above surfactant critical micelle concentration. The aggregation properties of conventional surfactant, CTAB, examined by nuclear magnetic resonance (NMR) and molecular dynamic (MD) simulation, were compared with the properties of double-tail analog, N,N,N′,N′-tetramethyl-N,N′dihexadecyl-1,4-butan di-ammonium di-bromide (BCTA). Both NMR and computer simulation methods suggest that micellization is a stepwise process and the pre-micellar aggregates take place in a solution at concentration below critical micelle concentration. According to MD simulation Gemini surfactant, BCTA, forms worm-like micelles, whereas CTAB, which may be considered as its “monomer”, forms only elongated micelles.

One pot synthesis, physicochemical and photophysical investigation of biologically active pyridine-3-carboxylate (ECPC) as probe to determine CMC of surfactants in organized media

Ethyl 4 - (9-ethyl-9H-carbazol-3-yl) – 2 – methyl - 5- oxo - 4, 5 - dihydro-1H-indeno[1,2-b]pyridine-3-carboxylate (ECPC) was prepared by one pot multi component reaction of 9-ethyl-9H-carbazole-3-carbaldehyde, indane-1,3-dione, ethyl acetoacetate and ammonium acetate in the presence few drops of piperidine. Structure of the title compound (ECPC) was confirmed by the spectroscopic techniques (FT-IR, 1H-NMR, 13C-NMR, EI-MS) and elemental analysis. Photophysical parameter of the ECPC such as transition dipole moment, extinction coefficient, stokes shift, oscillator strength, fluorescence quantum yield were calculated by using the different solvents (based on the polarity). The results of compound ECPC showed red shift in absorbance and emission spectrum with increase in the polarity of the solvents from n-heptane to DMSO. Emission spectrum of ECPC effected with anionic and cationic surfactant so ECPC can be used as probe to determine critical micelle concentrations (CMC) of anionic sodium dodecyl sulfate (SDS) and cationic cetyltrimethylammonium bromide (CTAB) surfactant. The in-vitro antibacterial activity of ECPC was examined by the disk diffusion and minimum inhibitory concentration (MIC) methods against two Gram-positive and two Gram-negative bacteria by using Tetracycline as reference drug.

Determination of the Critical Micelle Concentration of Neutral and Ionic Surfactants with Fluorometry, Conductometry, and Surface Tension—A Method Comparison

Micelles are of increasing importance as versatile carriers for hydrophobic substances and nanoprobes for a wide range of pharmaceutical, diagnostic, medical, and therapeutic applications. A key parameter indicating the formation and stability of micelles is the critical micelle concentration (CMC). In this respect, we determined the CMC of common anionic, cationic, and non-ionic surfactants fluorometrically using different fluorescent probes and fluorescence parameters for signal detection and compared the results with conductometric and surface tension measurements. Based upon these results, requirements, advantages, and pitfalls of each method are discussed. Our study underlines the versatility of fluorometric methods that do not impose specific requirements on surfactants and are especially suited for the quantification of very low CMC values. Conductivity and surface tension measurements yield smaller uncertainties particularly for high CMC values, yet are more time- and substance consuming and not suitable for every surfactant.

Evaluation of a sulfanilic acid based surfactant in crude oil demulsification: an experimental study

In this study, 2-Amino-5-Dodecyl Benzene Sulfonic Acid (ADBSA) surfactant has been prepared and tested as crude oil emulsions demulsifier. Electrical conductivity method was used to obtain the critical micelle concentration of surfactant. A critical micelle concentration value of 0.225 wt.%. Bottle testing measurements were conducted to investigate the effect of several parameters including temperature, water content, salinity, demulsifier dosage and type of crude oil (composition) on the demulsification efficiency in presence of new synthetic demulsifier. A reduction of emulsion stability was observed with increasing water content or demulsifier dosage decrease. A water separation index of 98% was recorded at 60 °C.

Nuclear Magnetic Resonance Investigation of the Effect of pH on Micelle Formation by the Amino Acid-Based Surfactant Undecyl l-Phenylalaninate.

Micelle formation by the anionic amino acid-based surfactant undecyl l-phenylalaninate (und-Phe) was investigated as a function of pH in solutions containing either Na+, l-arginine, l-lysine, or l-ornithine counterions. In each mixture, the surfactant's critical micelle concentration (CMC) was the lowest at low pH and increased as solutions became more basic. Below pH 9, surfactant solutions containing l-arginine and l-lysine had lower CMC than the corresponding solutions with Na+ counterions. Nuclear magnetic resonance (NMR) diffusometry and dynamic light scattering studies revealed that und-Phe micelles with Na+ counterions had hydrodynamic radii of approximately 15 Å throughout the investigated pH range. Furthermore, l-arginine, l-lysine, and l-ornithine were found to bind most strongly to the micelles below pH 9 when the counterions were cationic. Above pH 9, the counterions became zwitterionic and dissociated from the micelle surface. In und-Phe/l-arginine solution, counterion dissociation was accompanied by a decrease in the hydrodynamic radius of the micelle. However, in experiments with l-lysine and l-ornithine, micelle radii remained the same at low pH when counterions were bound and at high pH when they were not. This result suggested that l-arginine is attached perpendicular to the micelle surface through its guanidinium functional group with the remainder of the molecule extending into solution. Contrastingly, l-lysine and l-ornithine likely bind parallel to the micelle surface with their two amine functional groups interacting with different surfactant monomers. This model was consistent with the results from two-dimensional ROESY (rotating frame Overhauser enhancement spectroscopy) NMR experiments. Two-dimensional NMR also showed that in und-Phe micelles, the aromatic rings on the phenylalanine headgroups were rotated toward the hydrocarbon core of micelle.

Characterizations of surfactant synthesized from palm oil and its application in enhanced oil recovery

The present work deals with synthesis of an anionic biodegradable surfactant for application in chemical enhanced oil recovery (EOR) process. Alpha sulfonated ethyl ester (α-SEE) was synthesized from palm oil via trans-esterification process. The surfactant was characterized by FTIR, GC, TGA, FE-SEM and EDX analyses. Critical micelle concentration (CMC) was determined by surface tension measurement at air–aqueous interface. The efficiency of α-SEE was studied by calculating the interfacial tension (IFT) between crude oil and surfactant solution, and by investigating the ability of the surfactant to alter the wettability nature of carbonate and quartz surfaces. Salt effect was studied at CMC of surfactant to obtain an ultra-low IFT value of the order of 10−3 mN/m at optimal salinity. Addition of organic alkali also showed synergistic effect on IFT between crude oil and surfactant solution. The surfactant favorably altered the wettability of oil-wet carbonate and quartz surfaces to water-wet, which is desirable for oil recovery. The surfactant showed potential application in EOR owing to its enhanced interfacial properties and rock-wetting characteristics. Flooding experiments were conducted with surfactant slugs at different α-SEE concentrations to achieve about 25%–27% additional oil recoveries after conventional flooding.

Effect of pH and surfactant on the protein: A perspective from theory and experiments

In the present study, we are aimed to explore the bovine serum albumin (BSA) and sodium dodecyl sulfate (SDS) the interaction that can readily show the consequence of the change in concentration of protein with surfactants’ various concentration and the different pH’s, 4.0, 4.7, 7.0 of the medium through the many spectroscopic techniques. The BSA and SDS denaturation fully influenced by the pH. The results interpreted in terms of electrostatic and hydrophobic contributions to the stability of different phases formed in the system. Critical Micelle Concentration (CMC) of surfactant is influenced the protein folding and unfolding. The molecular docking supports the experiment data. This study demonstrates that the above and below the CMC of surfactant can significantly alter the binding interaction with the protein.

Determination of Critical Micelle Concentration of Surfactants by Headspace Gas Chromatography

AbstractThis paper presents a new method for the determination of the critical micelle concentration (CMC) of surfactants in water at elevated temperatures using headspace gas chromatography (HS‐GC). A volatile compound (e.g., toluene) was spiked into the studied solution. At a given temperature, the concentration of volatile species in the vapor phase (measured by HS‐GC) is proportional to the surfactant concentration in the liquid phase in a closed vial. This tendency will change after the solution is saturated with surfactant. By plotting the GC signal of the vapor species vs the concentration of the surfactant concentration in the solution, a breakpoint, i.e., corresponding to the CMC of the surfactant. The present method has good precision (RSD < 10%) and good accuracy. It is also very simple and can be used as an automated method for determining the critical micelle concentration of surfactants at the temperature of interest.

Evaporation kinetics of surfactant solution droplets on rice (Oryza sativa) leaves.

The dynamics of evaporating sessile droplets on hydrophilic or hydrophobic surfaces is widely studied, and many models for these processes have been developed based on experimental evidence. However, few research has been explored on the evaporation of sessile droplets of surfactant or pesticide solutions on target crop leaves. Thus, in this paper the impact of surfactant concentrations on contact angle, contact diameter, droplet height, and evolution of the droplets’ evaporative volume on rice leaf surfaces have been investigated. The results indicate that the evaporation kinetics of surfactant droplets on rice leaves were influenced by both the surfactant concentrations and the hydrophobicity of rice leaf surfaces. When the surfactant concentration is lower than the surfactant CMC (critical micelle concentration), the droplet evaporation time is much longer than that of the high surfactant concentration. This is due to the longer existence time of a narrow wedge region under the lower surfactant concentration, and such narrow wedge region further restricts the droplet evaporation. Besides, our experimental data are shown to roughly collapse onto theoretical curves based on the model presented by Popov. This study could supply theoretical data on the evaporation of the adjuvant or pesticide droplets for practical applications in agriculture.

Aggregation properties and thermodynamics of micellization of gemini surfactants with diamide groups and rigid spacers in water and water-organic solvent mixed media

The aggregation behavior and thermodynamics of micellization of three gemini surfactants with diamide groups and rigid spacers in various volume percentages of ethylene glycol (EG-WR) and three different types of 10% organic co-solvents (MA-WR, DMF-WR and AN-WR) across a range of temperature from (281.15 to 331.15) K, have been investigated by electrical conductivity measurements. The properties parameters including critical micelle concentration (CMC), degree of counter ion dissociation (α) were determined and discussed. The thermodynamic parameters of micellization including standard Gibbs energy (ΔG°m), Gibbs energy of transfer (ΔG°trans), Gibbs energy of micellization per alkyl tail (ΔG°tail), standard enthalpy (ΔH°m) and standard entropy (ΔS°m) were calculated from the temperature dependence of the CMC and α values. With the increasing chain length, the CMC of gemini surfactants decreased. At a given temperature, The CMC and α values are found to increase with the increasing percentage of EG content in the mixed solvent. The temperature dependence of ΔG°m and CMC values for the gemini surfactants in all the investigated systems is U-shaped. Compensation of enthalpy and entropy of micellization of all three gemini surfactants were observed in all the studied mixed media.

Influence of surfactant and electrolyte concentrations on surfactant Adsorption and foaming characteristics

Surfactant adsorption and foaming characteristics are influenced by surfactant concentration and presence of inorganic electrolytes. Hence, it should be possible to optimize the performance of the surfactants in subsurface applications by understanding the influence of these parameters on surfactants. This study investigates the adsorption of sodium dodecyl sulfate (SDS) on kaolinite as a function of surfactant concentration and added electrolyte (NaCl, CaCl2 and AlCl3) concentration. Influence of temperature on the electrolyte and surfactant interactions was also examined. Adsorption isotherms were obtained using surfactant concentrations higher and lower than the critical micelle concentration (CMC). Surfactants adsorption on kaolinite was determined using a surface tension technique and two phase titration methods. Adsorption data were analyzed by fitting with Langmuir and Freundlich adsorption isotherms. The foam was generated by dispersing CO2 gas into the surfactant solution through a porous stone. Foam half-life and the rate of foam collapse as function of time was monitored. The adsorption of SDS by kaolinite increases with the increasing concentration of NaCl and CaCl2 and decreasing temperature. However, adsorption in presence of AlCl3 shows different behavior. The adsorption remains constant irrespective of the increasing AlCl3 concentration. Results show that the adsorption of SDS onto kaolinite in presence and absence of salts follows the Langmuir isotherm models. Salts containing trivalent ions and divalent ions (AlCl3 and CaCl2) were found to increase SDS adsorption on kaolinite and decrease bubbles stability compared to salts containing mono ions (NaCl). The order of increase in surfactant adsorption and bubble coalescence in presence of salts is as follows: AlCl3>CaCl2>NaCl. There was an optimum surfactant concentration corresponding to maximum foam stability beyond which there was either a reduction or no significant changes in foam stability. This concentration decreases in presence of salts, except for AlCl3 and high concentrations of NaCl (5wt%) and CaCl2 (1wt%). The presence of salt improved foam generation and bubble stability at SDS concentration below the CMC. Above CMC, the bubble coalescence inhibition and foam stability decreased in the presence of salt. Decrease in surfactant surface tension and CMC, the screening effect of electrostatic double layer (EDL) by salts and the ability of SDS to form a complex with divalent (Ca2+) and trivalent (Al3+) cations are critical factors affecting SDS adsorption and foaming behaviors in presence of AlCl3, CaCl2 and NaCl salts. The results of this study have wide applications in the design, implementation and optimization of chemical EOR in the field.

SURFACTANT INDUCED INTERFACIAL ANCHORING TRANSITIONS IN NEMATIC LIQUID CRYSTAL DROPLETS ON GLASS SURFACES

The interfacial adsorption of surfactants at planar nematic liquid crystal (NLC)–water interface induces an ordering transition from a tilted to perpendicular anchoring with the increase in surfactant concentration at [Formula: see text], where [Formula: see text] is the Critical Micelle Concentration of surfactants in water. In this study, we show that depending upon the surfactant structure a tilted to perpendicular NLC anchoring transition is observed at [Formula: see text] in 5CB droplets of size 50–70[Formula: see text][Formula: see text]m. Micrometer sized 5CB droplets are deposited on glass surfaces using flow coating of 5CB-in-ethanol solutions. When placed on 5CB drop decorated glass surfaces, the aqueous surfactant solutions of aliphatic chain surfactants (SDS, CTAB and CPBr) at [Formula: see text], result in an optical transition to a bright-cross texture attributed to the tilted anchoring of 5CB molecules at 5CB–water interface. At [Formula: see text], perpendicular anchoring of 5CB molecules at 5CB–water interface results in a droplet texture with a hedgehog defect. In contrast, aqueous solutions of SDBS lead to 5CB droplets with a bright-cross texture regardless of the surfactant concentration in the aqueous phase. These results indicate that the orientation of 5CB molecules is independent of the nature of the surfactant headgroup. In addition, 5CB droplet decorated OTS treated glass substrates show a hedgehog texture which disappears completely on exposure to organic vapors with the response time-dependent on the polarity of the vapor molecules.

Influence of a Zwitterionic Surfactant on the Surface Properties of Electroless Ni–P Coating on Mild Steel

Abstract3‐(N,N‐Dimethyl myristyl‐ammonio) propane sulfonate zwitterionic surfactant (C14‐SB) which possessed both positive and negative charges was evaluated in the electroless Ni–P coating process. It was observed that the deposition rate, morphology and microhardness of the deposits were enhanced by the addition of C14‐SB surfactant. The excess attractive forces from the negative head of C14‐SB were strong enough to draw metallic nickel particles towards the substrate. Ni particles attempting to deposit on the electrolyte container were eliminated by the repulsive force from the positive head of the surfactant monomers. Thus, the deposition rate of the coating process was improved. The surfactant at its critical micelle concentration (CMC) doubles the deposition rate when compared to the substrate without surfactant. In addition, the microhardness of the deposit at the surfactant CMC increased by 62 %. The corrosion rate of the substrate without surfactant was 7.15 mpy, while it was 3.97 mpy for the substrate deposited with C14‐SB zwitterionic surfactant at the CMC.

Determination of Critical Micelle Concentrations of Surfactants by Terahertz Time-Domain Spectroscopy

A novel detection method is presented to determine the critical micelle concentrations (CMCs) of surfactants by using a simple and convenient terahertz time-domain spectroscopy (THz-TDS) technology which has been successfully used to study the picosecond dynamics of water in reverse micelles. Absorption coefficients or refractive indices of aqueous solutions of an ionic surfactant sodium dodecyl sulfate and a nonionic surfactant nonaethylene glycol monododecyl ether in 0.2–1.8 THz frequency range at different concentrations are measured through the THz-TDS. A clear intersection of THz absorption coefficient or refractive index caused by the micelle formation is observed. This phenomenon is strongly associated with the change of component ratio between surfactant, bulk water and hydration water during the formation of micelles. The corresponding concentration is determined as the CMC of the surfactant which is in good agreement with the most existing results using traditional methods. For comparison, absorption coefficients of the two surfactants in hydrochloric acid solutions are also measured and differences in contrast to those in aqueous solutions are observed and explained. Our findings indicate that THz spectroscopy technology can be applied to determine the CMCs of surfactants in a sensitive, accurate, and label-free way.

Synergistic Interaction in the adsorbed mixed surfactants film of Sodium Dodecyl Sulfate and Cocamidopropyl Betaine on Liquid – Air Interfacial

In the present work, the critical micelle concentration (CMC) of the solution of Sodium dodecyl sulfate (SDS) as anionic surfactant, Cocamidopropyl Betaine (CAPB) as amphoteric surfactant, and their mixture have been determined using surface tension and conductivity measurements at a temperature range 293 -323 K. The adsorption and thermodynamic micellization parameters (?G?m, ?G?ads, ?max ,Amin,?cmc ) for individual surfactants was calculated. Rosen model which is focuses on the adsorbed mixed surfactant film at the air/solution interface was used to calculate the interaction parameter ( ?? ) at the interface and the activity coefficients g1 and g2. The results indicate that the CMC of the individual surfactants was affected by the temperature at the temperature range studied. Also, the results indicate a synergistic effect present at the air – solution mixed film of surfactants.

Determination and calculation of micro-structural parameters of SDS/BA/H2O micelle

The micro-structural characteristics of sodium dodecyl sulfate (SDS)/benzyl alcohol (BA)/H2O micelle were studied by the methods of theoretical calculation, viscosity, dynamic light scattering, and Raman spectra. The radius of the spherical micelle was about 22.86–35.85 nm. The maximum length of rod-like micelle could reach up to 250.95 nm. In the micelle, the carbon chain of SDS was not completely stretched but still bended. The total chain length of SDS molecule in the micelle appeared to be 2 nm approximately. Both the axis ratio and the shape factor of rod-like micelles increased with the increase of aggregation number. The intrinsic viscosity and relative viscosity of spherical micelles were less than those of the rod-like micelles. The relative viscosity and critical micelle concentration values decreased gradually with the increase of BA content. The second critical micelle concentration of surfactant could also be determined through viscosity method.

A novel approach to the measurement of surfactant parameters in arthropod digestive juices

In arthropods, the determination of two important parameters of digestive juices, i.e. the total surfactant concentration and the critical micelle concentration (CMC), is challenging due to small sample volumes and low surfactant concentrations. In this work, we report a successful implementation of potentiometric titrations using the surfactant ion-selective electrode (SISE) and the pyrene fluorescence method (PFM) for the determination of the total surfactant concentration and CMC in the digestive juice of terrestrial isopod crustaceans Porcellio scaber. Pooled digestive juice extracts of four (SISE) or two (PFM) animals were used per measurement run. In both cases, digestive juice extracts in 100μL of deionized water were sufficient for one measurement run. The total surfactant concentration of P. scaber digestive juice was determined to be 9.2±3.5mM and the CMC was approximately 90μM. Our work presents an important improvement towards easy CMC determination in small volume samples in comparison with the commonly used stalagmometric technique, where much larger sample volumes are usually needed. To date, the total surfactant concentration was not measured in the digestive juices of arthropods other than Homarus vulgaris, Astacus leptodactylus and Cancer pagurus, for which complex separation and analytical techniques were required. Our results obtained by SISE and PFM therefore present the first successful quantification of surfactants and their CMC in small volumes of arthropod digestive juice without prior separation or purification techniques.

Experimental study of surfactant driven nematic liquid crystal (NLC) anchoring transitions at solid surfaces: role of solid surface energy and anisotropic NLC – solid interfacial energy

The fundamental understanding of nematic liquid crystal (NLC) alignment on solid surfaces is significant for the operation of liquid crystal displays and devices. We investigate the effect of structure and concentration of surfactants on the anchoring of NLC 4-cyano-4′-pentylbiphenyl (5CB) at 5CB–solid interface. 5CB molecules undergo an ordering transition from parallel to normal anchoring near the surfactant critical micelle concentration depending on the chemical structure of the surfactant tails. In contrast to the previous studies which point to the solubilization of 5CB in surfactant micelles, our experiments indicate that NLC anchoring transition occurs due to the ability of 5CB molecules to penetrate into the surface surfactant layer. The surfactant-driven 5CB anchoring is further related to the surface tension of 5CB (γ5CB), surfactant adsorbed substrate surface energy (γsa), anisotropic 5CB–surface interfacial energy (Δγsl), and spreading coefficient (S). Our experimental results agree with the Creagh–Kahn criterion of predicting surface anchoring from relative values of γsa and γ5CB. A change in the sign of Δγsl and S is also observed along with the anchoring transition. Using a semi-empirical approach, we show that dipolar interactions dominate over dispersion interactions when 5CB molecules exhibit perpendicular anchoring at the solid–5CB interface.